INFRARED AND MOLECULAR-DYNAMICS STUDY OF D2O ROTATIONAL RELAXATION INSUPERCRITICAL CO2 AND XE

Supercritical fluid solvents allow continuous tuning of solvent densit y and therefore continuous control of the number and distance of solve nt-solute interactions. Such interactions exert torque on molecules in solution and thereby influence the way in which the rotational orient ation 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 interaction s between the solute and solvent. Since the nonelectrostatic interacti ons of D2O are very nearly centrosymmetric about the center of mass, X e is expected to exert tittle torque on the rotating solute until liqu id densities are reached. For D2O dissolved in CO2, the dipole-quadrup ole interaction can exert torque on the rotating D2O, thereby hinderin g its rotation. Molecular dynamics simulations of these systems were u sed to calculate dipole autocorrelation functions containing only cont ributions from the solute rotational motion. These were then used to p redict 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 r otation of D2O was increasingly hindered as the solvent density was in creased from gaslike to liquidlike densities. At low densities the cor relation 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 exh ibited simple, monotonic decay, indicating a persistence in the direct ion of the vector with time and substantially hindered rotation.