| Abstract
| - The identity carbon-to-carbon proton transfer betweenoxygen-protonated acetaldehyde (syn and anti) andacetaldehyde enol (syn and anti) has been studied by abinitio 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 transferbetween acetaldehyde and its enolate ion have been extended to theMP2/6-311+G**//MP2/6-311+G** level. Onthe basis of Mulliken and natural population analysis charges, thetransition states of all reactions under study showa strong imbalance in the sense that charge shift in the product enollags behind proton transfer and charge shift inthe reactant enol is ahead of proton transfer. The imbalance inthe reactions of CH3CHOH+ is largerthan in thereaction of CH3CHO, and larger for the syn than theanti configuration of CH3CHOH+.At the highest level ofcalculation, the enthalpy difference, ΔH, between thetransition state and separated reactants is about −5kcal/mol(anti) and −2 kcal/mol (syn) for the reactions ofCH3CHOH+, which compares withΔH ≈ 0 kcal/mol for thealdehyde reaction. When basis set superposition error correctionsare applied, these ΔH values become −2.6,0.5,and 3.3 kcal/mol, respectively. The trend in these ΔHvalues can be understood mainly as the result of aninterplaybetween the effect of the increased acidity of the carbon acid, whichmakes ΔH more negative, and the effect of alarge imbalance, which makes ΔH less negative or morepositive. Electrostatic or hydrogen-bondingstabilizationof the transition state is also likely to play a role by attenuatingthese effects. Specifically, the lower ΔH forthereactions of CH3CHOH+ compared toCH3CHO is attributed to the much stronger acidityof CH3CHOH+which more than offsets the effect of the larger imbalance and the lossof electrostatic or hydrogen-bonding stabilization;on the other hand, the higher ΔH for the reaction ofCH3CHOH+ (syn) compared to that ofCH3CHOH+ (anti)can be explained by the dominance of the imbalance factor. Thereaction paths through the imbalanced transitionstates can be represented by means of a six-corner MoreO'Ferrall−Jencks type diagram with separate axes forproton transfer and electronic/structural reorganization. Thelarger imbalance for the reaction ofCH3CHOH+(syn) compared to CH3CHOH+ (anti) isconsistent with the relative energies of the intermediate corners ofthediagram in the two reactions, but this is not the case for the largerimbalance in the reactions ofCH3CHOH+compared to that of CH3CHO. This latterdiscrepancy is probably a consequence of an overinterpretation oftheMore O'Ferrall−Jencks diagram when applied to largeperturbations.
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