Effects of Molecular Charge and Methyl Substitution on Proton Transfers between Oxygen Atoms

Proton transfers in (HO-H-OH)-, (H2O-H-OH2)+, and (CH3OH-H-HOCH3)+ are compared via ab initio molecular orbital calculations by using a 4-31G basis set. In all three systems, lengthening the interoxygen H bond length leads to increases in the energy barrier to proton transfer. In the range of bond lengths studied, up to 2.95 Å, the barrier heights for (H2O-H-OH2)+ and (HO-H-OH)- are quite similar while the barriers in (CH3OH-H-HOCH3)+ are somewhat higher. These observations are explained on the basis of equilibrium OH bond lengths in the protonated subsystems and spatial extent of electron-density clouds. Angular deformations of the H bonds generally lead to enlargements of the transfer barriers. These enlargements are qualitatively similar for the two cationic systems whereas the anion behaves quite differently. It is shown that considerations of electrostatic interactions may account for the disparities between the two types of systems in a straightforward manner. Examination of the electron-density rearrangements that accompany the proton transfer lead to insights into the effects of overall charge and methyl substitution upon the process. The polarizability of the methyl group facilitates large contributions to the local shifts of density within the system while the negative charge of OH- permits a greater net transfer of density from the proton-accepting molecule across the H bond to the donor