Carbon-to-carbon identity proton transfers from propyne, acetimide, thioacetaldehyde, and nitrosomethane to their respective conjugate anions in the gas phase. An ab initio study.

Gas-phase acidities of CH3Y (Y: NO, C identical to CH, CH=NH, and CH=S), barriers to the identity proton-transfer CH3Y + CH2=Y- reversible CH2=Y- + CH3Y, as well as geometries and charge distributions of CH3Y, CH2=Y- and the transition states of the proton transfers were determined by ab initio methods 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. To make more meaningful comparisons, the same quantities for previously studied systems (Y: H, CH=CH2, CH=O, CN, NO2) were recalculated at the levels used in the present work. The geometric parameters as well as the group charges indicate that the transition states for all the reactions are imbalanced, although there is no correlation between the degree of imbalance and the pi-acceptor strength of the Y group. Based on multi-parameter correlations with the field (sigma F), resonance (sigma R), and polarizability effect (sigma alpha) substituent constants, the contributions of each of these effects to the acidities and barriers were evaluated. For the Y groups whose sigma F, sigma R, and sigma alpha are unknown (CH=NH, CH=S, C identical to CH), a method for estimating these substituent constants is proposed. The barriers for the CH3Y/CH2=Y- systems are all lower than for the CH4/CH3- system; this contrasts with the situation in solution where the Y groups lead to an increase in the barrier. The reasons for this reversal are analyzed. We also make an attempt to clarify the issue as to why the transition states of these reactions are imbalanced, a question which continues to draw attention in the literature.