Kinetics of the Reactions of Water, Hydroxide Ion and Sulfide Species with CO2, OCS and CS2: Frontier Molecular Orbital Considerations

Abstract

The kinetics of the reactions of water, hydroxide ion and sulfide species with CO2, OCS and CS2 are investigated using the molecular orbital approach and available kinetic data. Although these reactions are symmetry allowed, the lowest unoccupied molecular orbital (LUMO) for CO2 is a poor electron accepting orbital as it has a positive potential energy. At low pH, hydration of CO2 requires that the waters interact with CO2 via hydrogen bonding for subsequent formation of H2CO3 in an effort to overcome the high energy of activation. These factors are significant for the slow kinetics of hydration and the persistence of CO2 in water. The reaction of hydroxide ion with CO2 has a much smaller energy of activation. For the isoelectronic species OCS and CS2, their LUMO orbitals are good electron acceptor orbitals, and the energy of activation is less than that for the corresponding CO2 reactions. The LUMO orbitals for OCS and CS2 have less carbon character whereas the LUMO for CO2 has more carbon character. The relative rates of these reactions (CO2 > OCS > CS2) reflect the increased carbon character of the π* LUMO orbital for CO2 over CS2 and the fact that the LUMO for OCS is σ*, which when filled can readily break the C—S bond leading to sulfide (even though the C character of the LUMO is less than those for CO2 and CS2). Also, the higher hydrogen bonding interactions with nearest water molecules is in the order CO2 > OCS > CS2 indicating that hydrolysis via water catalysis is retarded as the number of S atoms increases. Solid phase FeS has a highest occupied molecular orbital (HOMO) with a potential energy similar to that of CO2 and can activate (or bond with) the carbon atom in CO2 so that organic compounds can be produced under hydrothermal vent conditions.