Theoretical Analysis of the Methane Elimination from Oxonium Cations [R 3 O] +, R ) H, CH 3

Transcription

1 J. Phys. Chem. 1996, 100, Theoretical Analysis of the Methane Elimination from Oxonium Cations [R 3 O] +, R ) H, CH 3 Minh Tho Nguyen and Guy Bouchoux*, Department of Chemistry, UniVersity of LeuVen, Celestijnenlaan 200F, 3001 LeuVen, Belgium, and Département de Chimie, Laboratoire des Mécanismes Réactionnels CNRS/URA 1307, Ecole Polytechnique, Palaiseau Cedex, France ReceiVed: August 21, 1995; In Final Form: NoVember 2, 1995 X Molecular orbital calculations up to the MP4/ G(2df,2p)//MP2/6-31G(d,p) + ZPE level have been undertaken on the isomerization/dissociation processes of oxonium cations [CH 3 OH 2 ] +, [(CH 3 ) 2 OH] +, and [(CH 3 ) 3 O] +. In addition to the classical oxonium cation structure, two ion-neutral complexes were identified: [H 2 CHO(R) CH 3 ] + and [H 2 CdO(R) CH 4 ] + (R ) H, CH 3 ). The former is only a weakly stabilized structure while the latter (stabilized by kj/mol) is a reaction intermediate during the direct hydride ion abstraction [CH 3 ] + + CH 3 OR f CH 4 + [CH 2 OR] +. The barrier height associated with the 1,2-elimination reaction [H 2 CHO(R)sCH 3 ] + f [H 2 CdO(R)] + + CH 4 is calculated to be 267 and 295 kj/ mol for R ) H and CH 3, respectively. This energy barrier is mainly due to the heterolytic CsO bond dissociation energy, and as a result, the corresponding transition structure looks like a [CH 3 ] + CH 3 OR complex. Calculation has also been used to predict a heat of formation of the trimethyloxonium cation: H f 300K [O(CH 3 ) 3 ] + ) 536 ( 15 kj/mol. Introduction A growing amount of accurate thermochemical data concerning gas phase ions has been collected in recent years. 1 In spite of this rapid progress, the evaluation of activation parameters of elementary reactions involving these chemical species remains a great challenge for the spectroscopists. As far as mass spectrometry is concerned, reports on energy barriers of gas phase reactions of species generated by relevant techniques are rather scarce. Moreover, when available, the accuracy of reported results may be a matter of discussion (see for example refs 2 and 3). In this context it is of primordial interest to carry out a systematic comparison between the known experimental estimates and theoretical results obtained by, for example, highlevel quantum mechanical calculations, in order to establish the limit of confidence of both approaches in determining activation parameters. In the present contribution, we consider the energy barriers for methane elimination from oxonium cations (eqs 1 and 2), for which a theory-experiment comparison could be undertaken. [(CH 3 ) 2 OH] + f CH 4 + [CH 2 OH] + (1) [(CH 3 ) 3 O] + f CH 4 + [CH 2 OCH 3 ] + (2) Deuterium- and 13 C-labeling experiments showed that both reactions 1 and 2 are 1,2-eliminations of CH 4 involving one methyl group and one hydrogen of a vicinal methyl group; the proton bonded to oxygen in [(CH 3 ) 2 OH] + is not lost with the methane molecule. In addition, no hydrogen scrambling seems to occur prior to the decomposition and the carbon atoms play identical roles in each oxonium cation. 4,5,8,9 Gas phase ionmolecule reactions of [CH 3 ] + with methanol and dimethyl ether have been recently carried out in an ion cyclotron resonance spectrometer. 9 Formation of ions [CH 2 OH] + or [CH 2 OCH 3 ] + University of Leuven. Ecole Polytechnique. X Abstract published in AdVance ACS Abstracts, January 1, was observed, suggesting an initial condensation process leading to the oxonium cations [(CH 3 ) 2 OH] + and [(CH 3 ) 3 O] +, respectively. From labeling experiments, however, the following findings were reported: [ 13 CH 3 ] + + CH 3 OH [ 13 CH 3 ] + + (CH 3 ) 2 O f 13 CH 4 + [CH 2 OH] + 43% f CH 4 + [ 13 CH 2 OH] + 57% f 13 CH 4 + [CH 2 OCH 3 ] + 50% f CH 4 + [ 13 CH 2 OCH 3 ] + 50% The reaction of [ 13 CH 3 ] + with methanol leads to a 57/43 ratio of 13 CH 4 /CH 4 elimination whereas a statistical 1/1 ratio is expected from the oxonium cation. Similarly, the reaction of [ 13 CH 3 ] + with dimethyl ether leads to a 50/50 ratio of 13 CH 4 / CH 4 elimination whereas a statistical 1/2 ratio is expected. These results are readily explainable by the existence of two parallel reaction channels: the first one involves the expected oxonium cation while a second route is responsible for the exclusive 13 CH 4 elimination. For the latter process (which participates to the composition to 14% and 25% for CH 3 OH and (CH 3 ) 2 O, respectively), Audier et al. 9 propose the intermediacy of an ionneutral complex, but it is clear that a direct hydride ion abstraction may also account for the observations. Recently, we have reported a theoretical estimate for the classical energy barrier of the CH 4 loss from [(CH 3 ) 2 OH] + (reaction 1) using ab initio molecular orbital calculations. 6 At the MP4/ G(2df,2p)//MP2/6-31G(d,p) + ZPE level, the barrier height of reaction 1 has been calculated to be 267 ( 15 kj/mol. In a subsequent spectrometric study, Holmes 7 deduced a value of 243 ( 10 kj/mol from the appearance energy measurement of [CH 2 OH] + ions coming from metastable [(CH 3 ) 2 OH] + ions. The latter have been generated by dissociative ionization of 1-methoxy-2-hydroxypropene (eq 3) /96/ $12.00/ American Chemical Society

2 2090 J. Phys. Chem., Vol. 100, No. 6, 1996 Nguyen and Bouchoux CH 3 -CH(OH)-CH 2 -OCH 3 f [(CH 3 ) 2 OH] + + OCH 3 (3) Although the experimental value is smaller than our theoretical estimate, the agreement between both theoretical and experimental values can be regarded as satisfactory as long as their respective lower and upper limits are overlapping each other. Concerning reaction 2, Wang et al. 8 estimated an activation energy of 357 ( 19 kj/mol on the basis of the observed [CH 2 - OCH 3 ] + ion appearance curve from collision-induced dissociation of [(CH 3 ) 3 O] + under single-collision conditions in a flowing afterglow apparatus. From a crude evaluation based on the trend of [CH 3 ] + dissociation energy following methyl substitutions in the oxonium cations, we also suggested an upper limit of about 328 ( 15 kj/mol for the energy barrier of reaction 2. This estimate is smaller than the experimental value of Wang et al., 8 but again, the theory-experiment comparison for both estimates is acceptable if we consider that upper and lower limits coincide. However, the theory-experiment difference for both oxonium ions is apparently not systematic. In this context, it is desirable to carry out ab initio calculations on the [(CH 3 ) 3 O] + system in order to obtain a more reliable theoretical estimate. It is the main purpose of the present work. As a necessary calibration, the simpler system [CH 3 OH 2 ] + has also been considered. On the other hand, we have searched for possible weakly-bonded complexes and their role in the methane elimination reaction. Method of Calculation All ab initio molecular orbital calculations were performed using a local version of the GAUSSIAN 90 set of programs. 10 Stationary points have initially been located by geometry optimization using single-reference Hartree-Fock (HF) wave functions with the split valence 3-21G basis set and characterized by harmonic vibrational analysis at this level. Geometrical parameters of relevant structures have been reoptimized at the second-order Moller-Plesset perturbation theory (MP2) using a dp-polarized 6-31G(d,p) basis set. Improved relative energies have subsequently been obtained from single-point electronic energy calculations which utilize MP2-optimized geometries, larger G(d,p) and G(2df,2p) basis sets, and s incorporating valence-electron correlation up to full fourth-order perturbation theory (MP4SDTQ). Calculated total and relative energies are summarized in Table 1. To facilitate comparison, we have used the same notation for each of the three oxonium systems considered, namely 1 for the oxonium cation, COM1 for the complex of [CH 3 ] +, TS1 for the transition structure converting COM1 to 1, TS for the transition structure for H 2 or CH 4 elimination, and COM2 for the complex of CH 4. Results and Discussion [CH 3 OH 2 ] + System. We first consider the methyloxonium cation, [CH 3 OH 2 ] +. Since structures and dissociation pathways of this cation have been the subject of two previous studies, 11,12 we only mention here the essentials of our results summarized in Figure 1. The two low-energy fragmentation modes of [CH 3 OH 2 ] + are losses of water and molecular hydrogen. It appears from Figure 1 that extension of the one-electron basis function, in particular the polarization space, induces rather small changes in relative energies. Comparison with available experimental values is satisfactory. Note that the proton affinity of methanol is calculated to be 763 and 752 kj/mol at the MP4/ G- (d,p)//mp2/6-31g(d,p) and MP4/ G(2df,2p)//MP2/6-31G(d,p) + ZPE levels, respectively. The corresponding experimental value is 760 kj/mol at 300 K, 14 i.e. 753 kj/mol at 0 K, as deduced from thermal correction. 12 Our results confirm two important points. The first is that the initial hydrogenbonded complex COM1 ([H 2 CH H 2 O] + ) does exist as a local minimum but the energy barrier for converting this complex to the methyloxonium cation, which corresponds to a small reorientation of both fragments, is tiny (about 1 kj/mol). The result suggests that such complexes are only transient species during ion-molecule reactions between [CH 3 ] + and H 2 O. The second observation is that both the fragments ([CH 3 ] + + H 2 O) and the transition structure for H 2 loss, TS, seem to possess comparable potential energy. Within the expected uncertainty of ab initio s, it is rather hard to calculate accurately the small energy difference between them. It may be recalled however that a value of 250 kj/mol has been experimentally determined by Huntress and Bowers 15 for the barrier [CH 3 OH 2 ] + f H 2 + [CH 2 OH] +, in satisfactory agreement with our MP4/ G(2df,2p)//MP2/6-31G(d,p) + ZPE and Uggerud s 12 estimate of 265 kj/mol. In the same experimental study 15 it has also been demonstrated that the loss of H 2 O needs more energy than the H 2 elimination and occurs at its thermochemical threshold. [(CH 3 ) 2 OH] + System. The dissociation of the dimethyloxonium cation [(CH 3 ) 2 OH] + has been examined in detail in a recent paper. 6 Figure 2 summarizes and completes the potential energy profile related to this ion. Additional results obtained in the present work must be mentioned: (i) A hydrogen-bonded complex between [CH 3 ] + and CH 3 - OH, COM1, persists. The complexation energy amounts to 85 kj/mol. Nevertheless, this complex disappears upon slight reorientation of both fragments to form the covalent C-O bond. This weak complex lies 12 kj/mol (MP4/ G(2df,2p)/ /MP2/6-31G(d,p)+ZPE) below the transition structure for CH 4 loss, a value much smaller than that of about 60 kj/mol in the methyloxonium ion (Figure 1). (ii) Dimethyloxonium cation is also the protonated form of dimethyl ether. The corresponding proton affinity is calculated to be 800 and 791 kj/mol using MP4/ G(d,p)//MP2/ 6-31G(d,p) and MP4/ G(2df,2p)//MP2/6-31G(d,p) s with zero point corrections, respectively. The latter value compares quite well with the experimental estimate for the 0 K proton affinity of dimethyl ether, PA((CH 3 ) 2 O) ) 788 kj/mol, which may be deduced from the experimental 300 K value of 793 kj/mol 14 and a correction of 5 kj/mol for thermal energies. 16 (iii) Starting from [CH 3 ] + and CH 3 OH, a hydride transfer from the methyl group of methanol to the methyl cation is also a possible reaction channel yielding methane. Due to the large difference in energy between ([CH 3 ] + + CH 3 OH) and ([CH 2 - OH] + + CH 4 ), the hydride transfer occurs without an energy barrier, resulting in a lone complex (COM2, Figure 2). In fact, this complex has been identified several years ago by Swanton et al. during a theoretical investigation of the decomposition pathways of ethyloxonium. 16 The hydride transfer corresponds thus to a situation having a single potential energy well. It is likely that this mode of CH 4 formation, exclusively from the incoming [CH 3 ] + reactant, contributes to the observation of a nonstatistical distribution of the carbon atoms in the eliminated methane. [(CH 3 ) 3 O] + System. Selected optimized geometries of species relevant to the [(CH 3 ) 3 O] + system (1, COM1, TS1, TS, and COM2) are recorded in Figure 3. Parameters of the four fragments are omitted but are available upon request from the authors. A schematic potential energy profile is illustrated in Figure 4.

3 Methane Elimination from Oxonium Cations [R 3 O] + J. Phys. Chem., Vol. 100, No. 6, TABLE 1: Calculated Total (hartrees), Zero Point Vibrational (kj/mol), and Relative Energies (kj/mol) of the Oxonium Systems Considered [CH 3OH 2] + 1 COM1 TS1 TS [CH 3] + + H 2O [CH 2OH] + + H 2 experimental e MP2(F)/6-31G(d,p) b MP4/ G(d,p) MP4/ G(2df,2p) ZPE c [(CH 3) 2OH] + 1 COM1 TS1 TS COM2 [CH 3] + + CH 3OH [CH 2OH] + + CH 4 experimental e MP2(F)/6-31G(d,p) b MP4/ G(d,p) MP4/ G(2df,2p) ZPE c [(CH 3) 3O] + 1 COM1 TS1 TS COM2 [CH 3] + + (CH 3) 2O [CH 2OCH 3] + + CH 4 experimental e MP2(F)/6-31G(d,p) b MP4/ G(d,p) ZPE c a Based on MP2(F)/6-31G(d,p) optimized geometries. Core orbitals are frozen in MP4SDTQ calculations. b Using full sets of molecular orbitals. c Zero point vibrational energies from HF/3-21G calculations and scaled by 0.9. d All relative energies are corrected for ZPEs. e Experimental H, see text for references and discussion. Figure 1. Schematic potential energy profile for the two main reactions of [CH 3OH 2] +. The energy entries are MP4/ G(d,p) (upper) and MP4/ G(2df,2p) (lower) values, both based on MP2(F)/ 6-31G(d,p) geometries. In parentheses, are experimental values derived from available heats of formation at 300 K (ref 1 except H f 300([CH 3- OH 2] + ) 568 kj/mol deduced from the PA(CH 3OH) value of ref 14). The trimethyloxonium ion 1 possesses a C 3V symmetry in its most stable conformation and a strongly pyramidal oxygen atom. The O-C distance is consistently shortened upon methylation of the [H 3 O] + ion, namely in going from 1.51 Å in [CH 3 OH 2 ] + to 1.49 Å in [(CH 3 ) 2 OH] + and to 1.48 Å in [(CH 3 ) 3 O] +. The COC angle is also closed up: in [(CH 3 ) 2 OH] + and in [(CH 3 ) 3 O] +. Calculations (at the MP4/ G(d,p)//MP2/ 6-31G(d,p) + ZPE level) indicate that trimethyloxonium ion 1 is 364 and 54 kj/mol below ([CH 3 ] + + O(CH 3 ) 2 ) and (CH 4 + [CH 2 OCH 3 ] + ), respectively. The calculated difference in energy between the latter products is thus equal to 310 kj/mol, a value Figure 2. Schematic potential energy profile for the two main reactions of [(CH 3) 2OH] +. The energy entries are MP4/ G(d,p) (upper) and MP4/ G(2df,2p) (lower) values, both based on MP2(F)/ 6-31G(d,p) geometries. In parentheses are experimental values derived from available heats of formation at 300 K (ref 1 except H f 300([(CH 3) 2OH] + ) ) 553 kj/mol deduced from the PA((CH 3) 2O) value of ref 14). corresponding with the experimental estimate of about 309 kj/ mol (values at 0 K 1,13 ). Together with available experimental heats of formation of the fragments, 1,13 the above relative energies allow the heat of formation of trimethyl oxonium cation to be evaluated: H f 0K [O(CH 3 ) 3 ] + ) 568 ( 2 kj/mol and H f 300K [O(CH 3 ) 3 ] + ) 537 ( 8 kj/mol. Another way to estimate the heat of formation of the [O(CH 3 ) 3 ] + cation is to consider isodesmic reactions 4 and 5:

4 2092 J. Phys. Chem., Vol. 100, No. 6, 1996 Nguyen and Bouchoux calculated at the same level, i.e hartrees, one obtains E 4 ) -7 kj/mol and E 5 ) 2 kj/mol, and consequently, if we consider that for isodesmic processes the difference in heat capacities between reactants and products is negligible, H f 300K [O(CH 3 ) 3 ] + ) 535 ( 5 kj/mol. Finally, we note that the two preceding estimates are in correct agreement with a 300 K value of 530 ( 8 kj/mol, which may be calculated by a group equivalent analysis 17 using the known heat of formation 1,13 and proton affinity 14 of H 2 O, CH 3 OH, and (CH 3 ) 2 O. Consequently, if we put trust in MP4/ G- (d,p)//mp2/6-31g(d,p) calculations and assign an uncertainty of (15 kj/mol, our suggested H f 300K [O(CH 3 ) 3 ] + value is 536 ( 15 kj/mol. Concerning the dissociation processes of oxonium cations 1, it is interesting to visualize the effect of methyl group substitution on the heterolytic bond energies associated with the C-O bond cleavage. The following dissociation energies ( E at 0 K) have been obtained from MP4/ G(d,p)//MP2/6-31G- (d,p) + ZPE calculations: [CH 3 OH 2 ] + f [CH 3 ] + + H 2 O [(CH 3 ) 2 OH] + f [CH 3 ] + + CH 3 OH E ) 264 kj/mol E ) 328 kj/mol Figure 3. Selected MP2(F)/6-31G(d,p) optimized geometries for structures relevant to the [(CH 3) 3O] + system. Bond lengths are given in angstroms and bond angles in degrees. Figure 4. Schematic potential energy profile for the two main reactions of [(CH 3) 3O] +. Relative energies are obtained at the MP4/ G- (d,p)//mp2(f)6-31g(d,p) level. [(CH 3 ) 3 O] + + [H 3 O] + f [CH 3 OH 2 ] + + [(CH 3 ) 2 OH] + E 4 (4) [(CH 3 ) 3 O] + + [CH 3 OH 2 ] + f 2[(CH 3 ) 2 OH] + E 5 (5) Using the MP4/ G(d,p)//MP2/6-31G(d,p) total energies reported in Table 1 together with the total energy of [H 3 O] + [(CH 3 ) 3 O] + f [CH 3 ] + + CH 3 OCH 3 E ) 365 kj/mol Thus, an increase of 100 kj/mol in dissociation energy follows two successive methyl substitutions. This remarkable effect is due to an increase of the number of (polarizable) methyl groups on the ionic reactant. In contrast, but for a similar reason, while the C-O bond cleavage becomes more difficult to achieve, the methane expulsion is getting less endothermic in going from 74 kj/mol in reaction 1 to 54 kj/mol in reaction 2. For these two reactions the stabilizing effect of the polarizable methyl group occurs on both the reactant and ionic product. A stronger ion-induced dipole interaction seems to be operative on the latter species, probably due to a more concentrated positive charge. Similar to the two previous cases, a hydrogen-bonded [H 2 - CH O(CH 3 ) 2 ] + complex (COM1) is found to lie 85 kj/mol below the [CH 3 ] + + O(CH 3 ) 2 fragments, but it rearranges almost barrier-free, giving oxonium ion 1 (Figure 4). In many aspects the transition structure TS for 1,2-elimination of CH 4 from 1 closely resembles that from [(CH 3 ) 2 OH] +. 6 As postulated by Wang et al., 8 TS can be regarded as a loose interacting complex between [CH 3 ] + and CH 3 OCH 3. In structure TS (Figure 3), one carbon-oxygen bond is virtually broken (C 3 -O 1 ) 2.71 Å) while the hydrogen atom H 5 is not yet transferred to C 3 (C 3 -H 5 ) 2.40 Å) but prefers to remain attached to C 2 (C 2 -H 5 ) 1.10 Å). Again there appears a short distance between O 1 and H 4 (1.72 Å), indicating a certain hydrogen bond of the type H 2 C + H O(CH 3 ) 2. The latter is expected to bring about a stabilization partly compensating for the energy cost due to the C-O bond breaking. Note that unconstrained hydrogen bonding involving O 1 and H 4 interaction leads to complex COM1, for which a stabilization energy of 86 kj/mol is calculated. In the case of transition structure TS a stabilization energy of ca. 69 kj/mol with respect to ([CH 3 ] + + O(CH 3 ) 2 ) is calculated. In fact, TS is close in energy to the complex COM1, being only 15 kj/mol above the latter. When TS is passed, the supersystem goes downhill, rearranging to the complex COM2 before dissociating to CH 4 + [CH 2 OCH 3 ] +. Our best estimate (MP4/ G(d,p)//MP2/6-31G(d,p) + ZPE calculations) suggests an energy barrier of 295 ( 15 kj/ mol for methane loss from trimethyl, a value smaller than our earlier crude estimate of 328 ( 15 kj/mol,

5 Methane Elimination from Oxonium Cations [R 3 O] + J. Phys. Chem., Vol. 100, No. 6, which was expected to be only an upper limit. 6 The present calculated value turns out to be 30 kj/mol higher than the barrier height of 264 kj/mol calculated for dimethyloxonium ion (reaction 1, Figure 2). This is consistent with the difference mentioned above in the heterolytic C-O bond dissociation energies in both cations which amount to 36 kj/mol. These results lend support for the view that the barrier height for the elimination process from oxonium ions originates essentially from the heterolytic dissociation of the relevant bond, achieved by a specific hydrogen bond interaction which is almost equal to the fully developing hydrogen bond established in complex COM1. On the other hand, it is clearly shown that the experimental value of 357 ( 19 kj/mol 8 is definitely overestimated. Note also that the heterolytic bond dissociation energy of 1 determined by Wang et al., 8 i.e. 386 ( 19 kj/mol, is also overestimated. The activation entropy accompanying the methane departure remains small but positive, S q ) 14 J/mol K including a negative entropic contribution of about 4 kj/mol to the free energy of activation at 298 K. Again, a single hydride transfer from ether to methyl cation is found to be possible, yielding the complex COM2, which foreshadows the formation of CH 4 (Figure 4). We believe that this transfer constitutes the second reaction route to form CH 4 directly and exclusively from the reacting methyl cation in the bimolecular process [CH 3 ] + + (CH 3 ) 2 O f CH 4 + [CH 2 OCH 3 ] +. Conclusion In summary, MO calculations reported in the present paper bring several pieces of information about the mechanism and the energetics of 1,2-elimination reactions of the type [H 2 - CHsO(R)sCH 3 ] + f [H 2 CdO(R)] + + CH 4 : (i) In addition to the classical oxonium cation structure 1, two ion-neutral complexes may be identified: [H 2 CHsO(R) - CH 3 ] + (COM1) and [H 2 CdO(R) CH 4 ] + (COM2). (ii) COM1 is only a weakly stabilized structure; a slight reorientation of the two partners of the complex leads with an essentially negligible activation energy to structure 1. (iii) The responsibility of COM1 in the preferred loss of a methane molecule containing the initial methyl cation in the bimolecular process [CH 3 ] + + (CH 3 ) 2 O f CH 4 + [CH 2 OCH 3 ] + is only conceivable if cation 1 produced by the reaction COM1 f 1 dissociates before internal energy redistribution (nonergodic behavior). (iv) The direct hydride abstraction involving COM2 as reaction intermediate seems to provide a more classical interpretation of the nonstatistical distribution of labeled products. It is noteworthy that a larger participation of the hydride ion abstraction occurs for the system in which COM2 and 1 are closer in energy. This is in keeping with the intuitive idea that a competition between two association reactions is (partly) guided by the exothermicity of both processes. (v) The transition structure TS associated with the methane elimination from 1 looks like a [CH 3 ] + CH 3 OH (or CH 3 OCH 3 ) complex. Its binding energy is equal to 64 and 69 kj/mol, respectively. In fact, the energy barrier for the methane elimination from (calculated values: 267 and 295 kj/mol, respectively) is due to the C-O bond dissociation energy; this energy demanding process is compensated by a favorable interaction between one hydrogen of the methyl cation and the oxygen atom. The binding energy of TS1 compares well with the stabilization energies of the ion-neutral complex COM1 (78 and 85 kj/mol, respectively) in direct relation with the basicity of the neutral (PA ) 760 and 793 kj/mol, respectively 14 ). (vi) The heat of formation of trimethyloxonium cation [O(CH 3 ) 3 ] + is estimated to be 567 kj/mol at 0 K and 536 kj/ mol at 300 K with a probable error of (15 kj/mol. Acknowledgment. The authors thank the Belgian National Fund for Scientific Research (NFWO), Belgian Government (DWTC), KU Computing Centre, and CNRS for continuing support. References and Notes (1) (a) Lias, S. G.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. Positive Ion Energetics Database, SRD Database 19A, (b) Bartmess, J. E. Negative Ion NIST Database, SRD Database 19B, (2) Nguyen, M. T.; Rademakers, J.; Martin, J. M. L. Chem. Phys. Lett. 1994, 221, 149. (3) Burgers, P. C.; Holmes, J. L.; Terlouw, J. K. J. Am. Chem. Soc. 1984, 106, (4) Farcasiu, D.; Pancirov, R. G. Int. J. Mass Spectrom. Ion Processes 1986, 74, 207. (5) Jarrold, M. F.; Illis, A. J.; Kirchner, N. J.; Bowers, M. T. Org. Mass Spectrom. 1983, 18, 388. (6) Nguyen, M. T.; Vanquickenborne, L. E.; Bouchoux, G. Int. J. Mass Spectrom. Ion Processes 1993, 124, R11. (7) (a) Holmes, J. L. 41st ASMS Conference on Mass Spectrometry, San-Francisco, (b) Sirois, M.; George, M.; Holmes, J. L. Org. Mass Spectrom. 1994, 29, 11. (8) Wang, D.; Squires, R. R.; Farcasiu, D. Int. J. Mass Spectrom. Ion Processes 1991, 107, R7. (9) (a) Audier, H. E.; McMahon, T. B. 41st ASMS Conference on Mass Spectrometry, San-Francisco, (b) Audier, H. E.; Koyanagi, G. K.; McMahon, T. B.; Tholman, D. To be published. (10) Frish, M. J.; Head-Gordon, M.; Trucks, G. W.; Foresman, J. B.; Schlegel, H. B.; Raghavachari, K.; Robb, M.; Binkley, J. S.; Gonzalez, C.; DeFrees, D. F.; Fox, D. J.; Whiteside, R. A.; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. P.; Topiol, S.; Pople, J. A. 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