Organic Mass Spectrometry, 1969, Vol. 2, pp. 1085 to 1101. Heyden & Son Limited. Printed in Northern Ireland

KINETIC STUDIES IN MASS SPECTROMETRY-V:

EVALUATION OF WIDE RANGE ELECTRON ENERGY KINETICS AND METASTABLE ION RELATIVE ABUNDANCE

TECHNIQUES, AND THE STRUCTURES OF [XC,H,CH,]+ IONS GENERATED FROM SUBSTITUTED BENZYL

PHENYL ETHERS AND TOLUENES*

PETER BROWN Chemistry Department, Arizona State University,

Tempe, Arizona 85281, USA

(Received 16 May 1969; accepted (revised) 7 July 1969)

Abstract-The problem of ion structure and energy is discussed briefly and attention directed to the existence of two types of structural information. Studies of ion formation reactions bear on the initial structure of the ion, whereas decomposition reactions relate to the ion at a later time, after structural rearrangement has had chance to occur. The desirability of conducting both types of investigation concurrently is emphasized. Of the available experimental methods at this time, kinetic measurements for formation studies and metastable ion relative abundances for decomposition studies have been utilized, although other techniques may eventually prove preferable in the future. The [M - Y] reaction of a series of substituted benzyl substrates XC8H,CH,Y, where Y = OC,H, and H, has been examined, and the present results are compared with previous work on ion structure and energy in this area, with the aim of evaluating the potential of the newer techniques.

I N T R O D U C T I O N

A MAJOR problem in mass spectrometry that is becoming increasingly acute is the general question of ion structure and its relationship to ion energy. Although the assumption that molecular and fragment ions still contain the same arrangements of atoms that are present in the original neutral sample molecules has proved to be extremely valuable in the interpretation of mass spectra of organic molecules,l to

it remains nevertheless only an assumption for the vast majority of cases. Actual determination of the structures of ions produced in the mass spectrometer remains a formidable task.

A much more thorough knowledge of ion structures is highly desirable for a number of reasons, such as (i) to extract the maximum amount of structural informa- tion for analytical applications of mass spectrometry; (ii) for valid evaluation of structurelreactivity relationships, such as substituent efTects5 and stereochemical effects;, (iii) for the detailed study of mechanism in gaseous ion reactions; and (iv) to permit more critical comparison of electron-impact induced reactions with those initiated by thermal,'^^ photochemical,ss9 or other forms of excitation.1° The ions produced by electron-impact in a mass spectrometer cannot be isolated and character- ized by conventional chemical and physical means, however, but only by their char- acteristic unimolecular (e.g. fragmentation pattern, appearance potentials, metastable

* Presented in part at the ASTM Committee E-14, 17th Annual Conference on Mass Spectrom- etry and Allied Topics, Dallas, Texas, May, 1969.

1085

1086 h T E R BROWN

ion peaks) or bimolecular (e.g. ion/niolecule reactions by tandem mass spectrometerll or ion cyclotron resonance12 techniques) reactions. Thus at present, gaseous ion structures are virtually impossible to 'prove' and the best that can be done is to design critical experiments to distinguish between all chernically reasonable alter- native structures. Even then, one must be prepared for complex situations in which a given mass spectral peak is due to ions of more than one particular composition, energy or energy distribution, and structure, and where ions of the same molecular composition may be formed from the molecular ion by multiple pathways.

DISCUSSION

SCHEME 1

[MI Tie > [MI+* -%- [A]+ * fragments

To determine the energy/structure characteristics of daughter ion [A]+, two distinct types of structural information can be 0btai11ed.l~~~~ In the first place, decomposition reactions of the ion such as [A]+ 4 fragments, as evaluated by (i) the fragmentation pattern, and including isotope distribution in the fragment ions resulting from labelled substrate^;^^.^^ (ii) metastable ion peak relative intensities;15 and (iii) ion/molecule reactions,l1,l2 give structural information that must be extrapolated back to the original ion structure [A]+ via at least one activated complex. This kind of data permits conclusions about only the [A]+ ions which are sufficiently energetic to de- c o m p o ~ e , l ~ . ~ ~ and says nothing about the more stable ions that are actually collected and measured.17 In the second place, formation reactions of the daughter ion by [MI+. + [A]+ as assessed by (i) appearance potentials13J8 of [A]+; (ii) kinetic substituent effect^;^,^^ and (iii) isotope effects,2O give structural information that must be extrapolated forward to the original structure of ion [A]+ via other transition states.

By judicious selection of experimental methods then, it is possible in principle to observe both formation and decomposition of [A]+ ions.* The significance of such approach is emphasized by the possibility that [A]+ ions might conceivably rearrange after initial formation, but before further decomposition. For this reason, structural implications derived from decomposition and formation studies of [A]+ ions need not necessarily be the same.13 Also the transition state for formation of [A]+ ions is taken to be a better model for the actual structure of the initially formed [A]+ ions than is the transition state for their fragmentation for a second reason. It is well known that most unimolecular ion decomposition reactions, and especially cleavages, are endothermic,18 and according to Hammond's Postulate?l a transition state resembling the product rather than the reactant in structure is to be expected.

To characterize an ion fully, one must be able to specify (i) elemental composition, (ii) structure, and (iii) internal energy (including energy distribution)22 of the ion. If one is interested in comparing the structures of two isoineric ions produced from different substrate^,^^ then both structure and energy of the two ions will be reflected in such measurable quantities as appearance potentials and heats of formation, and also in the relative rates of fragmentation reactions of the ions, as reflected by relative

* This dual approach has been pioneered by Meyerson et u I . ' O ' ~ ~ using ionization and appearance potentials, and isotope label distribution techniques in their classic studies on the structure of [CBH5CH# ions.

Kinetic studies in mass spectrometry-V 1087

peak intensities for both and metastable ions23 appearing in the fragmentation pattern. Thus structure and energy of an ion are not independent of each other as judged by the above criteria. Variation of either structure or energy while the other remains constant will each lead to changes in appearance potential and heat of forma- tion of the ions, and in the normal and metastable rates of decomposition of the ion. For this reason, attempts to decide whether two isomeric ions of different energies from different sources have the same or different structures are likely to be diffi~ult.2~

As a minimum effort, then, directed towards ion structurelenergy elucidation, it is necessary to investigate activation energies (i.e. AP-IP)25 for the formation of [A]+ ions, and activation energies for those that decompose, and to attempt to eliminate frequency factor differences22 in the systems under investigation. The main experimental methods available for such endeavors have recently been discussed critically.22 For the formation of [A]+ ions, ionization and appearance potentials* or rate measurements should in principle provide convenient estimates of the activa- tion energies involved. There are, however, some major difficulties in the determina- tion, calculation and interpretation of the widely used parameters ionization and appearance potentials,22*2G and their insensitivity to relatively small energy differences (i.e. up to 0.3 eV) as determined on mass spectrometers with conventional energy spread of the ionizing electron beam is a further drawback. In sufficiently sensitive measurements, fine structure in both ionization and appearance potential curves has been 0bserved,2'*~*,~~ and it is no simple matter to decide which state in the molecular ion [MI+. correlates with which state in the daughter ion [A]+. It seemed important therefore at this time to attempt some systematic evaluation of kinetic methods19 of estimating activation energies, using carefully chosen systems which have been previously studied by ionization and appearance potential techniques.

Severe objectionsz2*30 to the original simple kinetic treatment19 of comparing log Z/Zo values with cr constants can be largely overcome by (i) selecting model systems in which the molecular [MI+. and daughter [A]+ ions comprise the major portion (ideally > 90%) of the total ionization at all electron energies (neither

and (ii) by comparing only the effect of the smallest feasible change in isomers on activation energy for the same reaction.

It has been suggested% that (ii) might be realized by comparison of the rates of decomposition of m- and p-X substituted isomers with the same substituent X,? as measured by Z = [A]/[M],19 over the entire electron energy range from 70 eV to threshold for [A]+ formation. This approach has been termed33 'wide range electron energy kinetics' , and is essentially a kinetic comparison of the appearance potentials of isomeric or identical [A]+ ions, formed from m- and p - X isomer pairs of neutral compounds of essentially the same heats of formation.16 By comparison of only the rates for the one major reaction of m- andp-X isomeric substrates, factors such as (i)

* The appearance potential of a daughter ionza is the energy necessary to reach the activated complex for its formation, and to make the reaction proceed with a rate constant of the order of lo5 sec-'.

t X = D would provide the minimum structural change, but the smaller the complicating effects of introducing a substituent X, the smaller will be the difference between the activation energies of interest.

[MI+* nor [A]+ should suffer other decomposition to any significant extent5.30s31'32 >;

1088 PETER BROWN

different bond strengths of all bonds cleaved or formed in the reaction;32 (ii) competi- tive reactions in the molecular ions;32 (iii) secondary decompositions of the daughter ions [A]+;32 (iv) frequency factor differences;22 and (v) degree of freedom differences are reduced to a minimum. A further remaining objection to using the kinetic approach is that due to m- and p-X derived molecular ions of different ionization potentials, which might generate [A]+ ions of common structure but different energy distribu- t i o n ~ . ~ ~ . ~ ~

recently that Z = [A]/[M] is not a true measure of the absolute rate of the simple reaction [MI+. ---f [A]+, in that relative ion intensities as measured at the collector are not the same as those in the source.1g As a consequence of unimolecular decay kinetics, the ratio [AJ/[M] was shown30 to be dependent on accelerating voltage, and hence on the time of flight of the ions between the source and the collector. A case was also madew for the existence of a certain fraction of ions [MI+. at any time with insufficient energy to fragment further to [A]+. Such an energy distribution of reactant species is a common kinetic situation.

The reason for the current use of the approximation 2 = [A]/[M] as a measure of reaction rate in wide range electron energy kinetics is that one is interested in comparing the energetics of the one major reaction of two isomers, rather than obtaining ‘absolute reaction rates’. For example, as discussed below, if molecular ions of identical ionization potentials and energy distributionszz of m- and p-X isomers each rearrange to some common ion prior to decomposition in the ion source, then these common ions will behave identically, and Zp/Zm is as good a measure as any of the relative similarities and differences between them. For other energy/ structure differences between isomeric molecular ions and their activated complexes, as discussed below, the use of 2 values is clearly a poorer approximation, but in view of the paucity of experimental techniques presently available for ion structure] energy studies, it is felt that evaluation of wide range electron energy kinetics, together with metastable ion relative abundance techniques, for the simplest possible cases is still desirable. Obviously, with the above limitations on systems that may be studied, the approach described here will never be a general one, but only applicable to a few carefully selected systems where the approximations involved in the kinetic treatment are most likely to hold.

For each pair of m- andp-X isomers with the same substituent X undergoing the reactions in Scheme 1, four possible extreme energetic situations* can be visualized, and are shown in a simplified way in Fig. 1. In energy diagrams A and B, activation energies? (Em and Ep) and therefore reaction rates, if frequency factor differences can be neglected,22 are different for each m- andp-X pair, whereas in diagrams C and D these parameters are the same. In energy diagrams A and C, appearance potentials for [A]+ are the same for each m- andp-X pair, whereas in diagrams B and D they are different. Thus in principle, comparison of reaction rates Zp and Zm, and of

* A spectrum of energy surfaces intermediate between each pair of diagrams in Fig. 1 is of course probable, but the four cases shown facilitate discussion.

t The approximation of one activated complex and one activation energy only holds at best a t threshold. At higher electron energies, a distribution of excited states may each contribute to the overall rate. Irrespective of the distribution and population of excited states in the m- and p-X ions with the same substituent, however, the effects of the substituent X (provided that they are not isolated from the reaction site) on activation energy for a decomposition reaction are not expected to be identical, as long as substituent positional identity is maintained.

It has been pointed

Kinetic studies in mass spectrometry-V 1089

t Potential Energy

Transition States

Molecular Ions

Substrate Molecules

i

FIG. 1. Idealized Energy Diagrams of Activation Energies for Reaction Rates of rn- and p-X Substituted Isomer Pairs of Molecular Ions.

appearance potentials for [A]+ for each m- andp-X isomer pair will permit differentia- tion of the situations in energy diagrams A, B, C and D (Fig. l). This would allow assignments of equality or difference in molecular ion and transition state energy, which in turn might be structurally significant.16 Ionization and appearance potential measurements have been widely used for such p u r p o s e ~ ~ 0 ~ ~ ~ ~ ~ ~ but, as stated previously, the usual precision of such data leaves something to be desired.22.26 Future develop- ments might well make the more direct techniques the methods of choice eventually, especially in conjunction with or m a t h e m a t i ~ a l ~ ~ * ~ ~ devices designed to reduce the magnitude of the energy spread of the ionizing beam.

For a kinetic distinction between energy situations A, B, C and D (Fig. l), it has been proposed33 that Zp/Zm be compared as a function of electron energy from 70 eV to threshold for [A]+ formation. Thus in the simplest possible case, different rates for m- and p-X isomer pairs of compounds at 70 eV will result in the same Zp/Zm ratio as threshold is approached for diagram A (same transition state energies) but variation of Zp/Zni as threshold is approached for B (different transition state energies), as the reaction with the higher appearance potential (m- orp-X) is dis- criminated against preferentially. Similarly, the same rates Zp = Zm at 70 eV for in- and p-X isomers will result in Zp/Zm = 1.0 as threshold is approached for energy diagram C (same transition state energies), but variation of Zp/Zm as threshold is approached for D (different transition state energies).

Molecular ions of rn- and p-X isomers may therefore have the same (energy diagrams B and C) or different (A and D) energies, and therefore ionization potentials also, and initially at least different structures. Regardless of whether the molecular ions subsequently rearrange to a common structure or not for each isomeric pair of compounds, the negotiation of a common energy barrier means identical activated complex energies en route to [A]+ (Fig. 1, diagrams A and C), and therefore constant

1090 PETER BROWN

ZpjZnz over the entire electron energy range. It is of course also conceivable that different molecular ions may react via structurally dissimilar transition states of fortuitously the same energy, and hence still give rise to constant Zp/Znz ratios.

A further probe into molecular ion energy and structure is provided by metastable ion relative a b u n d a n c e ~ , l j * ~ ~ . ~ ~ which will always be used in conjunction with wide range electron energy kinetics measurements. In this way, the parameters [A]/ [MI, [ml*]/ [ml*]/[A],2z’23 [m,*]/ and [~II,*]/[A]~~ can be used to maximum effect. It has been pointed outlS that ions [MI+. identical in structure, energy and energy d i s t r i b ~ t i o n ~ ~ * ~ ~ will give identical [m,*]/[M] (Scheme 1) ratios. Of the four limiting energy situations (Fig. l), only that described by diagram C , with rearrange- ment of the molecular ions to some common structure of the same energy distribu- tion* prior to traversing the transition state for decomposition to [A]+ ions, would be expected to give identical [m,*]/[M] or [m,*]/[A] ratio^^^.^^ for m- and p-X isomers over the whole electron energy range. Coincidence at just one electron energy is clearly less rigorous. Other energy situations A, B and D, whether the cause of molecular ion energy difference between m- and p-X isomers is structural or not, would not be expected to give identical [m,*]/[M] ratios over the entire electron energy range. The useful findings have been reportedz3 that ions common in structure and energy distribution, but different in internal energy, exhibit higher values of [A]/[M], [ml*]/[M] and [A]/[m,*] for the higher energy ion.

As methods of characterizing the daughter ions [A]+ that can decompose, appear- ance potentials of further fragment ions,la isotope label distribution in both normallo and m e t a ~ t a b l e ~ ~ e ~ ~ fragment peaks, and comparison of relative metastable ion a b u n d a n c e ~ l ~ . ~ ~ may be cited. Since compounds in which [A]+ ions do not fragment further to any great extent are required (see above) and because of reservations con- cerning appearance potential determinations, metastable ion relative intensities are the method of choice at the present state of the art. Again, future developments may eventually enable utilization of some other more powerful technique for characterizing the energy/structure of decomposing daughter ions [A]+.

Of the four limiting energy situations (Fig. l), again only that depicted in diagram C, with molecular ions of the same energy distribution traversing identical activated complexes structurally and energetically and leading to identical daughter ions [A]+, would be expected to give identical [mz*]/[A] (or [m2*]/[m3*]15 if there is more than one metastable decomposition pathway of [A]+) for each m- and p-X pair over the whole electron energy range. Identical [mz*]/ [MI ratios have been proposed3’ as evidence for common, rearranged molecular ions of the same energy and energy distribution. If the structures of the identical energy transition states for isomer pairs as in diagram C are actually different, then structurally different molecular and [A]+ ions must be involved, and different [m,*]/[A] ratios are to be expected. Different [mz*]/[A] ratios are also predicted for energy situations A, €3 and D, irrespective of whether or not the energy differences are due to structural differences between rn- and p-X pairs.

It is proposed therefore that observation of Zp/Zm, and m,* and m2* relative abundances, over the electron energy range 70eV to threshold for the one major

* It has been notedz2 that ions of the same structure but different energy distributions may well give different relative metastable ion abundances.

Kinetic studies in mass spectrometry-V 1091

reaction (Scheme 1) of m- and p-X isomer pairs of compounds will provide informa- tion as to the equality or difference in energy of the relevant isobaric ions and their activated complexes. In order to then permit structural conclusions to be drawn from this 'relative energy' data, it will be necessary to apply wide range electron energy kinetics and metastable ion relative abundance techniques to several test systems that have been thoroughly investigated by conventional methods, and to make the necessary correlations. If structural inferences can indeed be made, then it might be possible, for example, to detect the timing of the rearrangement step in a rearrange- ment reaction, i.e. at the molecular ion or daughter ion stage. Such deductions hould only serve as indications of the actual situation, however, and never as absolute proof.

With these considerations in mind, it was decided to study first the [M - Y] reactions of a series of m- and p-X substituted benzyl compounds XC6H,CH,Y (Scheme 2).

SCHEME 2

[XC6H4CH,Y]+- a - y. [XC6H4CH2]+ fragments

M A

Among the reasons for this choice was the fact that decomposing [C,H,CH,]+ ions generated from a wide variety of benzyl substrates have been shown by the extensive studies of Meyerson ef to be higher symmetry than a benzyl structure, and presumably therefore the tropylium ion.3s This information, however, provides no clue as to the structure of the non-decomposing [C6H,CH2]+ ions, nor indeed to the nature of any of the initially formed [C,H,CH,]+ ions,13 which should be refiected in the transition state energy for the [MI + [M - Y] reaction in Scheme 2 for X = H.

Unfortunately, the heats of formation of benzyl and tropylium ions appear to be very similar.39 To circumvent this obstacle, Harrison et af.16*40 have described ingenious applications of m- and p-X substituent effects on appearance potentials of [XC6H4CH,]+ ions, generated both from substituted benzyl radicals40 and compounds ofthe general type16 XC6H4CH2Y, where Y = H, CH,, C1 and Br, and X = CH,, F, OH and OCH,. These experiments strongly suggested16 that the structure of [XC6H,CHz]+ ions at threshold is substituted tropylium when X = CH,, F and OH, but benzylic when X = OCH,. In addition, t o l ~ e n e , 3 ~ , " , ~ ~ methoxyt~luenes,~~ cy~loheptatriene,4~*~~ p-~ylene,4~ e t h y l b e n ~ e n e ~ ~ . ~ ~ benzyl chloride13 and benzyl a l ~ o l i o l ~ ~ ~ ~ ~ have been thoroughly studied by isotope labelling techniques. I t was felt that this area contains sufficient reported data to provide an essential check on experimental methods to be employed in the ion formation1decomposition studies described here and e l s e ~ h e r e , ~ ~ * ~ ~ * ~ ~ but also contains sufficient blanks to make extended investigation worthwhile.

At first, substituted benzyl phenyl ethers (Y = OC6H,, Scheme 2) were selected 33*49

to provide an excellent mass spectrometric leaving group for cleavage,* and for all substituents used, M and [M - Y] peaks accounted for at least 90% of the total ionization at all electron energies. As a case where cleavage of the leaving group is much less facile," substituted toluenes (Y = H) were chosen.48 For substituents X = OCH,, F and CN, again the only major peaks Mere at [MI and [M - Y], but

* As simple model systems, the heats of formation of [C,H5CH,]+ ions from benzyl bromide, ethylbenzene and toluene are 208, 236 and 232 Kcal. mole-' respectively.s0

4

1092 PETER BROWN

for X = CH,, C1 and Br competing reactions occurred giving peaks at [M - CHJ, [M - Cl] and [M - Br] respectively. As a possible intermediate situation, the [M - CH,] reaction of m- and p-X substituted ethylbenzenes (Y = CH,) has also been studied* by wide range electron energy kinetics and metastable ion relative abundance techniques.

RESULTS

Benzyl phenyl ethers The previous ob~ervat ion~~ of rate differences for the M -+ [XC,H4CH2]+ reaction

(Scheme 2, Y = OC,H5) between m- and p-X pairs of substituted benzyl phenyl ethers at 70 or 20 eV did not distinguish between molecular ion energy differences (Fig. 1 , diagram A) and/or transition state energy differences (diagram B). The wide range electron energy kineticSS data (Table 1) seems to indicate a difference in transi- tion state energies for each isomer pair of compounds, which could be due to either

TABLE 1. zp/z?ft RATIOS FOR THE REACTION [XC,H,CH,-OC,H,] + {XC,H,CH,] + IN BENZYL PHENYL ETHERS AT VARIOUS ELECTRON ENERGIES

70 50 30 25 20 15 12

8 9 5 9.20 9.35 9 5 0 8.66 8.74 8 8 2 1-72 1.78 1.98 1.97 1.98 2.07 1.98 2.82 2.70 2.81 2.75 2.59 3.98 4.82 2.15 2 2 5 2.20 2.30 2.45 2.76 3.00 2.31 2.30 2.25 2.30 2.41 2.44 2.70 0.51 0.48 0.49 0-50 0.58 0.71 0.83 0.97 1-00 0.99 0.98 0 8 1 0.71 0-56

1 1

9.74 1.73 5.99 3-04 2.96 0.87 0.46

- 10 9

10.9 11-5 1.89 2.48 6.68 7.73 3.51 3.70 3.27 3.22 0.89 0-86 0.27 0.06*

8 eV.

15.2 2.74

5.62 4.29

-

* Measurement made from very low rates,

structural or energetic dissimilarities, or both. Thus ZplZrn varies for each substituent X over the entire electron energy range, and Zp # Zrn, i.e. energy diagram B (Fig. 1) is apparently the best model.?

derived from m- and p-methoxylbenzyl chlorides and m- and p-nitrobenzyl bromides respectively seem to be benzylic, the very similar variation in Zp/Zm (a factor of 1.5 to 3.6) with electron energy observed for the ethers for all substituents used inclu- ding X = OCH, and NO2 could be attributed= to structural differences, and taken to indicate benzylic transition states throughout. However, [XC,H4CH2]+ ions have been shown16 to have the substituted tropylium structure at threshold when X = CH, and F, and Y = H and Br. If bromine and phenoxy radicals behave similarly as leaving groups, as might be anticipated, then the transition state energy differences as reflected by variation of Zp/Zrn with electron energy could be ascribed to vibrational energy differences in otherwise common [XC,H4CH2]+ ions derived from m- andp-X isomer pairs of the substituted benzyl phenyl ethers, when X = CH,, F, C1, Br, and CF,.

Since [CH3O.C,H4CH2It ions at threshold1, and [N0,.C,H4CH2]+

* P. Brown, unpublished results. t In fact some small differences between IP's for m- andp-X pairs might be expected. 1: We thank Prof R. H. Shapiro, University of Colorado, for informing us of his experimental

results in favor of different structures for [NOZC6H,CH2]+ ions generated from both nz- and p- nitrobenzyl bromides and phenyl ethers in advance of publication.61

Kinetic studies in mass spectrometry-V 1093

Metastable ion characteristics15~* for [MI + [XC,H4CH,]+ ions (Table 2, upper transitions for all substituents except X = OCH,)? indicate decomposing molecular ions of different energy and/or structure for each m- and p-X pair, as [ml*]/[M] ratios are different over the energy range 70 (Table 2) to 15eV.t. Metastable ion characteristics for further decomposition of [XC,H,CH,]+ ions (Table 2, lower

TABLE 2. METASTABLE ION CHARACTERISTICS FOR DECOMPOSITION REACTIONS IN SUBSTITUTED

BENZYL PHENYL ETHERS XC,H,CH,OC,H, AT 70eV

[metastable] x 103 x Transition Metastable Lvarentl

rn-X P-X

OCHa 121 -+ 77

198 -+ 105 105 -+ 103

CH, 105+ 79 105-t 78 105 -+ 77

202 -+ 109 F 109-t 89

1 0 9 4 83

2187 -+ 125 c1 125f -+ 99

125t -+ 89

Br 2643 -+ 171 171$- 90

252 -+ 159 CFS 159 -+ 139

159 -t 109

229 -+ 136 NO2 136-t 106

1 3 6 4 90

49.0*

55.7 101.0 59.4 57-9 56.5

58.8 72.7 63.2

71 -7 78.4 63.4

110.8 47.4

100.3 121-5 74.7

80.8 82.6 59.6

0.36

6.0 0.74 0.80 0.17 0.22

3.1 0.042 0.62

2.8 0.13 0.30

6.0 0.71

6.3 0 4 3 0.72

2.7 t0.05

0.87

0.14

7.7 0.78 0.82 0.18 0.21

4.4 0.040 0.72

4-0 0.15 0.34

8.1 0.63

5-8 038 0.69

4.0 2.8 0.83

* A further metastable ion peak at m/e 68.4 probably arises from both 121 -+ 91 and 214 +

t m/e values for a5C1 given. $ Values for slBr given.

121 transitions.

transitions) over the same range of electron energy now suggest decomposing species of common structure and energy for each m- andp-X isomer pair, except where X = OCH, and NO,.

Williams et aZ?2.23 have recently discussed the interelationship between ion energy and both normal and metastable fragmentation rates. Using isomeric ionic species [MI+. of different internal energy generated (i) by direct ionization and (ii) by fragmentation of other molecular ions, and assuming the same structure and energy distribution before introduction of excess vibrational energy for the test ions, it was found23 that the parameters [A]/[M], [ml*]/[M] and [A]/ [ml*] (Scheme 1) all increase

* These have been redetermined under better experimental conditions since preliminary data were reported.40

t See Footnote (*), Table 2. 3 Below 15 eV, metastable ion peaks were of too low intensity for accurate measurements.

1094 PETER BROWN

with increasing average internal energy of the [MI+- ions decomposing to [A]+ ions. Applying these criteria to the benzyl phenyl ethers data, if the molecular ions for each m- and p-X pair of compounds have different ionization potentials, and if isomerization to common structures of different energies (i.e. energy diagram A, Fig. 1) for each substituent X were occurring, then for X = OCH,, CH,, F, C1 and Br, the ratios [A]/[M], [ml*]/[M] and [A]/[m,*] should be larger for the p-X isomer, whereas for X = CF, and NO, they should be larger for the m-X isomer. This assumes that the isomer of higher internal energy reacts faster, which is perfectly reasonable if only vibrational energy differences are involved.

In fact the observed [A]/[M] ratios (Table 1) are in agreement with predicted trends for all substituents X. For [m,*]/[M] (Table 2) and [A]/[m,*] ratios, the predicted order obtains when X = CH,, F, C1, Br and CF,, but not when X = OCH, or NO,. This evidence is consistent with (but does not prove) structurally common, rearranged molecular ions of different internal energy for each pair of isomers, decomposing via activated complexes of similar but not identical energy (tropylium- like, intermediate between energy diagrams A and B in Fig. 1) for X = CH,, F, C1, Br and CF,, but of different energy and very probably structure (i.e. benzylic) when X = OCH,16 and N02.51

It is felt that the balance of evidence at this time for the benzyl phenyl ethers is in favor of rearranged molecular ions, tropylium-like transition states and substituted tropylium ions as the immediate cleavage products, for substituents X = CH,, F, C1, Br and CF,; and unrearranged molecular ions, and benzylic transition states and immediate product ions, when X = OCH, and NO,.* These conclusions, therefore, modify the earlier claim33.49 that benzylic transition states and immediate product ions were involved for all substituents. With this interpretation, it is apparent that just Zp/Zm values alone did not in this case distinguish clearly between isomeric transition states, but do so only in conjunction with metastable ion characteristics.

SCHEME 3

X GYCHzY - I X GfHg c J'Q + fragments

X

(IV)

The data are most simply interpreted structurally in terms of Scheme 3. For substituents X = CH,, F, C1, Br and CF,, molecular ions of different internal energies

* Similar conclusions have been reached in the [M - CH,] reaction of substituted ethylbenzenes (P. Brown, unpublished results).

Kinetic studies in mass spectrometry-V 1095

I1 (Y = OC6H5) rearrange to some common structure such as IV (Y = OC,H,)* of different energies for each m- and p-X pair of compounds. Hydrogen44 and carbon randomization in IV are to be expected by analogy with documented thermal proc- e ~ s e s , ~ ~ the latter necessarily equilibrating the substituent position. Even if carbon scrambling did not occur, the effect of substituent position in the ring of IV on the activation energy for cleavage to V is likely to be insignificant, since in any position X can interact by conjugation with the reaction site. Therefore, tropylium-like activated complexes of slightly different energies leading to substituted tropylium ions V are envisaged, which then, due to their symmetry, decompose identically for each m- andp-X pair of isomers. Actually, similar rather than identical [m2*]/[A] ratios might have been expected, since the molecular ions have negotiated transition states of differing energies, depending on original substituent position. Apparently, from the [m,*]/[A] ratios in Table 2, this transition state energy difference is insufficient to produce a clearly defined energy difference in the common decomposing substituted tropylium ions V.

When X = OCH, and NO,, the data are consistent with (but do not prove) molecular ions I1 (Y = OC,H,) of differing internal energy and structure cleaving OC,H5 via transition states of different energies (probably benzylic), and affording substituted benzyl ions 111 for each pair of isomers. That the difference in structure and/or energy of decomposing [XC6H4CH,]+ ions I11 is retained is evident from their different r e a c t i o n ~ ~ ~ , ~ ~ and [mz*]/[A] ratios (Table 2) for each m- and p-X pair of substrates. For X = OCH,, benzylic structureP for [XC6H,CH2]+ ions can be rationalized in terms of favorable contributing canonical forms for only the para- isomer. The explanation of different structures for X = NO, is less obvious, but it is noted that NO2 is the most powerful electron withdrawing (by resonance) group employed, whereas OCH, is the best electron donor (by resonance).

TABLE 3. RELATIVE RATE RATIOS Zp/Zm FOR THE [M - HI REACTION OF SUBSTITUTED TOLUENES

X* 70 50 30 25 20 15 12 11 1OeV.

OCH,$ 2.94 2.91 2-83 285 274 2.65 2.22 1.80 1.17 1.04 1.02 1.04 0.99 1.00 0.94 0.91 0.95 0.9t CHIl

F 099 1-03 1.02 1.00 1.02 098 1.00 1.00 CI 1.47 1-47 1.39 1-40 1.36 1.49 1.5i - - Br 1.75 1.85 1.81 1.77 1.70 1.78 1.77 - - CN 1.00 1.00 0.98 0.99 0.98 0.97 1.00 l * O t -

-

* X = NOIl and COCH, did not show [M - HI peaks. t Measurements made from very low rates. 3 [M - HI measurements made from d,-methoxytoluenes.

Toluenes Wide range electron energy kinetic data (Table 3) for the [MI + [XC,H,CH,]+

reaction (Scheme 2, Y = H) betrays three distinct types of behavior. (i) When X = CH,, F and CN, Zp/Zm = 1.0 over the whole electron energy range, consistent with energy diagram C (Fig. l), and identical transition state energies for each

* Such a ring expansion has been proposed10 previously for toluene itself, to account for the statistical equivalence of all hydrogen atoms in the molecular ions of toluene and cy~loheptatriene~**~" undergoing an [M - HI reaction, and also to explain the identical heats of formation of the [C,H,]+ product ion from each

1096 PETER BROWN

isomeric pair of compounds. (ii) When X = CI and Br, Zp f Zm, but the ratio Zp/Zm does not vary with electron energy, consistent with energy diagram A (Fig. l), and again with identical transition state energies for each m- andp-X pair of isomers. (iii) When X = OCH,,* Zp f Zm, and the ratio Zp/Zm varies with electron energy, consistent with some energy diagram intermediate between A and B (Fig. I ) , and with different transition state energies for the m- and p-OCH, isomers.

TABLE 4. METASTABLE ION CHARACTERISTICS FOR DECOMPOSITION REACTIONS IN SUBSTITUTED

TOLUENES XCoH6CHS AT 70 eV -

[metastable] x 103

X Transit ion Metastable - [parent] m-X P-X

125* --f 107 91-6 0-70 0.81 OCH, 125*+. 93 6 9 1 4.7 0.63

124* +. 92 68-3 1 -4 0-39

106 -P 91 78.1 5-2 5-0 105 e l 0 3 101.0 1 -7 1.6

CH3 105 -+ 79 59.4 1 -9 1.9 105 -P 78 57.9 0 5 0 0.52 105 +. 77 56.5 0-22 0-25

F 109 +. 89 72.7 0.047 0.047 109 + 83 63.2 0.74 0.71

126t+. 91 65.7 3.6 1.8

125?+. 89 63.4 1.7 1.5

Br 172$ +. 91 48.1 1 .o 1.6

CN 117 -+ 90 69.2 4.6 4.6 116 +. 89 68.2 1.6 1.6

c1 125t- 99 78,4 0.48 0.50

* m/e values for d,-methoxytoluenes given. ? m/e values for s5C1 given. $ m/e values for *lBr given.

Metastable ion characteristics for [MI -+ [XC,H4CH2]+ ions could not be deter- mined for the substituted toluenes, since the relevant metastable ion peaks appeared at nominal mass [M - 21, where a relatively large normal peak provided serious interference. For this reason, metastable ion characteristics for other decomposition reactions of the molecular ions were measured (Table 4, upper transitions for all substituents except X = F). It must be clearly stated, however, that such alternative fragmentations do not necessarily proceed through the same form of the molecular ion,14*22*54 either in structure or energy. In this case, however, the metastable ion relative intensities [m,*]/[M] do in fact complement the kinetic results. Thus when X = CH,,' and CN, identical ratios [ml*]/[M] (Table 4) and [m,*]/[M] for m- and p-X isomer pairs indicate decoinposing molecular ions of identical energy and struc- ture, traversing transition states of identical energy, as in energy diagram C (Fig. I ) , at

* Unfortunately, nz- and p-nitrotoli~enes~~ did not give peaks at [M - HI in their mass spectra, but rather at [MI, [M - 01, [M - hTO], [M - NO,] and in/e 65.

Kinetic studies in mass spectrometry-V 1097

least for the [M - CH,] (X = CH,) and [M - HCN] (X = CN) reactions. When X = Cl and Br, different [m,*]/[M] ratios for each pair of isomers for [M - Cl] and [M - Br] reactions respectively are again consistent with decomposing molecular ions of different energy and/or structure, as in diagram A (Fig.1). When X = OCH,,* decomposing molecular ions for each isomer are of different energy and/or structure for the 125 [MI - 93 + CD,O process? (Table 4), but possibly quite similar energy and the same structure for the 125 -+ 107 + CD, reaction.?

Identical metastable ion relative abundances [m,*]/[A] for further fragmentation of [XC,H,CH,]+ ions?: (Table 4, lower transitions for each substituent except X = Br) again demonstrate the existence of common species of the same energy for each m- andp-X pair of substrates, except as usual for X = OCH,. It will be noted that absolute [m,*]/[A] values for the ethers (Table 2, X = CH,, F, C1, Br) and the corre- sponding toluenes (Table 4) are not the same, although essentially identical ratios were recorded for each pair of m- and p-X isomers in a given series. No significance can be attached to these differences between series at present, since they Mere found to be very sensitive to operating conditions in the mass spectrometer, whereas the relative values for m- and p-X isomers obtained consecutively were not (see Experimental Section),

The data are again consistent with (but do not prove) reaction pathways depicted in Scheme 3. For substituents X = CH,,37 F and CN, molecular ions I1 (Y = H) of closely similar ionization potentials rearrange to some common structure or struc- tures 5 such as the ionized substituted cyc lohep ta t r i ene~~~*~~ IV (Y = H), leading to tropylium-like activated complexes of the same energy and structure for each m- and p-X pair. Further decomposition of the identical substituted tropylium ions V is then indistinguishable for m- andp-X isomers with the same substituent X. When X = C1 and Br, molecular ions of different energy11 and either the same (IV, Y = H) or different (11, Y = H) structure (the former situation is preferred by analogy with toluene i t ~ e l f ) ~ * * ~ ~ cleave a hydrogen atom via transition states of the same energy (tropylium-like) leading to substituted tropylium ions V of the same energy and structure for each m- and p-X pair of compounds, which again decompose further independently of original substituent position.

When X = OCH,, molecular ions, transition states and [XC,H,CH,14- ions apparently have different energies for the [M - HI reaction of m- and p-OCH, isomers, and other evidence4, suggests a mixture of rearranged (e.g. I1 - IV - V) and unrearranged (e.g. I1 -+ 111) pathways. Isotope labelling3 revealed only [M - HI and no [M - D] in m- andp-CD,O.C,H,CH,, a result independently confirmed in the present work. With m- andp-CH,OC,H,.CD,, however, [M - HI: [M - D] is 60:40 forpara and 46: 54 for meta, indicatinY3 some scrambling of ring and methyl

* The metastable ion peaks discussed for m- andp-methoxytoluenes have already been rep0rted.4~ t Actually, d3-methoxytoluenes were used in this work, to permit distinction between methoxy

and methyl hydrogen $ Many of the metastable ion peaks for decomposition of [XC,H,CH,]+ ions derived from sub-

stituted toluenes have been documented previou~ly.~~ 5 Isotope labelling studies'j in p-xylene demonstrated complete randomization of all hydrogen

atoms in the molecule before the [M - HI reaction takes place. 11 A similar situation was found" for the [M - CI] reaction of m- and p-chlorotoluenes, where

molecular ions of different energy reacted by way of transition states of identical energy to yield common [M - Cl] ions.

1098 PETER BROWN

hydrogens,* Complete scrambling would give rise to a ratio of 43:57. Clearly different pathways are involved, or the same pathways to different extents.

It is apparent, therefore, that the small differences in appearance potentials for [CH,0.C,H4CH2]+ ions derived from m- andp-methoxytoluenes, i.e. 12.1 3 and 11.98 eV respect i~ely,~~ are energetically and possibly structurally significant in this instance, although they are described4, as 'identical' by Meyer and Harrison. These also comment on the considerable differences in the [M - HI and C, region of the mass spectra of m- andp-methoxytoluenes, and point out that ionization and appear- ance potentials of corresponding ions from the two isomers are practically identical. In order to account for the differences, steric factors are suggested.m Whatever combination of structural, internal energy or energy distribution effects these differ- ences are due to, wide range electron energy kinetics and metastable ion character- istics techniques clearly suggested the existence of different energies (and possibly different structures) for [CH,O*C,H,CH,]+ ions as produced by the [M - HI reaction of m- and p-methoxytoluenes.

The energy diagrams in Fig. 1 are obviously greatly oversimplified, in that contributions to the overall reaction rate of the [MI + [M - Y] process from many energy states of the molecular ions are to be expected. It is indeed possible, especially at higher electron energies, that the fragmentation of any one molecular ion may differ in mechanism from that of another, depending on the internal energy available and the energy surfaces accessible to that particular ion.55 Thus particularly at higher electron energies, a quoted 'rate' is actually the sum of these contributions, and an 'activation energy' is then the weighted mean of many such energies.

Considering the [MI -+ [M - HI reaction of the substituted toluenes, for X = CH,, F and CN, energy diagram C (Fig. 1) in its simplest possible application predicts identical ionization potentials for m- and p-X isomers. Values of 8-56 eV for both m- and p-xylenes have been listed,% but 8-91 and 8.78 eV for m- and p-fluoro- t ~ l u e n e s . ~ ~ * t Similarly, for X = Cl and Br, energy diagram A predicts different ionization potentials, with m-X < p-X, but reported values of 8.83 and 8.69 eV for m- and p-chlorotoluenes,"*j- and 8.81 and 8.67 eV for m- and p-bromotoluenes6'j*t are in the opposite direction. For X = OCH,, ionization potentials of 8.48 and 8.56 eV for m- and p-methoxyt~luenes~~*$ have been cited,

If the relative activation energies for m- and p-X isomers as estimated by wide range electron energy kinetics are indeed valid, and if the transition states are of almost identical energy, it follows then that the [MI 4 [M - HI reaction studied here need not necessarily proceed from the lowest vibrational energy state of a molec- ular ion reached directly by vertical ionization of the neutral molecule. 3 For example, when X = F, a slightly more detailed energy diagram may be imagined. The lowest vertical ionization process for both isomers might lead immediately to structurally distinct molecular ions of different energy, which cannot undergo the [M - HI reaction. Ionization by more energetic vertical transitions then would lead to occu- pancy of higher vibrational energy levels, in which molecular ion isomerization by

* Spectra obtained at 50 eV.4s t Determined by photoionization. t: Determined by electron-impact. 5 It is well known that ionization potentials as determined by electron-impact (vertical transitions

are generally slightly higher than those determined spectroscopically (adiabatic zero-zero transitions).s'

Kinetic studies in mass spectrometq-V 1099

ring expansion can occur subsequently, and which are continuous with the energy surface (common in this case for m- andp-F isomers) leading to the activated complex for cleavage of the hydrogen atom. Other similar types of energy diagram can be envisaged for substituents such as X = CI and Br, except that as noted above, the reactive states should not have the same energies for m- and p-X isomers, if the Zp/Zm ratios at threshold are indeed true representations of relative activation energies. A further corollary would appear to be that (first AP-first IP)25 is not necessarily a good measure of reaction activation energy.

C O N C L U S I O N

For the two extreme benzyl systems described here (Scheme 2, Y = OC6H, and H), it appears that reasonable correlations can be made between (i) the relative energies and possibly structures of molecular ioiis, transition states, and [XC,H,CH2]+ ions as estimated by wide range electron energy kinetics and metastable ion relative abundance techniques, and (ii) published results of other studies on closely related systems by ionization and appearance potentials, and isotope labelling methods. Contrary to first opinion^,^^^^^ it would seem that the leaving group Y actually has very little effect on the nature of the [MI -+ [M - Y] reaction. Similar satisfactory correlations* have been found in an investigation of the one major reaction of sub- stituted ethylbenzenes (Scheme 2, Y = CH,). In addition, the M -+ [M - Cl] reaction of a series of substituted chlorobenzenes has produced evidence14 cow sistent with isomerization of the m- andp-X pairs of molecular ions to some common structure before decomposition. This finding implies a carbon (and hence substituent) scrambling mechanism, possibly analogous to photochemical isomerizations of benzene derivatives via the intermediacy of valence bond isomers,% which has also been proposed to account for hydrogenldeuterium randomization in ionized, partially labelled benzenes59 and substituted benzene^.^^,^^.^^

E X P E R I M E N T A L Mass spectrd

All mass spectra were secured with a Varian-Atlas CH-4B instrument, with source temperature 230" and accelerating voltage 3 kV, in conjunction with a Leeds and Northrup Speedomax G single pen and 1 0 chart recorder. Samples were introduced into the ion source through a gold leak, from a ceramic-lined 0 8 1. reservoir of the high temperature inlet system maintained at 160". The analyzer pressure was <lo-' torr (Penning page), and just sufficient sample was admitted to the reservoir via a sampling valve to give 75 % of full scale deflection on the 10 V. scale of the recorder for the most intense peak in the spectrum, with trap current 20 PA stabilized, wide slits and 70 eV electron energy.

Normal scans were taken at the 'medium slit' setting (source slit 0.03 mm, collector slit 0-07 mm, approx. resolution 1500), while metastable scans and all peak relative intensity measurements were secured at the 'wide slit' setting (source slit 0.1 mm, collector slit 0.3 mm, approx. resolution 600) to ensure flat-topped normal peaks. For data obtained between 20 and 70 eV, 20 pA stabilized trap current and ion source draw-out potentials were employed. Below 20 eV, the filament trap current was adjusted to 2 to 5 btA manually, and the source draw-out potentials removed.

The relative intensities of all peaks including metastables were determined by slowly scanning them at least three times in a given run for the same compound, and by appropriate attenuation all peaks including metastables were recorded between 2 to 7" high on the chart. In order to minimize changes in supposedly constant instrumental parameters such as voltages and temperatures, the rn- and p-X isomers were run consecutively, as soon as background from the previous sample had reached an acceptable level. The entire series of compounds was then rerun several months later to check re- producibility. While relative peak intensities (i.e. Z-values) for a given compound showed variations

* P. Brown unpublished results.

1100 PETER BROWN

of up to 15% in the worst instances, the ratios of normal (i.e. Zp/Zm) and metastable ion peak relative intensities for the m- andp-X isomer pairs were always in the range f2%.

Compounds Substituted benzyl phenyl ethers were prepared from appropriate commercially available benzyl

chlorides or bromides and phenol, using a conventional Williamson ether synthesis procedure. Phenol (10 mmole) was dissolved in a stirred solution of potassium carbonate (15 mmole) in water (15 ml) and a solution of the substituted benzyl halide (10 mmole) in 95 % aqueous ethanol (40 ml) added. The reaction was heated to reflux for 4 hours, allowed to cool, and then poured into water (150 ml). Three ether extracts were combined, washed with water and dried over anhydrous sodium sulfate. Removal of solvent by rotary evaporation gave crude substituted benzyl phenyl ether, typically in 60 to 90% yield. Purification was effected by repeated injection/collection cycles on a preparative g.1.c. fitted with a 6‘ x &” column of 5 % XE-60 on Chromosorb W .

Substituted toluenes were all commercial samples, except the labelled m- and p-CD,OC,H,CH, isomers, which were prepared as above from m- andp-cresol and d,-methyl iodide.* All substituted toluenes were also purified by repeated injection/collection on the preparative g.l.c., using a 14’ x &” column of 10% XE-60 on Chromosorb W .

Acknowledgements-We are greatly indebted to the National Science Foundation for funds (Grant No. GB-4939) to purchase the Varian-Atlas CH-4B mass spectrometer, and to Arizona State University for a Faculty Research Grant. Acknowledgement is also made to the donors of the Petroleum Research Fund, for support of this research.

R E F E R E N C E S 1 . K. Biemann, Mass Spectrometry, McGraw-Hill, New York, 1962, 2. F. W. McLafferty, Mass Spectrometry of Organic Ions, Academic Press, New York, 1963,

Chap. 7. 3. H. Budzikiewicz, C. Djerassi and D. H. Williams, Mass Spectrometry of Organic Compounds,

Holden-Day, San Francisco, 1967. 4. H. Budzikiewicz, C. Djerassi and D. H. Williams, Structure Elucidation of Natural Products by

Mass Spectrometry, Vol. I , Alkaloids; Vol. 2 , Steroids, Terpenoids and Sugars, Holden-Day, San Francisco, 1964.

5 . M. M. Bursey, Org. MUSS Spectrom. 1, 31 (1968). 6. S. Meyerson and A. W . Weitkamp, Org. MdSS Spectrom. 1, 659 (1968). 7. For example: F. Weiss, A. Isard and G. Bonnard, Bull. SOC. Chim. France, 2332 (1965); E. I<.

8. A. Maccoll, in R. I. Reed (Ed.) Modern Aspects ofMass Spectrometry, Plenum Press: New York,

9. For example: N. J. Turro, D. C. Neckers, P. A. Leermakers, D. Seldner and P. D’Angelo, J.

10. H. M. Grubb and S. Meyerson, in F. W. McLafferty (Ed.) Mass Spectrometry of Organic Ions,

11. For example: T. 0. Tierrnan and J. H. Futrell, J. Phys. Chem. 72, 3080 (1968). 12. J. D. Baldeschwieler, Science 159,263 (1968); J. L. Beauchamp,L.R.Anders and J. D. Baldesch-

13. S. Meyerson, P. N. Rylander, E. L. Eliel and J. D. McCollum, J. Am. Chem. Soc. 81,2606 (1959). 14. P. Brown, J. Am. Chem. Soc. submitted for publication. 15. T. W. Shannon and F. W. McLafferty, J. Am. Chem. SOC. 88,5021 (1966), and subsequent papers

16. J. M. S. Tait, T. W. Shannon and A. G. Harrison, J. Am. Chem. SOC. 84 ,4 (1962). 17. J. L. Occolowitz and G. L. White, Australian J. Chem. 21, 997 (1968). 18. F. H. Field and J. L. Franklin, Electron Impact Phenomena, Academic Press, New York, 1957,

19. M. M. Bursey and F. W. McLafferty, J. Am. Chem. SOC. 88,529 (1966), and subsequent papers in

Fields and S. Meyerson, J. Am. Chem. SOC. 88, 3388 (1966).

1968, pp. 143 to 168.

Am. Chem. Soc. 87,4097 (1965).

Academic Press, New York, 1963, Chap. 10.

wieler, J. Am. Chem. SOC. 89,4569 (1967).

in this series.

Chap. 4.

this series.

* Supplied by Stohler Isotope Chemical, NJ, 99.8 atom %D.

Kinetic studies in mass spectrometry-V 1101

20. J. K. MacLeod and C . Djerassi, J. Am. Chem. SOC. 89, 5182 (1967). 21. G. S. Hammond, J. Am. Chem. SOC. 77,334 (1955). 22. R. G. Cooks, I. Howe and D. H. Williams, Org. Mass Spectrom. 2, 137 (1969). 23. D. H. Williams, R. G. Cooks and I. Howe, J. Am. Chem. SOC. 90,6759 (1968). 24. P. Brown and M. M. Green, J. Org. Chem. 32, 1681 (1967). 25. R. A. W. Johnstone and D. W. Payling, Chem. Commun. 601 (1968). 26. P. Bommer and K. Biemann, Ann. Rev. Phys. Chem. 16,481 (1965). 27. J. D. Morrison, J. Chem. Phys, 39, 200 (1963). 28. S. Tsuda and W. H. Hamill, in W. L. Mead (Ed.) Advances in Muss Specfrometry, Elsevier,

29. R. E. Winters, J. H. Collins and W. L. Courchene, J . Chem. Phys. 45, 1931 (1966). 30. I. Howe and D. H. Williams, J. Chem. SOC. (B) , 1213 (1968). 31. M. M. Bursey and F. W. McLafferty, J. Am. Chem. SOC. 88,4484 (1966). 32. F. W. McLafferty, Chem. Commun. 956 (1968). 33. P. Brown J. Am. Chem. Soc. 90,4459 (1968). 34. K. Watanabe, J. Chem. Phys. 26,542 (1957). 35. H. Hurzeler, M. G . Ingraham and J. D. Morrison, J . Chem. Phys. 28,76 (1958). 36, D. H. Williams, S. W. Tam and R. G. Cooks, J. Am. Chem. SOC. 90,2150 (1968). 37. F. W. McLafferty and T. A. Bryce, Chem. Commun. 1215 (1967). 38. P. N. Rylander, S . Meyerson and H. M. Grubb, J . Am. Chem. SOC. 79,842 (1957). 39. A. G. Harrison, L. R. Monnen, H. J. Danken and F. P. Lossing, J. Am. Chem. SOC. 82, 5593

40. A. G. Harrison, P. Kebarle and F. P. Lossing, J. Am. Chem. SOC. 83, 777 (1961). 41. P. N. Rylander and S . Meyerson, J. Chem. Phys. 27,1116 (1957). 42. S. Meyerson and P. N. Rylander, J. Chem. Phys. 27, 901 (1957). 43. F. Meyer and A. G. Harrison, Can. J. Chern. 42,2008 (1964). 44. S. Meyerson, J. Am. Chem. SOC. 85, 3340 (1963). 45. S. Meyerson and P. N. Rylander, J. Phys. Chem. 62,2 (1958). 46. S. Meyerson and P. N. Rylander, J. Am. Chem. SOC. 79,1058 (1957). 47. E. L. Eliel, J. D. McCollum, S. Meyerson and P. N. Rylander, J . Am. Chem. SOC. 83,2481 (1961). 48. P. Brown, J. Am. Chem. SOC. 90,4461 (1968). 49. P. Brown, J. Am. Chem. SOC. 90,2694 (1968). 50. K. R. Jennings and J. H. Futrell, J. Chem. Phys. 44,4315 (1966). 51. R. H. Shapiro and J. W. Serum, Org. Mass Spectrom. 2, 533 (1969). 52. J. A. Berson and M. R. Willcott, J. Am. Chem. SOC. 88,2494 (1966), and refs. cited therein. 53. J. H. Beynon, R. A. Saunders and A, E. Williams, The Mass Spectra of Organic Molecules,

54. D. H. Williams, R. S. Ward and R. G. Cooks, J. Am. Chem. Soc. 90,966 (1968). 55. H. M. Rosenstock and M. Krauss, in F. W. McLafferty (Ed.) Mass Spectrometry of Organic

56. R. W. Kiser, Introduction fo Mass Spectrometry, Prentice-Hall, Englewood Cliffs, 1965, Ap-

57. F. H. Field and J. L. Franklin, Electron Impact Phenomena, Academic Press, New York, 1957,

58. K. E. Wilzbach, A. L. Harkness and L. Kaplan, J. Am. Chem. SOC. 90, 11 16 (1968). 59. K. R. Jennings, 2. Naturforsch. 22a, 454 (1967). 60. R. G. Cooks, R. S. Ward and D. H. Williams, Chem. Commun. 850 (1967); A. N. H. Yeo, R. G .

Amsterdam, 1966, p. 249.

( 1960).

Elsevier, Amsterdam, 1968, p. 326.

Ions, Academic Press, New York, 1963, Chap. 1.

pendix 4.

Chap. 3.

Cooks and D. H. Williams, Org. Muss Spectrom. 1, 910 (1968).