Water inertial reorientation: Hydrogen bond strength and the angular potential

Abstract

The short-time orientational relaxation of water is studied by ultrafast infrared pump-probe spectroscopy of the hydroxyl stretching mode (OD of dilute HOD in H2O). The anisotropy decay displays a sharp drop at very short times caused by inertial orientational motion, followed by a much slower decay that fully randomizes the orientation. Investigation of temperatures from 1°C to 65°C shows that the amplitude of the inertial component (extent of inertial angular displacement) depends strongly on the stretching frequency of the OD oscillator at higher temperatures, although the slow component is frequency-independent. The inertial component becomes frequency-independent at low temperatures. At high temperatures there is a correlation between the amplitude of the inertial decay and the strength of the O-DOO hydrogen bond, but at low temperatures the correlation disappears, showing that a single hydrogen bond (ODOO) is no longer a significant determinant of the inertial angular motion. It is suggested that the loss of correlation at lower temperatures is caused by the increased importance of collective effects of the extended hydrogen bonding network. By using a new harmonic cone model, the experimentally measured amplitudes of the inertial decays yield estimates of the characteristic frequencies of the intermolecular angular potential for various strengths of hydrogen bonds. The frequencies are in the range of Ϸ400 cm ؊1 . A comparison with recent molecular dynamics simulations employing the simple point charge-extended water model at room temperature shows that the simulations qualitatively reflect the correlation between the inertial decay and the OD stretching frequency.ultrafast IR experiments ͉ dynamics ͉ motion ͉ MD simulations ͉ harmonic model A lthough a great deal is known about water's bulk thermodynamic properties, the complex intermolecular forces that govern its nanoscopic structural arrangements and dynamics have made a detailed picture of the instantaneous local structures of water and their evolution elusive. Numerous models for the structure of water have been proposed to account for its anomalous behavior at different temperatures and pressures (1-6). These models all have some degree of success in reproducing the observed radial distribution function and other properties of water.Here, we present results on the fast, local (inertial) angular motions of dilute HOD molecules in water within their initial hydrogen-bonding structure through a study of how the orientational motions of water depend on the OD stretching frequency and temperature. The short-time orientational motions of the HOD molecule depend on the potential energy surface established by neighboring water molecules. Measurements of the orientational relaxation as a function of OD stretching frequency and temperature provide insight into how the motions of the HOD molecule depend on the strength of the O-DOO hydrogen bond (nature of the potential energy surface) and how the local structure of water changes with temperature. The measurem...

Non-equilibrium thermodynamics and collective vibrational modes of liquid water in an inhomogeneous electric field

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