Polarizability anisotropy relaxation in liquid ethanol: A molecular dynamics study

Paolantoni, Marco; Ladanyi, Branka M.
August 2002
Journal of Chemical Physics;8/22/2002, Vol. 117 Issue 8, p3856
Academic Journal
Molecular dynamics simulation is used to study the relaxation of the polarizability anisotropy in liquid ethanol at temperatures of 298 and 348 K. Ethanol molecules are represented by a four-site semi-flexible model in which the internal degree of freedom, corresponding to the torsional motion around the C-O bond, is taken into consideration. The molecular polarizability is calculated using an interaction-site model, based on the modified dipole-induced dipole model developed by Thole. The collective polarizability induced by intermolecular interactions is included using first-order perturbation theory and calculated considering both center-center and site-site models. Results are analyzed in terms of projected variables that allow the decomposition of the total relaxation into orientational and collision-induced components, both of which are influenced by molecular flexibility. We compare our data with the results of low-frequency depolarized Rayleigh light scattering experiments, examining the possibility of separating different relaxation processes from spectroscopic signals in the time and frequency domains. We find that even though the largest contributor to polarizability anisotropy dynamics is orientational relaxation, collision-induced contributions are important, especially at shorter times. Moreover, we show that torsional motion also plays a significant role in the fast decay of the polarizability anisotropy of the system. Although liquid ethanol is strongly associated and its dipolar relaxation is highly collective, we find orientational pair correlation effects on polarizability relaxation to be negligible. A comparison with simulation data previously obtained for methanol suggests that increasing the alcohol chain length decreases the relative importance of OH dynamics in polarizability relaxation. We further show that, as expected for such a structured liquid composed of strongly asymmetric molecules, polarizability and dipole relaxation...


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