Transition State Imbalances in Gas Phase Proton Transfers. Ab Initio Study of the Carbon-to-Carbon Proton Transfer from the Protonated Acetaldehyde Cation to Acetaldehyde Enol

The identity carbon-to-carbon proton transfer between oxygen-protonated acetaldehyde (syn and anti) and acetaldehyde enol (syn and anti) has been studied by ab initio methods at the 6-311+G**//6-311+G**, MP2/6-311+G**//6-311+G**, and MP2/6-311+G**//MP2/6-311+G** levels. Previous calculations on the proton transfer between acetaldehyde and its enolate ion have been extended to the MP2/6-311+G**//MP2/6-311+G** level. On the basis of Mulliken and natural population analysis charges, the transition states of all reactions under study show a strong imbalance in the sense that charge shift in the product enol lags behind proton transfer and charge shift in the reactant enol is ahead of proton transfer. The imbalance in the reactions of CH 3 CH OH + is larger than in the reaction of CH 3 CH O, and larger for the syn than the anti configuration of CH 3 CH OH + . At the highest level of calculation, the enthalpy difference, Δ H, between the transition state and separated reactants is about −5 kcal/mol (anti) and −2 kcal/mol (syn) for the reactions of CH 3 CH OH +, which compares with Δ H ≈ 0 kcal/mol for the aldehyde reaction. When basis set superposition error corrections are applied, these Δ H values become −2.6, 0.5, and 3.3 kcal/mol, respectively. The trend in these Δ H values can be understood mainly as the result of an interplay between the effect of the increased acidity of the carbon acid, which makes Δ H more negative, and the effect of a large imbalance, which makes Δ H less negative or more positive. Electrostatic or hydrogen-bonding stabilization of the transition state is also likely to play a role by attenuating these effects. Specifically, the lower Δ H for the reactions of CH 3 CH OH + compared to CH 3 CH O is attributed to the much stronger acidity of CH 3 CH OH + which more than offsets the effect of the larger imbalance and the loss of electrostatic or hydrogen-bonding stabilization; on the other hand, the higher Δ H for the reaction of CH 3 CH OH + (syn) compared to that of CH 3 CH OH + (anti) can be explained by the dominance of the imbalance factor. The reaction paths through the imbalanced transition states can be represented by means of a six-corner More O'Ferrall−Jencks type diagram with separate axes for proton transfer and electronic/structural reorganization. The larger imbalance for the reaction of CH 3 CH OH + (syn) compared to CH 3 CH OH + (anti) is consistent with the relative energies of the intermediate corners of the diagram in the two reactions, but this is not the case for the larger imbalance in the reactions of CH 3 CH OH + compared to that of CH 3 CH O. This latter discrepancy is probably a consequence of an overinterpretation of the More O'Ferrall−Jencks diagram when applied to large perturbations.