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.

Structure of liquid carbon dioxide at pressures up to 10 GPa

“…Theoretical studies of simple quadrupolar liquids [45,46] have shown that this arrangement originates from the electric quadrupole-quadrupole (EQQ) interactions, since the larger the quadrupole is, the higher is the probability to find T-shape configurations. Our findings agree very well with previous works based on neutron diffraction data [25] or ab initio calculations [41] at similar densities (14.9-19.17 molecules/nm 3 ). The fact that the same molecular configurations are obtained with ab initio calculations and with the classical potential used in the present paper gives further credit in the ability of the latter to describe the structure of liquid CO 2 .…”

“…[41], this configuration of molecules in the first shell is remarkably similar to that found in the P a3 crystal structure of phase I. In the latter indeed, each CO 2 molecule has 12 first neighbors at the same C-C distance (r = 0.398 nm at the melting point 0.57 GPa, 300 K [47]), six of which are in the equatorial plane and the other six are around the poles [see Fig.…”

“…[9], but the difference remains within 3% below 10 GPa, i.e., close to experimental uncertainties. We also compared the predicted site-site radial distribution functions (SS-RDF) g CC (r), g CO (r), and g OO (r) to those obtained using first-principles calculations [41,42] at the density of 19.17 molecules/nm 3 (1.4 g/cc), corresponding to a pressure of 0.47 GPa at 300 K. The close agreement between the two sets of SS-RDF (see Supplementary Fig. S1 in Ref.…”

Electronic absorption spectra and nonlinear optical properties of CO2 molecular aggregates: A quantum chemical study

“…CO 2 molecule in supercritical phase shows unique characteristics of liquid-like density and gas-like diffusivity in addition to its nontoxic behaviour, as observed in neutron diffraction experiments. [14][15][16] It has been observed in some earlier studies that, due to close proximity of the monomers in these dense phases, the CO 2 molecules no longer remain linear, [17][18][19][20] giving rise to instantaneous dipole moment. Thus we believe, which can be understood in the light of their linear and nonlinear optical response functions.…”

On the ν1/2 ν2 Resonance in the Complex of Carbon Dioxide with Dimethyl Ether

“…In the top panel of Figure 4 18 O). The presence of that impurity, and of its complex with DME, is confirmed by very weak bands in the 3490-3460 cm À1 region.…”