Keto-enol tautomers and distonic ions: The chemistry of [C,H,,Ol radical cations. Part I

G . Bouchoux Departement de Ckimie DCMT, Ecole Polytechnique, 91 I28 Palaiseau Cedex, France

I. INTRODUCTION

During recent years, technological progress in mass spectrometry, particularly the analysis of metastable ions and collisional activation (CA) have made more accurate the determination of ion structures and reaction mechanisms. Compu- tational capabilities involving molecular orbital (MO) methods have also been developed during the same time, so that a more confident picture of the relation between structure and reactivity of isolated ions has emerged. In this context, the purpose of the present review is to recapitulate the major aspects of the chemistry of oxygenated radical cations of the general formula C,H2,0. A number of the [CnH2nO]+' ions presented hereafter results from direct ionization of al- dehydes, ketones, and unsaturated alcohols and ethers. However, it is notewor- thy that some of the most stable [C,lH2nO]+' ions have no stable neutral coun- terpart. These stable ions are ionized enols (e.g., vinylalcohol, [CH2CHOH] +'), ionized carbenes (e.g., [HCOH] +'), and unsaturated distonic (1) ions (e.g., [CH2CH2CHOH+]), whose importance in the chemistry of [C,H2,0] +' ions will be evidenced in the next paragraphs.

Few other papers have been devoted to a large survey of [CnHznO]+' ions, and they were mainly concerned with the McLafferty rearrangement (2) and the keto enol tautomerism (3). The present compilation includes literature up to October 1986; it emphasizes the energetic aspects of the isomerization and disssociation reactions.

11. [CH201+'

Three [CH20] +. ion structures may be considered: ionized formaldehyde 1, hydroxymethylene 2, and hydrated carbon 3:

1 3

Mass Spectrometry Reviews 1988, 7, 1-39 0 1988 John Wiley & Sons, Inc. 0277-7037/881010001-39$04.00

2 BOUCHOUX

Several molecular orbital treatments were devoted to these ions (4-9); they all point to the greater stability of 1 with respect to the other two isomers 2 and 3. The calculation, however, also establishes that the energy level of ionized hy- droxfmethylene 2 is only slightly above that of 1. At the MP4/6-31G**//6-31G* level and including the zero-point vibrational energy correction (ZPE), the energy difference between 1 and 2 in its trans conformation is only 14 kJ mol-* and attains 32 kJ mol-I for the cis conformer (6-9). The barrier for the cis-trans isomerization of 2 lies 80 kJ mol-* above 1. One may emphasize the large difference in behavior between radical cations 1 and 2 and their respective neutral counterparts: for- maldehyde is calculated to be more stable than hydroxymethylene by an amount as large as 233 kJ mol-* (7).

The third ionic structure, 3, until now unobserved, is calculated to lie 230-226 kJ mol-' above 1 [MP3/6-31G**//6-31G* + ZPE, (6,8)]. However, 3 is predicted to be stable toward isomerization (calculated critical energy 3 + 2 = 138-149 kJ mol-') and toward dissociation (calculated critical energy for 3 + COH' + H = 140 kJ mol-l) (8,9).

Because of its small number of internal degrees of freedom, it is not obvious whether the [CH20] +' system fulfills the conditions of statistical behavior. Ac- cordingly, the most recent experimental results concerning 1 and 2 reveal unusual trends as will now be presented briefly.

A. Dissociation of formaldehyde molecular cation 3

Formaldehyde has been studied experimentally using electron ionization (10,l l), photoionization (12-14), and photoelectron spectroscopy (15,16). A theoretical assessment of the reactions of excited cation 1 by ab-initio quantum mechanical calculation was made by Lorquet and co-workers (17).

The lowest energy dissociation pathway corresponds to the loss of one H atom to give, presumably, the formyl cation, HCO+. The appearance energy (AE) of HCO + ions measured by photoionization spectroscopy (Table I) is in good

Table I. Thermochemistry of [CH,O]+. ions. AH7298 (CHzO) IE(CHZ0) AHPm [CHDI +

kJ mol-' kJ mol-' eV H,C=O 1 - 109 (106) 10.87 ? 0.01(12,14,15) 940 2 1 HC-OH 2 962 ? 8 (13,a)

1000 (18,a) 996 t 5 (22,a)

1034 2 30 (20,a) (trans) 954 (7-9,b) (cis) 972 (8,9,b)

C-OH2 3 1168 (8,9,c)

and electron ionization with (22) and without monochromator (20).

mol-I (7-9).

"Experimental value, deduced from AE[CH,O] + * from methanol, photoionization experiment (13,18),

hCalculated (MP4/6-31G**//6-31GX + ZPE) relative energy of 2 versus 1 added to AF [l] = 940 kJ

'Same procedure as footnote b with the MP3/6-31G*'//6-31G* + ZPE relative energy of 3 (8,9).

KETO-ENOL TAUTOMERS AND DISTONIC IONS 3

agreement with the reaction: 1 * HCO+ + H occurring at the thermochemical threshold.

However, this dissociation limit corresponds to an energy region situated be- tween the ground ( X ) and the first excited (A) electronic states, where no ionic levels are detected in photoelectron experiments (15,16). Thus, the suggested interpretation is that autoionization of the X state precedes dissociation (14).

Under electron ionization, the same explanation applies for fast dissociations (occurring in the ion source of the mass spectrometer) in the 12-14 eV range (i.e./ below the A state) (11). At approximately 14 eV, a second process occurs involving formation of 1 in its A state, and internal conversion to X state occurs before

VI 0

C

U E

m

a n

+ 0 0 X

E l ('V)

Figure 1. [XCO] + ion abundances (arbitrary units) coming from metastable dissociations of [XYCO]+' (X, Y = H, D), as a function of ionizing electron energy (EI, ref. 11). (a) [HDCO]+'-t [DCO]' + H (To.s = 150 5 20 meV), (b) [HICO] +'-+ [HCO] + + H ( To.s = 130 * 30 meV), (c) [D2CO] +'-+ [DCO] +

+ D (TOS = 100 & 40 meV), (d) [HDCO]+'+ [HCO]+ + D (To.5 = 325 f 20 meV). (T0.s values obtained at EI = 15.6 eV.)

4 BOUCHOUX

dissociation (11,17). For the metastable transitions (slow decay processes), two experimental results are underlined.

First, the unimolecular dissociation

[x~CO]+' + XCO+ + x' (X = H, D)

was investigated for three isotopically substituted molecules (12) CH20, CHDO, and CD20. Figure 1 shows the abundance of the product ion as a function of the electron energy.

It clearly appears that an important H/D isotope effect pertains to both the hydrogen loss and the kinetic energy released during metastable decompositions. The second point is that the appearance energy of XCO+ ions from metastable molecular ions is = 14 eV for the three precursors: H2C0, HDCO, and D2C0 (12,14,16). This value is greater than the dissociation limit by -2 eV and sur- prisingly very close to the A state energy level. After a careful examination of these data and of intermolecular and intramolecular isotope effects, the occurrence of metastable transition is interpreted to be a slow dissociation of [X2CO] + * in its X state produced by autoionization (12).

The thermochemistry of [H2CO] +' -+ HCO+ + H is summarized in Figure 2: the quoted AH; values come from the recent photoionization study by Traeger

- 1247 A state

1 2 3 1 A E ( m ' )

Figure 2. (experimental values from refs. 11, 12, and 16).

Potential energy profile for H loss from ionized formaldehyde 1

KETO-ENOL TAUTOMERS AND DISTONIC IONS 5

B. The hydroxymethylene radical-cation [HCOH] +' 2

Radical cation [HCOH]+*,, 2, was first suggested to be the product of H2 loss from low internal energy [CH30H] + * ions by Berkowitz in a photoionization study (19). At photon energies below 16 eV and from the threshold of [CH20]+' for- mation, the main process for the loss of H2 is given by Eq. (1); a substantial abundance for process described by Eq. (2) is observed only at energies greater than 17 eV.

Three years later, Wesdemiotis and McLafferty showed that 2 may also be pre- pared by loss of C2H4 from ionized cyclopropanol and characterized using colli- sional activation (CA) mass spectrometry (21). It was more recently observed that the CA mass spectra of [CH20]+' ions from methanol and cyclopropanol are identical, thus confirming that [CH30H+'] yields essentially ions of structure 2 (22). The measurement of AE[CH20] +' from methanol by photoionization (13,18) and by electron ionization (20,22) leads to a range of A% [CH20]+' situated between 962 kJ mol-l to 1034 kJ mol-' (using A% (CH30H) = -201 kJ mol-'). Assuming that these measurements pertain to the structure 2, a reasonable agree- ment with the MO predictions is only obtained (Table I) by using the lower limit: 962 2 8 kJ mol-'.

The dissociation of metastable [HCOH]+' ions coming from CH30H was studied by several authors (9,20,22-24). Beynon, Fontaine, and Lester (23) and Donchi and Derrick (24) reported the observation of a large isotope effect upon energy release for H or D losses from [XCOX]+' (X = H, D). Whereas T0.5 is approximately 280 meV (24) for H losses [HCOH]+'+ CHO+ + H and [DCOH]+' + CDO+ + H , this value is about twice (To.5 = 450 meV) (24) for D losses, [HCOD] +' - CHO+ + D and [DCOD]+' - CDO+ + D .

These differences are discussed in dynamical terms by using a mechanical model and assuming a 0-H(D) bond cleavage from structure 2. Momigny and co-workers (20) and more recently Holmes and co-workers (9,22) observed that the metastable peak for loss of H from [HCOH] +' is composite. Expressed as peak heights, the ratio of the low/high kinetic energy release components is 1/5. Starting with met- astable [DCOH]+' ions (from CD30H), the two competing channels are disclosed as H and D losses:

DCO+ + H

HCO+ + D

P [DCOH] +'

The H loss is accompanied by a flat-topped peak ( To.5 = 370 meV); for D loss, = 160 meV) is observed (20,22). The first expla- a broad "gaussian" peak

6 BOUCHOUX

nation is that two distinct daughter ions, DCO+ and COH+, are produced from [DCOH] +' (22). However, by using a VG-ZAB-4F mass spectrometer and its ability to identify by CA the product of a metastable dissociation, it is now known that a unique structure of XCO+ is produced (9). The other experimental indications concerning decomposition of metastable [DCOH] +' ions are: (i) H loss is favored by a factor of approximately 5 over D loss (20) and (ii) the appearance energies of HCO + and DCO + from metastable [DCOH] +' are similar and correspond to a transition state enthalpy of 1130 r: 60 kJ mol-' for the D loss and 1100 * 50 kJ mol-I for the H loss (20). (A common value of 1140 * 20 kJ mol-1 is quoted in ref. 22.)

Isomerization of ionized hydroxymethylene 2 into 1 may be proposed to ra- tionalize the formation of a formyl cation by elimination of a hydrogen atom of [HCOH]+' from either the carbene or the hydroxyl [Eq. (3)].

+. /o ,c-0

+. ,c=o - c-

H H 1 2

@O HZO HGO (3)

The loss of hydroxyl H may occur by direct 0-H cleavage from 2 or after 1,2 transfer to the carbon atom producing a vibrationally excited formaldehyde cation 1. The loss of carbene H can only proceed after isomerization 2 + 1. The cor-

Figure 3. of [CH20]+' ions (ab-initio MP3/6-31G**//6-31G + ZPVE, refs. 8 and 9).

Calculated potential energy diagram for isomerizationldissociation

KETO-ENOL TAUTOMERS AND DISTONIC IONS 7

responding part of the potential energy surface has been examined with the aid of the ab-initio MO calculations at the MP3/6-31G**//6-31G* + ZPE, (8,9) level of theory. The results are illustrated in Figure 3. Note that the participation of struc- ture 3 is excluded from this description due to the high transition state energy for 2 += 3 (252 kJ mol-* with respect to 1). The very different activation energies for H addition at the carbon and oxygen of HCO +, leading to 1 and 2, respectively, have been established and this phenomenon seems to be of more general appli- cability (60,61). The addition to the carbon atom is favored because the low- est unoccupied molecular orbital (LUMO) possesses a larger coefficient at this center (61).

It appears from examination of Fig. 3 that direct 0-H cleavage and 1,2-H mi- gration 2 -+ l require comparable amounts of energy. In addition, the calculations match the experimental AE determinations nicely. Thus, one interpretation is that there is competition between the two reactions (9). The large kinetic energy release is associated with the 0 - H cleavage, whereas the narrow signal corresponds to the reaction involving rearrangement to ionized formaldehyde 1.

In this case, however, only the original carbene hydrogen atom seems to be lost, suggesting, according to Bouma et al. (9) that the rearrangement fragmen- tation process 2 -+ 1 += HCO+ + H is non-ergodic.

111. [C2H40]+'

The chemistry of [C2H40] +' ions presents two interesting aspects: it provides the first possibility of ring opening of an ionized cyclic ether and the simplest case of keto-enol tautomerism. On the other hand, the small number of atoms still makes possible an accurate ab-initio molecular orbital treatment. From this latter point of view, the most complete description of the [C2&0]+' ions was done in 1979 by Bouma, Macleod and Radom (25). This molecular orbital study establishes the order of stability of the eIeven isomers 4-14. Using the RHF/631G*//STO-3G (25) relative energies and the experimental AH& (5) = 820 kJ mol-I, one obtains estimates of the AH;298 for structures 4-14; these values are indicated in brackets in Table 11.

0- CH

4 5 6 7

9 10 11

. * CH l= c - OH 1 &it- CH,- 0

1 2 13 14

H - C-CH20H

8 BOUCHOUX

Table 11. Thermochemical data of neutral and ionized CzH40. AH7298 (C2H40) IE(C&W) mi298 [c2ho l+ 'Q

Structure kT mol-I eV ki mol-I 4 CHFCH-OH - 125 b

- 129 b - 132 b - 136 b

5 CH3-CHO - 166 (106)

6 CH,--C-OH 7 CH-O-CHZ 8 H-C-OCH3 - 9 CH2CHZO

10 CH=CH-OH2

- 53 (106)

9.14 (37) 757 2 4 (26) 9.18 (40) (no) 9.22 (38) 9.26 (39)

10.21 (26) 820 10.22 (27,47) (820)

(890) (904)

(950) (979)

865 f 20 (30)

10.56 (26,32) 967 2 4

11 'CH2CHOH (987) 12 CH-C-OH, (1000) 13 CH-CH-0 (1004) 14 CH-CH-OH (1079)

"In brackets, estimated AH& in kJ mol-' using ab-initio relative energies of ref. 25 (6-31G*//STO-3G) added to AH;298 [CH,CHO]+. = 820 kJ mol-I.

bCalculated using AH: [CHrCHOH]'. = 757 kJ mol-' (26) combined with the related IE(C2H40) value (37,34,38,39).

Some of these values were refined by using molecular orbital calculations with more sophisticated basis sets and including the electron correlation effect, as will be discussed later. However, the differences in relative energies agree to within 15 kJ mol-l from RHF/6-31G*//STO-3G to UHF/MP3/6-31G*//4-31G(ZPE).

Thus, a satisfactory outline on the structurelstability relationship of [CzH40] +.

radical cations emerges from ref. 25. The first point arising from this study is that the most stable ion is ionized vinyl alcohol. The study confirms the reversed order of stability of the keto-enol tautomers 4-5 with respect to their neutral counter- parts, observed experimentally and discussed theoretically (3b,28). The second observation is that, besides the "classical" structures 4, 5, and 9, there are two other types of stable species: the carbenoid structures 6 (CH3-C -OH) and 8 (H-

COCH3) and the C . . . C open form of ethylene oxide 7 (CH2-O-CHz). Some of these structures were experimentally characterized. In 1975, Van de

Sande and McLafferty (29) presented the CA spectra of [C*H40] + * ions produced by electron ionization of 27 precursor molecules. The three "classical" cations 4, 5, and 9 were identified. The two latter cations were produced by direct ionization, whereas 4 originates from dissociation of aldehydes (via a McLafferty rearrange- ment), cyclic alcohols, alkyl vinyl ethers, aliphatic epoxides, 2-haloethanol, glyc- erol, and 1,4-butanediol.

In a more recent study, Terlouw et al. (30) presented part of the CA spectrum of the [C2H40]+' ion generated via metastable dissociation of pyruvic acid:

.+ .+ +

- co2 [CH3COCOZH] +' [C2H40lf'

KETO-ENOL TAUTOMERS AND DISTONIC IONS 9

Following an earlier suggestion (31), the authors proposed the hydroxymethyl- carbene 6 to be the structure, of this daughter ion.

No assignment of a CA spectrum is actually established for [CH2-O-CH2] +' 7, although this ion was identified as a stable species in ion cyclotron resonance experiments (32-35).

Unimolecular dissociations of [C2H40] +' ions were studied by Pritchard (36) and by Holmes and Terlouw (31). Identification of structures 4, 5 , and 9 (and mixtures of these structures) was made using the metastable peak profile for [C2H40]+'+ C2H30+ + H dissociation (31).

The experimental thermochemistry of [C2H40] +' ions is actually limited to four structures 4, 5, 6, and 9 (Table 11). The measurement of the appearance energy of [C2H40] +' ions produced by dissociative ionization of cyclobutanol, butanal, and 3-methylbutanal was made using energy selected electrons (26). A heat of formation AH; [C2H40]+' = 757 & kJ mol-' is derived and assigned to ionized vinyl alcohol 4. A second method of preparation of 4 was used in both mass spectrometric (37-39) and photoelectron (40) experiments. It consists of direct ionization of neutral vinyl alcohol generated by pyrolysis of cyclobutanol (37,39,40) or exo-5-bornenol(38). The measured ionization energy (IE) lies between 9.14 and 9.26 eV using electron spectrometry and 9.18 eV (40) (adiabatic value) in the photoelectron spectrum. By combining IE = 9.14-9.26 eV and AH; (4) = 757 kJ mol-', an estimate of AH; (neutral vinyl alcohol) = - 128 kJ mol-I with a probable error of k10 kJ mol-l is obtained. Note that this allows the attribution of a 0-(Cd)(H) increment in Benson's formalism (140) equal to -190 k 10 kJ mol-' as suggested in ref. 37.

The measurements of IE of acetaldehyde and ethylene oxide combined with AH;(CH3CHO) = - 166 kJ mol-' and AHT(ethy1ene oxide) = - 53 kJ mol-' lead to the estimates of the heats of formation of 5 and 9 presented in Table 11. The mono-energetic electron ionization (26) and photon ionization (27) give the same IE values within only a few hundred eV, thus giving AH; of 5 and 9 with a precision better than k 4 0 kJ mol-'.

The heat of formation of 6 [CH3COH]+' was also proposed by Terlouw et al. (30), assuming that this ion is obtained pure by dissociative ionization of pyruvic acid. By using a comparative method (41), the appearance energy of the metastable peak [CH3C0C02H] +' + [C2H40 ] +' + C02 was measured; the value (10.7 2 0.2 eV) leads to an estimate of AH; (6) to be 866 & 20 kJ mol-'.

A. The ring opening of ethylene oxide

Ion cyclotron resonance mass spectrometry (32,33,35,42,43), time-resolved pho- toionization mass spectrometry and photoelectron spectroscopy (32) have been employed to study the ethylene oxide ion-molecule reactions. A major conclusion is that a structurally modified [C2H40] +' specie is collisionally relaxed and reacts with the neutrals encountered (ethylene oxide itself, nitrogen bases such as pyr- idine and nitriles) by transferring CH;. The formation of the ring-opened struc- tures 7 and 13 by C-C (32,35,42) or C-0 (43) bond cleavage may explain these observations [Eq. (4)]:

10 BOUCHOUX

13a 13 b (4)

A first experimental indication in favor of a C-C bound rupture was given by Bouma, Macleod and Radom (33). They observed that, like ethylene oxide, 1,3- dioxolane 15 produces a m/z 44 ion that transfers a CH2+ group to acrylonitrile in the ICR cell. Moreover, by using [4,4,5,5-d4]-lf3-dioxolane, 16, only an m/z 46 ion is observed, indicating the exclusive formation of structure 7, [CD20CH2] +'

and not its isomer 13. Another attempt to characterize structure 13 was made using ethylene carbonate as a potential precursor (34). By using deuterium and '*O labelling of compound 17 and double resonance experiments in the ICR spec- trometer, it was demonstrated that [C2H40] +' ions possess two equivalent meth- ylene groups in clear disagreement with the formation of ion 13. The photodis- sociation spectra of [C2H40] +' ions from ethylene oxide, 9, and 1,3-dioxolan, 16, obtained in an ICR spectrometer exhibit the same characteristics and were inter- preted by the formation of vibrationally excited ions of open structure 7 (35).

The photoionization efficiency curves for the molecular and fragment ions of 1,4-dioxane 18 were determined by Fraser-Monteiro et al. (44). The [C2H40] +' ion is of lowest appearance energy, and assuming that in the reaction: lf4-dioxane [C2H40]+. + C2H40, the neutral is acetaldehyde, the authors derived AH& [C2H40]+' = 883 -+ 8 kJ mol-' and observed that 7 may well be the structure of the ion. Their experimental A q 2 9 8 is very close to the estimate of 890 kJ mol-' from ab-initio calculations (Table 11).

In a different context, the ring opening 9 + 7 was recently established for the radical cation derived radiolytically in the condensed phase (45,46). The electron spin resonance spectra of 9 and higher homologs were interpreted to indicate the occurrence of the planar structure 7a in which most of the spin density is localized by the two parallel carbon p . orbitals.

The opening of the oxirane ring was reinvestigated recently in two molecular orbital studies (48,49). Both studies include electron correlation effects and con- sider the two conformers 7a and 7b. The first conclusion is that C-C bond elon- gation of 9 needs little energy, if any. At the MP2/6-31G*//6-31G* level, the acti- vation energy is 15 kJ mol-' and may disappear at higher levels of theory (49). The product of this ring opening is the planar Czv structure 7a which lies 82 kJ mol-I below 9 (48). A full vibrational analysis of the rotamer 7b indicates that this

KETO-ENOL TAUTOMERS AND DISTONIC IONS 11

structure is in fact the transition state for the methylene group rotation (47,48). Finally, a reexamination of the C-0 ring-opened ion 13 (50) confirms the earliest results (25): 13a is less stable than 9 (its energy level is 253 kJ mol-' higher than 4 at the 6-31G*//4-31G level) and 13b is not a minimum of the potential energy surface and collapses to 9 during geometry optimization. The energetic aspect of Scheme 2 is illustrated by Figure 4, in which the values in brackets refer to the above mentioned ab-initio results combined with the experimental heat of for- mation of 9.

The AE for the C2H3O+ fragment ion coming from 9 is equal to 11.53 2 0.05 eV (51); this value leads to a transition state heat of formation of 1058 kJ mol-' (Figure 4). In the same work, the source-produced C2H30+ ions are attributed a mixture of acylium CH3CO+ and oxiranyl CH2CHO+ structures.

One may observe that the C-0 open form 13a is below the transition state energy for H loss and may reasonably account for the CH3CO+ formation.

Another comment results from the partial potential energy profile of Fig. 4. By considering the very low-energy barrier for isomerization 9 + 7a and the large energy range between 9 and the dissociation threshold (i.e., -- 1 eV), one con- cludes that the CA spectra should reflect a mixture of structures 9 and 7, and perhaps also other low-energy [C2H40]+* forms such as 4, 5, 6, 8, . . . that may be connected to them. This is indeed observed (29), and it also extends to uni- molecular dissociation [C2H40] +' + C2H30f + H, for which composite metasta- ble peaks were observed (31).

1. l*

Figure 4. bracket, an-initio MP2/6-31G* energy values, refs. 48-50).

Potential energy profile for ring opening of ethylene oxide 9 (into

12 BOUCHOUX

B. Acetaldehyde-vinyl alcohol tautomerism

The major dissociation pathway of metastable molecular ions of vinyl alcohol, 4, and acetaldehyde, 5, leads to C2H30+ + H; only weak signals are associated with eliminations of HZ, CH;, and HCO (31,33). At shorter lifetimes, the CH; loss becomes more competitive with the hydrogen atom elimination (27,38). These two dissociation channels afford most of the information about the keto-enol tautomerism of the molecular ions 5 and 4.

The loss of H' from metastable ion 5 is accompanied by a narrow gaussian peak (To.5 = 43 meV) (31). Deuterium labelling demonstrates that the hydrogen atom is eliminated from C(l) exclusively, both in the source or in the field-free region of the mass spectrometer (31). In addition, the resulting fragment ions were identified as acylium CH3C0 + cations either for the source produced ions (51) or in the case of metastable precursor 5 (52). Finally, the appearance energy, AE [CZH30]+, was also determined; the values [10.67 eV (53), 10.90 eV (27), 11.00 eV (41)] obtained by photoionization spectroscopy (27,53) or by measuring the AE of the metastable peak by using electron ionization (41) lead to the range for the threshold energy: 863-895 kJ mol-'. These estimations are only slightly above the heat of formation of the products (875 kJ mol-*) and thus demonstrate that the dissociation 5 - CH3CO+ + H occurs with a small reverse activation energy if any (Figure 5).

The formation of HCO+ ion from metastable molecular ion 5 occurs exclusively by elimination of the intact C(2)H3 group (31). However, a photoionization study reported that the major losses of CH; from [CH3CDO]+' and of CD3 from [CD3CHO]+' are also accompanied by 10% of CH2D and CDzH eliminations, respectively (27). The appearance energies of these two fragmentation routes differ by ca. 80 kJ mol-'. The threshold energy for the major CH; loss is equal to 970(53)- 990(27) kJ mol-' in good agreement with AH; [HCO]+ + AH;(CH3) = 973-957 kJ mol-'. The minor CH3 loss is associated with a threshold energy of 1057 kJ mol-' (Fig. 5) which may not be attributed to the formation of another product ion ( A q [COH]+ + AH4f(CH3) = 1110 kJ mol-I is largely higher in energy) but to the critical energy needed to initiate the hydrogen atom exchange in ionized acetal- dehyde (27). This mechanistic aspect of the exchange will be discussed after examination of the case of vinyl alcohol.

The metastable ion peak associated with the fragmentation 4 C2H30+ + H is dish-topped (To.5 = 580-620 meV) (31,39) and thus strongly different from that of precursor 5 . Only the hydroxylic H elimination is observed by dissociation of metastable ion 4 (31,57); however, at higher internal energies, the hydrogen atom comes not only from the hydroxyl group (68%) but also from C(1) (25%) and C(2) (7%) (38). In both cases, the hydroxyl hydrogen eliminations give the acylium ion CH3COf as indicated by CAD experiments (51,52). The AE of the metastably generated ions CH3CO+ was measured first by Burgers and Holmes (41) and, more recently, by Lifshitz and co-workers (39), giving the threshold energy values of 994 and 960 kJ mol-', respectively. A similar determination on the C2H2DOf ions coming from D-l-ethenol in the source of the mass spectrometer (38) leads to a threshold energy of 963 kJ mol-' in clear agreement with ref. 39. On the basis

KETO-ENOL TAUTOMERS AND DISTONIC IONS 13

Figure 5. Dissociation energetics of ionized acetaldehyde 5, experimental AE values taken from refs. 27, 41, and 53 (dotted line : threshold for %H(1)H2(2) elimination).

of these AE measurements, it appears that the transition state for 4 + CZHJO+ + H is 85-120 kJ mol-' above the products CH3CO+ + H .

This result may be interpreted by a rate-determining isomerization step 4 -+ 5 followed by the simple C-H bond cleavage 5 -+ CH3CO+ + H .

Three keto-enol isomerization mechanisms 4 -+ 5 must be considered: (i) a 1,3- sigmatropic shift of one H atom [Eq. (5), process (a)], (ii) two successive 1,2 hydrogen migrations via the C . . . 0 open form of ethylene oxide 13 [Eq. (5), process (b)], and (iii) two successive 1,2 hydrogen migrations via hydroxyme- thylcarbene 6 [Eq. (5), process (c)].

14 BOUCHOUX

Clearly, deuterium labelling results exclude the possibility of H loss from 4 after processes (a) and (b) [Eq. (5)]. However, it cannot be established from these experiments whether the hydrogen atom is expelled from the tautomer 5 or di- rectly from the intermediate structure 6 . This point was examined using molecular orbital calculations.

A first investigation of mechanisms (a), (b), and (c) with the semi-empirical MIND013 (55) method reveals that (c) is the energetically favored process (56). However, MIND0/3 overestimates the stability of ion 6 and, more recently, two ab-initio investigations were undertaken (50,57,58). The main conclusion of these studies is that the keto-enol tautomerism 4 Ft 5 may involve two mechanisms: the same amount of energy is required by the 1,3-hydrogen migration (a) (234 kJ mol-*, with respect to 4, MP3/6-31G*//4-31G + ZPE, refs. 50 and 57) and by the second step of the process (c); that is, the 1,2 hydrogen transfer 6 + 5 (239 kJ mol-' with respect to 4, MP3/6-31G"//4-31G + ZPE, refs. 50 and 57). Mechanism (b) is excluded because the energy level of the intermediate ion 13 relative to 4 (253 kJ mol-', 6-31G*, ref. 50) is too high.

It may be further concluded that the loss of H from ionized vinylalcohol 4 is not preceeded by its isomerization to ionized acetaldehyde 5, but the H loss must occur from the ionized hydroxymethylcarbene 6:

12-H'

[CH2CHOH]+' * [CH,COH]+'+ CH3COf + H (4 migration

The calculations are in accord and indicate that (d) is the lowest energy route connecting 4 and the dissociation products CH3CO+ + H , the rate-determining step being the 1,2-H migration 4 6 (50,57,58). The corresponding potential energy diagram is presented in Figure 6. The exact assignment of the highest energy transition state for process (d) is not yet clearly established from experi- ment.

The observation that the release of kinetic energy during the dissociation to CHKO' + H is slightly greater for 4 (To.s = 600 ? 20 meV) than for 5 (TOS = 500 ? 20 meV) (Fig. 5) may be interpreted by a 4 -+ 6 rate-determining step (31). The loss of D from [CH2CHOD]+' exhibits a very small isotope effect ( k ~ l k ~ = 1.07), whereas a much greater effect was observed for the H loss from [CH,CDOH]+' ( k ~ l k ~ = 1.28) (38). The conclusion is again that the rate-deter- mining step involves rupture of the C(l)-H bond as in 4 * 6.

In contrast, a recent observation was made by Burgers and co-workers (59), who estimated, from AE measurements, that simiIar reverse activation energies are associated with dissociation of 4 and 6. This would imply that the two steps of (d) need the same amount of energy or that the rate-determining step is the dissociation 6 -+ CH,CO+ + H .

The loss of a methyl group from metastable ion 4 is only barely detectable (31). The extent of this fragmentation is, however, comparable to the H elimination in the 75 eV mass spectrum of vinyl alcohol (38). Deuterium labelling results indicate that the main fraction of CHO+ ions (75%) retains the C(1) hydrogen atom; the AE of CDO+ ion coming from D-1-ethenol is found to be equal to 12.96 k 0.07 eV, thus allowing the estimate of 1120 kJ mol-* for the corresponding threshold

KETO-ENOL TAUTOMERS AND DISTONIC IONS 15

Figure 6. Potential energy diagram for isomerization and dissociation of ionized acetaldehyde, 5 and vinyl alcohol, 4. Experimental AE values are taken from refs. 27, 39, 41, and 53 and (into brackets) MP3/6-31G*//4- 31G + ZPVE results from refs. 50 and 57.

energy (38). This energy barrier has been attributed to isomerization of 4 to 5 via the 1,3-hydrogen migration (a) [Eq. (5)] (38). This experimental determination is largely higher than the estimate of 991 kJ mol-l based on MP3/6-31G*//4-31G + ZPE molecular orbital calculations (50,57).

Returning now to the case of acetaldehyde 5, two channeIs may account for the elimination of a methyl group containing the C(l) hydrogen: 5 + 4 + 6 - , 5 + H C O + + C H 3 0 r 5 - 6 + 4 + 5 + H C O + + CH3.Theex- perimental threshold energy of 1057 kJ mol-l (27) is probably associated with either 5 - 4 or 5 + 6 hydrogen migrations. The two routes cannot be distin- guished as indicated by the similar values of the theoretical critical energies (50,57).

One should observe that these AE measurements provide two different esti- mates of the transition state energy for CH3 loss after 1,3-H migration (and isom- erization 5 + 6 in the case of 5). This may be due to different competitive shifts as long as the lowest energy channel is the H elimination from both 4 and 5. Thus, the 1120-1057 kJ mol-* range of threshold energy certainly constitutes an upper limit for the transition state energy of the 1,3-hydrogen migration 5 - 4 and for the 1,2-H transfer 5 -+ 6 (see Table VII for a comparison with other systems).

IV. [C3H60]+'

A thorough examination of the stable [C3H60] +' ions was done in 1975 by Van de Sande and McLafferty (62); the seven structures 19, 20, 21, 23, 25, 33, and 35 may be characterized from their CA mass spectra.

16 BOUCHOUX

OH *. OH +.

L 20

A 19

+. PI +.

/ 9 / / 23 24

+

d b m

21 20

pi- 21 22

+. oL CH,-C-OCH,

25 26

+. a% C,H,-C-OH

+ 20 30

+. H -C- OC&I,

33 34

4 In a theoretical investigation of the [C3H60]+' system, a number of unusual

structures in addition to the above mentioned ions were found to be of remarkable stability (63). These ions are: (i) the three, as yet not identified, carbene ions 26, 30, and 34 and (ii) distonic ion 28 and ring-opened structures 29 and 32.

Two other distonic ions of special interest are also given as 22 and 27. Their stability, relative to the carbonyl homolog 25, was estimated by ab-initio calcu- lation at the 3-21G//3-21G level.

The available experimental thermochemical data or estimations based on ab- initio calculations of [C3H60] +' ions are presented in Table 111. It may be observed that, with the exception of 24, the calculations reproduce experiment to within tr 30 kJ mol-'.

The discussion concerning the chemistry of [C3H60]+' ions will be divided in three sections, each one dealing with a limited set of connected structures and illustrating a particular mechanistic aspect of their reactivity. The keto-enol tau- tomerism of acetone 21 @ 19 is first illustrated by a summary of the abundant literature covering this process. The keto-enol tautomerism of propanol, 25 S 20, is discussed in conjunction with structures 22, 24, and 31. Finally, the open and closed forms of oxetan, 32, 33, and of propene oxide, 29, 35, radical cations are examined together with methyl vinyl ether structure 23.

KETO-ENOL TAUTOMERS AND DISTONIC IONS 17

Table 111. Thermochemical data of neutral and ionized C3H60 species. my (C3H60) IE(C&O) AV [C3H60]+

Structure kJ mol-' eV kJ mo1-I - 177 (c) 8.68 k 0.05 (64) 660 (66)

- 158 (c) - 169 (c) (trans) 8.64 2 0.02 (65) 665 (66)

19 CH,C(OH)CHz - 171 (c) 8.61 & 0.10 (39) (682)

- 174 (c) (cis) 8.70 f 0.03 (65) (695)

(703)

(782) 23 CHpCHOCH3 - 100 (106) 8.95 & 0.05 (107) 764

(736)

(895)

(773) - (791)

(778) (795) (795) (803)

(791)

8.48 ? 0.05 (37)

20 CH3CHCHOH

21 CH3COCH3 - 218 (106) 9.70 k 0.01 (107) 720

22 CH2CHZCHOH - - 736 (67) 742 (b)

767

773 24 - 111 (68) 9.10 i 0.05 (68)

25 CH2 CHZCHO - 187 (106) 9.95 f 0.05 (107) 26 CH-C-OCH3 - 27 CH2CHCHOH2 - - 28 CHzC(OH2)CHz - 29 CH3CHOCH2 - - 30 CzHSCOH - -

31 CHi==CHCH,OH - 134 (68) 9.67 ? 0.05 (68)

-

799

32 CH2CHzOCHz 828 (44) (828) - '848 (824) (858)

33 .CHZCHZOCHz - 81 (106) 9.63 & 0.03 (107)

34 HCOC2HS - - - 894 (887) 35 CH 3CHCHZO - 95 (106) 10.25 (69) . .

"In brackets AH: estimated using ab-initio 4-31G//STO-3G energies (63), (except for 22 and 27, where RHF/3-21G//3-21G total energies are used, (22 = - 190.554018; 25 = - 190.557699; and 27 = - 190.556720 Hartrees). The energy scale is adjusted to ATzsa [25]" = 773 kJ mol-I.

bEstimated using the thermochemical cycle described in the Appendix. 'Calculated using AV [CH,C(OH)CH,]+. = 660 kJ mol-' (66) or AH7 [CH3CHCHOH]+ = 665 kJ

mol-' (66) combined with the related IE(C3H60) value (64,39,37,65).

A. Keto-enol tautomerism of acetone

An impressive number of studies were concerned with ionized acetone 21 and its enol tautomer 19 during the last 20 years. However, few of them are concerned with the metastable molecular ion 21 produced by electron ionization of acetone (70-73). Only one peak is found to be of significant intensity in metastable spectra and it is due to the loss of a methane molecule [CH3COCH3] +' + [CH2CO] +'+ CH4.

A conspicuously small amount of kinetic energy is released during this frag- mentation (To.5 = 3 4 meV) (71,77). A complete analysis of the corresponding energy release distribution demonstrates that metastable ions 21 dissociate with a very small amount of (3 kJ mol-') internal energy above its dissociation limit

18 BOUCHOUX

(77). There is also an indication from a photoionization appearance energy de- termination that the loss of CH4 occurs close to its thermochemical threshold (77,78). (The heat of formation of the products is 791 kJ mol-’, using AH; (ke- tene) = - 61 kJ mol-’, IE (ketene) = 9.61 ‘-c 0.01 eV and AH;2g8 (CH4) = - 75 kJ mol-I . )

A second reaction, observed to not be significant for the metastable decom- positions, is the loss of a methyl radical: 21 + CH3CO+ + CH3.

This is the most important of the fast dissociation processes, and it was studied by photoionization (53,79) and photoelectron (69,74-76) spectroscopy. Recent col- lisional activation studies show that the acetyl ion CH3CO+ is produced in a pure state in the source of the mass spectrometer by the dissociative ionization of acetone (51,52). Formation of CH3CO+ + CH: at the thermochemical threshold is verified by photoionization appearance energy measurements (53,69), which yield a transition state energy within 2 kJ mol-’ of the heat of formation of the products (i.e., AH& = 804 kJ mol-I).

In summary, no reverse activation energy seems to be associated with the two most facile dissociation channels of 21 (i.e., losses of CH;. and of CHI. Although favored by its slightly lower critical energy (AE [CH2CO]+’ < AE [CH3CO]+ by = 0.03 eV, ref. 77), the loss of CH4 is observed only in a very narrow energy range (-0.15 eV) (77). This may be interpreted by a less favorable frequency factor for CH4 loss (rearrangement reaction) than for CH3 loss (simple rupture) (71,72,77). This view seems reasonable, although the detailed mechanism of alkane loss from metastable ketone molecular ions is not known (78,80).

The metastable fragmentation of the ionized enolic isomer of acetone, 19, has received much attention (37,39,64,66,70,72,73,77,81-90,92). In most of the inves- tigations, 19 was generated from various methylketones via a McLafferty rear- rangement or by dissociation of ionized 1-methylcyclobutanol. Several authors reported their studies on ion 19 formed by ionization of the neutral enol. Holmes and Lossing (37) and, more recently, Lifshitz and co-workers (39) prepared 2- hydroxypropene by low-pressure pyrolysis of 1-methylcyclobutanol. Turecek and Hanus (64) prepared neutral 2-hydroxypropene by a retro-Diels-Alder reaction from 5-exo-methyl-5-norbornenol36 at 800°C and under low pressure (10-6 torr):

The metastable ion 19 loses one methyl group, and no unimolecular elimination of CH, occurs. The distribution of kinetic energy releases is of a bimodal character; the high-energy component is ca. 7 times greater than the low T component [from ratio of peak areas, (77)].

Several careful examinations of the CA spectrum of C2H3O+ ions reveal that the acetyl structure CH3C0 + is exclusively produced at the threshold (51,89,90). At higher energy (70 eV), approximately 10% of the C2H30+ ions consist of the CH2COH+ structure.

Appearance energy measurement of the C2H3O+ ions gives an apparent heat

KETO-ENOL TAUTOMERS AND DISTONIC IONS 19

of formation of the products that is greatly in excess of the sum AH;298 [CH30]+ + A q 2 9 8 (CH3 ) = 804 kJ mol-l. The value lies between 890 (64,92) and 930 (37,77,88) kJ mol-'. In addition, Turecek and Hanus (64) interpreted a break in the C2H3O+ ionization efficiency curve as formation of structure CH3COH+ at 1000 k 4 kJ mol-'.

All these results were interpreted by a ketonization process 19 4 21 (84,85), the acetone molecular cation 21 being the precursor of CH3CO+ ions, whereas the direct methyl loss from 19 leads to the CH2COH+ structure. Isomerization 19 + 21 is the energy-requiring step responsible for the high value of AE [C2H30] +

from 19. The formation of a vibrationally excited ion 21 also explains the inability to observe the CH4 loss, the low-energy dissociation channel of the acetone radical cation (see above).

What is not explained is the bimodal aspect of the kinetic energy release dis- tribution for methyl loss, although only one product ion, CH3CO+, is observed. Furthermore, specific deuterium (64,73,84,85,89) and 13C (64,77,86,87,90) labelling experiments demonstrate that the two methyl radicals are eliminated from the excited "symmetrical" ion 21 at unequal rates. Moreover, it is the newly formed CH3 [encircled on Eq. (6)] that is lost more readily.

+.

19 21

+. 9

19 21

Source 10 % 34 % 56 %

70 .V)

m * 0 % 42 % 50 % (6)

This observation remains valid even taking into account the further compli- cation arising from the occurrence of the transenolization process 19 -+ 19':

19 1 9 '

in competition with the fragmentation via 21 (64). No alternative mechanisms other than direct 1,3-hydrogen transfers have been retained for isomerization 19 4 21 and 19 --+ 19' (56,89,90). In particular, a double 1,2-H migration (93) 19 4 [CH3CH(0)CH,] +' .+ 21 was ruled out on the basis of MIND0/3 calcula- tions (56).

However, it must be remembered that the lower homolog [CH2CH20]", 13a,

20 BOUCHOUX

is 250 kJ mol-’ higher than its enol isomer [CHzCHOH] +’, 4. This value is not far from the experimental energy barrier for 19 + 21 isomerization, and clearly a more detailed theoretical investigation is needed for this system.

Nevertheless, a potential energy diagram associated with the isomerization of 19, 19‘, and 21 may be proposed (Figure 7). The corresponding data come from Tables I11 and VIII (to be published in Part I1 of this article), the AE CHzCOH+, and the estimate of the activation barrier 19 + 19’ are from ref. (64).

The remaining question concerns the different rates for elimination of the two methyls from 21 coming from 19 and the two components of the corresponding kinetic energy release distributions. These observations have been attributed to an incomplete vibrational energy relaxation of the ”chemically” excited acetone ion 21 (64,73,77,85-90); a detailed review of the subject and a consideration of the possible intervention of the first excited state (A, Fig. 7) of 21 was done by Lifshitz (88). Briefly, the isomerization 19 + 21 produces a highly excited acetone ion that decomposes more rapidly than the vibrational energy randomizes. The 1,3-H migration 19 + 21 causes the vibrational energy to be localized in a few modes. In particular, a C-C antisymmetric stretching is probably strongly involved because the C=C double bond in 19 becomes the new single C-C bond in 21. This excitation may induce loss of the “new” methyl group before vibrational energy randomization occurs. In line with this reasoning, the large excitation energy localized in the reaction coordinate would give rise to a large release of kinetic energy. Thus, the “chemically” excited ions 21 that escape vibrational

6 6 0 6 6 0

9 2 # .. A stat.

cn,Eo + ‘cr

I 0 4

21 I

Figure 7. Energy diagram for keto-enol tautomerism of ionized acetone 21, experimental data from refs. 37,53, 64, 69, 77, 88, and 92). The AE [CH2COH]+ and the estimate of the activation barrier 19 19’ are from ref. (64).

KETO-ENOL TAUTOMERS AND DISTONIC IONS 21

energy randomization are responsible for the high-energy component of the bi- modal kinetic energy release distribution.

B. Keto-enol tautomerism of propanal

The exclusive decomposition of metastable molecule ion 25 and its enol 20 [ionized 1-hydroxypropene produced either via a McLafferty rearrangement of 2- methylpentanol(56,72), dissociative ionization of 3-pentanol(94) or by retro-Diels- Alder decomposition of 3-methylborn-5-en-2-01 (97)] is the loss of one hydrogen atom. A similar observation was made for metastable molecular ion of allyl alcohol 31 and for the [C3HGO]+' ion originating from cyclopentanol after C2H4 loss; the latter was originally suggested to be the distonic ion 22 (72), but more recent CA results seem to indicate that such an ion is not produced alone in the dissociation of metastable cyclopentanol (95).

The loss of H from the metastable ion of propanal, 25, involves only the carbonyl bonded H atom (94) and is accompanied by the release of a large amount of kinetic energy = 160-180 meV (56,72,94,97)]. Curiously, the measurement of AE [C3H50] + from propanal by using photoionization spectroscopy points to a thresh- old energy precisely equal to the heat of formation of C2H5COf + H (98). The CAD spectra of c&o+ ions produced at low ionizing energy (97) or from met- astable ion 25 (91) are consistent with the generation of C2H5CO+ structure; at high ionizing energy (70 eV), a non-negligible fraction of the CH2CHCHOH + ions is competitively produced (97).

The relative abundances of metastable peaks for the losses of H and D from specifically deuterated 20 ions (94,97), and 31 ions (94), show that the loss of the hydroxylic hydrogen is only a minor process (less than 7% of the total H elimi- nation). In addition, for both precursors 20 and 31, positions C(l) through C(3) are involved in a nearly statistical manner; a slight excess of H loss from the methyl group C(3) is observed in the case of 20. This extensive randomization of the labelled atoms is insensitive to the lifetime or to the internal energy of the molecular ion as pointed out in (97) for 20 and as emerging from a comparison between the data given in (94) and (96) for 31. A value of ca. 170 +- 10 meV is also associated with the metastable transitions 20 -+ C3H50+ + H and 31 + C3H50+ + H (72,94). On the basis of their CA spectra, the structure of the resulting C3H50+ ions produced in the first field-free region of a triple sector mass spectrometer was established to be C2H5CO+ for precursors 20, 22 (from cyclopentanol), and 31 (91). In contrast, the C3H50+ ions prepared from 20 in the source are constituted by a mixture of acylium C2H5CO+ and protonated acrolein CHZCHCHOH + structures; the abundances ratio [C2H5CO] +I [CH2CHCHOH] + is equal to 80/20 at 12 eV, whereas protonated acrolein becomes dominant at 70 eV (97).

The energetics for the dissociation of [CH,CHCHOH] +*, 20, and [CHzCHCH2OH] +*, 31, radical cation were also explored. From neutral 1-hy- droxypropene, the AE[C3H50]+ is equal to 10.63 2 0.06 eV, thus giving an es- timate of 854 & 6 kJ mol-' for the corresponding transition state energy (97). In a comparative study of allyl alcohol and cyclopropanol by photoelectron-photoion

BOUCHOUX 22 '

coincidence, indications in favor of interconversion of 31 and 24 were obtained (68). The breakdown diagrams of the two cations 31 and 24 are indistinguishable, and the common threshold energy for dissociation into C3H50+ + H is 860 ? 7 kJ mol-' (68) (using as reference the experimental AH;298 (ally alcohol) = -124 kJ mol-I, Table 111).

In summary, most of the actual experimental results strongly suggest a facile interconversion of structures 20, 22, 24, and 31 before ketonization into 25; the lowest energy dissociation route involves the passage over a common energy barrier situated 45-51 kJ mol-I above the fragments C2H5CO+ + H . Equation (7) presents a possible set of isomerization steps involving the abovementioned struc- tures. The two most stable structures for the C a 5 0 + product are acylium C2H5COf and oxygen protonated acrolein CH2CHCHOH+ ions (see Table VII which will be published in Part I1 of this review); the former may be produced by dissociation of 25 and 30, whereas the latter may arise from 20, 22, and 31.

The MIND0/3 exploration of the isomerization of ionized propanal 25 into its enol tautomer 20 suggests that the energetically favored pathway is the double 1,2-H migration 25 * 30 s 20 (56). As noted previously, the MIND013 method seriously overestimates the stability of carbenoid species such as 30. In particular, MIND0/3 reverses the order of stability of 22 and 30 by as much as 80 kJ mol-* with respect to ab-initio results. Accordingly, the observation that the hydroxyl hydrogen is lost to only a small extend (<7%) is a clear indication that 30 is not significantly involved in the isomerization preceding H loss from 20.

+ c,n,io H' c,n,co + n'

A 24

I / \ / O n 25 50 +.

+ H'

KETO-ENOL TAUTOMERS AND DISTONIC IONS 23

As will be seen later, the two 1,2-H shifts 31 7rt 22 s 20 seem to be of general applicability for a, @-unsaturated alcohols. On the other hand, spontaneous ring opening of hydroxy substituted cyclopropanes such as 24 has been experimentally proved (99). The specific case 22 z$ 24 was explored using the MNDO semi- empirical method (100) as a prototype of 1,2-HCOH migration on a radical site (67). At this level of theory, the ring opening 24 + 22 needs no activation barrier.

The facile isomerization of ions 20 and 31 is clearly substantiated by the results of deuterium labelling (97) and by the identity of their CA spectra (62). One may conclude that 20 31 are not the steps of the highest energy re- quirement in the process described in Eq. (7). Thus, the transition state energy 854-860 kJ mol-I associated to the H loss from 20, 31, and 24 must be attributed to the 1,3-H shift 20 -+ 25 or (and) to the 1,4-H migration 22 + 25. There is actually no reason to exclude one of these possibilities, although the overall data quoted in Table VII (to be published in Part 11) seem to indicate that the five-center process needs less critical energy. In conclusion, a convergent set of data support Eq. (7) in describing the lowest energy path, but several questions remain concerning its energetic aspects.

C. Cyclic and open forms of "C3H601+' ethers

The [C3H60]+. arising from dissociation of 1,4 dioxane or from ionization of oxetane possess similar CA spectra at high collisional energies (62). In contrast, data obtained using low collision energies (i.e., 10-100 eV) show differences interpretable in terms of different ion structures (101). Obviously, the most logical candidate for CH20 loss from ionized 1,Cdioxane is the distonic ion 32, which is predicted by molecular orbital calculations to be close in energy to ionized oxe- tane 33.

A proof in favor of the formation of 32 from 1,Cdioxane was obtained using ion cyclotron resonance experiments (63,102). By analogy with C-C opened eth- ylene oxide, which easily transfers CH;' to various neutral substrates, ion 32 is expected to transfer a C2H4f' moiety more readily than 33. Indeed the rnlz 58 ion from 1,4-dioxane shows a much more efficient transfer of C2H4f' to acetonitrile than does 33 (63). Starting with deuterated 1,4-dioxane 37 [Eq. (S)], two fragment ions of rnlz 60 and mlz 62 are produced in equal amounts. Their reaction with acetonitrile in the ICR cell shows that the ion of rnlz 60 transfers exclusively C2H4f' and that rnlz 62, C2D4f'. The specificity of these reactions demonstrates that no equilibration (even partial) 32 33 occurs in the millisecond time scale and that ionized 1,4-dioxane only generates the distonic ion 32 upon fragmentation.

The [1,4-dioxane-CH20] +' ion has one of the lowest AE and is also produced from metastable 1,4-dioxane molecular ions (44). From the photoionization effi- ciency curve and from a QET (Quasi Equilibrium Theory) statistical calculation of the dissociation rate constant, an onset energy of 10.65 eV is estimated for 1,4- dioxane [C3H60]+' + CH20 (44). A similar value is obtained by extrapolation of the mean kinetic energy release value to zero: there is, thus, no appreciable kinetic shift associated with the determination of AE [C3H60]+' and probably also no reverse activation energy (44). By assuming the neutral ejected during dissociation is formaldehyde, a AH& [C3H60]+' = 828 kJ mol-' is obtained.

22 and 22

24 +.

BOUCHOUX

+.

This value is assigned to structure 32 in Table 111, where a fair agreement appears with the computed AH& value.

This evidence demonstrating the stability of the open form of oxetane parallels the situation of ionized propylene oxide 35. Recent results obtained by Turecek and McLafferty (103) and by Lifshitz et al. (104) concerning the CH3. loss from 35 are best explained by an initial isomerization step involving the C-C ring opening 35 -+ 29.

The first observation is that the metastable molecular ions of propylene oxide, 35, and also of methyl vinyl ether, 23, lose a methyl group with the same kinetic energy release = 750 +- 35 meV). Both precursors yield only the CH3CO+ fragment ion as shown by the CA spectra (51,52,103). For methyl vinyl ether, 23, deuterium labelling shows that the original methyl group is preferentially lost. At lower energy, extensive scrambling involves the original methyl and meth- ylene, but not methine hydrogens (103). In the case of metastable propylene oxide molecular ion 35, carbon-13 labelling demonstrates that the eliminated methyl group contains 98% of the carbon atom originally present in the intracyclic meth- ylene (104).

To account for these results, it was proposed (103) that 23 isomerizes to the carbenoid ion 26 by a 1,2-hydrogen shift analogous to that occurring before H loss from ionized vinyl alcohol (see Section 111). This isomerization is followed by a direct cleavage of the C-0 bond, leading to the expected products

The AE [C2H30]+ measurement from 23 gives a threshold energy of 908 kJ mol-’ (i.e., 109 kJ mol-I greater than the products) and thus is consistent with the large amount of kinetic energy released during the metastable decomposition (103,104). The observed hydrogen exchange between the two opposite carbons of 23 may be explained by a 1,4-H migration producing ion 29, the C-C open form of propylene oxide.

CH,CO+ + CHS: 23 + 26 + CH3COf + CH3.

KETO-ENOL TAUTOMERS AND DISTONIC IONS 25

The fact that the methine hydrogen is not involved in the scrambling before CH3 loss from 23 shows that the energy barrier for 23 + 29 is lower than that of 23 + 26 + CH3CO+ + CH3. (A value of = 879 kJ mol-' has been proposed for the energy at the top of the barrier 23 * 29 in ref. 103.)

The connection of 29 with CH3CO+ + 'CH3 affords a simple explanation of the behavior of ionized propylene oxide 35 itself. Its ring opening would lead to 29 and the isomerizations of 23 and 26 would be followed by loss of a methyl group containing the original methylene. This is indeed found at high energy in the spectra of labelled compounds.

The total isomerization process 35 3 29 $ 23 is also demonstrated to require less energy than 23 + 26 + CH3CO+ + 'CH3.The first argument is that the scrambling of all hydrogens, except the methine H, is complete for metastable dissociation of 35. The second point is that a common rate-determining step for dissociations of 35 and 23 (such as 23 -+ 26) rationalizes the observation of the same kinetic energy release values. Finally, the measurement of AE[C2H30] +

from propylene oxide by photoionization spectroscopy to give a threshold energy of 925 ? 8 kJ mol-' (69) compares favorably to the estimate of 908 kJ mol-1 obtained with methyl vinyl ether as precursor (103). One may note that AH& 35 = 897 kJ mol-' and that consequently the AE[C2H30]+ value implies that the critical energy for the ring opening 35 + 29 is less than 20 k 10 kJ mol-l. This may be compared with the opening of ethylene oxide, which is theoretically predicted to require less than 16 kJ mol-'. It is also clearly in line with ESR results in the condensed phase (45,46) and with the behavior of higher homologs of gaseous epoxide molecular ions (see Sections VI and VII in Part I1 of this review).

Figure 8. Energy diagram for ring opening and dissociation of propylene oxide 35 (69, 103). Dotted curves are proposed to lie below 908-925 kJ mol-'.

26 BOUCHOUX

Note that a minor process, responsible for 2% of the CH3 loss from metastable ion 35 and suggested to involve isomerization into the acetone molecular ion (69,104), and non-ergodic behavior for the latter (104), is not discussed further here.

The potential energy profile associated with isomerization and dissociation of ionized propylene oxide 35 and methyl vinyl ether 23 is presented in Figure 8.

v. [C4H80]+' An appreciable amount of mass spectral and thermodynamic data are known

for the [C4H80] +' ion structures, 38-57. Concerning the chemistry of low-energy [C4H80]+' ions, much effort has been made to clarify the behavior of enols, ketones, aldehydes, and alcohols, species 38-49. Data concerning linear or cyclic ethers 50-57 are sparse (31,88,108,109), and their study using actual experimental and theoretical capabilities remains to be done (105). Thus, the discussion will be limited to (i) the keto-enol tautomerism of butanone (38 e 42 F1: 39) and the possible connection with ionized methyl vinyl alcohol 47 and (ii) the rearrange- ment of ionized butanal 44 and 2-methylpropanal 43 into their enol(40,41) or alco- hol (45-49) isomers. The relevant thermochemical information is summarized in Table IV.

The heats of formation of positively charged enols 38-41 were *obtained from appearance energy measurements using energy resolved electrons (66). The enol

Table IV. Thermochemical data of some neutral and ionized C4H80 species. AH; (C,H80)" IE(CJW) A H ; [C,H80]"

Structure kJ mol-' eV kJ mol-'

38 CH,C(OH)CHCH, - 202 582 (66) 39 CHZC(0H)CzHs - 197 8.55 2 0.1 (39) 628 (66) 40 CHOHC(CH3)z - 193 607 (66) 41 CHOHCHCZH5 - 178 628 (66) 42 CH3COC2Hs - 241 9.52 (110,111) 678 43 (CH3)ZCHCHO - 218 9.72 (110,111) 720 44 H-C~H~CHO - 207 9.83 (110,111) 741 45 CHjCHCHCHzOH - 159 9.13 (111) 724 46 CHZC(CH3)CH2CHzOH - 159 9.24 (111) 728

48 CH2CHCHZCH20H - 151 9.2 736 47 CH2CHCH(CH3)OH - 167 9.53(111) 753

49 CH2CH2CH2CHOH

51 CH2CHCH20CH, 50 CHZCHOC2H5

- 141 9.25 (111) 753 - 146 9.55 (112) 774 - 105 9.84 (113) 845

54 CZH5CHCH20 - 112 10.15 (108) 866 - - -

55 CH3CHCHOCH3 130 9.98 (108) 833 -~

56 (CH3)ZCCH20 - 131 10.00 (108) 833

57 'CH2CH2CH2CH20 - 184 9.38 (107) 720 "Benson's incremental method (140).

KETO-ENOL TAUTOMERS AND DISTONIC IONS 27

structures were assumed to be produced entirely from the suitable aldehydes or ketones (3-methyl-2-hexanone and 3-methyl-2-heptanone for 38; 3-hexanone and 3-heptanone for 39; 2,2-dimethylbutanal for 40; 2-ethylbutanal and 2-ethylpen- tanal for 41). As already discussed, the derived AH; [enol]+' values are lower than the AH; of the corresponding keto isomer. Enol radical cations 40 and 41 are more stable than [aldehydes]+'43 and 44 by 113 kJ mol-'. In contrast, the AH;2981~ of 38 and 39 are only 96 and 50 kJ mol-' less than the AH& of the ionized ketone 42.

Photoionization spectroscopy provides ionization energies for the neutral forms of 42-51, 54-57 (110-113) allowing for the estimates of AHpss's of their molecular ions as indicated in Table IV.

A. Keto-enol tautomerism of butanone

The two major fragmentations of the molecular ion of butanone 42 are losses of CH;. and C2H5.

Photoionization mass spectrometry was used to derive the thermodynamics of these two dissociation channels (53,79,98,111). Thus, it was clearly established from appearance energy measurements that the two fragmentations occur at the

28 BOUCHOUX

thermochemical threshold for production of acylium ions CZHSCO+ and CHSCO +, (i.e., 736 and 774 kJ mol-', respectively; see Figure 9). Collisional activation studies reveal that C2HjCO+ is indeed the major C3Hj0+ fragment ion structure produced by electron ionization in the source of the mass spectrometer; however, at 70 eV, a non-negligible proportion (10-25%) of CH2CHCHOH+ is also obtained from 42 (1 14,115).

Metastable ions 42 give similar amounts of C3Hj0+ and C2H3O+ ions (71,114- 119). The kinetic energy releases are equal to To,, = 13 ~fr 2 meV and To.s = 32 & 2 meV, respectively. Interestingly enough, dissociation of the metastable 1,1,1,3,3- ds-butanone molecular ions leads exclusively to C3H3D20 + and C2D3O +, indicat- ing that no hydrogen exchange occurs between the methyl group and the ethyl chain. The structure of the C3H50+ ions coming from metastable ion 42 is exclu- sively C2H5CO+ as shown by the CA spectra obtained on a triple sector mass spectrometer (114,115).

The apparently simple behavior of metastable ion 42 was, however, questioned by McAdoo and Barbalas (118). Following the general trends of metastable ketone molecular ions, 42 would be expected to lose the alkane molecules CH4 or C2H6 (71). The energy levels of the two corresponding sets of products are shown in the potential energy diagram presented in Fig. 9 (by using AH& (ketene) = - 61 kJ mol-', (methylketene) = -84 kJ mol-I (98), IE (ketene) = 9.61 eV (75), IE (methyl ketene) = 8.95 (120) eV, A q 2 9 8 (CH,) = -75 kJ mol-l, d q 2 9 8

(C2H6) = -84 kJ mol-I). Elimination of CH4 appears to be the lowest endother- micity process (AH" = 27 kJ mol-I). For a critical energy of 27 kJ mol-', the rate constant for 42 -+ [CH3CH = C = O]+' + CI& would be largely greater than

7 1 2

Figure 9. Energy diagram for dissociations of ionized butanone 42 (53,79,98,111).

KETO-ENOL TAUTOMERS AND DISTONIC IONS 29

lo6 s-' at all internal energy values of 42, (118,119). Therefore, the formation of ionized methyl ketene is exp,ected to be the dominant fast process; in fact, this is not observed experimentally. Even though no reverse activation energy seems to be associated with the similar reaction [CH3COCH3] +' + [CH2 = C = 01 +. + CH4, the endothermocity of the methane loss from 42 certainly must not be equated to the critical energy.

The enol ion 38 is the most probable product of (R-H) elimination from [CH3COCH(CH3)R] + * molecular ions via a McLafferty rearrangement. This was proposed for cases when R = C2H5 (121), C3H7 (66), C4H9 (66,115), COCH3 (116), and C12H25 (116). Dissociations of metastable 38 are losses of CH;. and C2H; radicals (116,117,121), which occur to a similar extent. Extensive deuterium la- belling shows that the eliminated CH: contains exclusively (>99%) the original 1-methyl group; conversely the expelled C2H; group contains the CHCH3 moiety and the hydroxyl hydrogen (116). Therefore, no hydrogen exchange occurs be- tween the two parts of the hydrocarbon chain of 38, before dissociation.

The structures of the C2H3O+ and C3H50+ fragment ions produced in the first field-free region of a triple sector mass spectrometer were characterized by their collisional activation spectra (115,121). It was established that loss of C2H; leads to the CH3CO+ structure; no trace of CH2COH+ was detected (115). The loss of the methyl group from metastable ion 38 gives a majority of C2H5CO+ ions and a minor amount of the CH2CHCHOH+ structure (115,121).

The interpretation of these results is summarized in Eq. (9). The ketonization 38 + 42 explains the formation of the two acylium ions, whereas direct bond cleavage of ionized 3-buten-2-eno1, 47, leads to the oxygen-protonated acrolein specie CH2CHCHOH + .

+.

In a careful analysis of deuterium labelling results, McAdoo and co-workers (116) concluded that the direct 1,3-hydrogen migration, 38 + 42, is negligible. This point was confirmed more recently: the C3H4DO+ ion coming from meta- stable ion 38 O-dl, gives a dominant loss of CH2D under collisional activation, indicating that the deuterium atom is preferentially located at the end of the ethyl chain after isomerization to 42 (115).

Finally, thermodynamic data obtained from photoionization experiments allow the description of the tautomerism 38 S 42 to be refined. The appearance energy of C3H50 + from 3-methyl-2-hexanone indicates that the high point on the energy surface associated with dissociation of 38 is 775 ? 5 kJ mol-l (i.e., 38 kJ mol-'

30 BOUCHOUX

above the products C2H5COf + CH3) (111). Thus, a rate-determining isomeri- zation step precedes the CH; loss; it may be either the 1,2-H shift, 38 -+ 58, or the 1,4-H transfer 58 + 42. Deuterium labelling results indicate that HID exchange between positions 3 and 4 is of minor importance before decomposition of 38 (116). Consequently, it was suggested that the higher critical energy step is isom- erization 38 +. 58. An approximate energy diagram illustrates this situation (Fig- ure 10). According to ab-initio molecular orbital calculations of the [C3H60] +'

system, the heat of formation of the distonic ion 58 is expected to be close to that of the corresponding ketone. (i.e., 678 kJ mol-I, a value of 690 kJ mol-' was proposed in ref. 115 and an estimate of 650 kJ mol-' is obtained when using the procedure described in the Appendix of this article to be published in Part 11.)

The second enolic form of butanone, ion 39, is also produced by alkene elim- ination from suitable precursors: 3-hexanone (66,121), 3-heptanone (66,117), 3- dodecanone (116), 1-ethylcyclobutanol (39,92,116). It was also demonstrated, us- ing CA spectra (122), that 39 is the [C4H80]+' ion coming from dissociative ion- ization of 1-hepten-3-01 (123). Again, ion 39 eliminates competitively a methyl radical and an ethyl group; the former elimination is slightly favored in the met- astable time frame (116,117,121). Deuterium labelling shows that CH3 and C2H; losses from metastable 39 are preceded by one hydrogen transfer from C(4) to C(1) and by a second 1,4-H migration involving the hydroxyl and C(4) positions [Eq. (10)l.

Distonic ion 58 is the key intermediate between the two enol structures 38 and 39. Note that the direct isomerization 38 @ 39 and the 1,3-H transfer 39 + 42 may be excluded in view of the deuterium labelling data (116).

The CA spectra of metastably generated C2H3O+ and C3H50+ ions show that the former is CH,CO+, whereas the latter is a mixture of C2H5CO+ and CHzCHCHOH' ions (115,121). Interestingly enough, a greater amount of oxygen protonated acrolein is produced from 39 than from 38 (115,121). A bimodal kinetic energy release distribution was observed for 39 -j CH3CO+ + C2H5 and attrib- uted to incomplete intramolecular energy redistribution (92). However, the cal- culated statistical mean lifetime of excited 39 is ca. s (i.e., approximately three orders of magnitude greater than the time required for vibrational relaxation).

The appearance energies of C2H;O + and C3H50 + ions produced by metastable dissociation of 39 are very similar (39). The appearance energy of C2H30f ions originating from 3-heptanone measured by photoionization spectroscopy gives a

31

Figure 10. 42 (111). Dotted curve is proposed to lie below 775 kJ mol-'.

Energy diagram for keto-enol tautomerism of ionized butanone

value of 799 f 4 kJ mol-l for the energy barrier separating 39 from its dissociation products CH,CO+ + C2H5 (111). Comparable results are obtained after mea- surement of AE[C2H30]+ and AE[C,H,O]+ from metastable ion 39 (806 and 791 kJ mol-', respectively, ref. 39).

This barrier is higher than that connecting 38 with the same final state, and one must draw the conclusion that the barrier is for 39 + 58, the only different step between the isomerization schemes presented in Eqs. (9) and (10). The fact that 39 + 58 is the rate-determining step in dissociation is also in accord with the non-reversibility of this hydrogen migration as demonstrated by deuterium la- belling (116). Finally, the greater amount of CH2CHCHOHf structure in the mixture of C3H50+ ions coming from metastable ion 39, compared to 38, is also in agreement with a higher internal energy content in the intermediate structures 58, 42, and 47. The corresponding potential energy profile is illustrated in Fig. 10.

The molecular ion of 3-buten-2-01, 47, is proposed to be the precursor of the CHzCHCHOH+ ions obtained from 38 and principally from 39 [Eqs. (9) and (lo)]. Ion 47, produced by direct ionization, loses a methyl radical in the source of the mass spectrometer to give essentially CHzCHCHOH+ . This point is established from collisional activation spectra (114), metastable peak analysis (1 14,124), and appearance energy determinations using an electron ionization source (125) and a photoionization source (111). Ethyl loss is also observed, giving rise to CH3CO+ ions. The photoionization AE measurements lead to threshold energies equal to 793 kJ mol-l and 801 kJ mol-I for [47 - CH3]+ and [47 - C2H5]+, respectively (111).

32 BOUCHOUX

The metastable ions 47 behave differently. The CA spectra indicate the existence of a mixture of CH2CHCHOH+ and C2H5COf structures; the latter acylium ion being the major component (114,115). This result may be well explained by the occurrence of the ketonization process 47 + 58 + 42 depicted in Eqs. (9) and (10). This is also confirmed by the metastable decomposition of the deuterium- labelled ions 47, 0-dl, 2-dl, and l,l,l-d3 (117). The methyl groups eliminated contain exclusively the three H atoms initially in position 1, in keeping with the direct CH3 loss from 47 and the sequence of reactions 47 + 58 -+ 42 + C2H5C0 +

+ CH3. Part of the potential associated with direct dissociation of 47 and its isomeriza-

tion into distonic ion 58 is presented in Figure 11. One should observe that the dominant formation of C2H5CO+ ions from metastable ions 47 is only explicable if the critical energy associated with the isomerization barrier 47 + 58 is lower than 40 kJ mol-'. It must also be remembered that the energy of the transition state associated with the subsequent step 58 + 42 is lower than 774 kJ mol-' (see Figure 10). It is consequently not clear why the measured threshold energy for C2H; loss is as high as 801 kJ mol-'.

In summary, the keto-enol tautomerism of the butanone radical cation 42 seems to involve principally the intermediate distonic cation 58. The critical energy for the probable thermoneutral 1,4-hydrogen migration 42 + 58 is lower than 97 kJ mol-l. Starting from the "turn-table" 58, this reaction constitutes the easier pro- cess. The other reactions such as 1,2-H migration (58+ 38, Fig. 10) and 1,4-H migration (58 + 39, Fig. 10) to the radical site require 125 and 149 kJ mol-',

Figure 11. 1-butene-3-01 47 (111).

Energy diagram for isomerization and dissociation of ionized

KETO-ENOL TAUTOMERS AND DISTONIC IONS 33

respectively, assuming AH; [58] = 650 kJ mol-'. These two reactions should be exothermic by 68 and 22 kJ molt*. In contrast, the 1,2-H migration toward the charged site (58 - 47, Fig. 11) is endothermic by 108 kJ mol-'; this endothermicity accounts for the largest part of the critical energy of the reaction (i.e., 141 kJ mol-l) .

B. Keto enol tautomerism of butanal and 2-methylpropanal radical cations

Metastable [C4H80]+' ions bearing the oxygen atom at one terminus of the carbon skeleton, 40,41,4346,48, and ionized cyclobutanol, 49, behave similarly. A facile loss of methyl is observed in competition with eliminations of C2H5 and H20 (117). The elimination of a methyl radical also occurs in the source of the mass spectrometer. We first present the description of this fast process before discussing the slightly different behavior of metastable ions.

The complexity of the CH3 loss from terminal [C4H80]+' ions was first raised by Kingston and Tannenbaum (126). These authors explained the mass spectra of variously deuteriated ions 45 by a specific methyl elimination involving position 4 in addition to a second mechanism involving statistically three hydrogen atoms from positions 1, 2, and 3. More recently it was shown that MIKE and CA spectra of C3H50+ ions produced by dissociation not only of 45, but also of 43, 44, 46, 47, and 48, in the source of the mass spectrometer are identical (114). The CA spectra are similar to that of CH,CHCHOH+ ions obtained, for example, by protonation of acrolein (115) and distinct from C2H5Of ions obtained by disso- ciation of methyl propionate.

Appearance energies of C3H50+ ions originating from 43, 44 (111) and 45, 46, 47 (111,125) lead to the same threshold energy of 794 2 3 kJ mol-'. In addition, this value is in accord with the formation of CH,CHCHOH+ ions without any appreciable reverse activation energy, in keeping with the small amount of kinetic energy released during this decomposition (Taverage = 57-60 meV) (111). Thus, it seems established that loss of CH3 from ions 4 3 4 8 gives predominantly oxygen protonated acrolein CH2CHCHOHf in the source of the mass spectrometer.

These observations were rationalized by the mechanistic pathways summarized in Eq. (11).

The scheme involves essentially 1,2- or 1,4-H migrations and a 1,2-HCOH shift (67) connecting the two distonic ions 59 and 60.

This reaction scheme is incomplete if we wish to account for the behavior of metastable [C4H80] +' ions. Collision-induced dissociations of C3H30 + ions pro- duced in the first field-free region of a triple sector instrument were studied for 40 (115); 41 (115,121); 42, 43 (114,121), 46, 48; and 44, 45 (114,115,121). It was concluded that besides the CH2CHCHOH+ ion, the acylium structure C*HsCO+ is competitively produced. This parallel formation of acylium in the C3HsO+ ion mixture and the detection of C2H3O+ ions of structure CH3COf (verified for at least 44 and 47, ref. 115) suggest a partial ketonization of terminal [C4HsO] +' ions into ionized butanone 42. Studies of partial deuterium labelled 44,45,46, and 48 demonstrate that numerous H/D exchanges precede these dissociations. Before

34 BOUCHOUX +.

-;* -OH 44 /

/ +' - ' I

+. - -on 45 Mo"-- 59 41 OH

I I 46 60

&+. OH

40 4 3 41

the metastable loss of a methyl group occurs, statistical HID scrambling takes place for 44.

The 0-dl derivatives of 45,46, and 48 all undergo essentially CH3 loss; a minor elimination of CH2D is noted for 48 (117). A study of the I3C labelled ions (2-13C) 41, (3-13C) 41, and (l-13C) 45 was recently reported (127). The first conclusion is that CH3 loss from the metastable enol ion 41 involves position 2 for 40% and not position 3; the carbon 3 is always present in the eliminated C2Hj. Secondly, the metastable molecular ion of 2-butene-1-01, 45, retains the carbon atom origi- nally in position 1 in its ionic fragments C2H30+ and C3H50+. The majority of these mechanistic clues is compatible with Eq. (11). One must remember that none of these isomerization steps may be above 794 kJ mol-' (111). This implies that:

(a) Critical energies for 1,2-H migrations 45 + 59 and 46 + 60 are lower than 67 kJ mol-l; similar rearrangement, 47 + 58 needs less than 40 kJ mol-' (Fig. 11).

(b) Critical energies for 1,4-H migration to one oxygen atom, 44 + 59 and 43 + 60, are lower than 54 kJ mol-' and 75 kJ mol-', respectively; a maximum critical energy of 96 kJ mol-' was deduced for 42 + 58 (Fig. 10).

(c) A transition state energy for 1,2-HCOH migration is lower than 794 kJ mol-'; MNDO calculation predicts a critical energy of 54 kJ mol-' (67), allowing a max- imum value for A H ; [59] or A H ; [60] of 740 kJ mol-l, in agreement with the expectation that the carbonyl structures and their distonic counterparts possess similar heats of formation. ( A H ; [59] = AH; [60] = 700 kJ mol-' may be estimated using the method described in the Appendix.)

KETO-ENOL TAUTOMERS AND DISTONIC IONS 35

(d) The critical energy for 1,2-H migration 41 + 59 is lower than 167 kJ mol-'; this point is not necessarily fulfilled if one observes that the analogous reaction 38 + 58 requires 192 kJ mol-1 (Fig. 10).

Finally, questions concerning the isomerization of 40, 41, 43-46, and 48 to 42 in order to produce CH3CO+ and C2H5CO' remain open. The recent AE[C2H30] +

measurements done using photoionization spectroscopy lead to a threshold en- ergy equal to that of CH; loss for 43-46.

In the case of 47, similar AE[C2H30]+ measurements give a threshold energy value of 801 kJ mol-' (i.e., 6 kJ mol-' above that of 43-46). This observation caused the authors of ref. 111 to rule out the intermediacy of 47 in the conversion of 43- 46 to 42 via a methyl transfer as depicted by Eq. (12).

60 41 42

Another interconversion process suggested by MIND013 calculations (128) may also be excluded. This is the 1,3-sigmatropic shift of the hydroxyl group which is not compatible with the I3C labelling of 45 in position 1 (127).

To conclude this discussion, two other [C4HsO]+' ions are briefly discussed: cyclobutanol, 49, and its C-C open form [CH2CH2CH2CHOH]+', 61.

The fragmentation mechanism of ionized cyclobutanol49 was first explored by Holmes and Rye who reported spectra of D-labelled derivatives (129). Its major fast dissociation gives [CH2CHOH] +' + C2H4 (26,129); eliminations of CH;, C2H5 and H 2 0 become significant for metastable ions (117). The appearance ener- gies were determined using monoenergetic electrons for [CH2CHOH] + * ions (26) and by photoionization spectroscopy for C2H30+ and C3H50+ (111). Within ex- perimental error, the same transition state energy is obtained for the three dis- sociations (816 k 4 kJ mol-I). This may be interpreted to indicate a barrier for ring opening 49 + 61 [Eq. (13)] of 63 kJ mol-', a surprisingly large value compared to the absence of such a barrier for the cyclopropanol analog.

61 4

q+.- 49 OH

Wo +. - - CH, C3H,0+

44

c2n,o+

36 BOUCHOUX

As for the terminal [C,H,O]+’ ions, the metastably generated C3H50+ fragment ions are a mixture of CH2CHCHOHf and C,H,CO+ structures (115). Thus it is probable that 49 and 61 are connected to the pathways presented on Scheme 12. A pertinent link is offered by 1,5-H migration 61 + 44.

This latter reaction has been explored in the course of an ab-initio study of the McLafferty rearrangement of ionized butanal (130). The MP2/6-31*//3-21G esti- mations of relative energies give 61 more stable than 44 by only 2 kJ mol-’. This theoretical prediction is comparable to the difference between heats of formation of 44 (741 kJ mol-’, experimental value, Table IV) and of 61 (713 kJ mol-I, Eq. (13), estimated value, see Appendix). The second piece of information reported in the ab-initio study of Ha and co-workers (130) is that the y-hydrogen transfer 44 S 61 occurs with barely no critical energy. Consequently, connection between isom- erization processes presented in Eqs. (11) and (13) via ionized butanal, 44, is energetically allowed.

The remainder of this review, “Keto-Enol Tautomers and Distonic Ions. The Chemistry of C,H2,0 Radical Cations. Part 11” will appear in Mass Spectrometry Reviews Volume 7, Number 2.

1.

2. 3.

4.

5.

6. 7.

8. 9.

10.

11.

12. 13. 14. 15.

REFERENCES

(a) Yates, B. F.; Bouma, W. J.; Radom, L. J. Am. Chem. SOC. 1984, 106, 5805-5808. (b) Radom, L.; Bouma, W. J.; Nobes, R. H.; Yates, B. F. (c) Hammerum, S. Muss Spectrom. Rev. in press. The term “distonic” (from the Greek ~ L E U T W U or the Latin distans meaning ”separate”) was introduced to describe the class of ions in which, in the formalism of Lewis electronic structure, the charge and the radical cannot be localized on the same atom. To avoid ambiguities, it is used here only to designate ions in which charge and radical are separated by at least one saturated carbon atom. Kingston, D. G.; Bursey, J . T.; Bursey, M. M. Chem. Rev. 1974, 74, 215-242. (a) Zwinselmann, J. J., Nibbering, N. M. M.; Ciommer, B.; Schwarz, H. In “Tandem Mass Spectrometry”, F. W. Mclafferty, Ed., Wiley, New York, 1983; pp. 67-104. (b) Schwarz, H. Adu. Mass Spectrom. 1985, 20, 13-34. Bouma, W. J.; Macleod, J. K.; Radom, L. Int. J. Muss Spectrom. Ion Phys. 1980, 33, 87- 93. Osamura, Y., Goddard, J . D.; Schaefer, 111, H. F.; Kim, K. S. J, Chem. Phys. 1981, 74, 617-621. Nobes, R. H.; Bouma, W. J.; Radom, L. Chem. Phys. Lett. 1982, 89, 497-500. Frish, M. J.; Raghavachari, K.; Pople, J . A,; Bouma, W. J.; Radom, L. Chem. Phys. 1983,

De Frees, D. J., McLean, A. D. and Herbst, E. Astrophys. J. 1984, 279, 322-334. Bouma, W. J.; Burgers, P. C.; Holmes, J. L.; Radom, L. (a) Adv. Muss Spectrom. 1986, 10, 743-744. (b) I. Am. Chem. SOC. 1986, 208, 1767-1770. (a) Guyon, P. M.; Tronc, M. J. Chim. Phys. 1969, 66, 3542 (b) Guyon, P. M. J. Chim.

75, 323-329.

Phys. 1969, 66, 429. Wankenne, H.; Caprace, G.; Momigny, J . lnt. J. Mass Spectrom. Ion Proc. 1984, 57, 149- 158. rraeger, J. C. lnt. J. Mass Spectrom. lon. Proc. 1985, 66, 271-282. Warneck, P. Z. Nafurforsh 1971, 26H, 2047. Guyon, P. M.; Chupka, W. A.; Berkowitz, J . J. Chem. Phys. 1976, 64, 1419. Baker, A. D.; Baker, C.; Brundle, C. R.; Turner, D. W. lnt. J. Mass Spectrom. lon PIlys., 1968, 1, 285.

KETO-ENOL TAUTOMERS AND DISTONIC IONS 37

16.

17.

18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

29. 30.

31. 32.

33. 34. 35. 36. 37. 38. 39. 40.

41. 42. 43.

44.

45. 46. 47. 48.

49. 50.

51.

52. 53.

54. 55. 56. 57.

Bombach, R.; Dannacher, J.; Stadelmann, J. P.; Vogt, J . (a) Chem. Phys. Lett. 1980, 76, 429-432. (b) Chem. Phys. Lett. 1981, 77, 399402. (c) Znt. J. Mass Spectrom. Ion Phys. 1981,

(a) Vaz Pires, M.; Galloy, C.; Lorquet, J. C. J . Chem. Phys. 1978,69,3242-3249. (b) Lorquet, J. C. Org. Mass Spectrom. 1981, 26, 469482. (c) Barbier, C.; Galloy, C.; Lorquet, J . C. J. Chem. Phys. 1984, 82, 2975-2980. Refaey, K. M. A.; Chupka, W. A. J. Chem. Phys. 1968,48, 5204-5219. Berkowitz, J. J. Chem. Phys. 1978, 69, 3044-3054. Momigny, J.; Wankenne, H.; Krier, C. Int. J. Mass Spectrom. Ion Phys. 1980, 35, 151-170. Wesdemiotis, C.; McLafferty, F. W. Tetrahedron Lett. 1981, 22, 3479-3480. Burgers, P. C.; Mommers, A. A.; Holmes, J. L. J. Am. Chem. SOC. 1983, 205, 5976-5979. Beynon, J. H.; Fontaine, A. E.; Lester, G. R. Int. J. Mass Spectrom. Ion Phys. 1968, 2, 1. Donchi, K. F.; Derrick, P. J. Org. Mass Spectrom. 1983, 28, 538-541. Bouma, W. J.; Macleod, J . K.; Radom, L. J. Am. Chem. SOC. 1979, 201, 5540-5545. Holmes, J. L.; Terlouw, J. K.; Lossing, F. P. J. Phys. Chem. 1976, 80, 2860-2862. Jochims, H. W.; Lohr, W.; Baumgartel, H. Chem. Phys. Lett. 1978, 54, 594-596. (a) Frenking, G.; Heinrich, N.; Schmidt, J.; Schwarz, H. Z. Natuforsch 1982, 37b, 1597- 1601. (b) Henrich, N.; Koch, W.; Frenking, G.; Schwarz, H., J. Am. Chem. SOC. 1986, 208,

Van de Sande, C. C.; McLafferty, F. W. J. Am. Chem. SOC. 1975, 97, 4613-4616. Terlouw, J. K.; Wezenberg, J.; Burgers, P. C.; Holmes, J. L. J. Chem. SOC. Chem. Commun. 1983, 1121-1123. Holmes, J . L.; Terlouw, J. K. Can. J. Chem. 1975, 53, 2076-2083. Cordeman, R. R.; Lebreton, P. R.; Buttrill, Jr., S. E.; Williamson, A. D.; Beauchamp, J . L. J. Chem. Phys. 1976, 65, 49294939. Bouma, W. J.; Macleod, J. K.; Radom, L. J. Chem. SOC. Chem. Commun. 1978, 724-725. Baumann, B. C.; Macleod, J. K. J . Am. Chem. SOC. 1981, 203, 6223-6224. Van Velzen, P. N. T.; Van der Hart, W. J. Chem. Phys. Lett. 1981, 83, 56-58. Pritchard, J. G. OUR. Mass Spectrom. 1973, 8, 103-108. Holmes, J. L.; Lossing, F. P. J. Am. Chem. SOC. 1982, 204, 2648-2649. Turecek, F.; Hanus, V. Org. Mass Spectrom. 1984, 29, 423427. Iraqi, M.; Pri-Bar, I.; Lifshitz, C. Org. Mass Spectrom. 1986, 21, 661-664. Albrecht, B.; Allan, M.; Haselbach, E.; Neuhaus, L.; Carrupt, P. A. Helv. Chim. Acta

Burgers, P. C.; Holmes, J . L. Org. Mass Spectrom. 1982, 27, 123-126. Blair, A. S.; Harrison, A. G. Can. J . Chem. 1973, 52, 703. Kumakusa, M.; Arakawa, K.; Sigiura, T. Int. J. Mass Spectrom. Ion Phys. 1978, 26, 303- 320.. Fraser-Monteiro, M. L.; Fraser-Monteiro, L.; Butler, J. J.; Baer, T.; Hass, J. R. J. Phys. Chem. 1982, 86, 739-747. Qin, X. Z.; Snow, L. D.; Williams, F. J . Am. Chem. SOC. 1985, 107, 3366-3368. Qin, X. Z.; Snow, L. D.: Williams, F. J . Phys. Chem. 1985, 89, 3602-3606. Feller, D.; Davidson, E. R.; Borden, W. T. J . Am. Chem. SOC. 1983, 205, 3347-3348. Bouma, W. J.; Poppinger, D.; Saebo, S.; Macleod, J. K.; Radom, L. Chem. Phys. Lett. 1984, 204, 198-202. Clark, T. J. Chem. SOC. Chem. Commun. 1984, 666-667. Apeloig, Y.; Karni, M.; Ciommer, B.; Depke, G.; Frenking, G.; Meyn, S.; Schmidt, J.; Schwarz, H. Int. J, Mass Specfrom. Ion Proc. 1984, 59, 21-37. Burgers, P. C.; Holmes, J. L.; Szulejko, J. E.; Mommers, A. A,; Terlouw, J. K. Org. Mass Spectrom. 1983, 18, 254-262. Turecek, F.; McLafferty, F. W. Org. Mass Spectrom. 1983, 28, 608-611. Trager, J. C.; McLoughlin, R. G.; Nicholson, A. J . C. J. Am. Chem. SOC. 1982, 204, 5318- 5322. Vogt, J.; Williamson, A. D.; Beauchamp, J. L. J. Am. Chem. SOC. 1978, 100, 3478-3483. Bingham, R. C.; Dewar, M. J. S.; Lo, D. H. J. Am. Chem. SOC. 1975, 97, 1285-1293. Hoppiliard, Y.; Bouchoux, G.; Jaudon, P. Noun. J. Chim. 1982, 6 , 43-52. Apeloig, Y.; Karni, M.; Ciommer, B.; Depke, G.; Frenking, G.; Meyn, S.; Schmidt, J.; Schwarz, H. J. Chem. SOC. Chem. Commun. 1983, 1497-1499.

40, 275--285.

593-600.

1984, 67, 216-219.

38 BOUCHOUX

58.

59.

60.

61. 62. 63. 64. 65. 66. 67. 68.

69. 70. 71. 72. 73. 74. 75 * 76. 77. 78.

79. 80. 81.

82.

83. 84. 85.

86.

87. 88. 89. 90. 91. 92. 93 * 94 * 95. 96. 97. 98. 99.

100. 101. 102. 103. 104.

105.

Bouchoux, G.; Flament, J . P.; Hoppilliard, Y. Int. J. Mass Spectrom. lon Proc. 1984, 57, 179-190. Burgers, P. C.; Terlouw, J. K.; Holmes, J. L. lnt. J. Mass Spectrom. lon Proc. 1985, 65, 91- 95. Burgers, P. C.; Holmes, J . L.; Terlouw, J. K. J. Chem. SOC., Chem. Commun. 1984, 642- 643. Frenking, G.; Heinrich, N.; Koch, W.; Schwarz, H. Chem. Phys. Lett. 1984, 105, 490494. Van de Sande, C. C.; McLafferty, F. W. J. A m . Chem. SOC. 1975, 97, 46174620. Bouma, W. J.; MacLeod, J . K.; Radom, L. J. A m . Chem. SOC. 1980, 102, 2246-2252. Turecek, F.; Hanus, V. Org. Mass Spectrom. 1984, 19, 631-638. Turecek, F. J. Chem. SOC. Chem. Commun. 1984, 1374-1375. Holmes, J. L.; Lossing, F. P. J. Am. Chem. SOC. 1980, 202, 1591-1595. Bouchoux, G.; Hoppilliard, Y. Int. J. Mass Spectrorn. Ion Proc. 198311984, 55, 47-53. Bombach, R.; Dannacher, J.; Honegger, E.; Stadelman, J. P.; Nier, R. Chem. Phys. 1983, 82, 459-470. Bombach, R.; Stadelman, J. P.; Vogt, J. Chem. Phys. 1982, 72, 259-266. Beynon, J. H.; Caprioli, R. M.; Cooks, R. G. Org. Mass Spectrom. 1974, 9, 1-11. Bouchoux, G.; Hoppilliard, Y. Can. J. Chem. 1982, 60, 2107-2112. McAdoo, D. J.; Witiak, D. N. J. Chem. SOC., Perkin Trans. 1981, 770-773. Lifshitz, C.; Tzidony, E. Int. 1. Mass Spectrom. Ion Phys. 1981, 39, 181-195. Mintz, D. M., Baer, T. lnt. J. Mass Spectrom. Ion Phys. 1977, 25, 3945. Stockbauer, R. Int. I. Mass Spectrom. Ion Phys. 1977, 25, 89-101. Powis, I.; Danby, C. J. lnt. J. Muss Spectrom. lon Phys. 1979, 32, 27-33. Lifshitz, C. Int. 1. Mass Spectrom. lon. Phys. 1982, 43, 179-193. Traeger, J. C.; Hudson, C. E.; McAdoo, D. J. presented at the American Society for Mass Spectrometry Meeting, Cincinnati, 1986. Murad, E.; Inghram, M. J. Chem. Phys. 1964, 42, 404409. Hudson, C. E.; McAdoo, D. J. lnt. J. Mass Spectrom. lon Proc. 1984, 59, 325-332. Diekman, J.; MacLeod, J . K.; Djerassi, C.; Baldeschwieler, J . D. J. Am. Chem. SOC. 1969,

Eadon, G.; Diekman, J.; Djerassi, C. (a) J, Am. Chem. SOC. 1969, 91, 3986-3987; (b) J. Am. Chem. S5c. 1970, 92, 6205-6212. Bouma, W. J; MacLeod, H. K.; Radom, L. Org. Mass Spectrom. 1981, 16, 301-302. McAdoo, D. J.; McLafferty, F. W.; Smith, J. S. J. Am. Chem. SOC. 1970, 92, 6343-6345. McLafferty, F. W.; McAdoo, D. J.; Smith, J. S.; Kornfeld, R. J. Am. Chem. SOC. 1971, 93, 3720-3730. Depke, G.; Lifshitz, C.; Schwarz, H.; Tzidony, E. Angew. Chem. Int. Ed. Engl. 1981, 20,

Heyer, R. C.; Russel, M. E. Org. Muss Spectrom. 1981, 26, 236-237. Lifshitz, C. J. Phys. Chem. 1983, 87, 2304-2313. Turecek, F.; McLafferty, F. W. 1. Am. Chem. SOC. 1984, 106, 2525-2528. McAdoo, D. J.; Hudson, C. E. Int. J. Mass Spectrom. lon Proc. 1984, 59, 77-83. Hudson, C. E.; McAdoo, D. J. Org. Mass Spectrom. 1982, 17, 366-368. Lifshitz, C.; Berger, P.; Tzidony, E. Chem. Phys. Lett. 1983, 95, 109-113. Splitter, J. S.; Calvin, M. J. Am. Chem. SOC. 1979, 201, 7329-7332. Holmes, J. L.; Burgers, P. C.; Mollah, Y. A. Org. Mass Spectrom. 1982, 17, 127-130. Hudson, C. E.; McAdoo, D. J. Org. Mass. Spectrom., 1984, 29, 1-6. Kurland, J. J.; Lutz, R. P. J. Chem. Soc., Chem. Commun. 1968, 1097-1098. Turecek, F.; Hanus, V.; Gaumann, T. lnt. J. Muss Spectrom. Ion Proc. 1986, 69, 217-231. Traeger, J. C. Org. Mass Spectrom. 1985, 20, 223-227. Bouchoux, G.; Flammang, R.; Maquestiau, A. Org. Mass Spectrom. 1985, 20, 154-155. Dewar, M. J. S.; Thiel, W. J. Am. Chem. SOC. 1977, 99, 4899-4907. Verma, S.; Ciupek, J. D.; Cooks, R. G. Int. J. Mass Spectrom. Ion. Proc. 1984, 62, 219-225. Baumann, B. C.; Macleod, J . K.; Radom, L. J. Am. Chem. SOC. 1980, 102, 7927-7928. Turecek, F.; McLafferty, F. W. J. A m . Chem. Soc. 1984, 206, 2528-2531. Lifshitz, C.; Peres, T.; Ohmichi, N.; Pri-Bar, I. Int. J. Mass Spectrom. Ion Proc. 1986, 72, 253. Bouchoux, G.; Tortajada, J, in preparation.

91, 2069-2084.

792-793.

KETO-ENOL TAUTOMERS AND DISTONIC IONS 39

lob.

107. 108. 109. 110.

111. 112.

113. 114.

115. 116. 117. 118. 119. 120. 121. 122. 123. 124. 125. 126. 127. 128. 129. 130.

Pedley, J. B.; Rylance, J . Sussex-NPL, Computer Analysed Thermochemical Data Organic and Organometallic Compounds, University of Sussex, 1977. Levin, R. D.; Lias, S. G. Natl. Stand. Ref. Data Ser., Natl. Bur. Stand (U.S) 71, (1982). McAlduff, E. J.; Houk, K. N. Can. J. Chem. 1977, 55, 318-332. Smakman, R.; DeBoer, Th. J. Org. Muss Spectrom. 1968, 1, 403416. Hernandez, R.; Masclet, P.; Mouvier, G. J. Elec. Spectrosc. Relat. Phenom. 1977, 10, 333- 341. Traeger, J . C.; McAdoo, D. J. Int. J . Mass Spectrom. Ion Proc. 1986, 68, 35-48. The adiabatic IE was obtained by a linear extrapolation of the low-energy side of the first band on the PES spectrum (ref. 125). McAlduff, E. J.; Caramella, P.; Houk, K. N. J. Am. Chem. SOC. 1978, 100, 105-110. Bouchoux, G.; Hoppilliard, Y.; Flammang, R.; Maquestiau, A.; Meyrant, P. Org. Mass Spectrom. 1983, 18, 340-344. McAdoo, D. J.; Hudson, C. E. Org. Muss Spectrom. 1983, 18, 466473. McAdoo, D. J.; McLafferty, F. W.; Parks, T. E. J . Am. Chem. SOC. 1972, 94, 1601-1609. McAdoo, D. J.; Hudson, C. E.; Witiak, D. N. Org. Mass Spectrom. 1979, 14, 350-359. McAdoo, D. J.; Barbalas, M. P. Int. 1. Mass Spectrom. Ion Phys. 1980, 36, 283-284. Hoppilliard, Y.; Bouchoux, G. Int. J. Mass Spectrorn. Ion Phys. 1983, 47, 109-112. Bock, H.; Hirabayashi, T.; Mohmano, S. Chem. Ber. 1981, 114, 2595-2608. Meyrant, P. Thesis, September 1983, Universite de 1’Etat a Mom, Belgium. McAdoo, D. J.; Hudson, C. E. Org. Mass Spectrom. 1981, 16, 294-296. Grant, B.; Djerassi, C. J . Am. Chem. SOC. 1974, 96, 3477-3481. Krenmayr, R. Monatslz. Chem. 1975, 106, 925-939. Bouchoux, G.; Flament, J . P.; Hoppilliard, Y. Nouv. J. Chirn. 1983, 7, 385-390. Kingston, D. G. I.; Tannenbaum, H. T. Org. Muss Spectrom. 1975, 10, 263-272. Hudson, C. E.; McAdoo, D. J . Org. Mass Spectrom. 1985,20, 402405. Hoppilliard, Y.; Bouchoux, C. Org. Muss Spectrom. 1982, 17, 534-536. Holmes, J . L.; Rye, R. T. B. Can. J . Chem. 1973, 51, 2342-2346. Ha, T. K.; Radloff, C.; Nguyen, M. T. J. Phys. Chem. 1986, 90, 2991-2994.