Role of steric and electrostatic effects in the short-range order of quasitetrahedral molecular liquids

Abstract

The study of how both steric and electrostatic interactions affect the structure of liquids formed by quasitetrahedral molecules has been undertaken in this work. We have studied trichlorobromomethane (CBrCl${}_{3}$) and dibromodichloromethane (CBr${}_{2}$Cl${}_{2}$), both displaying a dipole along their C${}_{3\mathrm{v}}$ and C${}_{2\mathrm{v}}$ molecular symmetry axes, respectively. The short-range order of the liquid state has been determined using neutron diffraction experiments that were modeled through the reverse Monte Carlo (RMC) technique. To study changes in steric effects due to the distortion of the tetrahedral symmetry, we have compared our results with a previous RMC modeling of carbon tetrachloride (CCl${}_{4}$). The subtle effects of the dipole in the structure of the liquid have been determined using a set of molecular dynamics simulations with and without atomic partial charges, being the force field validated via comparison with the diffraction data. In a first approximation, neither steric nor electrostatic interactions are able to modify the molecular ordering of a fully tetrahedral liquid such as CCl${}_{4}$. A more detailed analysis indicates that, although the interaction between dipoles does not have appreciable effects when aligned along the C${}_{3\mathrm{v}}$ molecular axes, as for the CBrCl${}_{3}$, it enhances the antiparallel orientation of dipoles when it is oriented along the C${}_{2\mathrm{v}}$ axes, as in the case of CBr${}_{2}$Cl${}_{2}$.