Molecular origin of the difference in the HOH bend of the IR spectra between liquid water and ice.

The intensity of the HOH bend in the infrared (IR) spectrum of ice is significantly smaller than the corresponding one in liquid water. This difference in the IR intensities of the HOH bend in the two systems is investigated using Molecular Dynamics (MD) simulations with the flexible, polarizable, ab initio based TTM3-F model for water, a potential that correctly reproduces the experimentally observed increase of the HOH angle in liquid water and ice from the water monomer value. We have identified two factors that are responsible for the difference in the intensity of the HOH bend in liquid water and ice: (i) the decrease of the intensity of the HOH bend in ice caused by the strong anti-correlation between the permanent dipole moment of a molecule and the induced dipole moment of neighboring hydrogen bond acceptor molecules, and (ii) the weakening of this anti-correlation by the disordered hydrogen bond network in liquid water. The presence of the anti-correlation in ice is further confirmed by ab initio electronic structure calculations of water pentamer clusters extracted from the trajectories of the MD simulations with the TTM3-F potential for ice and liquid water.