Proton Transfers from Carbon Acids Activated by π-Acceptors. Changes in Intrinsic Barriers and Transition State Imbalances Induced by a Cyano Group. An ab Initio Study

We report an ab initio study of the identity carbon-to-carbon proton-transfer NCCH₂Y + NCCHY^- ⇄ NCCHY^- + NCCH₂Y in the gas phase, where Y = H, CHCH₂, CHO, CHS, CN, NO, and NO₂. The main focus is on a comparison with the previously reported systems CH₃Y + CH₂Y^- ⇄ CH₂Y^- + CH₃Y, i.e., on the effect of the cyano group on acidities, proton-transfer barriers, and transition state structures. The conclusions of this study are as follows:  (1) The transition state for the NCCH₂Y/NCCHY^- systems is more imbalanced than that for the CH₃Y/CH₂Y^- systems. (2) The cyano group leads to an increase in the acidities but to a decrease in the proton transfer barriers. This barrier reduction results from the fact that the stabilizing effect of the cyano group on the transition state is greater than that on the anion. (3) Within a reaction series, the barriers are largely dominated by the π-acceptor strength of Y, i.e., the strongest π-acceptors lead to the highest barriers. This is similar to proton transfers in solution but quite different from the CH₃Y/CH₂Y^- systems in the gas phase; in these latter systems π-acceptor effects play a minor role while the barrier lowering field effect of Y is dominant.