Determination of Carbon Dioxide Cluster Structures and Binding Energies from Quantum Chemistry: Magic Number and Temperature Effects in (CO2)n with 2≤n≤16

Abstract:

Weak intermolecular interactions play an important role in nature and are involved in the stabilization of a variety of different molecular aggregates. Carbon dioxide clusters (CO₂)_n are a good example in which monomers interact through London dispersion forces. The ability to accurately describe these types of interactions is crucial in understanding fundamental molecular-scale processes controlling the chemistry of carbon dioxide, ranging from CO₂ self-organization into monolayer films on metal and mineral surfaces, formation of CO₂ clouds and molecular interactions in supercritical CO₂. Weak interactions in (CO₂)_n clusters pose a challenge for experimental techniques, and are therefore in many cases either difficult or impossible to explore. Density functional theory with dispersion correction (DFT-D), on the other hand, can provide insight into intermolecular interactions among CO₂ molecules, provided that dispersion correction is properly accounted for. In this presentation results from dispersion sensitive DFT (M05-2X, B97-D, B2PLYPD) and MP2 theory will be shown, that describe interactions in (CO₂)_n clusters over a broad range of temperatures, and in particular, in those clusters with magic number sizes 6 and 13. Briefly, structure determinations and thermodynamic calculations for (CO₂)_n clustering reactions by DFT-D compare well against benchmark MP2 and CCSD(T)/CBS results, and therefore may be extended to significantly larger systems than accessible with highly correlated methods. The stepwise free energies of CO₂ cluster formation at temperatures from 60-400K reveal valuable new insights, the most important being that the stacked cyclic hexamer and tridecameric cluster, consisting of a 3-6-3 ring structure with a centrally enclosed CO₂ monomer, are highly stable clusters and therefore should be spectroscopically detectable. These results indicate that DFT-D provides an accurate and cost effective description of non-covalent interactions in (CO₂) clusters, and thus may provide important information on nucleation phenomena in CO₂ phases with gas-like densities.