Structural changes upon ionization of simple hydrogen bonded hexagonal dimers

Abstract

The geometry of hexagonal dimers, (QH) 2, derived from small molecules, containing two electronegative atoms, QH=H 3COH, H 3CF, H 2NOH, N 2H 4, NH 2F, H 2O 2 and HOF are investigated by ab initio quantum chemical geometry optimizations. All the considered (neutral) dimers, have some symmetry and two hydrogen bonds of equal length. The geometry of the corresponding cations, (QH) 2 +, are determined in order to investigate the structural changes upon ionizatioin. The structural changes can be divided into two groups. One, where the structural changes include migration of H/H + and one, without such migration. In the former group the cations can be considered as adducts of QH 2 + and Q. In the latter group the cations can be described either as adducts of QH + and QH or as [QH, QH] + cations. In each group there may be ring openings or ring reductions. Generally, the cations have fewer and shorter hydrogen bonds than the corresponding neutral dimers, but there are exceptions. All the hydrogen bonds investigated here are found to be unsymmetrical, in the sense that the H atom in the hydrogen bond have different distances to the neighbouring electronegative atoms.