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J. Phys. Chem. 1996, 100, 8220-8223
Evidence for Gas-Phase Hydrogen-Bonded Complexes of the Form [CH3+/CH3OH] and [CH3+/CH3OCH3] H. E. Audier,*,† G. K. Koyanagi,‡ T. B. McMahon,‡ and D. Tholmann‡ Laboratoire des Me´ canismes Re´ actionnels, URA CNRS 1307, Ecole Polytechnique, 91128 Palaiseau Cedex, France, and Department of Chemistry, UniVersity of Waterloo, Waterloo, Ontario, Canada N2L 3G1 ReceiVed: July 31, 1995; In Final Form: January 2, 1996X
Studies of unimolecular dissociation (MIKES) of ions formed in a chemical ionization ion source show that under CI conditions, the reaction of CH3+ with CH3OH or CH3OCH3 leads to the covalent structures (CH3)2OH+ or (CH3)3O+. In contrast a FT-ICR study indicates that these reactions lead either to covalent structures by C-O bond formation and to [CH3+/CH3OH] or [CH3+/CH3OCH3] ion-neutral complexes. Ab initio calculations confirm that such complexes correspond to minima on the potential energy surface. Their geometries correspond to a species in which a hydrogen of the CH3+ cation is weakly bonded to the oxygen of the neutral. The interaction energies are ∼20 kcal/mol.
Introduction In the course of the last several years, many gas-phase ionmolecule processes, both unimolecular and bimolecular, have been reexamined. Several recent works show that noncovalent ion-neutral complexes may play an important, if not determinant, role in the reaction mechanism.1-5 Demonstration of the existence of such complexes is not straightforward and, very frequently, must be inferred through the use of one or more of three complementary methods. The first of these involves the use of ab initio calculations to demonstrate that the hypothetical intermediate proposed to explain the experimental evidence does indeed correspond to an energetic minimum on the potential energy surface for the reaction. Mass spectrometric methods, such as collisional activation (CA) or neutralization-reionization (NRMS) can also be employed to distinguish between ionneutral complexes and their, usually more stable, covalently bound isomers. Finally, chemical methods in which reactivity differences between various isomeric forms should exist can be brought to bear. These reactivity differences may involve the use of isotopic labeling, stereochemical influences of ligand exchange and other ion-molecule reactions. A significant body of recent work by several groups has demonstrated the particular efficacy of this latter method. The unimolecular dissociation behavior of the dimethyl- and trimethyloxonium ions, 1 and 2,6-11 represents a particularly interesting case in point. Each of these ions undergoes both metastable and collision-induced dissociation to lose CH4 yielding CH2OH+ or CH3OCH2+ respectively, reactions 1 and 2. While several studies of these dissociations have been carried
(CH3)2OH+ f CH2OH+ + CH4
(1)
(CH3)3O+ f CH3OCH2+ + CH4
(2)
out, no conclusive evidence has yet been presented to implicate the intervention of noncovalent ion-neutral complexes on the potential energy surface. On the basis of threshold collision induced decomposition studies, Wang et al.6 proposed that the †
Ecole Polytechnique. University of Waterloo. X Abstract published in AdVance ACS Abstracts, April 1, 1996. ‡
S0022-3654(95)02174-5 CCC: $12.00
fragmentation of (CH3)3O+ occurs via a heterolytic cleavage mechanism with the critical transition state strongly resembling a [CH3+/(CH3)2O] complex. Further, Nguyen et al.,7 using ab initio calculations, showed that such as asynchronous concerted mechanism is indeed possible for the loss of CH4 from (CH3)2OH+. However neither these authors nor others have successfully located at energy minimum corresponding to a noncovalent complex on the potential energy surface. In a very early study of bimolecular reactions of CH3+ with CH3OH and several deuterium-labeled isotopic variants, Smith and Futrell8-10 provided the first experimental evidence that some form of ion-neutral complex might play a role in the reaction. The labeling studies revealed that at low ion kinetic energies the dominant process in the reaction of CH3+ with methanol is the formation of the covalently bound oxonium ion, as a transient intermediate, which subsequently undergoes loss of CH4 to yield CH2OH+, reaction 3.
CH3+ + CH3OH f [(CH3)2OH+]* f CH2OH+ + CH4 (3) However, using labeled species, they also showed that a small fraction of such reactions must proceed by a “direct” mechanism corresponding to hydride abstraction by the initial methyl cation from the methyl group of methanol. More recently Farasiu and Pancirov11 have studied this and the analogous reactions for dimethyl ether and have analyzed the oxonium ion products using CID techniques in a triple quadrupole mass spectrometer. Using deuterium isotopic labeling they concluded that only covalent adducts are formed and that no hydrogen scrambling occurs during the dissociation event. These results were taken to infer that the dissociation involved a concerted, but not necessarily synchronous, 1,2 methane elimination. They cautioned however that the results are somewhat uncertain since the deuterium isotope effect on the unimolecular dissociation itself is unknown. In the present work,12 we report a combined unimolecular dissociation (MIKES) and FT-ICR study of the unimolecular dissociation of the dimethyl- and trimethyloxonium ions. These methods provide complementary entrances onto the oxonium ion potential energy surfaces by providing significantly different oxonium ion internal energies. In addition a large number of both 13C and deuterium-labeled variants of the relevant species involved have been used to determine the specific isotope effects © 1996 American Chemical Society
Evidence for [CH3+/CH3OH] and [CH3+/CH3OCH3]
J. Phys. Chem., Vol. 100, No. 20, 1996 8221
involved in the metastable dissociations and to verify that a “direct” mechanism does exist in the bimolecular reactions of methyl cation with both methanol and dimethyl ether. Such a mechanism, involving a noncovalent ion-neutral complex, is proposed and suggests therefore, that an ion-neutral complex may in fact play a significant role on each of the oxonium ion potential energy surfaces. Finally ab initio calculations will be presented which demonstrate that a stable, noncovalent, ionneutral complex does exist as an isomer for each of the oxonium ions.
TABLE 1: Reactions of Labeled CH3+ Cations with Labeled Methanols and Dimethyl Ethers (MIKE)
Experimental Section
13
Unimolecular dissociations of metastable oxonium ions (MIKES) were studied using a VG Instruments ZAB 2F spectrometer. Adduct ions were generated in a chemical ionization source using an excess of a methyl cation donor reagent such as a mixture of H2O and CH3I. In this way specifically labeled species could be prepared using either labeled CH3I or labeled substrate [CH3OH or (CH3)2O]. The unimolecular dissociation of magnetically mass-selected parent ions was studied using an electric field scan of the electrostatic analyzer. Bimolecular reactions of methyl cation with methanol and dimethyl ether were examined using a Bruker CMS 47 FTICR spectrometer equipped with an external ion source constructed at the University of Waterloo. Mehtyl iodide in the external ion source provides a mixture of ions by electron impact which includes the methyl cation of interest. These ions are then transferred to the FT-ICR cell where all ions, except the desired methyl cation, are ejected using a sequence of RF pulses. A high-pressure pulse of CH4 serves to collisionally relax any excess kinetic or thermal energy of the isolated methyl cation. These mass-selected ions are then allowed to react with the neutral of interest present in the FT-ICR cell at a pressure of (1-2) × 10-8 mbar. In addition a constant pressure of ∼2 × 10-7 mbar of Ar was also present to ensure thermalization of all product ions formed by ion-molecule reaction. Ab initio calculations were carried out using the Gaussian 92 Revision A program system13 running on an SGI 4D.210 platform. Geometry optimizations were performed at the MP2 FU/6-31+G (2d,2p) level followed by single-point energy calculations at the QCISD(T) FU/6-31+G (2d,2p) level. Vibrational analysis at the lower level provided corrections for zero-point energies and thermal corrections to 298 K to give overall enthalpy exchanges. As noted in the potential energy profiles, where experimental data were available agreement with calculated enthalpy changes was within 1 kcal mol-1.
CH3+ + CD3OCD3
Results and Discussion Initially the metastable unimolecular dissociations of the adducts generated by association reactions of CH3+ with either CH3OH or (CH3)2O in the chemical ionization (CI) source of a reverse-geometry double-focusing mass spectrometer were investigated (MIKES). For example, when a 1:5:1 mixture of CH3OH/H2O/CH3I is introduced into the CI source to a sufficiently high-pressure methylation of methanol by the CH3 group initially in the iodide occurs. The metastable dissociation in the second field free region of the mass spectrometer of the adduct ion thus formed, (CH3+/CH3OH) can be monitored. This dissociation involves the elimination of CH4 to yield CH2OH+ as the ionic product. Three results demonstrate that the two moieties become equivalent prior to dissociation, most probably by invoking the covalent structure I. First, exactly the same behavior is observed whether the parent ion is formed by the association reaction described above or by the protonation of
reactants CH3 + CD3OD(H) CD3+ + CH3OH CD2H+ + CD2HOH CDH2+ + CH2DOH CH3+ + CH3OH
CD3+ + CH3OCH3 CD3+ + CD3OCHD2 13CH + 3
+ CH3OCH3
loss
yields
m/z
CH3D CD3H CD3H CH3D CD3H CH2D2 CH2D2 CH3D 13 CH4 CH4 CH3D CD3H CD4 CD3H CH3D CH4 CD4
CD2dOD(H)+ CH2dOD(H)+ CH2dOH+ CD2dOH+ CHDdOH+ CD2dOH+ CH2dOH+ CHDdOH+ CH2dOH+ 13CH dOH+ 2 CD2dOCD3+ CH2dOCD3+ CD2dOCH3+ CH2dOCH3+ CD2dOCH3+ CH2dOCD3+ CD2dOCHD2+ orCHDdOCD3+ CD2dOCD3+ CH2dOCH3+ 13CH dOCH + 2 3
34 (33) 32 (31) 31 33 32 33 31 32 31 32 50 48 47 45 47 48 49 49 50 45 46
CHD3 13CH 4 CH4
exp (%)
stat (%)
55 45 45 55 63 37 33 67 50 50 34.5 38 27.5 32.5 27.5 40 55 45
50 50 50 50 66.7 33.3 33.3 66.7 50 50 33.3 33.3 33.3 33.3 33.3 33.3 55 45
33.3 33.3 66.7 66.7
dimethyl ether. In addition, the adducts (CD3+/CH3OH) and (CH3+/CD3OH) give the same products (Table 1). Finally, the adduct (13CH3+/CH3OH) undergoes, dissociation with equal probability to give loss of either CH4 or 13CH4 (Table 1), conclusively demonstrating the equivalence of the two methyl groups prior to dissociation. Similarly, the reactions of CH3+ with dimethyl ether lead to the same conclusions. For example, the adduct (13CH3+/CH3OCH3) leads to a statistical (2:1) loss of CH4/13CH4 thus demonstrating the probable intermediacy of the trimethyloxonium ion, (CH3)3O+, II. The study of the deuterium-labeled ions I and II (Table 1) thus formed indicates the following: (i) The methane loss from I is accompanied by a significant secondary isotope effect (is ≈ 1.3). The primary isotope effect is negligible (ip ≈ 1.05). (ii) These isotope effects are greater when ion II fragments. The primary isotope effect can be evaluated either by the ratio [m/z 48]/[m/z 47] for the reaction between CD3+ and CH3OCH3 or by the ratio [m/z 48]/[m/z 50] for the reaction between CH3+ and CD3OCD3. Therefore ip is about 1.4. Similarly the ratio [m/z 50/]/[m/z 47] when CH3+ reacts with CD3OCD3 gives is ) 1.25. Since the secondary isotope effects for the decomposition of ions I and II are signifiant, these results provide a direct proof of the asynchonous concerted process previously proposed.6,7-11 The parallel FT-ICR experiments (Figure 1) of reactions of methyl cations with methanol and dimethyl ether carried out in the present study (Table 2) provide strong evidence for the role of ion-neutral complexes in these reactions. For example, the reactions of CD3+ with CH3OH and CH3+ with CD3OH lead to losses of CH3D and CD3H in ratios which are significantly different. Analogously, the reaction of 13CH3+ with CH3OH gives losses of 13CH4 and CH4 in a ratio of 57:43. This strongly suggests that roughly 86% of the products observed arise from the covalently bound oxonium ion ((CH3)2OH+) while the remaining ∼14% are the result of some other mechanism, either spectator stripping or one involving an ion-neutral complex which directly eliminates exclusively 13CH4. In the same way, reaction of 13CH3+ with dimethyl ether (Figure 2) gives a roughly 1:1 ratio of 13CH4/CH4 losses, whereas statistically a 1:2 ratio is expected. This then leads to the conclusion that ∼25% of the products arise from a second mechanism again conceivably one involving an ion-neutral complex.
8222 J. Phys. Chem., Vol. 100, No. 20, 1996
Audier et al.
Figure 1. Reaction of CH3+ cations with CH3OCD3.
TABLE 2: Reactions of Labeled CH3+ Cations with Labeled Methanols and Dimethyl Ethers (FT-ICR) reactants
loss
CH3+ + CD3OD(H) CH3D CD3H CD3+ + CH3OH CD3H CH3D CH3+ + CD2HOH CH4 CH3D CH2D2 CD3+ + CH2DOH CD4 CD3H CH2D2 13CH + 13CH 3 + CH3OH 4 CH4 + CH3 + CD3OCD3 CH3D CD3H CD4 CD3+ + CH3OCH3 CD3H CH3D CH4 CH3+ + CD3OCH3 CH4 CH3D CD3H CD3+ + CD3OCH3 CD4 CD3H CH3D 13 CH4 13CH + CH OCH CH4 3 3 3
yields
m/z
exp (%)
stat (%)
CD2dOD(H)+ CH2dOD(H)+ CH2dOH+ CD2dOH+_ CD2dOH+ CHDdOH+ CH2dOH+ CH2dOH+ CHDdOH+ CD2dOH+ CH2dOH+ 13CH dOH+ 2 CD2dOCD3+ CH2dOCD3+ CD2dOCH3+ CH2dOCH3+ CD2dOCH3+ CH2dOCD3+ CH2dOCD3+ CD2dOCH3+ CH2dOCH3+ CD2dOCH3+ CH2dOCD3+ CD2dOCD3+ CH2dOCH3+ 13CH dOCH + 2 3
34 (33) 32 (31) 31 33 33 32 31 31 32 33 31 32 50 48 47 45 47 48 48 47 45 47 48 50 45 46
50.6 49.4 57.1 42.9 20.0 34.1 46.0 15.8 39.1 45.1 57.2 42.8 50.6 28.6 20.8 50.4 21.3 28.3 44.2 32.6 23.2 31.8 43.6 24.6 49.5 50.5
50 50 50 50 16.7 33.3 50 16.7 33.3 50 50 50 33.3 33.3 33.3 33.3 33.3 33.3 33.3 33.3 33.3 33.3 33.3 33.3 33.3 66.7
The probable nature of an important ion-neutral complex possible in these reactions may be inferred from recent ab inito calculations by Uggerud14 in which he showed that a stable hydrogen-bonded complex (III) exists between CH3+ and H2O. This isomer of protonated methanol is bound by 15 kcal mol-1 with respect to methyl cation and water but is less stable than its covalent isomer by 47 kcal mol-1. Following this example, ab initio calculations by us have located minima corresponding to the hydrogen-bonded complexes IV and V for the interactions between CH3+ and methanol and dimethyl ether respectively (Figure 2). The former is bound by 19.5 kcal mol-1 and the latter by 20.3 kcal mol-1. The similarity between these two values as well as that for the analogous interaction involving H2O strongly implicates an electrostatic, rather than covalent binding, in these complexes since the gas-phase basicity of the neutral appears to play an extremely minor role in the strenght of the interaction. In addition to the noncovalent complexes calculations for the covalent adducts and final products for each case were also carried out allowing the potential energy surfaces shown in Figures 3 and 4 to be constructed. The calculated enthalpy
Figure 2. Structure of the [CH3+/CH3OH] and [CH3+/CH3OCH3] ionneutral complexes (IV and V).
Figure 3. Potential energy profile for the reaction of CH3+ with CH3OH.
differences between reactants and covalent adducts and between reactants and products agree with the experimental values for the same quantities to within 1 kcal mol-1, lending considerable confidence to the energetics and structures derived for the noncovalent complexes. A search for the transition states for transformation of IV to the dimethyloxonium ion and for loss of CH4 from IV revealed that either the two transition states are very similar or that the two reaction paths share a common transition state. However, attempts to locate the transition states for these reactions via ab inito calculations were unsuccesful, due, likely, to the very flat nature of the potential energy surface
Evidence for [CH3+/CH3OH] and [CH3+/CH3OCH3]
J. Phys. Chem., Vol. 100, No. 20, 1996 8223 methanol and dimethyl ether. These complexes lie below the energy of the transition state for loss of CH4 from the corresponding oxonium ions and may possibly be accessed from the oxonium ion prior to metastable decomposition. Such an interconversion however would be undetectable in the MIKES experiments. However it is in the low-pressure bimolecular reactions where the most compelling evidence for these complexes is found. The alternative, a spectator stripping mechanism, appears unlikely for this thermal ion energy reaction in view of the successful location of the weakly bound ionneutral complexes by ab initio calculation. References and Notes
Figure 4. Potential energy profile for the reaction of CH3+ with CH3OCH3.
in this region. Previous calculations by Nguyen et al.7 revealed that the transition state for elimination of CH4 from the dimethyloxonium ion does indeed have the character of a hydrogen-bonded species. In this transition state the methyl cation anchored to the oxygen via a hydrogen bond as it is picking up a hydrogen from the other methyl group. This is also a strong argument in favor of the intermediary of hydrogenbonded complexes when CH3+ reacts with methanol or dimethyl ether. Conclusion The evidence thus obtained to date from the combined results of metastable dissociation, bimolecular reactions, and ab initio calculations strongly implicates an important role for a hydrogenbonded ion-neutral complexes in the reactions of CH3+ with
(1) McAdoo, D. J. Mass Spectrom. ReV. 1988, 7, 363. (2) Hammerum, S. J. Chem. Soc., Chem. Commun. 1988, 858. (3) Bowen, R. D. Acc. Chem. Res. 1991, 24, 364. (4) Longevialle, P. Mass Spectrom. ReV. 1992, 11, 157. (5) Morton, T. H. Org. Mass Spectrom. 1992, 16, 423. (6) Wang, D.; Squire, R.; Farcasiu, D. Int. J. Mass Spectrom. Ion Processes 1991, 107, R7. (7) Nguyen, M. T.; Vanquikenborne, L. G.; Bouchoux, G. Int. J. Mass Spectrom. Ion Processes 1993, 124, R11. (8) Smith, R. D.; Futrell, J. H. Org. Mass Spectrom. 1978, 13, 688. (9) Smith, R. D.; Futrell, J. H. Int. J. Mass Spectrom. Ion Phys. 1977, 24, 173. (10) Smith, R. D.; Futrell, J. H. Chem. Phys. Lett. 1976, 41, 64. (11) Farcasiu, D.; Pancirov, R. G. Int. J. Mass Spectrom. Ion Processes 1986, 74, 207. (12) (a) Audier, H. E.; Berthomieu, D.; McMahon, T. B. Proc. of the 41th ASMS Conference, San Francisco, 1993. (b) Audier, H. E.; Koyanagi, G. K.; McMahon, T. B.; Tholmann, D. Proc. of the 42th ASMS Conference, Chicago, 1994. (13) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. I.; Raghavachari, K.; Binkley, J. S.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian 92, Revision A. Gaussian, Inc.: Pittsburgh, PA, 1992. (14) Uggerud, E. J. Am. Chem. Soc. 1994, 116, 6873.
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