Molecular modeling approach for contrasting the interaction of ethane and hexafluoroethane with carbon dioxide

Interactions between carbon dioxide and ethane or hexafluoroethane were examined using ab initio calculations which were performed at the restricted Hartree-Fock level of theory using the STO-3G and 6-31G* basis sets. Computations at the 6-31G* level have identified key differences between the interaction of hydrocarbons and fluorocarbons with carbon dioxide. The interaction of the fluorocarbon with carbon dioxide is predominantly electrostatic in nature, with the positively charged CO2 carbon atom having a strong attraction to the negatively charged fluorine atoms of the fluorocarbon, resulting in a favorable interaction energy of 0.75-0.8 kcal/mol for each CO2 molecule in the first solvent shell. The interaction of CO2 with hydrocarbons is minimal due to the neutral nature of the hydrocarbon molecule. Calculations on CO2/hydrocarbon systems show a clustering of CO2 molecules away from the hydrocarbon, whereas the calculations on the CO2/fluorocarbon systems indicate that the CO2 molecules orient around the C2F6 by sandwiching the positively charged CO2 carbon between two negatively charged fluorine atoms. These molecular modeling computations have brought to light differences between the interaction of hydrocarbons and fluorocarbons with carbon dioxide which may help to explain the different solubilities of these types of molecules in supercritical carbon dioxide.