Ab initio study of reactions of hydroxyl radicals with chloro- and fluoro-substituted methanes

Ab initio calculations at the unrestricted Hartree-Fock (UHF) level have be en performed to investigate the hydrogen abstraction reactions of (OH)-O-. radicals with methane and nine halogen-substituted methanes (F, Cl). Geomet ry optimization and vibrational frequency calculations have been performed on all reactants, adducts, products, and transition states at the UHF/6-31G * level. Single-point energy calculations at the MP2/6-31++G* level using t he UHF/6-31G* optimized geometries have also been carried out on all specie s. Pre- and postreaction adducts have been detected on the UHF/6-31G* poten tial energy surfaces of the studied reactions. Energy barriers, DeltaE(doub le dagger) reaction energies, DeltaE(r), reaction enthalpies, DeltaH(r), an d activation energies, E-a, have been determined for all reactions and corr ected for zero-point energy effects. Both E-a and DeltaH(r), come into reas onable agreement with the experiment when correlation energy is taken into account and when more polarized and diffuse basis sets are used. The E-a va lues, estimated at the PMP2/6-31++G* level, are found to be in good agreeme nt with the experimental ones and correctly reproduce the experimentally ob served trends in fluorine and chlorine substitution effects. A linear corre lation between E-a and DeltaH(r) is obtained, suggesting the presence of an Evans-Polanyi type of relationship. ((C) 2001 John Wiley & Sons, Inc.