Anal. Cheni. 1986, 58, 2001-2009
and only (M2- H)+ and MzH+appear in the mass spectrometer for both the Ni /3 and laser ionization source even a t the lowest concentration levels studied. Below 150 "C the laser ionization signal becomes very weak due to lack of hot-band population. Although direct ionization of the neutral dimer is possible, at low concentration, formation of M+ followed by MH+ + M MzH+ is believed to be the mechanism of dimer formation.
-
ACKNOWLEDGMENT We thank Michael Tierney for technical assistance given during the course of this work. Registry No. P-10,39390-76-6;DMMP, 756-79-6;COP,12438-9; N2, 7727-37-9; Ar, 7440-37-1; 63Ni,13981-37-8; benzene, 71-43-2; phenol, 108-95-2;toluene, 108-88-3;p-xylene, 106-42-3; rn-xylene, 108-38-3;o-xylene, 95-47-6;p-cresol, 106-44-5;m-cresol, 108-39-4; o-cresol, 95-48-7; aniline, 62-53-3; azulene, 275-51-4; naphthalene, 91-20-3; 2-methylnaphthalene, 91-57-6; indene, 95-13-6;4-picoline, 108-89-4;indoline,496-15-1; pyridine, 110-86-1; pyrazine, 290-37-9; triazine, 290-87-9; pyridazine, 289-80-5; quinoline, 91-22-5;isoquinoline, 119-65-3;quinoxaline, 91-19-0; 1-methylnaphthalene,90-12-0. LITERATURE CITED Siegel, M. W.; Fite, W. L. J. Phys. Chem. 1978, 80, 2871-2881. Spangbr, G. E.; Lawless, P. A. Anal. Chem. 1978, 5 0 , 884-892. HIII, H. H., Jr.; Baim, M. A. In P&sma Chromatography Carr, T. W. Ed.; Plenum Press: New York, 1984; Chapter 5, pp 143-176. Mlsui, Y.; Kambara, H.; KoJima, M.; Tomita, H.; Katoh, K.; Satoh, K. Anal. Chem. 1983, 5 5 , 477-481. Kambara, H.; Ogawa, Y.; Mlsui, Y.; Kanomata, I.Anal. Chem. 1980, 5 2 , 1500. Iribarne, J. V.; Dziedzic, P. J.; Thomson, 8 . A. Int. J. Mass. Spectrom. Ion Phys. 1983. 5 0 , 331-347. Karasek, F. W. Int. J. Envlron. Anal. Chem. 1972, 2 , 157-166. Karasek; F. W.; Hill, H. H.; Kim, S. H. J. Chromatcgr. 1978, 117, 327-336. Meier, R. W. Am. Ind. Hyg. Assoc. J . 1978, 3 9 , 233-239. Carroll, D. I.; Dzidic, R. N.; Stillwell, M. G.; Hornlng, M. G.; Horning, E. C. Anal. Chem. 1974, 46, 706-710.
2001
Griffin, 0. W.; RzMlc, I.; Carroll, D.; Stillwell, R. N.; Hornlng, E. C. Anal. Chem. 1973, 45, 1204-1209. Lubman, D. M.; Kronlw, M. N. Anal. Chem. 1982. 5 4 , 1546-1551. Lubman, D. M.; Kronick, M. N. Anal. C h m . 1982, 5 4 , 2289-2291. Lubman, D. M.; Kronick, M. N. Anal. Chem. 1982, 54, 660-665. Dietz, T. 0.; Duncan, M. A.; Liverman, M. 0.; Smaiby. R. E. Chem. Phys. Len. 1980, 7 0 , 246. Carr, T. W. I n Pksma Chromatography; Can, T. W., Ed.; Plenum Press: New York, 1984; Chapter 7, pp 215-235. Dam. R. J. I n Plasma Chromatwaphy; Carr. T. W., Ed.; Plenum Press: New York, 1984; Chapter 6, pp 177-213. Karasek, F. W. Anal. Chem. 1974, 46, 710A. Altshuller, A. P.; Cohen. J. R. Anal. &em. 1980, 32, 802. Carr, T. W. Anal. Chem. 1979, 51, 705. Revercomb, H. E.; Mason, E. A. Anal. Chem. 1980, 32, 802. Kim, S. H. PhD. Dissertation, 1978, The Guelph-Waterloo Centre for Graduate Work in Chemistry, Unhrerslty of Waterloo, Waterloo, Ontario, Canada N2L 3G1. Lubman, D. M. Anal. Chem. 1984, 5 6 , 1298. Tembreuii, R.; Sin, C. H.; Pang, H. M.; Lubman, D. M. Anal. Chem. l985i 5 7 , 2911-2917. Ellis, H. W.; Pai, R. Y.; Gatiand, I.R.; McDaniel, E. W.; Werniund, R.; Cohen. M. J. J. Chem. Phys. 1978, 64, 3935-3941. Lubman, D. M.; Naaman. R.; Zare. R. N. J. Chem. Phys. 1980, 72, 3035. Boesl, U.; Neusser. H. J.; Schiag, E. W. J. Chem. Phys. 1980, 72. 4327. Kim, S. H.; Spangler, 0. E. Anal. Chem. 198S, 5 7 , 567. Spangler, 0. E.; Campbell, D. N.; Carrlco, J. P. 1983 Pittsburgh Conference, Paper 641. Spangler, 0. E.; Suh, S. W.; Carrico, J. P. 1980 Pittsburgh Conference, Paper 549. Preston, J. M.; Karasek, F. W.; Kim, S. H. Anal. Chem. 1977, 49, 1746. Lubmah, D. M.; Kronick, M. N. Anal. Chem. 1983. 5 5 , 867-873. Lubman, D. M., unpublished results, The University of Michigan, 1985.
RECEIVED for review January 13, 1986. Accepted April 14, 1986. We gratefully acknowledge support of this work from the Army Research Office under Grant DAAG 29-85-K-1005 and also the Department of Defense through the US.Army Research Office under Grant No. DAAG 29-85-G-0018 for purchase of the equipment used in this work. We also acknowledge partial support under NSF Grant CHE 83-19383.
Methane High Pressure Collisional Activation Mass Spectrometry of Aromatic Hydrocarbons Karl F. Blom and Burnaby Munson*
Department of Chemistry, University of Delaware, Newark, Delaware 19716
The hlgh-pressure colllslonal actlvatlon mass spectra wlth methane as the reagent/colllslon gas are reporled for benzene and eight alkylbenrenes. Colllslon energy resolved spectra are used to deduce dlssoclatlon pathways and structures of fragment Ions. The structures of fragment Ions are indicated by thelr colilslonally actloated decomposltlon patterns and by their chemlcal reactlvlty wlth methane. Fragmentationsof the protonated afkylbenzenes appear to be controlled by both thermochemical and klnetlc factors. Hlghgressure col#slonal acttvatkm spectra are compared wlth conventional CID mass spectra and the C I spectra resulting from exothermic proton transfer reactlons.
Chemical ionization mass spectrometry (CIMS) often produces fragmentation that is diagnostic of the structural features of the sample substrate as well as ions that provide 0003-2700/86/0358-2001$01.50/0
molecular weight information (1-4). Much of the research in CIMS has been toward the development of low-energy reagent ions which give very simple mass spectra (3-6). These spectra are useful for the qualitative and quantitative analysis of mixtures and for the determination of the molecular weight of the sample compound (7, 8). But, since these spectra contain essentially no fragmentation they usually provide no structural information. A few special cases have been reported in which the CI reagent ions react selectively with a particular functional group or combination of functionalities (8-1 1). These spectra can provide information about specific structural features of the sample substrate, but interpretation requires some prior knowledge of the sample. It is often necessary to use additional techniques that produce fragmentation to obtain the desired structural characterization. Selective CI reagent ions have frequently been used with collisional activation techniques to obtain the molecular weight and structural information (12-1 7). 0 1986 American Chemical Soclety
2002
ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986 e
I
1
Grid
1
I
-1
I
2
I COLLISION
C I REGION
I
REGION
II
.
I
0
Flgure 2.
No. of Collisions +
Diagram of the major processes occurring under HPCA
conditions. e
1 I I
1
I
I
2
I
CI R E G 1 0 1 1
1
3
1 COLLISION
I I
REACTION
1
I
REGION
I
KEGION
I
I
I
MuRiregion high-pressure collisional activation Ion sources: (a) two-region source: (b) three-region source.
in region 3 under low field conditions before mass analysis. This configuration is useful for studying the reactions of fragment ions with the reagent gas. The first region is long enough (>2 cm) so that the CI ions react with the sample essentially to completion at moderate sample sizes (sample pressure -1% of the total pressure). Thus, only sample ions enter region 2 to be collisionally activated. This arrangement eliminates competing bimolecular decomposition channels, such as reactions 1 or 2. Fragment
+
[CHS+]* M
[CHS+]*
Flgure 1.
Early in the development of CIMS it was realized that the extent of fragmentation in CI spectra could be increased by increasing the repeller voltage or field strength within the high-pressure source (6, 18). The increased fragmentation results from inelastic collisions of sample ions (which have been accelerated by the applied field) with molecules of the reagent gas. Both collisional activation and decomposition occur within the ion source. The fragment ions produced by high-pressure collisional activation (HPCA) decompositions often correspond closely to the structural features of the sample molecule (19,20). When HPCA is combined with a suitable low-energy CI reagent gas, the extent of fragmentation can be varied from no fragmentation a t low field strength to complete decomposition of the sample ions at high field strength. The applied field strength can be varied easily and continuously to obtain energy-resolved fragmentation data or breakdown curves. Such information from low-pressure collisionally activated decompositions has been useful in determining decomposition channels and in characterizing structural features of the parent ions (21-23). Since the field strength can be changed very rapidly under HPCA conditions, it is possible to obtain mass spectra with no fragmentation and a large extent of fragmentation on consecutive scans of the mass spectrometer (20). This feature is particularly useful when used with GC sample introduction. Fragment ions may react with the high-pressure reagent gas and, in some cases, this chemistry is useful in characterizing the structure of the fragment ion (20, 24). We have reported the use of multiregion ion sources to study HPCA processes (20). Two multiregion sources are shown schematically in Figure 1. The regions of the sources are separated by fine mesh stainless steel grids so that different field strengths can be applied across each region. Sample ions are produced by ion/molecule reactions in region 1 under low field conditions (ca. 10 V/cm). These ions are then collisionally activated in the second region by field strengths as high as 200 V/cm. In the two-region configuration, Figure la, mass analysis of the ions leaving the second region gives the high-pressure collisional activation mass spectrum. In the three-region configuration, Figure l b , the fragment ions formed in region 2 interact with the high-pressure reagent gas
-
CH,+
+M
F+ + N
+ CH,
+ Hz F+ + N + ...
CH,+
(24
ions produced by increasing the field strength in the second region can be unambiguously attributed to collisionally activated decompositions of the sample ions in region 2. A qualitative model of the high-pressure collisional activation process is shown in Figure 2. The rate of unimolecular dissociation of an excited ion, P+,is a function of the internal energy content of the ion and, in the simplest terms, may be expressed as k = u [ ( E - E,)/E]"-', where E is the internal energy of the ion, E , is the minimum energy required for decomposition to occur and is related to the thermodynamic heat of reaction, u is a constant and is generally referred to as the frequency factor, and s is the effective number of oscillators in the ion (25,26). In the HPCA experiment, ions acquire kinetic energy from the applied electric field and convert some of this to internal energy via inelastic collisions with the high-pressure reagent gas. The ions undergo many collisions with the reagent gas and acquire only a small amount of kinetic energy between collisions (27). This results in the gradual accumulation of internal energy depicted in Figure 2. After many collisions the ions accumulate enough internal energy to decompose by the lowest energy dissociation channel (E = E, for reaction I in Figure 2). The time between activating collisions is large enough so that, in many cases, the ions will decompose via reaction I before the additional internal energy needed for a more endothermic pathway (reaction 11) to occur can be accumulated. Under these conditions, reaction I will be the dominant decomposition observed. If, however, reaction I is slow because of an unusually small frequency factor, v, then the excited ions may have lifetimes long enough to accumulate the internal energy needed for reaction I1 to occur. In this case, the higher energy channel can be a major decomposition pathway. Since fragment ions are formed within the ion source, they may also be collisionally activated. Under sufficiently energetic conditions this can result in consecutive or stepwise decomposition sequences (reaction I followed by reaction 111). Fragment ions may also undergo ion/molecule reactions with the high-pressure reagent gas (reaction IV in Figure 2) to form new ions. The purpose of this paper is to present the high-pressure collisional activation mass spectra of a series of simple aromatic hydrocarbons. Where possible, the HPCA fragmenta-
ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1966
2003
80
60
40
n
E 20 W
1
$ O ( c ~ H ~ c ~ H ~ + ~ ~ H ~ )
i 6
+
4 C*Hg+ 2
0 FIELD STRENGTH (VICM)
Flgm 3. Relathre abundances of the ions from benzene at a methane pressure of 0.42 torr as functions of the field strength in the second
region of the two-region source.
tions will be compared with those observed in conventional collisionally induced decomposition experiments and those resulting from exothermic ion/molecule reactions. the qualitative model for high-pressure collisional activation will be tested in the interpretation of these spectra.
EXPERIMENTAL SECTION All experiments were done with a Du Pont (CEC) 21-llOB double-focusing mass spectrometer that has been modified for high-pressure operation. The instrumentand the multiregion ion sources have been described in detail elsewhere (20). The length of region 1was 2.1 cm in the two-region source and 1.3 cm in the three-region source. The lengths of the second and third regions were 0.8 cm, each. The source pressures were measured with a Texas Instruments Model 144 quartz spiral precision pressure gauge connected directly to the ion source. The source was unheated and operated at approximately 60 "C. The ionizing potential was about 600 eV and the ionizing current was 200 PA measured to the outside of the source block. The aromatic hydrocarbons used in these studies were all obtained commercially and used as received. The CHI reagent gas was Matheson U1tra-high purity, 99.97% minimum purity.
RESULTS AND DISCUSSION The relative abundances of the major sample ions in the CH4 HPCA mass spectra of benzene, ethyl- and diethylbenzene, toluene, and the xylenes are shown in Figures 3-7 as functions of the field strength in the second region of the two-region ion source. At low field strengths, below about 20 V/cm, the distributions of sample ions are essentially independent of the field strength and the extent of collisionally activated decomposition is negligible. The distributions of sample ions a t these field strengths are similar to those reported previously for the CHI CI mass spectra of these compounds (28). The major sample ion for all of these compounds is the protonated molecule, (M + H)+. The decreases in the
50 100 FIELD STRENGTH (V CM)
150
Figure 4. Relative abundances of the malor ions from ethylbenzene at a methane pressure of 0.4 torr as a function of the field strength in region 2 of the two-region source. relative abundances of the CI ions with increasing field strength in region 2 and corresponding increases in the abundances of the fragment ions establish the occurrences of collisionally activated decompositions. The major high pressure collisionally activated decomposition for protonated benzene, Figure 3, is Hz elimination, reaction 3, which produces a C$5+ ion. The a-and 0-bond *
-
(M + H)+
(M - H)++ Hz
(3)
cleavages of cyclic c&+that produce the most stable acyclic C6H5+structures have activation energies estimated at 62 kcal/mol and 82 kcal/mol, respectively (29). Therefore, we assume that the C,&+ produced by reaction 3 retains its cyclic or phenylic structure. The reactivity of the C6H5+with the CHI reagent gas (see below) supports this assignment. The HPCA of protonated C6D6 under comparable conditions results in HD and Dzelimination. The ratio of decomposition products, C6D5+/C6D4H+, is 0.64 f 0.05. This ratio is approximately independent of the field strength in region 2 as well as the pressure of the CHI reagent gas. Thus, the ratio of HD to Dz elimination is approximately independent of the excitation energy and lifetime of the decomposing C6D6H+.The ratio of decomposition products is nearly the same as that reported previously for the MIKE spectrum (7 keV ions) of C6D&+, C6D5+/C6D4H+= 0.54 (30). Decomposition of the CJIeH+ through a 1,l elimination, as has been reported for the metastable transition (31),would result in 100% HD loss if there were no scrambling prior to decomposition. A 1,2 elimination would produce C6D5+and C6D4H+ in a 1 to 1 ratio in the absence of isotope effects and scrambling. Complete randomization of H and D prior to decomposition would produce C6D5+and C6D4H+in a ratio of 0.4 to 1 in the absence of any isotope effects. The present results
2004
ANALYTICAL CHEMISTRY, VOL. 58,
NO. 9,
AUGUST 1986
a
J7
80
60
40 h
H
I
v
20
$
i
C10H13+
a
(c~H~cH.,+H) +
+
-
-
0
2o
C5H5+
/
10
0 100
50
150
FIELD STRENGTH (V/CM)
50 100 FIELD STRENGTH (VlCM)
do not establish the mechanism of HPCA decomposition,but they are consistent with previous reports of hydrogen scrambling prior to decomposition and a small isotope effect that favors HD loss over D2 loss in the MIKE spectrum of C,&H+ (30, 31). The relative abundance of C6H6+ in Figure 3 increases slightly during the breakdown of C6H7+,indicating that the H elimination, reaction 4, is a minor process. The ratio of
*
M+ + H
100
150
FIELD STRENGTH (V/M)
150
Figure 5. Two-reglon CH, HPCA mass spectra of dlethylbenzene (mixed isomers) at a methane pressure of 0.4 torr: (a) relative abundances of major ions as a function of field strength in region 2; (b) first derivathres of the breakdown curves for protonated diethyibenzene and its decomposition products.
(M + H)+
50
(4) the increase in c,&+to the increase in C6H5+is approximately
ngWe 6. CH, HPCA mass spectrum of toluene at a methane pressure of 0.4 torr: (a) relative abundances of ions as a function of field strength in second region of two-region source; (b) first derivatives of breakdown curves for protonated toluene and its fragment ions.
0.1 (corrected for carbon-13contribution of C6H5+to the m / z 78). The major decompositions in the MIKE spectrum of C6H,+ are the losses of H and H, in a ratio of about 0.7 (30), a value significantly larger than that observed in the HPCA experiment. This disparity illustrates a fundamental difference between the high-pressure and single collision activation processes. The single, 7-keV collision used in the MIKES experiment transfers enough energy to activate any available decomposition channel. The fragmentation pattern is then determined by the flow of energy within the ion and the relative kinetics of the competing decomposition processes. Under high-pressure conditions the ions are activated grad-
ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986
t
a
y'I
50 100 F I n D STRENGTH (V/CM)
150
Figure 7. Relative abundances of the major ions from the three Isomeric xylenes at a methane pressure of 0.4 torr as a function of the field strength in region 2 of the two-reglon source.
ually in many collisions with the high-pressure buffer gas; see Figure 2. The time between collisions (ca. s) is long enough to permit excited ions to decompose via lower energy pathways before the additional energy needed to activate more endothermic processes can be accumulated. Thus, the relative endothermicities of competing decomposition processes are a major factor in determining the fragmentation pattern under HPCA conditions. The H2elimination from C6H7' is favored over H elimination in the HPCA experiment because it is 14 kcal/mol less endothermic than H loss (see Table I). The major collisionally activated decomposition of the protonated mono- and diethylbenzenes,Figures 4 and 5a, and protonated p-ethyl- and p-isopropyltoluene (see Table IV) is the olefin elimination to give the protonated arene, reaction 5 in which Y = H, CH3,or CzH5 and R = CzH5 or i-C3H7. The
Y
Y
loss of Hz, reaction 3, and the loss of CHI, readion 6, are minor dissociation pathways for the protonated alkylbenzenes. The
(M + H)+
*
(M - CHJ+
+ CH4
(6)
olefin eliminations have somewhat higher calculated endothermicities than either the loss of Hz or CH4 (assuming the (M+ H - Hz)+and (M+ H - CH4)+have the stable benzylic structures, see Table I). Consequently, for olefin loss to be favored the eliminations of H2and CHI must either have additional internal energetic barriers, which make them more endothermic than olefin elimination, or be unusually slow as
2005
a result of small frequency factors or large negative entropies of activation. The first possibility can be tested experimentally. The threshold field strength for a collisionally activated decomposition is related to the activation energy for the decomposition: the larger the endothermicity of the reaction, the higher the field strength for the onset of the decomposition (20,36).The onset of decompition may be more easily obtained from the first derivative of the breakdown curve for the fragment ion. Figure 5b shows the first derivative curves for protonated diethylbenzene and the fragment ions produced by the olefin, CHI, and H2 elimination reactions. The onsets for the formation of CJI13' (H2elimination) and CgHll+(CH4 elimination) occur at lower field strengths (about 20 V/cm for both ions) than the onset for the olefin elimination product, C8H11+(approximately 60 V/cm). Analogous results are obtained for ethylbenzene, p-ethyltoluene, and p-isopropyltoluene. These results establish that the H2 and CHI eliminations are less endothermic than the olefin eliminations, in agreement with the calculated thermochemical heats of the reactions. Thus, olefin elimination must be the preferred decomposition channel because the H2 and CH4 losses have small frequency factors or negative entropies of activation. Nearly equal amounts of C7H7+and C6H5+are produced by the losses of H2 and CHI from protonated toluene, Figure 6a. If the C7H7+were the benzyl or tropylium ion, then H2 loss would be thermochemicallyfavored over CHI elimination by about 37 kcal/mol or 44 kcal/mol, respectively (see Table I). If, however, the C7H7+were the methylphenyl ion, then CHI elimination would be favored by about 8 kcal/mol. The first derivative plots in Figure 6b show that the onset for the formation of C7H7+is significantly lower than thaC for the formation of C6H5+,approximately 40 V/cm vs. 55 V/cm. Thus, H2 loss is the lower energy decomposition channel and C7H7+is probably the benzyl ion. This assignment is supported by the lack of reactivity of the C7H7+with CHI (see below). The formation of the abundant C6H5+fragment ion in the toluene spectrum suggests that the energetically favored Hz elimination is somewhat hindered by a low-frequency factor or negative entropy of activation. The distributions of ions in the HPCA mass spectra of the three positional isomers of xylene, Figure 7, are very similar at all field strengths. The reactions of the CH4 reagent ions with p-xylene produce a slightly larger relative abundance of C8H9+and less (C6H4(CH3),+ H)+than for the other isomers. The distribution of the fragment ions formed by collisionally activated decompositions are identical within experimental error. The decompositions of the protonated xylenes produce predominantlyC7H7+from the loss of CH4and a small m o u n t of C8Hg+from Hz elimination. It seems reasonable to assume that the Hz elimination is analogous to that for the protonated toluene and that the C8Hg+is methylbenzyl ion. Similarly, one might expect the CHI loss forming the C7H7+to be like that for protonated toluene and to produce a methylphenyl ion. If the C7H7+were the benzyl ion, the CHI elimination would be less endothermic than H2 elimination by about 6 kcal/mol while Hz loss would be the less endothermic channel by 38 kcal/mol if the C7H7+formed were a methylphenyl ion. The onset for the formation of C7H7+occurs at roughly 60 V/cm. While the onset for the formation of C8H9+is not sharp, it is clearly being formed at field strengths less than 60 V/cm. This observation suggests that the H, elimination is less endothermic than CHI loss and that the C7H7+formed is a methylphenylion. However, studies using the three-region ion source found the C7H7+to be unreactive with CHI (see below), which would suggest that it is a benzyl or tropylium ion (24, 37, 38). This apparent contradiction might be rationalized by a rapid isomerization of the methylphenyl
2006
ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986
Table I. Calculated Heats of Reaction for Decompositions of (M + H)+ Ions
ions will be much more endothermic than the H2 elimination, reaction 3. For example
8
AHr,, in kcal/mol" for the elimination of the following from (M + H)+
protonated molecule
-H
-Hq
benzene ethylbenzene diethylbenzene p-ethyltoluene p-isopropyltoluene o-xylene m-xylene p-xylene toluene
81 80 70 e e 75 74 71 79
67 19 17 e 17 24.5 26.9 24.7 28/21d
-CH,
CeH7'
C4H3+
+ CzH4
-(R-H)
-C&Yb
D = 115 kcal/mol
f
f
Consequently, any such reaction should be much slower than reaction 3 and should not occur to any signifcant extent under these conditions. These new ions probably result from the consecutive decompositions
f
17 16 11
35 23 24 23 f f f
e 63.5c 63.gC 62.Y 65
58 56 53 33 95 95 94 92
f
+
References 32-35. AHm for the formation of Rt. Assuming C7H7+is a methylphenyl ion. dFor C7H7+being benzyl and tropylium ion, respectively. e Data not available. f Not applicable.
C4H3+L..C4Hz+ H
fragment ion (which is probably formed in an excited state) to one of the more stable benzyl or tropylium ion structures. The more endothermic CH4 elimination forming C7H7+is probably the favored decomposition channel because the H2 loss is hindered by a low-frequency factor or negative entropy of activation as was the case for toluene. The elimination of a neutral arene to produce an alkyl ion, reaction 7 , is thermochemically unfavorable for all of the alkylbenzenes in this study (see Table I). For protonated
C4Hz+
D = 93 kcal/mol
8
8
RC6H5Y+
R+
+ C6H5Y
(7) isopropyltoluene, however, reaction 7 is only 10 kcal/mol more endothermic than olefin elimination and a small amount of C3H7+(,
