| Abstract
| - Gas-phase acidities of CH3Y (Y: NO, C⋮CH, CHNH, and CHS), barriers to the identity proton-transfer CH3Y + CH2Y- ⇄ CH2Y- + CH3Y, as well as geometries and charge distributions ofCH3Y, CH2Y- and the transition states of the proton transfers were determined by ab initiomethods at the MP2/6-311+G(d,p)//MP2/6-311+G(d,p), B3LYP/6-311+G(d,p), and BPW-91/6-311+G(d,p) levels of theory. The acidities were also calculated at the CCSD(T)/6-311+G(2df,2p) level. Tomake more meaningful comparisons, the same quantities for previously studied systems (Y: H,CHCH2, CHO, CN, NO2) were recalculated at the levels used in the present work. The geometricparameters as well as the group charges indicate that the transition states for all the reactions areimbalanced, although there is no correlation between the degree of imbalance and the π-acceptorstrength of the Y group. Based on multi-parameter correlations with the field (σF), resonance (σR),and polarizability effect (σα) substituent constants, the contributions of each of these effects to theacidities and barriers were evaluated. For the Y groups whose σF, σR, and σα are unknown (CHNH, CHS, C⋮CH), a method for estimating these substituent constants is proposed. The barriersfor the CH3Y/CH2Y- systems are all lower than for the CH4/CH3- system; this contrasts with thesituation in solution where the Y groups lead to an increase in the barrier. The reasons for thisreversal are analyzed. We also make an attempt to clarify the issue as to why the transition statesof these reactions are imbalanced, a question which continues to draw attention in the literature.
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