Nucleophilic Substitution at Sulfur: SN2 or Addition− Elimination?

J. Phys. Chem. 1996, 100, 3535-3540

3535

Nucleophilic Substitution at Sulfur: SN2 or Addition-Elimination? Steven M. Bachrach* and Debbie C. Mulhearn Department of Chemistry, Northern Illinois UniVersity, DeKalb, Illinois 60115 ReceiVed: NoVember 13, 1995X

The thiolate-disulfide exchange reaction was studied at the HF/6-31+G*, MP2/6-31+G*, MP4SDTQ/631+G*//MP2/6-31+G*, and CCSD(T)/6-31+G*//MP2/6-31+G* levels for three identity reactions: R1S- + R2SSR3 f R1SSR2 + R3S-, where (1) R1 ) R2 ) R3 ) H, (2) R1 ) R3 ) Me, R2 ) H, and (3) R1 ) R3 ) H, R2 ) Me. Results indicate that at the HF/6-31+G* level the reactions proceed via an SN2 pathway. However, the potential energy surface at correlated levels has a triple-well structure, indicating an additionelimination pathway. The HF TS becomes a stable intermediate upon inclusion of electron correlation, and an asymmetric transition state connects the ion-dipole complex with the intermediate. Structural and energetic results do suggest however that as R2 becomes larger, the reaction may not be able to proceed via an additionelimination pathway and the SN2 mechanism operates.

Nucleophilic substitution is a fundamental reaction and played a central role in the development of modern physical organic chemistry.1 Interest in extending this strategy to heteroatoms is growing.2 However, the lack of stereochemistry at most heteroatoms removes one of the most powerful tools for determining the mechanisms of these reactions. There are indications that the mechanism is different between first- and second-row atoms. For example, a classic SN2 mechanism is evident for nucleophilic substitution at carbon,3 nitrogen,4,5 and oxygen,6,7 but an addition-elimination pathway occurs for reactions at phosphorus8 and is implied for reactions at silicon.9 Solution phase experiments suggest an SN2 mechanism for substitution at sulfur.10-16 We report here ab initio calculations that suggest that an addition-elimination mechanism for substitution at sulfur may actually occur in the gas phase. Experimental studies to date for nucleophilic substitution at sulfur have concentrated on reactions at disulfides. Reactions at an S-S bond are very interesting due to their occurrences in numerous biological systems and because the bond is highly reactive: the S-S bond energy is 63.8 kcal mol-1.17 The thiolate-disulfide exchange reaction has been studied with many substituents in various solvents. The kinetic results indicate a very fast reaction, which is dependent on the solvent, giving βnuc values (from a Bronsted plot) and entropy values that are indicative of an SN2 reaction.10-14 Although it has been suggested that nucleophiles generally prefer backside attack at sulfur,18 definitive evidence has not been provided. Since sulfur is a nonstereogenic atom, classic stereochemical studies are of no assistance. The existence of a stable intermediate along the substitution pathway contradicts the SN2 mechanism. In 1970, Painter and co-workers reported finding a short-lived glutathione trisulfide anion intermediate, G3S3-.19 Since then, there has not been any other report of a trisulfide anion, even in the recent gas phase study done by Grabowski and Zhang.20 They followed the reaction between various nucleophilic anions and dimethyl disulfide. When the nucleophile is a thiolate, substitution is the major reaction. If the nucleophile was a strong enough base, however, a proton would transfer to the base, forming a dimethyl disulfide ion-base complex that underwent further rearrangement. Theoretical studies can provide detailed structural information on the transition state, particularly the relationship of the X

Abstract published in AdVance ACS Abstracts, February 1, 1996.

0022-3654/96/20100-3535$12.00/0

SCHEME 1

nucleophile, central atom, and leaving group. Aida and Nagata21 examined the prototype thiolate-disulfide reaction, HS- + HSSH, at the HF level using the 6-31G* basis set augmented with a set of diffuse p functions on sulfur. They found a classic gas phase SN2 mechanism (Scheme 1) where the reactants form an ion dipole complex (IDC) which then passes through a symmetrical transition state (TS) to give another ion dipole complex before dissociating to the products. Analytical frequencies were used to confirm that the ion dipole complex was a local minimum and that the TS had one imaginary frequency. Previously, Pappas22,23 had claimed that the reaction took place through a stable intermediate, H3S3-. However, Pappas concluded that H3S3- was an intermediate and not a TS solely on the basis that H3S3- happened to be lower in energy than the reactants, and not from a frequency analysis. Therefore, it is unclear what the nature of the trisulfide anion is. In this paper, we report significantly more detailed theoretical studies of the thiolate-disulfide exchange reaction. Geometries are optimized at MP2/6-31+G* to account for the effects of electron correlation, along with single-point energies evaluated at higher levels. We examine reactions 1-3, whereby we can observe the effect of steric interactions. Reaction 1 is the prototype exchange. Reaction 2 places a methyl group on the terminal sulfurs, and reaction 3 places a methyl group on the central sulfur atom. Of most importance is the structure of the TS, specifically the approach angle of the nucleophile relative to the leaving group, and the presence of any intermediates in the reaction sequence.

HS- + HS-SH f HS-SH + HS-

reaction 1 -

MeS + HS-SMe f MeS-SH + SMe -

-

HS + MeS-SH f HS-SMe + HS

reaction 2 reaction 3

Computational Methods Ab initio calculations were performed for all reactants, intermediates, and transition states in reactions 1-3 using GAUSSIAN 9224 and GAUSSIAN 94.25 Structures were fully optimized at HF/6-31+G* and reoptimized at MP2(full)/631+G*. The lowest energy conformer for each of the reactants © 1996 American Chemical Society

3536 J. Phys. Chem., Vol. 100, No. 9, 1996

Bachrach and Mulhearn point energy calculations were performed with the MP2 optimized geometries at MP4SDTQ(full)/6-31+G* and CCSD(T)(fc)/6-31+G* for all three reactions. To further evaluate the effects of higher order electron correlation, the structures in reaction 1 were fully optimized at MP4SDQ(full)/6-31+G*. Numerical frequencies (unscaled) were obtained at this level. Results

Figure 1. HF/6-31+G* potential energy surface for reaction 1. Energies are in kcal mol-1.

Figure 2. HF/6-31+G* optimized geometries for reactions 1-3. Distances are in angstroms, and angles are in degrees.

was used: Cs for HS-, C3V for CH3S-, C2 for HSSH, and C1 for CH3SSH. Analytical frequencies were calculated at both the HF and MP2 levels to properly characterize the structures and obtain ZPE values. The HF frequencies were scaled by 0.89,26 while the MP2 frequencies were used unscaled.27 Single-

In their study of reaction 1, Aida and Nagata21 report a double-well potential (Figure 1) having a single transition state with a near linear S-S-S angle. They concluded that the thiolate-disulfide exchange occurs via an SN2 mechanism. At the HF/6-31+G* level, our calculations for reactions 1-3 confirm this finding. The HF/6-31+G* geometries for the iondipole complex (IDC) and transition state (TS) for the three reactions are drawn in Figure 2. The ion-dipole complexes all show the thiolate nucleophile to be interacting with the disulfide compound through a weak hydrogen bond. The S-H bond of the disulfide fragment lengthens about 0.02 Å when IDC-1 and IDC-2 are formed, while the C-H is unchanged in IDC-3. This is due to the fact that the S-H bond is much weaker (∆Hacid(H2S) ) 353 kcal mol-1) than the C-H bond (∆Hacid(CH4) ) 416 kcal mol-1).28 In the TSs, the S-S distances are longer than in the bare disulfide. The S-S distance is 2.469 Å in TS-1, compared to 2.065 Å in HSSH. The S-S distance is slightly shorter in TS-2 (2.451 Å) than in TS-1 but still longer than in MeSSH (2.060 Å). TS-3 has inequivalent S-S bonds of length 2.480 and 2.508 Å, both longer than the S-S distances in the other TSs, due to the increased steric bulk at the central sulfur atom. The S-S-S angle is 172.4° in TS-1, 172.6° in TS-2, and 183.6° in TS-3 (where the terminal sulfurs bend away from the methyl group on the central sulfur). The relative energies of the species in reactions 1-3 are listed in Table 1, with a graphical representation of reaction 1 shown in Figure 1. At HF/6-31+G*, all three reactions show the classical backside attack of the nucleophile and double-well potential energy surface of an SN2 reaction. Optimization of the structures for reaction 1 at MP2 proved to be surprising. The C2 structure is not a transition state, as it is on the HF surface, but rather it is a local minimum (which we will call INT), having no imaginary frequencies (determined using analytical MP2 frequency analysis). In fact, for all three reactions, the HF TS proved to be a local minimum on the MP2 surface! A transition state linking the ion-dipole complex with the intermediate was located for all three reactions. These

TABLE 1: Energies (Relative to the Reactants) in kcal mol-1 for Reactions 1-3 (Energies in Italics Are for the Syn Pathway; See Discussion in Text) level HF/6-31+G*a IDC TS MP2/6-31+G*b IDC TS′ INT MP4SDTQ/6-31+G*b IDC TS′ INT CCSD(T)/6-31+G*b IDC TS′ INT a

reaction 1 HS- + HSSH

reaction 2 CH3S- + HSSCH3

reaction 3 HS- + CH3SSH

-10.29 -0.10

-10.78 +1.95

-6.41 +7.10

-16.95 -10.68 (-10.63) -12.94 (-12.92)

-20.16a -11.33 (-11.57) -13.78 (-14.00)

-9.05 -8.27 (-8.23) -8.34 (-8.55)

-15.87 -10.73 (-10.70) -12.90 (-12.89)

-19.42a -11.36 (-11.63) -13.78 (-13.57)

-9.05 -8.76 (-8.71) -8.67 (-8.90)

-15.37 -10.40 -12.08

-19.02a -10.86 -12.29

-8.82 -8.08 -7.74

Energy is based on the HF/6-31+G* geometry and corrected with the HF ZPE (scaled by 0.89). b Energies are based on the MP2/6-31+G* geometries and corrected with the MP2 ZPE (unscaled), unless otherwise indicated.

Nucleophilic Substitution at Sulfur

Figure 3. MP2/6-31+G* potential energy surface for reaction 1. Energies are in kcal mol-1.

Figure 4. MP2/6-31+G* optimized geometries for reactions 1-3. Distances are in angstroms, and angles are in degrees.

transition states are designated TS′. Analytical frequencies at the MP2 level were performed for all of the new transition structures, giving one imaginary frequency, which corresponds to the nucleophile moving from its weakly hydrogen bonded position in the IDC to where it starts its attack on sulfur. The MP2/6-31+G* potential energy surface has three wells, as shown in Figure 3, and the geometries for the IDC, TS′, and INT for reactions 1-3 are drawn in Figure 4. For the first reaction, IDC-1 has a much stronger hydrogen bond than in the HF geometry, which is expected at the MP2 level. As the reaction proceeds to TS′-1, the nucleophile swings around to attack the sulfur, resulting in an Snuc‚‚‚S distance of 3.186 Å and a slightly elongated S-Slg bond of 2.123 Å. Most important is the Snuc-S-Slg angle of 153.2°, much smaller than the expected 180° for an SN2 transition state. The geometry of INT-1 is nearly identical to the geometry of the transition structure of the HF surface: the S-S distance of 2.467 Å (MP2) compared to 2.469 Å (HF) and the S-S-S angle is the same in both structures. In reaction 2, methylthiolate anion attacks the less substituted sulfur of methyldisulfide. Repeated attempts at locating IDC-2

J. Phys. Chem., Vol. 100, No. 9, 1996 3537 at the MP2 level resulted in a complex where the proton is transferred to the methylthiolate nucleophile, a reaction known to compete with the substitution reaction.20 Such a proton transfer from the disulfide to the nucleophile was not readily observed in reaction 1 due to the less basic HS- nucleophile. The MP2 optimized geometries of TS′-2 and INT-2 are very consistent with those in reaction 1. TS′-2 has an Snuc‚‚‚S distance of 3.089 Å and an S-Slg bond length of 2.118 Å. All three of these distances are shorter than those in TS′-1, due to CH3S- being a stronger base than HS-. For INT-2 (C2 symmetry) the S‚‚‚S distance is 2.448 Å, which is also shorter than in INT-1. The Snuc-S-Slg angle is 172.6°, nearly identical with INT-1. In reaction 3, a methyl group is placed on the sulfur being attacked, while the nucleophile and leaving group are both HS-. Determining the MP2 geometry of IDC-3 was not a problem, as it was in reaction 2, due to both the weaker base HS- (vs methylsulfide in reaction 2) and the stronger C-H bond (vs S-H bond in reaction 2) that the nucleophile interacts with in the complex, leading to a more difficult deprotonation. Since IDC-3 involves the strong C-H bond and a weak base, there is little change in the structure of the disulfide. TS′-3 differs from the other two TSs is that the bulkier methyl group attached to the central sulfur sterically hinders the nucleophile from attacking the disulfide at a smaller angle, resulting in a shorter S‚‚‚S distance, the nucleophile being nearly in line with the leaving group sulfur, and a later TS. The Snuc-S distance is 2.813 Å, while the leaving group is displaced slightly more than in the first two reactions, r(S-Slg) ) 2.232 Å. The Snuc-SSlg angle is 182.2°, much larger than in TS′-1 or TS′-2. For this substitution reaction to continue on to INT-3, the Snuc-S distance shortens from 2.813 to 2.480 Å while the Snuc-S-Slg angle remains relatively unchanged. Having the methyl group on the center sulfur forces the nucleophile to attack the disulfide later in the reaction, resulting in a TS geometry that is very close to the stable intermediate. Since the HF and MP2 potential energy surfaces differ so dramatically, we were concerned about the quality of the calculation. Characterization of the critical points on the potential energy surface requires analytical energy second derivatives, which are unavailable to us for computational levels beyond MP2. Therefore, to test the effects of higher order electron correlation, we have performed single-point energy calculations using the MP2/6-31+G* geometries at MP4SDTQ and CCSD(T) for reactions 1-3. The relative energies of the IDC, TS, and INT are reported in Table 1. At MP2/6-31+G*, INT-1 is 2.26 kcal mol-1 more stable than TS′-1. The gap between the two structures gets smaller upon inclusion of higher order correlation: INT-1 is 2.17 (MP4SDTQ) and 1.68 kcal mol-1 (CCSD(T)) more stable than TS′-1. Reaction 2 displays similar results; at MP2 INT-2 is 2.45 kcal mol-1 lower than TS′-2, and at MP4SDTQ and CCSD(T) this difference is 2.42 and 1.43 kcal mol-1, respectively. Despite the fact that the energy differences are small, the reaction coordinate still possesses three wells at the higher levels, supporting the MP2 results. INT-3 is only 0.07 kcal mol-1 more stable than TS′-3 at MP2/ 6-31G*, reflecting their geometric similarity. The MP4SDTQ and CCSD(T) energies contradict the MP2 results in that INT-3 is 0.09 and 0.34 kcal mol-1 higher in energy than TS′-3, suggesting the lack of a stable intermediate. However, since the energy differences are very small (less than 0.5 kcal mol-1), the geometries have not been optimized at these levels, and the frequencies are unknown, it is difficult to estimate the reliability of these results.

3538 J. Phys. Chem., Vol. 100, No. 9, 1996

Bachrach and Mulhearn

SCHEME 2

As an additional test of the reliability of the MP2 surface, the structures for reaction 1 were further analyzed (full optimization and numerical frequencies) at MP4SDQ/6-31+G*. All of the MP2 structures optimized at MP4SDQ with little change (maximum differences in bond lengths of 0.05 Å and angles of 0.5°) from their MP2 structures, proving that the MP2 geometries are sufficient. The relative energies at this level are IDC-1, -14.40 kcal mol-1; TS′-1, -9.76 kcal mol-1; and INT-1, -10.17 kcal mol-1. Furthermore, the numerical frequencies for IDC-1 and INT-1 are all real, while TS′-1 has one imaginary frequency. Therefore, the MP4SDQ surface is identical to the MP2 surface, one having three wells (Figure 3). We were also concerned about the adequacy of the basis set. The 6-31+G* basis set has been used effectively in discussing a number of substitution reactions;3,9 however, the bridging H in TS′ suggests that polarization functions on H may be important. Upon reoptimization of TS′-1 and INT-1 at MP2/ 6-31+G**, INT-1 is 2.56 kcal mol-1 lower in energy than TS′1. Thus, polarization functions on hydrogen are not critical. Upon close examination of the optimized structures for all three reactions, we realized that the structures correspond to one enantiomeric pathway. As shown in Scheme 2, there is a pair of enantiomeric reaction pathways which have the substituents R1 arranged anti. Another pathway, having the substituents arranged syn, which proceeds through an intermediate having a mirror symmetry plane, is also a possibility. To verify the existence of the syn path, we located the transition state (TS′-syn) and intermediate (INT-syn) for the three reactions at MP2(full)/6-31+G*. These structures were confirmed using the MP2 analytical frequencies. Single-point energies at MP4SDTQ(full)/6-31+G*//MP2/6-31+G* were also obtained. The syn structures are shown in Figure 5, and their relative energies are presented in italics in Table 1. The geometries of the syn transition structures are similar to the anti TS discussed above. At MP4SDTQ, TS′-1syn is isoenergetic to TS′-1 (only 0.03 kcal mol-1 higher); TS′-2syn is 0.27 kcal mol-1 lower in energy than TS′-2; TS′-3syn is 0.05 kcal mol-1 higher in energy than TS′-3. The syn intermediates for reactions 1 and 3 optimized to local minimum having Cs symmetry. Outside of the orientation of the terminal hydrogen atoms, INT-1syn is nearly identical in structure to INT-1. INT-1syn lies only 0.01 kcal mol-1 higher in energy than INT-1. INT-3syn also has similar structural

Figure 5. MP2/6-31+G* optimized geometries for the syn pathway of reactions 1-3. Distances are in angstroms, and angles are in degrees.

parameters as in INT-3, but lies slightly lower (0.23 kcal mol-1) in energy than INT-3. The Cs structure for reaction 2 proved to be a TS rather than an intermediate. Breaking symmetry to C1 and following the imaginary frequency led to the true intermediate, INT-2syn, which has the methyl groups staggered and lies 0.23 kcal mol-1 higher in energy than INT-2. INT-2 and INT-2syn have comparable geometries. The Cs TS structure lies 1.51 kcal mol-1 above INT-2syn and connects INT-2syn to its mirror image. This, in effect, changes the triplewell reaction coordinate to a quadruple-well reaction coordinate, but the mechanism remains addition-elimination. One other interesting structure was located on the surface of reaction 1. This structure, HILL-1, of C2V symmetry, has two imaginary frequencies. The two imaginary frequencies correspond to rotation about the two S-S bonds in the same

Nucleophilic Substitution at Sulfur direction (connecting the mirror image structures of INT-1syn) and rotation in the opposite direction (connecting the mirror image structures of INT-1). HILL-1 is approximately 3.25 kcal mol-1 higher in energy than INT-1 and 1.01 kcal mol-1 higher in energy than TS′-1 or TS′-1syn. Since the reaction only needs slightly more energy to reach the hilltop, it is very possible that interconversion of the pathways can and does occur. For all three reactions, the minimal energy differences between TSs and intermediates on the enantiomeric anti pathways and the syn pathway suggest that both are likely to occur. Since the mechanism is the same for both the anti and syn pathways, and their similar energetics, we need further discuss only one path, and we arbitrarily choose the anti path. Discussion The mechanism for the thiolate-disulfide reaction, for which reactions 1-3 are the simplest examples, has long been understood to be classic SN2, with backside attack of the nucleophile. Previous ab initio calculations21 and our HF calculations support this notion. The HF potential energy surface has two wells (the ion-dipole complex for the incoming and exit channels), as shown in Figure 1. The TS geometry has the incoming nucleophile approaching nearly collinear to the exiting sulfide. However, the surface is very sensitive to electron correlation. The ion-dipole complex is 3-10 kcal mol-1 more stable at MP2 than at HF, corresponding to the expected better description of the hydrogen bond in the complex. The HF TS, which is equivalent or higher in energy than the reactants, optimizes at MP2 to a stable intermediate 8-12 kcal mol-1 below the reactants. An asymmetric TS connects the ion-dipole complex with the stable intermediate, producing a triple-well potential, shown in Figure 3. The key to the change in the topology of the two surfaces is the true nature of the symmetric structure that is a TS on the HF surface and an intermediate on the MP2 surface. Singlepoint energy calculations at MP4SDTQ and CCSD(T) for reactions 1 and 2 indicate that the INT lies below the TS′, a necessary condition for the existence of a stable intermediate. Unfortunately, we do not have computational resources to obtain analytical frequencies at these levels to ascertain the true nature of these critical points. However, numerical frequencies at MP4SDQ indicate that INT-1 is a local minimum and TS′ is a transition state. There is strong evidence supporting the conclusion that the potential energy surface for reactions 1 and 2 has three wells with a stable intermediate, representative of an addition-elimination reaction and not an SN2 reaction. While the surface clearly contains three wells, the well corresponding to the stable intermediate is very shallow. The difference in energy between TS′-1 and INT-1 is 2.26 kcal mol-1 at MP2/6-31+G*, but only 2.17 at MP4SDTQ and 1.68 kcal mol-1 at CCSD(T). Similarly, INT-2 is 2.55 kcal mol-1 below TS′-2 at MP2, but only 2.42 kcal mol-1 at MP4SDTQ and 1.43 kcal mol-1 at CCSD(T). The shape of the potential energy surface for reaction 3 is less certain. At MP2, reaction 3 has a stable intermediate, lying only 0.07 kcal mol-1 below TS′-3. However, at MP4SDTQ, INT-3 lies 0.09 kcal mol-1 above TS′-3, and at CCSD(T) INT-3 lies 0.34 kcal mol-1 above TS′-3, clearly indicating that INT-3 is not a stable intermediate. One possibility is that reoptimization at MP4SDTQ or CCSD(T) will locate a true intermediate. While this is certainly possible, the geometries of the structures examined here have not been very sensitive to the level of electron correlation.

J. Phys. Chem., Vol. 100, No. 9, 1996 3539 An alternative explanation is that reaction 3 represents the substitutional change at which a mechanistic crossover occurs. All post-HF calculations for reactions 1 and 2 support an addition-elimination mechanism. The potential surface contains a stable intermediate. The geometry of the TS does not support backside attack since the S-S-S angle is too narrow (153.2° in TS′-1 and 155.4° in TS′-2). A critical feature is that the central sulfur, where the nucleophile attacks, is unhindered, bearing just a hydrogen atom. For reaction 3, a methyl group is attached to the central sulfur. This larger substituent now blocks some of the space about the central sulfur. The S-S-S angle in TS′-3 is 182.2°, essentially a linear attack, as expected for an SN2 reaction. Structurally, TS′-3 and INT-3 are almost identical, which leads to an energy separation of less than 1 kcal mol-1 at higher levels and the apparent disappearance of the intermediate on the MP4SDTQ and CCSD(T) surface. Therefore, the mechanism of the thiolate-disulfide exchange in the gas phase is dependent on the substitution at the central sulfur. Summary The results show that when the substituent on the sulfur under nucleophilic attack is small, like hydrogen and perhaps methyl, the reaction takes place through an addition-elimination reaction. The highest level calculations suggest that when the substituent on the central sulfur is larger, the substituent induces sufficient steric hindrance to prevent the formation of a stable intermediate, and the mechanism is SN2. We have previously noted that the substitution reaction H- + PH3 f PH3 + Hproceeds through an addition-elimination pathway.9 It appears that substitution reactions at second-row atoms can proceed through the addition-elimination path by expansion of its octet, which is prohibited for first-row systems. However, the results presented here suggest that the size of the substituents may preclude the addition-elimination pathway and that the preferred path when the substituents become sizable is SN2. These results are consistent with the gas phase study of Grabowski.20 Furthermore, the very shallow well of the intermediate suggests that, even with systems having little steric encumbrance at sulfur, the intermediate will be difficult to detect. We have also observed that enantiomeric anti pathways and the syn path are possible. The calculations presented here indicate that given the appropriate steric constraints, substitution at sulfur will proceed via an addition-elimination reaction. Acknowledgment is made to the National Science Foundation and the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. D.C.M. is a recipient of a Graduate Assistance in Area of National Need fellowship from a U.S. Department of Education grant. References and Notes (1) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in Organic Chemistry, 3rd ed.; Harper and Row: New York, 1987. (2) Beak, P. Acc. Chem. Res. 1992, 25, 215-222. (3) Gronert, S. J. Am. Chem. Soc. 1991, 113, 6041-6048. (4) Beak, P.; Li, J. J. Am. Chem. Soc. 1991, 113, 2796. (5) (a) Bu¨hl, M.; Schaefer, H. F., III. J. Am. Chem. Soc. 1993, 115, 9143-9147. (b) Glukhovtsev, M. N.; Pross, A.; Radom, L. J. Am. Chem. Soc. 1995, 117, 9012-9018. (6) Beak, P.; Loo, D. J. Am. Chem. Soc. 1986, 108, 3834. (7) Bachrach, S. M. J. Org. Chem. 1990, 55, 1016-1019. (8) (a) Li, J.; Beak, P. J. Am. Chem. Soc. 1992, 114, 9206-9207. (b) Bachrach, S. M.; Mulhearn, D. C. J. Phys. Chem. 1993, 97, 12229-12231. (9) Damrauer, R.; DePuy, C. H.; Bierbaum, V. M. Organometallics 1982, 1, 1553-1554.

3540 J. Phys. Chem., Vol. 100, No. 9, 1996 (10) Whitesides, G. M.; Lilburn, J. E.; Szajewski, R. P. J. Org. Chem. 1977, 42, 332. (11) Wilson, J. M.; Bayer, R. J.; Hupe, D. J. J. Am. Chem. Soc. 1977, 99, 7922-7926. (12) Freter, R.; Pohl, E. R.; Hupe, D. J. J. Org. Chem. 1979, 44, 17711774. (13) Szajewski, R. P.; Whitesides, G. M. J. Am. Chem. Soc. 1980, 102, 2011. (14) Whitesides, G. M.; Kouk, J.; Patterson, M. A. K. J. Org. Chem. 1983, 48, 112-115. (15) Hupe, D. J.; Pohl, E. R. Isr. J. Chem. 1985, 26, 395-399. (16) Singh, R.; Whitesides, G. M. J. Am. Chem. Soc. 1990, 112, 11901197. (17) Price, C. C.; Oae, S. Sulfur Bonding; Ronald Press: New York, 1962. (18) Rosenfield, R. E., Jr.; Parthasarathy, R.; Dunitz, J. D. J. Am. Chem. Soc. 1977, 99, 4860. (19) Painter, A.; Hunter, F. E., Jr. Biochem. Biophys. Res. Commun. 1970, 40, 387-395. (20) Grabowski, J. J.; Zhang, L. J. Am. Chem. Soc. 1989, 111, 11931203. (21) Aida, M.; Nagata, C. Chem. Phys. Lett. 1984, 112, 129-132. (22) Pappas, J. A. J. Am. Chem. Soc. 1977, 99, 2926-2930. (23) Pappas, J. A. J. Chem. Soc., Perkin Trans. 2 1979, 67-70. (24) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Jouhnson, B. G.; Schlegel, H. B.; Robb,

Bachrach and Mulhearn M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzales, C.; Martin, R. L.; Fox, D. L.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. GAUSSIAN 92; Gaussian, Inc.: Pittsburgh, PA, 1992. (25) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. L.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; HeadGordon, M.; Gonzales, C.; Pople, J. A. GAUSSIAN 94; Gaussian, Inc.: Pittsburgh, PA, 1995. (26) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; John Wiley and Sons: New York, 1986. (27) (a) Pople et al. have suggested a scaling factor of 0.9427 for MP2 frequencies and 0.9646 for MP2 ZPE.27b This study did not include any thiols. The ZPEs of the species in any given reaction we examined here are very similar, such that any scaling factor will not change the results. Therefore, we have elected to report unscaled values. (b) Pople, J. A.; Scott, A. P.; Wong, M. W.; Radom, L. Isr. J. Chem. 1993, 33, 345350. (28) Gas Phase Ion Chemistry; Bowers, M. T., Ed.; Academic Press, Inc.: New York, 1979; Vol. 2, p 346.

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