Structure and Fragmentation of [C,H,O]' Ions Formed By Chemical Ionization

Alex. G. Harrifon,? Tin0 Gaumam and Daniel Stahl Institut de Chimie Physique, kale Polytechnique FkdCrale, CH-1015 Lausanne, Switzerland

The unhnolecular metastable and collision-induced €ragmentation reactions of [C3I,O]' ions produced by gas-phase protonation of acetone, propanal, propylene oxide, oxetan and allyl alcohol have been studied. The CID studies show that protonation of acetone and allyl alcohol yield different stable ions with distinct structures while protonation of propanal or propylene oxide yield [C3H70]+ iom: of the same structure. Protonated oxetan rearranges less readily to give the same structure(s) as protonated propanal and propylene oxide. The [CJ&O]' ions fragmenting as metastable ions after formation by CI have a higher internal energy than the same ions fragmenting after formation by EX. Deuteronation of the C3&0 isomers using CD, reagent gas shows that loss of G H a proceeds by a different mechanism than loss of GK. The results are discussed in term of potential energy profile for the [C3H70]+' system proposed earlier.

WRODUCl'ION

Considerable progress has been made in recent years towards the development of a better understanding of the unimolecular fragmentation reactions of isolated organic ions. It is now quite clear that the use of a potential energy profile approach frequently affords considerable insight into the chemistry of the ions, particularly when the slow (k = 104-106 s-') reactions of relatively small cations are investigated.'-"

Among the ions which have been studied in detail in this fashion are the [C,H70]+ isomers 1 and 2,"12 produced primarily by electron impact ionization. These studies have led to the development of the potential energy diagram illustrated in Fig. 1 to

Figure 1. Potential energy profile for C,H,O+ system. From Ref. 13 as adapted from Refs 11 and 12.

rationalize the slow fragmentation reactions observed. As indicated in Fig. 1, the [C,H,O]' ions 3-5 are postulated to lie on the same potential energy surface; indeed, it has been reported" that ions initially

7 Author to whom correspondence should be addressed at: Depart- ment of Chemistry, University of Toronto, Toronto, Canada M5S 1Al.

m (CH,),C=OH+ CH3CH2CH=OHt CH,--CH--CH,-OH'

1 2 3

CH2=CHCH20H2+ CH,CH,CH,dH+ I

4 5

formed as 2 equilibrate C1 and C3 by way of 4 prior to metastable fragmentation, although more recent results14 are not in agreement with this conclusion. The chemistry of ions 3-5, which cannot be prepared by electron impact, has received much less study than the chemistry of ions 1 and 2. In a brief study, McLafferty and Sakai7 have reported that ions of nominal structures 3 and 4 prepared by protonation (CH4CI) of propylene oxide and oxetan, respectively, give collision-induced dissociation (CID) spectra very similar to the spectrum of 2 prepared by electron impact, implying isomerization of all three to a com- mon structure or mixture of structures. On the other hand, it has been reported" that ions 1-5 produced by self-protonation, give distinct CID spectra indicating distinct structures. The major features of the chemical ionization mass spectra resulting from protonation of C3H60 isomers have been interpreted in terms of the potential energy diagrams shown in Fig. l . 1 3 The present work reports a more detailed study of the structure and fragmentation of [C3H70]+ ions of ini- tial structures 1-5 produced by proton transfer chemi- cal ionization. In particular, the structures of the [C,H,O]+ ions formed in both the CH, and i-C4HIo chemical ionization mass spectra of the C,&0 isom- ers have been investigated in detail by way of collision-induced dissociation studies to explore the prevalence of interconversion of the isomeric ions.

Relatively recently, Holmes et ~ 1 . ' ~ have reported that the To,5 value for metastable loss of GH3D from 2-0-D is less than the To.5 value for metastable loss of GH4 from the same ion. They further reported that the To.5 value for the former reaction was identical to

0 Wiley Heyden Ltd, 1983

CCC-0030-493X/83/0018-0517$04.00

ORGANIC MASS SPECTROMETRY, VOL. 18, NO. 12, 1983 517

A. G. HARRISON, T. GAUMANN AND D. STAHL

CHz-CH2 I I CHz-QH

+ . sr C H ~ C Y C H = ~ H +CH~CHCH~OH+&CH~C%OH

I . G H 4 + CHFOH

2

'OH / \

CHJ-CH-CHz

3

J. CH3C+OCH;

6

CH2

Scheme 1

the To.5 value for metastable loss of C2H4 from the ion [CH3CH20CH2]+ (6) ; they have suggested a second mechanism for ethylene elimination as shown in Scheme 1 expanded to include other [C3H,0]+ ion structures. The present work extends the study of oxygen-deuterated ions to the other isomers shown in Fig. 1 and provides additional results supporting two mechanisms for elimination of ethylene.

Finally, as Holmes and Terlouw17 have pointed out, there have been relatively few studies of the metasta- ble fragmentation reactions following chemical ioniza- tion. The few studies to date indicate that the kinetic energy release in the metastable fragmentation reac- tion following chemical ionization is greater than the kinetic energy release in the same fragmentation reac- tion induced by electron impact14 and that, in chemical ionization systems, the kinetic energy release increases as the exothermicity of the ionization reaction forming the fragmenting ion increases.16 However, there is no satisfactory rationalization of these differences in chemical ionization and electron impact systems. Since 1 and 2 may be readily prepared by both electron impact and proton transfer chemical ionization methods, the present study affords the opportunity of comparing the metastable fragmentation reactions of the same ions prepared by the two methods.

EXPERIMENTAL

The unimolecular fragmentation reactions of ions ini- tially prepared as 1-5, and several deuterium-labelled variants thereof, were studied using a Vacuum Generators ZAB-2F double-focussing mass spec- trometer by examining the reaction occurring in the second field-free region between the magnetic anal- yser and the electrostatic analyser by mass analysed ion kinetic energy spectroscopy (MIKES). The ions of initial structures 1 and 2 were prepared by dissociative electron impact ionization of t-butanol and 3- pentanol, respectively, as well as by protonation

(CH4CI) of acetone and propanal. Ions of initial struc- tures 3-5 were prepared by protonation (CH,CI) of propylene oxide, oxetan, and ally1 alcohol, respec- tively. The oxygen-deuterated derivative of 1-5 were prepared by the CD, chemical ionization of the ap- propriate precursors; in addition, 1-0-D and 2-0-D were prepared by dissociative electron impact ionua- tion of t-butanol-0-D and 3-pentanol-0-D produced by admitting the alcohol and D20 simultaneously into the heated inlet system. The unimoleculaf dissociation reactions of [CH,CD,CH=OH]+, [CH3CHCD20Hl+ and [CH2CH2CD20H]+., prepared by protonation of the appropriate deuterated precursors, were also studied. The relative metastable ion abundances re- ported represent the averages of at least five measure- ments of the relative peak areas obtained by repeated scanning of the electrostatic analyser voltage with computer acquisition of data. The kinetic energy re- lease data were obtained by recording the peak shapes on an X-Y recorder; the To , values were evaluated from the peak widths at half-height after correction for the main beam width.

The collision-induced dissociation (CID) mass spectra of ions of initial structures 1-5 were obtained by admitting helium to the second drift region collision cell to reduce the parent ion signal to -1/3 of the original intensity. In the chemical ionization studies, the CI reagent gas pressure and collision cell pressure were kept constant while the CID spectra of all isomers were obtained. The ions 1-5 were prepared as de- scribed above as well as by I-C,H,, chemical ioniza- tion. The CID spectra represent the averages of at least five repeat measurements of the relative peak areas in the CID-MIKES spectra obtained with com- puter acquisition of data.

All spectra were obtained using a source tempera- ture of -100 "C with samples being introduced from a heated inlet system at -150°C. In the chemical ioni- zation experiments, reagent gas source pressures were -0.3 Torr. All unlabelled C3H60 compounds were commercially available or were synthesized by litera- ture procedures. The syntheses of the deuterium label- led compounds have been described previously. l3

8

- RESULTS AND DISCUSSION

CID mas spectra: structures of [GH,O]+ ions

Table 1 records the CID mass spectra of the ions of initial structures 1-5 produced by CH4CI of the ap- propriate C3H60 precursor. In addition, Table 1 in- cludes the CID spectra of the ions 1 and 2 produced by dissociative electron impact ionization. The agree- ment of the CID spectra for the EI-produced and CI-produced ions is quite satisfactory.

The ions of initial structures 1 and 5 show quite distinctive CID mass spectra indicating that these two ions represent distinct and separate stable structures. The potential energy diagram for the [C3H70]+ sys- tem (Fig. 1) shows that both 1 and 5 exist in deep potential energy wells and will not readily interconvert

518 ORGANIC MASS SPECTROMETRY, VOL. 18, NO. 12, 1983

STRUCTURE AND FRAGMENTATION OF [C,H,O]' IONS

Table 1. CID mass spectra of [C,H,O]' ions" (% of total ionization)

mfr 1El) 1(CI) 2(EI) 2(CI) 3(CI) 4(CI) 5(CI)

58 3.9 4.9 2.3 2.9 2.0 1.4 3.4 57 2.2 1.6 10.8 9.1 8.9 9.9 6.7 55 0.7, 0.6, 2.0 1.8 1.7 1.3 1.1 53 0.5, 0.4, 0.5, O.$ 0.5, 0.3, 0.2, 44 1.1 1.7 1.7 1.8 1.6 1.0 0.4, 43 32.8 29.7 8.6 7.7 9.0 6.2 3.7 42 7.2 7.4 3.7 3.5 3.6 2.3 0.5, 40 - - 1.0 0.8, 1.0 1.1 2.8 39 17.2 19.2 12.7 11.8 11.9 11.1 30.2 38 2.9 1.9 3.9 3.5 3.6 3.7 9.5 37 3.6 4.5 3.9 3.4 3.4 3.5 7.3 30 0.2, 0.2, 2.5 2.7 2.5 3.2 7.3 29 8.0 7.3 18.7 20.1 19.3 21.2 9.8 28 0.8, 0.G 3.6 3.9 3.9 6.0 2.1 27 6.6 6.5 13.1 13.9 13.6 14.6 9.1 26 3.9 3.6 6.9 7.3 7.5 8.4 5.6 25 1.2 1.1 1.2 1.0 1.2 0.8, 1.4 15 5.1 5.7 1.8 2.5 2.6 2.0 2.3 14 2.0 2.1 0.8, 1.1 1.1 1.3 0.9, 13 0.6, 0.G 0.3, 0.4, 0.4, 0.5, 0.4, 12 0.2, 0.2, 0.2, 0.2, 0.2, 0.2, 0.2,

All CI results obtained using CH, reagent gas. lntensities omit m/z 31 and m/z 41 which may have unimolecular contribution.

Table2. CID spectra following i-C4HloCP

mfz 1 2 3 4 5

58 7.2 5.4 3.4 1.2 2.6 57 1.3 8.4 8.4 13.7 5.9 55 0.7, 1.8 1.5 1.3 0.7, 53 0.4, 0.6, 0.5, 0.3, 0.3, 44 2.0 2.5 2.6 0.8, 0.5, 43 32.6 8.1 9.8 4.3 1.9

40 0.2, 1.2 2.5 1.6 4.5 39 16.8 11.4 12.1 12.9 33.6 38 2.4 2.9 3.3 1.3 9.7 37 2.7 2.9 3.1 2.7 8.7 30 0.2, 2.8 2.9 4.2 4.5 29 6.9 19.0 16.4 18.1 7.8 28 0.6, 4.0 4.4 8.8 1.3 27 6.2 13.4 12.5 13.8 6.9 26 3.7 6.5 6.5 8.8 5.2 25 0.1, 0.7, 0.4, 0.7, 1.6 15 5.0 2.3 2.4 1.6 1.9

13 0.4 0.3, 0.3, 0.4, 0.3,

a m/z 41 and m/z 31 omitted because of possi- ble unimolecular contribution.

(O/O of total ionization)

42 8.2 4.0 4.8 1.9 0.4,

14 1.8 0.9, 0 . q 1.1 0.7,

12 0.18 0.15 0.17 0.6, 0.18

or isomerize to 2-4. On the other hand, the ions of initial structures 2 and 3 show practically identical CID spectra while the spectrum of 4 is only slightly different. The potential energy diagram, Fig. 1, shows that, of these three structures, 2 is the most stable and that the critical energy for the isomerization 3+2 is -21 kJmole-', while the critical energy for the isomerization 4 -+ 2 is -92 kJ mole-'. By contrast, the exothermicity of protonation of the substrate molecules by CH5+ is >250 kJ mole-'; thus, the inter- conversion of 2-4 is energetically feasible and the ions sampled probably largely represent a common struc- ture or mixture of structures. These observations are in agreement with the earlier observations of McLafferty and Sakai7 who found that 1 and 2 (pro- duced by electron impact ionization) gave distinctly different CID spectra, while the [C,H,O]+ ions pro- duced by protonation (CH4CI) of propylene oxide and oxetan gave CID spectra very similar to the spectrum of 2. By contrast, it has been reported15 that when the [C,H,O]+ ions are prepared by self-protonation of appropriate C3&0 isomers, distinct CID spectra are obtained, although details are not available.

Compared to the spectra of 2 and 3 the CID spec- trum of 4 shows a number of small differences, such as lower abundances for m/z 43 and 42 and enhanced abundances for m/z 28 and 26; this suggests that some of the ions initially prepared as 4 may retain that structure. In an attempt to minimize isomerization we have prepared ions 1-5 by gentle protonation using i-C4Hl0 as reagent gas; the resulting CID mass spectra are recorded in Table 2. The CID spectra of ions initially prepared as 2 and 3 are still very similar suggesting a cormnon structure (or mixture of struc- tures) for the ions; it is unlikely that 2 will have sufficient excess energy (122 kT mole-') to isomerize to 3 and the common structure most likely is 2. The

differences in the spectrum of ions prepared as 4 are enhanced with the result, for example, that the m/z 43 : m/z 28 ratio is 0.49 for 4 but 2.0 and 2.3 for 2 and 3 while the mlz 57: m/z 42 ratio is 7.3 for 4 but only 2.1 and 1.8 for 2 and 3, respectively. We conclude that, under i-C4HI0 CI conditions at least a significant fraction of the ions initially prepared as 4 retain a distinct structure although it is possible that some of the ions may have isomerized to 2 and/or 3.

The discussion above is based on what is essentially a qualitative comparison of the CID spectra and it would be desirable to have a quantitative method for comparing CID spectra in order to establish when significant differences are observed. Lay er al.'' re- cently have proposed that CID spectra be compared on the basis of similarity indices where the similarity index (SI ) for two spectra is given by:

where i - io is the difference in intensities for a given ion where io is the smaller intensity and N is the total number of masses used in the comparison. In the computation of SI values they have used all resolved masses for which the relative standard deviations of the intensity measurements were below an arbitrarily acceptable level (RSD,,) and have concluded that if SI < RSD,,, the two ions have indistinguishable CID spectra, but that if SI>RSD,,, the ions have non- identical CID spectra and, very likely, also have differ- ent structures or mixtures of structures.

We have applied this approach to the CID spectra presented in Tables 1 and 2 using in the comparisons all ion intensities except those for m/z 38, 37 and 25,

ORGANIC MASS SPECTROMETRY, VOL. 18, NO. 12, 1983 519

A. G. HARRISON, T. G A U M A " AND D. STAHL

Table 3. Similarity indices

1(El) 22.3 35.4 l(CIM) 21.7 l(ClB) 2(EI) 2(CIM) 2(CIB) 3(CIM) 3(CIB) 4(CIM) 4(CIB) 5(CIM) 5(CIB)

l(El) l(CIM) W I B I 2(EI) Z(CIM) 2(CIBI 3(CIM) 3(CIE)

633 258 264 232 267 340 349 357 327 365 305 300 298 288 304

19.3 37.1 19.4 43.5 29.8 12.9 48.1

46.1 31.2 44.8

4(CIM)

802 440 394 43.5 41.9 85.9 32.9 70.2

3tCIB) 5(CIM)

468 684 610 894 556 712 84.3 170 81.1 164

128 194 74.7 165

111 218 53.4 122

123

5fCIB)

664 738 705 240 236 259 238 294 190 195 43.1

which were poorly resolved and showed poor repro- ducibility. The resultant similarity indices are pre- sented in Table 3, where the designations EI, CIM and CIB represent preparation of the relevant ion by electron impact, CH, CI and i-C4HI0 CI methods, respectively. The following observations are pertinent.

The ion 1 exists in a deep potential well (Fig. 1) and, consequently, retains a unique structure independent of the method of preparation. Thus, the comparison of 1 (EI), 1 (CIM) and 1 (CIB), which yields SI values in the range 22-35, presumably reflects the effects of instrumental parameters and changes in internal energy on CID spectra and the resulting similarity indices. It should be noted that spectra obtained in one day using a single ionization method showed reproducibility of ion intensities to better than 10% ; the spectra reported for different ionization methods were obtained over a period of approximately 1 month with consequent small changes in instrument parame- ters. A similar comparison of 2 prepared by different methods leads to S I values in the same range, al- though in this case one cannot discount the possibility of contributions from other structures, such as 3 and 4.

If one accepts, on a conservative basis, an SI > 40 as an indicator of significantly different CID spectra and, thus, as an indicator that different structures or mix- tures of structures are being compared, it is clear that 1 and 5 represent two distinct structures, a conclusion that is readily reached from more qualitative argu- ments. Ions of initial structures 2 and 3 show SI values in the range 13-48, depending on the method of preparation; the larger value is barely beyond the arbitrary criterion for significant differences. These results thus support the conclusion derived above that the ions initially prepared as 2 and 3 have isomerized to a common structure or mixture of structures, al- though some differences in the proportion of different structures dependent on the method of preparation may be indicated by the relatively large SI values. Ions of initial structure 4 remain problematical. Those pre- pared by CH, CI and by i-C4H10 CI show substan- tially different CID spectra (SI = 53). Ions prepared by CH, CI show CID spectra similar to the CID spectra for 2 and 3 prepared by E1 or CH, CI (SI = 33-44) but different from the spectra shown by 2 and 3 prepared by i-C4HI0 CI (SI=70-86). Ions of initial structure 4 prepared by i-C,Hlo CI show distinctly different CID spectra than ions prepared initially as 2

or 3 by any method (SI=75-128). Although these comparisons are not completely clear-cut they tend to reinforce the conclusions reached above that 2, 3 and 4 largely isomerize to the same mixture of structures following formation by CH, CI but 4 largely retains a distinct structure after formation by i-C4HI0 CI. 'The present comparison suggests that the use of similarity indices does not solve the basic problem of deciding when CID spectra are significantly different; one still must decide what value of the similarity index repres- ents a significant difference.

Unimolecular fragmentation reactions: kinetic energy releases

Two unimolecular met astable fragmentation reactions, (1) and (2), are observed for [C,H,O]+ ions.

[C,H,O]+ ---$ [CH,OH]'+ C,H, (1)

-+ [C,HsI+ + H,O (2) Table 4 records the relative fragment ion intensities

Table 4. Fragmentation of [C,H,O]+ ions

Initial structure ObseNation mode

1 rn* El rn* CI(CH,) MS CI(HJa CID CI CID El

rn* CI(CH,) MS CI(H,Ia CID CI CID El

3 rn * CI(CH,) M S CI(H21a CID CI

4 rn* CI(CH,) M S CI(H21a CID CI

5 m* CI(CH,) MS CI(H,Ia CID CI

2 m*El

Relative intensity [CH,OHI' I&H,I'

100 63 100 61 100 55 100 68 100 60 9.4 100 25 100

100 55 100 33 100 28 23 100 100 57 100 47 14 100

100 19 100 17 36 100 34 100 5 100

a Relative fragment ion abundances in H, CI mass wectra. from Ref. 13.

520 ORGANIC MASS SPECTROMETRY, VOL. 18, NO. 12, 1983

STRUCTURE AND FRAGMENTATION OF [C,H,O]+ IONS

observed in the unimolecular MIKES spectra of ions initially produced as 1-5 by CH, CI of the appropriate precursor (m* CI CH,). Also included are the relative intensities observed in the MIKES spectra of 1 and 2 prepared by dissociative electron impact ionization (m* EI), the relative fragment ion intensities observed in the H2CI mass spectra of the respective C3H60 precursors'" (MS CI H2) and the relative ion intensities observed in the CID spectra of the ions initially pre- pared as 1-5 by CH, CI (CID CI). The unimolecular MIKES (metastable ion) spectra represent the lowest energy fragmentation reactions observable with the H2 CI mass spectra and the CID mass spectra, repres- enting fragmentation reactions occurring at considera- bly higher average internal energies.

For 1, the relative fragment ion intensities are es- sentially insensitive to the reaction mode examined; hence, over the internal energy range examined, the relative rates of the competing reactions are essentially independent of internal energy. This is consistent with the that fragmentation of 1 by either reaction (1) or (2) involves a rate-determining isomeri- zation to structure 2 (Fig. 1). In contrast to 1, ions of initial structures 2-4 show reaction (2) as the domin- ant fragmentation reaction at low internal energies. This is consistent with the observation (Fig. 1) that reaction (2) has the lower critical energy while reac- tion (l), involving a less complex rearrangement, has a more favourable frequency factor and is favoured at high internal energies. The metastable ion intensity for reaction (1) is more intense for 2 formed by CI than for 2 formed by EI, indicating that the ions fragment- ing as metastables after formation by CI have, on average, a higher internal energy than those fragment- ing as metastables after formation by EI. Similarly, the relative intensities indicate that the ions fragmenting after collisional activation have average internal energy higher than those fragmenting in the H2 CI mass spectra. For 5, fragmentation reaction (2) is favoured at all internal energies consistent with the fact that this reaction not only has the lower critical energy but also has a favourable frequency factor since it nominally is a simple bond cleavage reaction (see, however, the discussions of the results for the metasta- ble fragmentation of 5-0-D discussed below).

The metastable ion intensities observed after

protonation of CH3CD2CH0, CH,CHCD,O and m

CH2CH2CD20 by CH4 CI also were recorded. These results are reported in Table 5 and compared, where available, with results obtained in E1 studies12 and with the relative fragment ion intensities observed in the H2 CI mass spectra of the same compounds.'3 The relative metastable ion intensities observed fol- lowing CI preparation of the fragmenting ions are in good agreement with the relative intensities observed in the E1 work. Extensive isotope mixing is observed; the E1 results have been modelled quantitatively by Bowen et al.12, the essential features of their model being that the oxygen-bonded hydrogen is largely, but not quantitatively, retained in the oxygen-containing fragment, while the hydrogens bonded to carbon be- come essentially randomized prior to fragmentation. Interestingly, the relative fragment ion intensities ob- served in the H, CI mass spectra of 2-d2 and 3-d2 are in essential agreement with the metastable ion inten- sity data; in these cases, the extent of H/D scrambling does not appear to depend on the internal energy of the fragmenting ions. By contrast for 4-d2, the H2 CI mass spectral results indicate a distinct preference for loss of GH2D2 and GH, compared to GH3D. This result has been interpreted13 in terms of preferential decomposition at high internal energies of the 'CH2CH2CD20H and 'CD2CH2CH20H ions formed in the initial ring-opening process.

The oxygen-deuterated forms of 1-5 also were pre- pared by CD, CI; in addition, 1-0-D and 2-0-D were prepared by E1 methods. Table 6 records the relative metastable ion intensities for loss of H 2 0 and loss of HDO from the labelled ions as well as the associated kinetic energy releases To.5 for these fragmentations and for loss of H20 from the unlabelled ions. The 1- 0-D ion shows predominant (9OOh) loss of HDO in metastable ions following formation by both E1 and CI; this is consistent with a rate-determining isomeri- zation 1 -+ 2 followed by a relatively rapid fragmenta- tion. The 2-0-D ion shows only -71% loss of HDO in metastable ions following both E1 and CI forma- tion; by contrast, the fragment ion intensities in the D2 CI mass spectra show13 -92% loss of HDO from

Table 5. Fragmentation of [C3H5D20]' ionsa Relative lossb of Relative lossb of

Initial structure Observation mode CzH2Dz CzH.D CZHI 4 0 DHO HzO

CH,CD,CH==~H m* EI 28 60 12 - 25 75 2- dz m* CI(CH,) 34.2 54.4 11.4 0.9 22.8 76.3

+ MS CI(H2) 33.2 54.7 12.1 - 23 77 CH,CH-CD2-OH m* CI(CH,) 32.6 54.6 12.8 0.9 24.6 74.5 3-d2 + MSH,CI 25.5 53.6 20.9 - 16.2 83.8 ~H,CH,CD;OH m* EI 32 56 12 - 23 77 Qd2 m* CI(CH,) 30.9 53.0 16.1 1.0 25.8 73.2

MS H2 CI 47.8 17.2 39.9 - 26.1 73.9 a El results from ref. 12. MS CI (H,) represents fragment ion intensities in H2CI mass spectra from ref. 13.

Expressed as a percentage of total ion signal in relevant mass region.

ORGANIC MASS SPECTROMETRY, VOL. 18, NO. 12, 1983 521

A. G. HARRISON, T. GAUMANN AND D. STAHL

Table 6. Abundance and To.5 for loss of (H, D),O from [C,&O(D, IQ]' ions

Ion (formation mode)

(CH,),C==OH+ (El) (CH,),C==OH+ (CI) (CH,),C==OD' (El) (CH,),C==OD' (CI) CH,CH,CH=OH' (El) CH,CH,CH=OH+ (CI) CH,CH,CH=OD+ (El) CH,CH,CH=OD+ (Cl) CH,CHCH,OH+ (CI) - -

CH,CHCH,O~ (CI) CH,-CH,-CH,--~~~ (CI) CH,CH,CH,O~ (cl) CH,=CH-CH,~H, (CI) CH,=CH-CH,-~HD (CI)

-

70 loss of H20 HDO

100 - 100 -

9.6 90.4 11.3 88.7

100 - 100 - 28 72 29 71

100 - 3 4 6 6

100 - 42 58

100 - -58 -42

a Composite metastable peak.

To.5 (rneV) for loss of HzO H W

21.6 - 25.8 - 21.2 21.5 23.8 23.8 17.8 - 19.1 - 18.1 18.2 19.1 18.8 20.3 - 19.4 17.2 16.0 - 23.5 17.3 12.0" - 22.2 0.1,16.88

2-0-D. The extent of H/D exchange prior to the metastable fragmentation of 3-0-D and 4-0-D is even greater with HDO loss being only 66% and 58%, respectively; in the D2 CI mass spectra HDO loss is -93%. Clearly, the extent of mixing of the oxygen- bonded hydrogen with the carbon-bonded hydrogens is strongly dependent on the internal energy (and, hence, lifetime) of the [C,H,O]' ion. Deuteronation of allyl alcohol on the oxygen should lead to a species which readily fragments by loss of HDO in a simple bond fission reaction and, in the D, CI mass spectrum of allyl alcohol elimination of HDO, accounts for -89% of the water loss reaction.13 By contrast, in metastable ions loss of H20 is more prevalent than loss of HDO. Further, as shown in Fig. 2, the metasta- ble peak for loss of HDO is clearly composite in shape. The narrow component (T,,,, - 0.1 meV) pre-

I I I I I 4 100 4120 4140 4160 4180

VOLTS

Figure 2. Metastable peak for HDO elimination from 5-0-D. Main beam transmitted at 6000 V.

sumably corresponds to a simple bond fission reaction of the 0-D species. The broad component (Tcl.5- 17 meV), as well as the metastable component for loss of H20 (To.5 = 22 meV), must represent fragmentation occurring after rearrangement (Scheme 2).

The To5 values associated wth elimination of H20 and HDO from 1-0-D to 4-0-D are all relatively small and show no significant trend with initial ion structure. The kinetic energy releases are greater for fragmentations of ions generated by CI than for frag- mentation of the same ion generated by EI. Similar results are observed for the ethylene-loss reaction (Table 7). Holmes et have reported similar obser- vations while Schwarz and Stahl16 have observed that the kinetic energy releases depend on the exothermic- ity of the protonation reaction forming the fragment- ing ion. In the present work, we have observed that the kinetic energy releases associated with loss of H20 from 1 is 21.6 meV following EI, 25.8 meV following CH, CI and 28.5 meV following H2 CI; similarly, the kinetic energy release associated with loss of C2H4 from 1 was 12.9 meV following EI, 18.6 meV follow- ing CH, CI and 22.2 meV following H, CI. The kinetic energy release originates from partitioning of the non- fixed energy of the decomposing ion among the availa- ble modes including translation along the reaction coordinate and the results must mean that the ions formed by CI methods and fragmenting as metastable ions have, on average, a greater non-fixed energy than the same ion formed by E1 methods; in addition, the non-fixed energy in the fragmenting ions appear to be larger the more exothermic the CI reaction forming the fragmenting ion. The higher average energy icon- tent of the metastable ions in CI compared to E1 has been a t t r i b ~ t e d ' ~ to shorter ion residence times in the CI experiments. However, this seems unlikely as a general explanation since the repeller voltage fre- quently is lower in CI experiments and the collisions undergone by the ions should also retard ion with- drawal; both effects would tend to increase source residence times in the CI experiments compared to E1 experiments. In addition, this explanation does not explain the dependence of the energy content on the exothermicity of the CI reaction; in the CI experi- ments with different reagent gases the lifetimes and, hence, the internal energies should be the same i n all cases. A more likely explanation is that the energy distribution within the energy band corresponding to metastable ions differs for the different methods of preparation of the fragmenting ions. The ions formed by CI are produced initially with a high internal energy (determined by the protonation exothermicity) and relax to the metastable region by collisional de- excitation with the result that a distribution skewed to favour higher internal energies (Fig. 3) is not unex- pected. By contrast, the ions produced by E1 are

-DHO CH3-6HCHzOD --* CH2=CH-CH,-6HD C,H,+

+ /" CH,=CHCH,OHD

- HzO CH2D--6H-CHzOH __* CDH=CH-CH,-&H, + C3H,D' \

Scheme 2

522 ORGANIC MASS SPECTROMETRY, VOL. 18, NO. 12, 1983

STRUCTURE AND FRAGMENTATION OF [CC,H,O]' IONS

Table 7. Abundance an4 To.5 for loss of G(M, D)., €rom IC3&O(H, D)1+ ions

Ion (formation mode)

(CH,),C=OH+ (El) (CH,),C=OH+ (Cl) (CH,),C=OD+ (El) (CH,),C==OD' (Cl) CH,-CHZ-CH=OH' (El) CH~-CH,-CH=OH' (Cl) CH3-CHz-CH=OD' (El) CH,CH,CH=OD+ (Cl) CH,--CH-CH,OH+ (CI) I

CH,-CH-CH,O~ (CI) CH,-CH,-CH,-O~I (CI) CH,-CH,-CH~-O~ (CI) CH,==CH-CH~-~H, (ci) CH,=CH-CH,--OAD (CI)

% loss of CzH4 C z b D

100 - 100 - 68 32 73 27

100 - 100 - 64 37

100 - 70 30 100 - 64 36

100 - 39 61

n 23

To.5 ( m V ) for loss of CzHe CzHdJ 12.9 - 18.6 - 13.8 12.5 20.7 15.0 13.8 - 28.0 - 15.2 10.2 35.9 17.3 21.3 - 24.4 14.2 16.0 - 19.7 13.1 24.0 - 22.5 28.8

formed in a dissociative ionization process and only a small fraction dissociate further, hence, a distribution skewed to favour lower internal energy ions is ex- pected (Fig. 3). The net result would be a higher average internal energy for metastable fragmenting ions in the CI experiments. In addition, it would be expected that the higher the initial internal energy in the CI experiments the greater the extent of skewing of the distribution and the higher the average energy content. This interpretation implies that the kinetic energy release in the CI experiments should depend on the reagent gas pressure since the extent of coili- sional de-excitation should increase with increasing pressure; systematic experiments to test this have not been performed.

The relative metastable ion abundances for loss of CZH4 and C2H3D from the [C3H6DO]+ ions produced by CD, CI are summarized in Table 7, which also includes the similar data for 1-0-D and 2-0-D pre- pared by E1 methods as well as the relevant metasta- ble ion kinetic energy release data for both labelled and unlabelled ions. The deuteronated ions 1-0-D-4- 0-D show 27-37% elimination of G H 3 D in metasta- ble fragmentation, irrespective of whether formed by E1 or CI methods. By contrast, in the source fragmen- tation reactions following D2 CI only 10-20°/0 elimi- nation of C2H3D is 0b~erved. l~ Deuteronated allyl alcohol shows 61% loss of GH,D, similar to the -64% observed13 in the D, CI mass spectrum.

METASTABLE , BAND ~

E

Figure 3. Internal energy distribution for ions prepared by El and by CI.

Deuteronation at the oxygen of allyl alcohol followed by elimination of ethylene by the path summarized in Fig. 1 would not lead to more than 50% elimination of GH,D and it is possible that there is some direct protonation (deuteronation) of the double bond.

The kinetic energy release data show that, for 1-0- D-4-0-D, To.5 for loss of GH,D is less than To.5 for loss of GH,, although the difference is relatively small for 1-0-D prepared by electron impact. This confirms results reported earlier14 for 2-0-D. It is apparent that, at least for 2-0-D-4-0-D, GH3D loss must be occurring by a different mechanism than c;?H4 loss. As discussed above, Holmes et al.', have proposed the mechanism outlined in Scheme 1 in addition to the mechanism implicit in Fig. 1. The present results sup- port this interpretation by showing that it applies also to ions 3 and 4.

In the CI studies the kinetic energy releases are greatest for ions initially prepared as 2 and smallest for ions initially prepared as 4. In view of the relative paucity of information concerning the effect of internal energy on kinetic energy release in metastable ion fragmentation following chemical ionization, the sig- nificance of these differences is not clear. Similarly, the significance of the difference ion the kinetic energy releases for loss of GH,D and GH, from deutero- nated allyl alcohol is not clear; the difference does not appear to be explainable solely on the bases of the mechanisms outlined in Scheme 1.

CONCLUSIONS

The present work has reported a detailed study of the unimolecular metastable and collisionally-induced fragmentation reactions of [C,H,O]+ ions initially pro- duced as 1-5 by proton-transfer chemical ionization. This represents the first study of 5 and the first de- tailed studies of 3 and 4. The results show clearly that 1 and 5 represent distinct stable ion structures but 2 and 3 readily interconvert. The structure 4 intercon- verts less readily with 2 and 3. The fragmentation reactions of [C3H,0]+ ions cannot be rationalized entirely on the basis of the potential energy diagram (Fig. 1) established from E1 studies but require a second mechanism for elimination of GH, (Scheme 1) proposed earlier.I4 The relative metastable ion abun- dances and metastable ion kinetic energy releases show clearly that metastable ions in chemical ioniza- tion systems have higher internal energies than metastable ions (of the same nominal structure) in electron impact systems. This difference is attributed to the difference in the method of preparation of the ions which results in different distributions in the energy band corresponding to metastable ions.

Acknowledgements A.G.H. gratefully acknowledges a sabbatical leave provided by the University of Toronto. The financial support of the Natural Science and Engineering Research Council of Canada and the Fonds Na- tional Suisse de la Recherche Scientifique is gratefully acknowl- edged.

ORGANIC MASS SPECTROMETRY, VOL. 18, NO. 12, 1983 523

A. G. HARRISON, T. GAUMANN AND D. STAHL

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Received 22 April 1983; accepted 22 July 1983

524 ORGANIC MASS SPECTROMETRY, VOL. 18, NO. 12, 1983