Intra- and Intermolecular Charge Fluxes Induced by the OH Stretching Mode of Water and Their Effects on the Infrared Intensities and Intermolecular Vibrational Coupling

Abstract

Intra- and intermolecular charge fluxes induced by the OH stretching mode of water and their effects on the infrared (IR) intensities and intermolecular vibrational coupling are analyzed theoretically. Density functional theoretical (DFT) calculations are carried out for the clusters consisting of 28 and 30 water molecules, and the change in the electron density induced by the OH stretching mode, i.e., the electron density derivative ∂ρ((el))(r)/∂Q(OH), is calculated and its effect is analyzed. It is shown that the electron density derivative extends to the spatial region of the water molecule that accepts the hydrogen bond donated by the vibrating OH bond, indicating that intermolecular charge flux is induced. Both the amount of vibration-induced change in the electric charge (the charge derivative) and the distance between charge centers are large for the intermolecular charge flux, giving rise to a significant enhancement of the dipole derivative, and hence the IR intensity. After showing quantitatively that it is possible to understand the intermolecular vibrational coupling between the OH stretching oscillators with the picture of the "electric field modulation by one vibration" and the "response by the other vibration to the electric field modulation", the effect of the intermolecular charge flux on the intermolecular vibrational coupling is analyzed. It is shown that the intermolecular charge flux plays an important role also in this relation, but the two factors determining the dipole derivative (the charge derivative and the distance between charge centers) should be carefully taken into account.