Comparative Studies on the Reactions of Acetyl and Thioacetyl

Feb 7, 2011 - These differences could be caused by the π-bond strength of C═O and C═S. In the case of acetyl fluoride, the T±-type species was n...
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Comparative Studies on the Reactions of Acetyl and Thioacetyl Halides with NH3 in the Gas Phase and in Aqueous Solution: A Theoretical Study In Suk Han,† Chang Kon Kim,† Chang Kook Sohn,‡ Eun Kyung Ma,‡ Hai Whang Lee,*,† and Chan Kyung Kim*,† † ‡

Department of Chemistry, Inha University, Inchon 402-751, Korea Department of Chemistry Education, Chonnam University, Kwangju 500-757, Korea

bS Supporting Information ABSTRACT: The reactions of acetyl halides, CH3C(dO)X and corresponding sulfur analogues, thioacetyl halides, CH3C(=S)X, where X = F and Cl, with NH3 nucleophile were studied theoretically, at the QCISD level of theory, in the gas phase and in aqueous solution. All reactions occurred via the tetrahedral species, and reactions through neutral intermediates both in the gas phase and in aqueous solution could be ruled out, except for the case of the gas-phase reaction of acetyl fluoride. The tetrahedral structure was a transition state (TS) in the reactions of acetyl chloride, while it was a stable intermediate in reactions of thioacetyl halides. These differences could be caused by the π-bond strength of CdO and CdS. In the case of acetyl fluoride, the T(-type species was neither a saddle point nor an energy minimum in the gas phase, but existed as a stable intermediate in aqueous solution due to solvation. Moreover, in reactions of thioacetyl chloride, the rate-limiting step changed from the first step in the gas phase to the second step in aqueous solution, since the zwitterionic intermediates become more stabilized in aqueous solution. However, lower activation energies (ΔG‡) in aqueous solution were not caused by the solvent effects, but smaller deformation effects, in going from reactants through the TS.

’ INTRODUCTION In acyl transfer processes, two distinct reaction mechanisms have been proposed, as shown in Scheme 1, that is, a one-step concerted mechanism via a tetrahedral transition state (TS; I) and a two-step addition-elimination process through a stable tetrahedral zwitterionic (denoted T() intermediate (II).1 Therefore, the reaction mechanisms of acyl transfers can be determined by the nature of the tetrahedral structure. It has been thought that in most cases, substitution reactions at a carbonyl carbon occur through the addition-elimination mechanism,2 the identity of the tetrahedral intermediate being proved by 18O labeling experiments in the hydrolyses of carboxylic esters.3 In addition, the concerted mechanism of acyl transfers has been demonstrated by a number of experimental works.4 In particular, in gas phase reactions using ion cyclotron resonance techniques, it has been found that the tetrahedral structure is not an energy-minimum but a saddlepoint, suggesting that the carbonyl transfer proceeds via a concerted mechanism.5 In our previous work, we also reported that acyl transfers for a variety of carbonyl compounds, including thiocarbonyl, r 2011 American Chemical Society

sulfonyl and phosphoryl derivatives, proceed via the concerted mechanism.6 Several theoretical works done in the gas phase have also shown that these processes occur through a concerted mechanism without a stable tetrahedral intermediate.7 These fundamental controversies on acyl transfer mechanisms might be caused by incomplete understanding of the nature of the tetrahedral structures. Therefore, if all the details of the factors influencing the stabilities of these species were known, the mechanistic changeover could be explained and predicted with ease. Unfortunately, this process is by no means clear, since the stabilities of the tetrahedral species might be governed by various factors such as nucleophilicity, leaving group ability, solvent effect, and so on. Accordingly, one efficient way to acquire such information could be in terms of theoretical works performed both in the gas phase and in solution. Received: May 17, 2010 Revised: January 2, 2011 Published: February 7, 2011 1364

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The Journal of Physical Chemistry A Scheme 1

Scheme 2

The reaction of acetyl halides with ammonia (or amines) is a general method for the preparation of amides,8 and is a typical acyl transfer reaction. In a recent theoretical work for the gasphase substitution reactions of acetyl chloride with the chloride ion, Fox and co-workers have shown that the reactions proceed concertedly without a stable tetrahedral intermediate.9 However, theoretical works for substitution reactions of acetyl halides with a neutral nucleophile have been seldom reported. Therefore, in this work, the nucleophilic substitution reactions of acetyl halides, CH3C(dO)X and their sulfur analogues, CH3C(=S)X, where X = F or Cl, with a simple neutral nucleophile NH3 were studied theoretically at the QCISD level of theory in the gas phase and in aqueous solution in order to elucidate acyl transfer processes in detail as shown in Scheme 2: (1a) one-step concerted mechanism via a tetrahedral TS (I), (1b) stepwise mechanism via a tetrahedral zwitterionic intermediate (II), (1c) stepwise mechanism via a neutral tetrahedral intermediate (III). In mechanism (1c), nucleophilic attack to the carbonyl carbon was concurrent with proton transfer when mechanisms (1a) and (1b) were not feasible.

’ COMPUTATIONAL DETAILS In the gas-phase reactions, all stationary species were fully optimized and characterized by frequency calculations at the MP2(FC) level of theory with a 6-31þG(d) basis set.10 The reactions in aqueous solution were studied using the self-consistent

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reaction field (SCRF) method of the conductor-like polarizable continuum model (CPCM)11 with the cavity using the Universal Force Field (UFF)12 model at CPCM-MP2/6-31þG(d). In an earlier work, Houk and co-workers have shown that the UAKS or UAHF cavity model could be better than the UFF cavity model employed in this work.13 As is well-known, the UAKS or UAHF model is basically the united atom topological model with spheres centered only on heavy atoms including the hydrogen atom(s).14 In this work, we have compared the relative reactivity of three different reaction paths (1a∼c), and the reaction path (1c), which proceeded via two TSs involved acidic proton transfers. For example, the TS1 of path (1c) involved a proton transfer from the attacking NH3 to the carbonyl oxygen (or thiocarbonyl sulfur) and thus the transferring proton was located approximately in the middle of the attacking nitrogen and carbonyl oxygen (or thiocarbonyl sulfur) atoms. If we calculated the solvation free energies of TS1 using the UAHF model at HF/6-31G(d) level or UAKS model at PBE0/6-31G(d), the spheres formed from these heavy atoms were somewhat questionable because the transferring hydrogen did not belong to any of these heavy atoms. This implies that the UFF cavity model is more appropriate in this work, even if the UAHF or UAKS model is computationally more efficient and reliable in many cases than the cavity models with explicit hydrogen(s). In CPCM method, the nonelectrostatic terms are important because the computed energies depend on the cavity size, one of the major components of the nonelectrostatic terms.13 In this work, however, the nonelectrostatic terms were not included in the energies because the contribution of these terms was almost constant in a series of reaction. For example, the changes in the nonelectrostatic terms from the reactants to the products were 1.3 and 0.6 kcal mol-1 in the reactions of acetyl fluoride and acetyl chloride, respectively. All the optimized structures and imaginary frequencies of the transitions states are summarized in the Supporting Information. Geometries of the stationary point species in aqueous solution were also fully optimized and characterized by frequency calculations. To obtain more accurate energetics, the single-point calculations were performed at the QCISD(T)/6311þG(3df,2p) level on the geometries optimized at the MP2 level in the gas phase. In aqueous solution, however, the single-point calculations were carried out at the CPCM-QCISD/6-311þG(3df,2p) level on the optimized geometries at the CPCM-MP2 level, because the QCISD(T) calculations were inapplicable to CPCM method. The gas-phase activation (ΔG‡) and reaction (ΔGR) Gibbs energies were obtained at 298 K by use of the electronic energies at the QCISD(T) level corrected for the zeropoint vibrational energies (EZPVE), thermal energies (ETh), and entropies (S) at the MP2 levels. Similarly, the ΔG‡ and ΔGR values in aqueous solution were also estimated from the energies at the CPCM-QCISD level corrected for the EZPVE, ETh, and S at the CPCM-MP2 levels. All calculations were performed with the Gaussian 9815 and 03 programs.16

’ RESULTS AND DISCUSSION (A) Nature of a Tetrahedral Structure. In the acyl transfer reactions, the reaction mechanism can be determined by the nature of the tetrahedral species. To do this, the characteristics of the tetrahedral species need to be well-understood. In the substitution reaction of acetyl chloride (X = Cl) with NH3, the tetrahedral structure in both the gas phase and in aqueous solution is not a stable intermediate but a saddle point. However, in the gas-phase reaction of acetyl fluoride (X = F), the 1365

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Table 1. Nature of T ( Species in Several Solvent Media for the Reaction of Acetyl Fluoride with NH 3 at CPCMMP2/6-31þG* Level

Table 2. Optimized Bond Lengths at Several Theoretical Levels for the T( Intermediate of Thioacetyl Chloride in the Gas Phase

εa

nature

gas phase

1

nonexistent

carbon tetrachloride

2.228

nonexistent

diethyl ether

4.335

intermediate

1.622

1.523

tetrahydrofuran

7.58

intermediate

1.600

1.529

6-311þG(d,p)

1.579

2.077

acetone acetonitrile

20.7 36.64

intermediate intermediate

1.585 1.583

1.531 1.531

MP2

6-31þG(d) 6-311þG(d,p)

1.576 1.572

1.878 1.875

water

78.39

intermediate

1.580

1.531

QCISD

6-31þG(d)

1.573

1.872

6-311þG(d,p)

1.567

1.869

solvent media

dC-Nb

dC-Fc

method RHF B3LYP

a

Dielectric constants taken from Frisch, Æ.; Frisch, M. J.; Truck, G. W. Gaussian 03 User’s Reference; Gaussian Inc.: Carnegie, PA, 2003. b Bond length between carbonyl carbon and nitrogen on NH3 in Å. c Bond length between carbonyl carbon and fluorine atom in Å.

tetrahedral structure cannot be located as a saddle point (I) or as a stable zwitterionic intermediate (II). In aqueous solution, on the other hand, this structure is a stable intermediate (II). This indicates that the solvent influences the nature of the tetrahedral species, and plays an important role in the reaction mechanism. Therefore, the nature of this structure was examined in several solvent media, and the results are summarized in Table 1. Table 1 shows that the tetrahedral species is neither a saddle point nor a stable intermediate in carbon tetrachloride (dielectric constant ε = 2.228) nor in the gas phase. However, this structure becomes a stable intermediate if the dielectric constant is higher than 4.335 (diethyl ether). Note that, as the dielectric constant increases, the dC-N bond lengths become shortened, but the dC-F bond lengths become elongated. The change in dC-N, however, is more evident. For example, when the solvent changes from diethyl ether to water, the dC-N is shortened by 0.042 Å, whereas the dC-F is lengthened by only 0.008 Å. This implies that structural variation in (II) is more pronounced in the bond formation between the reaction center (carbonyl carbon) and the nucleophile than in the bond cleavage between the reaction center and the leaving group. Unlike those for acetyl halides, the tetrahedral structures in the case of thioacetyl halides exist as a stable intermediate (II) both in the gas phase and in aqueous solution, probably because the CdS π-bond is weaker than the CdO π-bond. In the literature, it has been estimated that the CdO π-bond is stronger at least by 20 kcal mol-1 than the CdS π-bond, although these values are dependent upon the methodologies employed.17 Therefore, in forming (II), the energy required to break the CdS π-bond can be easily compensated by the energy gained by C 3 3 3 NH3þ bond formation. However, the energy required to break the CdO π-bond cannot be replenished, because the C 3 3 3 NH3þ bond is essentially a weak bond. These results accord well with our previous theoretical work on thiocarbonyl transfers, which proceed via a T(-intermediate.18 To select a proper theoretical method to study the reaction mechanism, structures of the T( formed from the reactions of thioacetyl chloride with NH3 in the gas phase were examined at various levels of theory, and the results are shown in Table 2. Table 2 shows that optimized bond lengths are very similar in all theoretical levels employed, although the bond lengths at B3LYP levels are slightly longer than those at the MP2 and/or QCISD levels. In particular, the bond lengths at MP2/6-31þG(d) level are very similar to those at QCISD/6-31þG(d) level. Thus, we

dC-Na

dC-Clb

6-31þG(d)

1.542

1.836

6-311þG(d,p)

1.538

1.842

6-31þG(d)

1.586

2.046

basis Set

a

Bond length between carbonyl carbon and nitrogen on NH3 in Å. b Bond length between carbonyl carbon and chlorine atom in Å.

chose to optimize the structures using the MP2/6-31þG(d) level of theory to study the reaction mechanism. (B) Reaction Mechanism of Acetyl Halides with NH3. Substitution reactions of acetyl halides with NH3 are likely to happen through the reaction paths shown in Scheme 2. The reaction paths (1a) and (1b) cannot occur competitively, because the tetrahedral structure should belong to either of the two structures, I or II. On the contrary, the reaction path (1c) via a neutral tetrahedral intermediate (III) is able to occur competitively with the reaction path (1a) or (1b). In the reaction path (1c), the TS leading to (III) is formed by the C 3 3 3 NH3 bond formation and concurrent proton transfer from NH3 to the carbonyl oxygen. However, the reaction path (1c) can be excluded in most cases both in the gas phase and in aqueous solution, because the activation barriers for the rate-limiting step (RDS) of (1c) are considerably higher than those for RDSs of (1a) and (1b). In the gas-phase reaction of acetyl chloride, for example, the activation barrier, ΔG‡, for the RDS of (1c) at the QCISD(T)/6-311þG(3df,2p) level is 20.1 kcal mol-1 higher than that of (1a). There is one exception for this mechanism: the gas-phase reaction of acetyl fluoride. In this case, (1c) is the only path since the tetrahedral species cannot be located as I or II due to the poor leaving ability of the fluoride ion in the gas phase as mentioned above. The potential energy diagrams for the respective reaction path are represented in Figures 1 and 2. As shown in Figures 1 and 2, the activation barriers, ΔG‡, in aqueous solution are lower than the corresponding barriers in the gas phase. For example, in the case of acetyl chloride, the ΔG‡ values for RDSs of (1a) and (1c) in aqueous solution are 6.0 and 1.7 kcal mol-1 lower than the corresponding ΔG‡ values in the gas phase. This indicates that the reactions of acetyl halides with NH3 become more favorable in aqueous solution. However, lowering of the ΔG‡ values in aqueous solution was not caused by the difference in the theoretical levels employed: the QCISD(T) level in the gas phase and the QCISD level in aqueous solution, when the gas-phase ΔG‡ values for TS1 in the reactions of acetyl chloride recalculated at the QCISD level, the ΔG‡ values for the reaction paths (1a) and (1c) were 27.7 and 49.1 kcal mol-1 at the QCISD level, respectively, comparable to the corresponding ΔG‡ values of 26.5 and 46.6 kcal mol-1 at the QCISD(T) level. Note that the reaction mechanism of acetyl chloride is different from that of acetyl fluoride in aqueous solution. As discussed above, the tetrahedral structure is a transition state in the reaction of acetyl chloride, but a stable intermediate in the reaction of acetyl fluoride. Therefore, the former proceeds via the one-step 1366

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Figure 1. Potential energy diagrams for the reactions of acetyl fluoride. Solid and dashed lines represent the reaction paths (1b) and (1c) in the gas phase, respectively, and bold-solid and bold-dashed lines represent the corresponding reactions in aqueous solution.

concerted process, but the latter occurs through the two-step process in which the second step is rate-limiting. These results agree well with the experimental results that acyl transfers with a poor leaving group occur through an associative T(-type species.19 The optimized structures of the stationary species for the reaction path (1b) of acetyl fluoride are shown in Figure 3 and those for the reaction path (1a) of acetyl chloride are shown in Figure 4. The optimized structures for reaction path (1c) are summarized in the Supporting Information. As shown in Figure 4, the gas-phase and aqueous optimized geometries of TSs and PCs are very different. TS(a) is a product-like structure in the gas phase, but changes to a reactant-like structure in aqueous solution; that is, both the C 3 3 3 NH3 bond formation and C 3 3 3 Cl bond breaking are more advanced in the gas phase. This is consistent with the results for the solvent effects on the SN2 Menshukin reactions, in which TS is shifted to an earlier position in solution and the solvent effects on TS are more pronounced as the solvent polarity increases.20 Such a structural variation can also be explained using the Hammond postulate.21 Moreover, the structure of PC is a molecular complex of acetamide and hydrogen chloride in the gas phase but is an ion-pair complex of N-protonated acetamide and chloride ion in aqueous solution. These results seem quite reasonable, because a charge-separated ion-pair structure is highly unstable in the gas phase, and hydrogen chloride, HCl, can fully ionize in aqueous solution to form the ion-pair complex. This also implies that, in the reaction of acetyl fluoride, the structure of PC is a molecular complex of acetamide and hydrogen fluoride, HF, because HF is a weak acid in aqueous solution.

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Figure 2. Potential energy diagrams for the reactions of acetyl chloride. Solid and dashed lines represent the reaction paths (1a) and (1c) in the gas phase, respectively, and bold-solid and bold-dashed lines represent the corresponding reactions in aqueous solution.

Figure 3. Optimized structures of the stationary point species at the MP2/6-31þG(d) level of theory for the reaction path (1b) of acetyl fluoride in aqueous solution. Bond lengths are in Å.

(C) Reaction Mechanism of Thioacetyl Halides with NH3. The potential energy diagrams for reaction paths of thioacetyl halides are depicted in Figures 5 and 6. As noted in section (A), the tetrahedral intermediates were stable in both phases. Therefore, the reaction mechanisms of thioacetyl halides are stepwise processes of (1b), similar to the reaction of acetyl fluoride in aqueous solution. As mentioned above, the reaction path (1c) is also excluded in the discussion because of higher barriers. 1367

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Figure 4. Optimized structures of the stationary point species at the MP2/6-31þG(d) level of theory for the reaction path (1a) of acetyl chloride in aqueous solution. Bond lengths are in Å and parentheses values are in the gas phase.

Figure 5. Potential energy diagrams for the reactions of thioacetyl fluoride. Solid and dashed lines represent the reaction paths (1b) and (1c) in the gas phase, respectively, and bold-solid and bold-dashed lines represent the corresponding reactions in aqueous solution.

Figure 6. Potential energy diagrams for the reactions of thioacetyl chloride. Solid and dashed lines represent the reaction paths (1b) and (1c) in the gas phase, respectively, and bold-solid and bold-dashed lines represent the corresponding reactions in aqueous solution.

In the reaction path (1b), the free energies of the transition states, TS1(b) and TS2(b), were lowered in going from the gas phase to solution phase. For example, in the reaction of thioacetyl fluoride, the solvent lowered the free energy by 3.9 and 5.6 kcal mol-1 for TS1(b) and TS2(b), respectively. Moreover, in the reaction of thioacetyl chloride, solvent stabilization in TS2(b) (-13.1 kcal mol-1) was much greater than that in TS1(b) (-3.1 kcal mol-1). As a result, the energy gap between TS1(b) and TS2(b) was reduced to 4 kcal mol-1 in aqueous solution (see Figure 5) or the RDS changed from the second step in the gas phase to the first step in aqueous solution (see Figure 6). Such changes are caused by the different stabilities of the T(-intermediates in the gas phase and in aqueous solution. In the gas phase reactions, the free energies of the T(-intermediates of the thioacetyl fluoride and thioacetyl chloride reactions are þ22.0 and þ24.0 kcal mol-1 higher, respectively, than the corresponding free energies of reactants. On the contrary, the T(-intermediates are stabilized by -14.5 and -15.1 kcal mol-1 in going from the gas phase to solution phase for the thioacetyl fluoride and thioacetyl chloride reactions, respectively. Because

Figure 7. Optimized structures of the stationary point species at the MP2/6-31þG(d) level of theory for the reaction path (1b) of thioacetyl fluoride in aqueous solution. Bond lengths are in Å, and parentheses values are in the gas phase. 1368

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Table 4. Structural Deformation Energies (ΔEdef in kcal mol-1) in Going from Reactants to TS1a on the Path (1b) at QCISD(T)/6-311þG(3df,2p) Level in aqueous solutionb

in the gas phasec

ΔEdef-

ΔEdef-

ΔEdef-

ΔEdef-

(substrate)

(NH3)

(substrate)

(NH3)

acetyl fluoride

12.5

0.1

acetyl chloride

10.2

0.1

48.6

1.4

thioacetyl fluoride

6.9

0.5

12.6

0.4

thioacetyl chloride

7.3

0.2

14.9

0.5

a

Figure 8. Optimized structures of the stationary point species at the MP2/6-31þG(d) level of theory for the reaction path (1b) of thioacetyl chloride in aqueous solution. Bond lengths are in Å and parentheses values are in the gas phase.

Table 3. Gibbs Free Energies of Solvation (ΔGsol in kcal mol-1) for Reactants, TS1, TS(a), and T(-Intermediate on the Paths (1a) and (1b) at CPCM-QCISD/6-311þG(3df,2p)//CPCMMP2/6-31þG(d) Level of Theory in Aqueous Solution reactantsa

TS1b

T(-intermediate -24.9

acetyl fluoride

-9.5

-12.2

acetyl chloride thioacetyl fluoride

-8.5 -7.5

-10.1 -8.2

-27.0

thioacetyl chloride

-6.5

-7.6

-25.0

Sum of ΔGsol(substrate) and ΔGsol(NH3). b For acetyl chloride, the TS1 refers to the TS(a) on the path (1a). a

the T( intermediates are intrinsically charge-separated ylides, it is natural that they are stabilized by solvation in aqueous solution. Therefore, the solvent plays an important role in the reactions of thioacetyl halides. The optimized structures for reaction path (1b) are represented in Figures 7 and 8, and those for reaction path (1c) are summarized in the Supporting Information. Figures 7 and 8 show that the C 3 3 3 NH3 bond lengths of TS1(b) become longer and the C 3 3 3 X bond lengths become shorter by solvation, which also indicates that the structures shift to earlier TSs in aqueous solution, as was already shown in Figure 4. (D) Solvent Effects on Reactivity. To examine the solvent effects on reactivity, the Gibbs free energies of solvation (ΔGsol) for the reactants, TS1, TS(a), and T(-intermediate on the paths (1a) and (1b) were estimated, and the results are summarized in Table 3. The ΔGsol values were estimated from the difference in QCISD energies obtained from the two separate calculations: Eaq [=E(CPCMQCISD/6-311þG(3df,2p)//CPCM-MP2(FC)/6-31þG(d))] in aqueous solution and Egas [=E(QCISD/6-311þG(3df,2p)// CPCM-MP2(FC)/6-31þG(d))], that is, ΔGsol = Eaq - Egas. Table 3 shows that the magnitudes of ΔGsol values for T(intermediates are much larger than those for reactants as discussed above. The difference in ΔGsol between reactants and T(-intermediates, δΔGsol(T() [=ΔGsol(T() - ΔGsol(Rea)], are estimated to be -15.4 ∼ -19.5 kcal mol-1. Interestingly, the difference in ΔGsol between reactants and the first transition state, TS1, δΔGsol(TSI) [=ΔGsol(TSI) - ΔGsol(Rea)],

For acetyl chloride, the TS1 refers to the TS(a) of the path (1a). b Values are estimated by using the optimized geometries at CPCMMP2 level, QCISD(T)/6-311þG(3df,2p)//CPCM-MP2/6-31þG(d). c Values are estimated by using the optimized geometries at MP2 level, QCISD(T)/6-311þG(3df,2p)//MP2/6-31þG(d).

are very small (-0.7-2.7 kcal mol-1), which indicates that the solvent exhibits little influence on the activation barriers (reactivity) in aqueous solution. However, this finding is inconsistent with our earlier findings: the ΔG‡ values are lowered by 3-6 kcal mol-1 by solvation (see Figures 1, 2, 5, and 6). Therefore, lowering of the activation barriers in aqueous solution should originate from some other effect. One way to explain this phenomenon is to examine the structural variation of the transition states by solvation. As mentioned above, the structures of TS1 shift to earlier (reactant-like) geometries in aqueous solution, and hence smaller structural deformations are expected in going from reactants to TS1 in aqueous solution. To estimate this effect, the deformation energy (ΔEdef),22 which is required to change the geometry in the reactants to its geometry in the transition state, is estimated at the QCISD(T) level, and the results are summarized in Table 4. As expected, the ΔEdef values in the gas phase are much larger than those in aqueous solution, which can explain why the activation barriers in aqueous solution are more favorable. Close examination of Table 4 also shows that the ΔEdef values of NH3 are negligibly small compared to those of the substrates. In other words, smaller activation barriers in aqueous reaction are mainly caused by smaller deformation of the substrates. Since the solvent effects influence structures of the TSs and stabilities of the T(-species, the solvation effects are one of the most important factors in determining the reactivity and reaction mechanisms of substitution reactions of carbonyl and thiocarbonyl halides.

’ SUMMARY The reactions of acetyl halides and thioacetyl halides with NH3 occur through the tetrahedral structures (I) or (II) instead of through the neutral intermediate (III) both in the gas phase and in the aqueous solution, except for the gas-phase reaction of acetyl fluoride. The tetrahedral structures are saddle points for acetyl chloride, but are stable intermediates for thioacetyl halides; the reactions of acetyl chloride occur via the concerted process, and the reactions of thioacetyl halides proceed through the stepwise process. These mechanistic differences could be caused by the π-bond strength of CdO and CdS. On the other hand, the gas-phase reaction of acetyl fluoride proceeds through (III), but the reaction in aqueous solution occurs through (II) because the tetrahedral intermediate is stabilized by the solvent. 1369

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The Journal of Physical Chemistry A The structures of the transition states are shifted to an earlier position, and the reactivity (ΔG‡) is more favorable in aqueous solution compared to those in the gas phase. However, the lowering of the activation barriers in aqueous solution does not originate from solvent stabilizing effects, but smaller structural deformation energies (ΔEdef) on going from reactants to TS1. In the reactions of thioacetyl chloride, the rate-limiting step changes from the first step in the gas phase to the second step in aqueous solution, since the intermediates become largely stabilized by solvation.

’ ASSOCIATED CONTENT

bS

Supporting Information. All the optimized structures and imaginary frequencies of the transitions structures. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Fax: þ82-32-867-5604. [email protected] (H.W.L.); kckyung@ inha.ac.kr (C.K.K.).

’ ACKNOWLEDGMENT This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD), Basic Research Promotion Fund (KRF-2008-314-C00207). ’ REFERENCES (1) Lee, I.; Lee, H. W. Collect. Czech. Chem. Commun. 1999, 64, 1529. (2) March, J. Advanced Organic Chemistry, 3rd ed.; John Wiley and Sons: New York, 1985; pp 290-295, and references cited therein. (3) Bender, M. L. J. Am. Chem. Soc. 1951, 73, 1626. (4) (a) Um, I. W.; Han, H. J.; Ahn, J. A.; Kang., S.; Buncel, E. J. Org. Chem. 2002, 67, 8475. (b) Bentley, T. W.; Liewellyn, G.; McAlister, J. A. J. Org. Chem. 1996, 61, 7927. (c) Williams, A. Acc. Chem. Res. 1989, 22, 387. (d) Ba-Saif, S.; Luthra, A. K.; Williams, A. J. Am. Chem. Soc. 1989, 111, 2647. (e) Kevill, D. N.; Kim., C. B. J. Chem. Soc., Perkin Trans. 2 1988, 1353. (f) Chrystiuk, E.; Williams, A. J. Am. Chem. Soc. 1987, 109, 3040. (g) Ba-Saif, S.; Luthra, A. K.; Williams, A. J. Am. Chem. Soc. 1987, 109, 6362. (h) Bentley, T. W.; Harris, H. C. J. Chem. Soc., Perkin Trans. 2 1986, 619. (5) (a) Craig, S. L.; Zhong, M.; Brauman, J. I. J. Am. Chem. Soc. 1999, 121, 11790. (b) Zhong, M.; Brauman, J. I. J. Am. Chem. Soc. 1999, 121, 2508. (c) Wilbur, J. L.; Brauman, J. I. J. Am. Chem. Soc. 1994, 116, 9216. (d) Wilbur, J. L.; Brauman, J. I. J. Am. Chem. Soc. 1994, 116, 5839. (b) Guthrie, J. P. J. Am. Chem. Soc. 1991, 113, 3941. (f) Kim, J. K.; Caserio, M. C. J. Am. Chem. Soc. 1981, 103, 2124. (6) Lee, I.; Kim, C. K.; Li, H. G.; Sohn, C. K.; Kim, C. K.; Lee, H. W.; Lee, B.-S. J. Am. Chem. Soc. 2000, 122, 11162. (7) (a) Blake, J. F.; Jorgenson, W. L. J. Am. Chem. Soc. 1987, 109, 3856. (b) Yamabe, S.; Minato, T. J. Org. Chem. 1983, 48, 2972. (8) Beckwith, A. L. J. In The Chemistry of Amide; Zabicky, J., Ed.; Interscience Publisher: New York, 1970; pp 73-185. (9) Fox, J. M.; Dmitrenko, O.; Liao, L.; Bach, R. D. J. Org. Chem. 2004, 69, 7317. (10) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; John Wiley and Sons: New York, 1986. (11) (a) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995. (b) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003, 24, 669. (12) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A., III; Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024. (13) Takano, Y.; Houk, K. N. J. Chem. Theory Comput. 2005, 1, 70.

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