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.