Proton Transfers from ZCH3 to ZCH2- (Z = F, Cl, Br, OH, SH, SeH). An ab Initio Investigation

Abstract

Energy profiles for the identity-reaction carbon-to-carbon proton transfers from carbon acids of the type ZCH3 to their conjugate bases ZCH2- have been studied by ab initio methods. Gas-phase acidities of ZCH3 species are reproduced well at MP2/6-31+G*//MP2/6-31+G*. The barriers to proton transfer relative to the separated reactants (ΔHTS) in kcal mol-1 are F (2.2), Cl (−4.2), Br (−8.2), OH (−2.1), SH (−5.7), SeH (−11.0) at MP2/6-31+G*//MP2/6-31+G*. Values at MP4/6-31+G**//MP2/6-31+G* are very similar. The orders of acidities and barrier heights suggest that the polarizability of Z is the dominant effect on both. That two or more ion−dipole complexes of similar energy exist for each system implies that the complexes will be fluxional. It is doubtful that these proton transfers could be observed experimentally. The dominant process for HYCH2- + CH3YH would almost certainly be the transfer of the YH proton, and calculations on the SN2 reactions ZCH2- + CH3Z → ZCH2CH3 + Z- (Z = F, Cl, Br) reveal them to be strongly exothermic with barriers 6−10 kcal mol-1 below those for the corresponding proton transfer processes.