Single water molecule reorientational dynamics

An important signature of the dynamics of water molecules is the reorientation of its dipole vector that can be probed by dielectric and NMR measurements. We have calculated the single molecule dipole-dipole time correlation function (TCF), defined as,... 

The hrst molecular dynamics computation of single molecule orientational correlation functions at liquid interfaces was reported by Benjamin. In bulk water, the water dipole correlation time (4 0.2 ps) and the water HH vector correlation time (1.5 0.1 ps, which can be approximately deduced from the NMR line shape) are in reasonable agreement with experiments. The reorientation was found to be faster at the water liquid/vapor interface. The reorientation dynamics of water molecules at the water/l,2-dichloroethane interface is, in contrast, slightly slower (to 6 0.3 and 2.3 0.2 ps for the dipole and the HH vectors, respectively).Similar results were found in a recent study by Chowdhary and Ladanyi of water reorientation near hydrocarbon liquids having different structure (different branching). The slower reorientation was limited to water molecules immediately next to the organic phase. Slower dynamics were observed when the reorientation was calculated in the intrinsic frame (thus eliminating the effect of capillary fluctuations). 

Even if we consider a single solvent, e g., water, at a single temperature, say 298K, depends on the solute and in fact on the coordinate of the solute which is under consideration, and we cannot take xF as a constant. Nevertheless, in the absence of a molecular dynamics simulation for the solute motion of interest, XF for polar solvents like water is often approximated by the Debye model. In this model, the dielectric polarization of the solvent relaxes as a single exponential with a relaxation time equal to the rotational (i.e., reorientational) relaxation time of a single molecule, which is called Tp) or the Debye time [32, 347], The Debye time may be associated with the relaxation of the transverse component of the polarization field. However the solvent fluctuations and frictional relaxation occur on a faster scale given by [348,349]... 

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