Rotational dynamics of CH3Cl and CH2CF2 in the fluid state

Abstract

The far infrared absorptions of compressed gaseous and liquid CH₃Cl and CH₂CF₂(1,1-difluoroethylene) have been measured in the frequency range 2-200 cm^–1 and the results interpreted in terms of two statistical models of molecular rotational dynamics in fluids. A description based on an extended Langevin equation of Brownian motion reproduces the experimental features adequately using empirical intermolecular torque and width parameters. However, it does not reduce to free rotation in the limit of vanishing torque. The J-diffusion model of collision interrupted rotation in symmetric top molecules reduces to free rotation as the interval between elastic impacts becomes infinite, but fails to predict torsional oscillation at short times in denser fluids, so that it compares badly with the far infrared data for pure liquid CH₃Cl and for a solution in ethane.