Thermodynamic and structural stability of (CO2)n n=2-4: An ab initio and QTAIM Study

Abstract

Quantum chemical and experimental studies of weakly bound molecular complexes are of fundamental importance in shaping our understanding of molecular interactions in the condensed phase. The gas phase carbon dioxide clustering reaction according to n(CO2) = (CO2)n provides a suitable model for such studies and, can as such provide valuabe insight into the stability of carbon dioxide clusters in the Earthís atmosphere, the occurrence of carbon dioxide clouds on Mars as well supercritical phenomena in the lower Venusian atmosphere [1]. The first experimental data on the CO2 multimer series stems from microwave spectroscopic work at 1.6K, that revealed the existence of a slipped parallel C2h symmetry for the carbon dioxide dimer [2]. Only one study exists where (CO2)2 bonding patterns, structures and their relationship to the theoretical binding energy have been discussed in detail [3]. Unfortunately, there are no reliable experimental thermodynamic data (Gibbs energy, enthalpy and entropy) for either the dimerization reaction or formation of higher clusters from starting CO2. Here we report new CO2 cluster structure parameters as well as new values for ∆G, ∆H and ∆S for (CO2)n clustering reactions up n=4 building on both QTAIM and ab initio approaches at MP2/cc-pVTZ level of theory and atmospheric T. For instance, MP2 level (CO2)2 geometric parameters from this study are in excellent with microwave data (Figure below) [2]. The corresponding CO2 dimerization enthalpy and entropy are -1.5 kJ·mol-1 and -70.7 J·K-1·mol-1, respectively, indicating that the dimer and higher clusters may be present in appreciable levels in the 60-80km altitude range of the Martian atmosphere and thus an important requirement for CO2 cloud nucleation. [1] Montmessin et al, 2006, Icarus, 183, 403; [2] Jucks et al, 1988, JCP, 88, 2185; [3] Bone et al, 1996, JPC, 100, 10892.