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 H(2)O). 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 degrees C to 65 degrees 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-D O hydrogen bond, but at low temperatures the correlation disappears, showing that a single hydrogen bond (OD O) 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 approximately 400 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.

Fig. 1.

Plot of long-time anisotropy decay curves for various frequencies at 25°C. The slow component of the orientational relaxation is wavelength-independent. (Inset) A decay curve on a semilog plot with single exponential fit. All decays are exponential within experimental error.

Fig. 2.

Amplitudes of the experimentally measured orientational correlation functions at t = 100 fs as a function of the OD stretching frequency (hydrogen bond strength, solid circles). The dashed line is a linear fit to the data. Also, the amplitudes of the orientational correlation function from MD simulations (open circles) are shown.

Fig. 3.

Results of the analysis based on the harmonic cone model. (A) Oscillator frequency as a function of OD stretching frequency. (B) Average angular deviations, 〈θ〉 = θ_avg, determined from the oscillator frequencies in A.

Fig. 4.

Temperature dependence of the experimentally measured orientational correlation function at t = 100 fs as a function of OD stretching frequency. The solid lines are linear fits to the data.