Longitudinal dielectric properties of molecular liquids: Molecular dynamics simulation studies of CH3CN, C6H6, and CO2

Molecular dynamics (MD) simulations of epsilon(L) (k, omega), the frequency (omega) and wave vector (k) dependent longitudinal component of the dielec tric permittivity tensor, a quantity of importance in several theories of s olvation dynamics and charge transfer reactions, is reported for three mole cular liquids: CH3CN, CO2, and C6H6, represented by nonpolarizable model po tentials. In order to study dielectric properties of nondipolar fluids we u se, instead of the conventional approach which relates epsilon(L) (k, omega ) to longitudinal dipole density fluctuations, a more general approach of R aineri and co-workers which expresses this quantity in terms of charge dens ity fluctuations. The two formulations are compared in the case of acetonit rile to assess the model dependence of epsilon(L) (k, omega). We find that at finite k, 1/epsilon(L) (k), where epsilon(L) (k) = epsilon(L) (k, 0) is the static longitudinal permittivity, exhibits several similar features for all three liquids: A partial cancellation between single-molecule and pair charge density fluctuation correlations at small k, their constructive int erference at intermediate k and the lack of molecular pair correlation cont ributions at large k. We also find that the extended reference interaction site model (XRISM) integral equations provide an excellent approximation to epsilon(L) (k) of all three liquids. We use the fact 1/epsilon(L) (k) is a polynomial in k(2) at small k to determine the static dielectric constant epsilon(0) = epsilon(L) (k=0) of acetonitrile and obtain a value in good ag reement with e0 evaluated by more conventional methods. We find that interm olecular correlations contribute the most to the dielectric properties of C H3CN and the least to those of CO2. In the range of k most relevant to solv ation (k less than or equal to 1 Angstrom(-1)), the pair component of the c harge-charge time correlation function Phi(qq) (k, t) is negative, partiall y cancelling the positive single-molecule component. The extent of cancella tion varies with k and the strength of intermolecular electrostatic interac tions, leading to significant qualitative differences in the behavior of Fq q (k, t) for polar and nondipolar liquids: In this k range, Fqq (k, t) in a cetonitrile decays more slowly as k increases, while the opposite k-orderin g is seen in the two nondipolar liquids. We use our results for epsilon(L) (k(min), omega), where k(min) is the smallest wave vector accessible in our simulation, to calculate the far-IR (infrared) absorption coefficient alph a(omega) of acetonitrile and find that it agrees well with alpha(omega) obt ained from the transverse permittivity component, epsilon(T) (k(min), omega ), indicating that the bulk limit for this quantity has been reached. (C) 1 999 American Institute of Physics. [S0021-9606(99)00813-2].