Robert W. Carr, Associate Fellow

Atmospheric Chemistry of the Chlorine-Containing Organic Compounds

Supercomputing Institute resources were used to complement experimental research on atmospheric chemistry of halogenated alkoxy radicals and the chemistry of GaN chemical vapor deposition. By performing ab initio calculations on reactions of CF₃CClHO, CF₃CH₂O and CH₂BrO radicals, the researchers predicted optimized geometries, vibrational frequencies, and total energies of reactants, products, and transition states. Reaction path following calculations will also be used to verify that the located transition states connect with two minima in each side of the reaction coordinate (reactant and products). The results of the ab initio computations will be used in Rice-Ramsperger-Kassel-Marcus (RRKM) estimates of rate coefficients via molecular theories of kinetics. Comparisons were made between the computations and experimental results. Comparison of the estimated and experimental rate coefficients permit development of rate coefficient models that are accurate over the entire range of atmospheric conditions, which is much wider than the range of conditions accessible by experimental methods. These calculations provide information on the effect of halogen substitution on the reactivity of these oxy radicals.

One study involved an ab initio molecular orbital study of the unimolecular elimination of HCl and Cl from the CH₂ClO radical. Geometry optimizations were carried out at the HF/6–31G(d), MP2(full)/6–31G(d), and MP2(full)/6–31G(d,p) levels, and total energies were calculated using G2 and G2(MP2) theories. The zero point energy (ZPE)-corrected energy barrier for HCl elimination is predicted to be 8 kcal/mol, and for Cl elimination it was predicted to be 10.5 kcal/mol. RRKM models for both unimolecular reactions were made from the ab initio vibrational frequencies (scaled), moments of inertia, and barrier heights. The RRKM predicted thermal rate coefficients for HC1 elimination are in good agreement with experimental data taken over the temperature range 265–306 K and the pressure range 5–35 torr (Wu and Carr, J.Phys.Chem. A, 105, p. 1423, 2001) when the barrier height is adjusted to 8.5 kcal/mol. Because of the low energy barrier and, to a lesser extent, the small size of the reactant, the fall-off curve is very broad, and the high pressure limit is predicted to be closely approached only when pressures of 107 torr and above are reached. A second research project conducted by this research group investigated homogeneous gas phase reactions that occur during chemical vapor deposition (CVD) of GaN from NH3 and trimethylgallium (TMG). Both experimental and theoretical studies are necessary to develop a physically based detailed CVD reactor model for GaN deposition. For the first half of 2002, these researcher will study the successive loss of CH₃ radicals from TMG, and decomposition of TMG:NH₃ via ab initio and RRKM methods.

Research Group

Christopher Duda, Graduate Student Researcher
Aleksey Pelekh, Associate Researcher
Fuxiang Wu, Associate Researcher