The effect of pinning the lone pair on the direction of the OH stretching transition moment of water

Abstract

When analyzing the experimental vibrational peak position of heavy light XH stretching vibrations of simple molecules, the local mode model has great success. In addition, the bond dipole approach has helped analyze the experimental intensities, where the transition moment of the XH stretching mode is assumed to be along the XH bond. We theoretically show that the calculated transition moment for the local mode OH stretching vibration of HDO is not aligned exactly along the OH bond. We performed theoretical calculations of the OH stretching vibrations of HDO…X for X = H2O, CH3OH, Na+, Li+, and found that when the alkali metal cation is bound to the lone pair of oxygen, the OH stretching transition moment is aligned along the OH bond. Furthermore, for H2O we use a simple model that relates the tilt angle of the OH stretching fundamental vibration transition moment to the ratio of the integrated intensity of the symmetric and antisymmetric OH stretching normal mode fundamental transition. Using this model, we utilized available experimental data to show that the OH stretching transition moment will stay aligned along the OH bond if the lone pair electron is pinned.