Wave vector dependent dielectric relaxation in hydrogen-bonding liquids: A molecular dynamics study of methanol

Abstract

Molecular dynamics simulation is used to study ε(k,ω), the frequency and wave vector dependent dielectric permittivity of a three-site model of methanol in which the methyl group is represented as a single site. The effects of induced dipoles are taken into account using perturbation theory and a three-site molecular polarizability model. The data are analyzed in terms of projected variables which allow us to distinguish between local field factors which renormalize the permanent-dipole contribution to the permittivity from the ‘‘collision induced’’ dipole relaxation. We find that induced dipoles significantly enhance the static permittivity, mainly through the local field factors. The time correlation functions for the longitudinal and transverse components of the collective dipole moments, evaluated at several of the smallest wave vectors in the system, present rapid oscillations at short times, followed by a nonexponential relaxation regime at intermediate times. At long times, for which the longitudinal component has essentially decayed to zero, the transverse component decays exponentially. Collision-induced dipole fluctuations are a major contributor to the longitudinal collective dipole relaxation and a much smaller one to transverse component dynamics. The far-infrared absorption spectrum, evaluated from the smallest k transverse component, exhibits three well resolved absorption bands, with maxima located at frequencies 60, 150, and 645 cm−1, in qualitative agreement with recent experimental measurements. We find that the fluctuations of the induced dipoles contribute in a nontrivial way to the shape of the spectra at frequencies above 50 cm−1. The permanent dipole contribution to ε(k,ω) was obtained also for partially deuterated methanol, CH3OD, in order to aid in the analysis of the hydrogen-bond dynamics. In addition, we briefly describe an approach to the study of solvation dynamics using the MD results for ε(k,ω) obtained here.

Short-Time Relaxational Dynamics of the “Strong” Glass-Former Methanol

“…The more recent data of ref 32 is consistent with earlier reported data in the frequency range 0.95-89 GHz by Barthel et al 33 The dielectric relaxation data depart from the Debye form starting approximately at frequencies higher than the loss frequency by a factor of 10, and this is taken as evidence of the existence of two additional dielectric relaxation processes at high frequencies. The existence of three distinct relaxation process in the dielectric spectrum was also found in MDS by Skaf et al 34 and in the dielectric spectrum calculated from simulated data by Bermejo et al 35 The origins of the three dielectric processes have been elucidated by MDS. 34 Among these three processes, the Debye relaxation has the largest dielectric strength, more than an order of magnitude larger than the others, irrespective of temperature.…”

“…The existence of three distinct relaxation process in the dielectric spectrum was also found in MDS by Skaf et al 34 and in the dielectric spectrum calculated from simulated data by Bermejo et al 35 The origins of the three dielectric processes have been elucidated by MDS. 34 Among these three processes, the Debye relaxation has the largest dielectric strength, more than an order of magnitude larger than the others, irrespective of temperature. 33 Naturally, the nearly Debye process is identified with the slow R-relaxation of real methanol, with a effectively equal to 1.…”

Dielectric properties of liquid ethanol. A computer simulation study

“…The value obtained by averaging the two runs, i.e., ⑀ϭ16Ϯ2, is lower than the experimental data, but the agreement is similar to that achieved for liquid methanol. [2][3][4] Since part of the discrepancies are due to the value of the high-frequency dielectric constant, we find it more significant to compare the difference (⑀Ϫ⑀ ϱ ). In this case, the agreement between experimental and MD simulation results is better.…”

“…Among these liquids, water has focused the interest of most of the computer simulation studies. In the case of alcohols, only the dielectric properties of liquid methanol [2][3][4] and methanol-water mixtures 5 have been analyzed by computer simulation. Many experimental studies of both static and dynamic dielectric properties of hydrogen bonded liquids have been carried out, but in the case of ethanol, the latter are restricted to the microwave region of the spectra 6 and to the infrared region 7 up to about 34 cm Ϫ1 .…”

The Anisotropy of the Molecular Reorientational Motions in Liquid Methanol

“…Pure liquid methanol under various conditions [19][20][21][22][23][24][25][26][27][28][29][30][31], methanolic mixtures [32][33][34][35][36] as well as ionic solutions in methanol [37] and solutions of argon in methanol [38] have been studied earlier by MC and MD simulations. In most instances, the methanol The total potential is assumed to be the sum of these inter-and intramolecular terms.…”