Infrared and molecular dynamics study of reorientational relaxation of liquid acetonitrile

Abstract

The infrared CN stretching bands of acetonitrile in neat liquid and in dilute solution in carbon tetrachloride have been measured at various temperatures. The reorientational and vibrational relaxation times obtained from temperature dependence of the ν 2 fundamental bandwidth are consistent with previous Raman results. The reorientational relaxation times are linear in the viscosity divided by the temperature and the proportional constants for the neat liquid and the dilute solution agree with each other within experimental uncertainty. This fact indicates that the effect of the dipole-dipole interaction on the molecular reorientation is included in the viscosity and the proportional constant is characteristic of the molecule, which is consistent with a hydrodynamic theory. Molecular dynamics calculations of liquid acetonitrile were performed at several temperatures with two types of pair potential, with and without Coulomb terms in addition to the atom-atom Lennard-Jones terms. Comparison of the calculated reorientational relaxation times with the experimental results shows that the dipolar interaction plays an important role in the molecular reorientation in liquid acetonitrile.