Solvation by benzene: molecular dynamics simulation of orientational motion, translational diffusion, and site–site radial distributions of the solutes di- and trichloromethane (chloroform)

Abstract

By a standard, proven molecular dynamics procedure we generated individual equilibrium systems of dichloromethane and trichloromethane (chloroform) in benzene, each at equimolar concentration, as well as those of the neat components. We analysed the orientational dynamics of the C–H bond direction of chloroform in benzene-solvated and neat chloroform by the tumbling motion of its l = 1 and l = 2 Legendre polynomial auto-correlations between 250 and 438 K at iso-density conditions specific to the solution and neat system, respectively. We find that solvent benzene exerts a hindering effect on the orientational motion of chloroform over its neat state under an activation energy of 3.8 kJ mol−1, although the density of the solution is less than that of the neat solute. The hindering effect diminishes with increasing kinetic energy to eventually disappear near the critical region. Probing benzene as solute within an equimolar solution with chloroform does not show a correspondingly increased hindering of the orientational motion of the (in-plane) axes of benzene. The short-time rotational regimes at ambient conditions were examined by simulating the auto-correlation functions of the three components of angular velocity, the results implying that the coherence in the correlation of the primary rotational variable is rather short-lived. The long-time translational–diffusional characteristics were examined by the mean-square distance of chloroform travelled during 100 ps, establishing that chloroform diffuses slightly faster in equimolar solution with benzene than in its neat state—most likely on account of more favourable free volume conditions in the solution. From various site–site radial distribution functions between solute and solvent species and from appropriate atom–atom distances between nearest-neighbour molecules, we found that chloroform aggregates, to a minimum, with two nearest-neighbour benzene shells in a three-member configuration. We conjecture that the hindering effect by benzene on the chloroform solute is caused by its temporary entrapment, on time scales of the order of 0.2 ps, in cages from momentary perpendicular and parallel configurations of nearest- and near-neighbour benzene molecules. We find no convincing evidence for the existence of a stable, symmetric-top 1:1 chloroform–benzene complex. Solute species dichloromethane in benzene exhibits similar, but less pronounced, solution dynamics.