Evolution of intermolecular structure and dynamics in supercritical carbon dioxide with pressure:  an ab initio molecular dynamics study

Abstract

The effect of pressure on supercritical carbon dioxide (scCO₂) has been characterized by using Car−Parrinello molecular dynamics simulations. Structural and dynamical properties along an isotherm of 318.15 K and at pressures ranging from 190 to 5000 bar have been obtained. Intermolecular pair correlation functions and three-dimensional atomic probability density map calculations indicate that the local environment of a central CO₂ molecule becomes more structured with increasing pressure. The closest neighbors are predominantly oriented in a distorted T-shaped geometry while neighbors separated by larger distances are likely oriented in a slipped parallel arrangement. The structure of scCO₂ at high densities has been compared with that of crystalline CO₂. The probability distributions of intramolecular distances narrow down with increasing pressure. A marginal but non-negligible effect of pressure on the instantaneous intramolecular OCO angle is observed, lending credence to the idea that intermolecular interactions between CO₂ molecules in an inhomogeneous near neighbor environment could contribute to the observed instantaneous molecular dipole moment. The extent of deviation from a perfect linear geometry of the carbon dioxide molecule decreases with increasing pressure. Time constants derived from reorientational time correlation functions of the molecular backbone compare well with experimental data. Within the range of thermodynamic conditions explored here, no significant changes are observed in the frequencies of intramolecular vibrational modes. However, a blue shift is observed in the low-frequency cage rattling mode with increasing pressure.