Proton transfers in hydrogen-bonded systems. VI. Electronic redistributions in (N2H7)+ and (O2H5)+

Abstract

Electronic rearrangements accompanying transfer of the central proton between the two XHn units of (H3NHNH3)+ and (H2OHOH2)+ are studied using ab initio molecular orbital methods. Electron density difference maps are calculated by subtracting the density of the equilibrium structure (X–H---X) from that of the midpoint geometry (X--H--X) using the split-valence 4-31G basis set. Some of the features revealed by the maps are common to both systems while others indicate significant differences between nitrogen and oxygen. Decomposition of the total electron density into contributions from individual occupied molecular orbitals (MOs) provides insight into the factors responsible for the overall charge migrations. The orbitals of a1 symmetry lead to density shifts in a direction parallel to the H bond axis. Among the features attributed to these MOs are the charge transfer across the H bond from one molecule to the other and characteristic density changes in the lone pair regions of the first-row X atoms. Internal polarizations of the XH bonds of each molecule arise from the density shifts perpendicular to the H bond axis associated with the MOs of non-a1 symmetry. Simple arguments involving electrostatic and covalent effects are used to explain the redistributions observed in the various MOs. Mulliken analyses provide information, complementary to the difference maps, concerning the relative involvement of various atoms and atomic orbitals in the electronic redistributions associated with each MO.