A study of molecular processes in calcium clusters, sarin, and calcium carbonate clusters

Extensive quantum mechanical calculations have been performed on systems of atoms and molecules to include the study of calcium clusters, the degradation of sarin with OH, and the study of calcium carbonate clusters.; The electronic structure of calcium clusters containing up to 13 atoms was studied within the General Gradient Approximation (GGA) of the density-functional formalism. For the calcium dimer it is found that the exchange functional in GGA overestimates the binding energy, while a hybrid approach including Hartree-Fock exchange gives a better agreement with the experimental results. It was found that Ca₁₂ undergoes a structural transition as a function of temperature, changing structure at T = 318 K.; The breakdown of Sarin (GB), isopropyl methylphosphonofluoridate, by H₂O and OH has been investigated using a molecular orbital approach, an extended basis set, and within the Hartree-Fock approximation. It was found that when Sarin is exposed to water, the bond between the fluorine and phosphorus atoms breaks and the water molecule dissociates into OH and atomic hydrogen. Two new bonds are formed between the OH radical and the phosphorus atom and between the freed fluorine and hydrogen atoms giving rise to isopropylmethylphosphonic acid and HF. When the GB degradation is attempted with OH, there are two alternative reaction paths. In one, HF and the radical CH₃PO[OCH(CH₃) ₂]O are formed. In the other reaction an intermediate radical is formed in which the hydroxyl sits in the former site of the fluorine atom and the latter moves to a position such that the P-F bond is almost perpendicular to the plane of the three oxygen atoms. Once this intermediate radical is formed, it proceeds by unimolecular decomposition to give either HF and CH _{accompanied by the formation of isopropylmethylphosphonic acid.; This ab-initio study was extended to the investigation of molecular clusters of calcium carbonate. It is found that the molecular clusters do not show local point symmetry consistent with the calcite lattice. (Abstract shortened by UMI.)...}