Infrared and molecular dynamics study of D2O rotational relaxation in supercritical CO2 and Xe

Supercritical fluid solvents allow continuous tuning of solvent density and therefore continuous control of the number and distance of solvent-solute interactions. Such interactions exert torque on molecules in solution and thereby influence the way in which the rotational orientation of a solute molecule varies with time. In this work, we examine the rotational relaxation of D2O in two supercritical fluid solvents: Xe and CO2. In the former case, there are no electrostatic interactions between the solute and solvent. Since the nonelectrostatic interactions of D2O are very nearly centrosymmetric about the center of mass, Xe is expected to exert tittle torque on the rotating solute until liquid densities are reached. For D2O dissolved in CO2, the dipole-quadrupole interaction can exert torque on the rotating D2O, thereby hindering its rotation. Molecular dynamics simulations of these systems were used to calculate dipole autocorrelation functions containing only contributions from the solute rotational motion. These were then used to predict the rotation wings of the asymmetric stretching band, v(3), of D2O. Infrared spectra for this band were obtained in both solvents at 110.0 degrees C as a function of density. In Xe, the experimental and simulated correlation functions indicated that the solute was free to rotate, even at the highest densities examined. In CO2, however, the rotation of D2O was increasingly hindered as the solvent density was increased from gaslike to liquidlike densities. At low densities the correlation functions had negative minima, indicating a partial reversal in the direction of the average transition dipole moment vector as the molecules freely rotated. In CO2, at high densities the functions exhibited simple, monotonic decay, indicating a persistence in the direction of the vector with time and substantially hindered rotation.