AN AB-INITIO STUDY ON REACTIVITY OF FLUOROETHANE WITH HYDROXYL RADICAL - APPLICATION OF G2 THEORY

The reaction of fluoroethane with hydroxyl radical has been investigat ed by ab initio molecular orbital theory, and two possible reaction pa thways were examined, alpha and beta abstractions. Optimized geometrie s and harmonic vibrational frequencies have been calculated for reacta nts, products, and transition states up to the (U)MP2/6-31G(d,p) level of theory. Accurate energies of reactants, products, and transition s tates are obtained using G2 theory. The calculated barrier height and reaction enthalpy of alpha abstraction are 1.35 and -18.36 kcal mol(-1 ), respectively. A very good agreement is found with available experim ental data. Two rotamers were found as the transition-state structures involved in the beta abstraction reaction. This reaction has higher b arrier heights, 2.66 and 3.73 kcal mol(-1), respectively, and it is le ss exothermic than alpha abstraction, with reaction enthalpy of -14.42 kcal mol(-1). One rotamer is stabilized by the delocalization of elec trons from a fluorine atom into the O-H bond. The transition-state int eractions between reacting units have been analyzed in terms of the na tural bond orbital (NBO) method. The main delocalization process for a ll transition-state structures seems to be between the oxygen nonbondi ng orbitals and the NBOs of the reactive C(1)-H(3) bond. The results o n fluoroethane reactivity with hydroxyl radical have been compared wit h previous results on chloroethane and ethane. The influence of haloge n substitution on the barrier heights and reaction enthalpies has been rationalized in terms of three different effects: the electron densit y redistribution in the reactive C(1)-H(3) bond, the stabilization of radical product, and the participation of halogen atom in breaking the reactive C(1)-H(3) bond. Finally, the C-H bond dissociation energies of fluoroethane and chloroethane have been calculated at G2, UMP4, and UMP2/6-311+G(2d,p)/UMP2/6-3 1G(d,p) levels of theory as well as the G 2 enthalpies of formation of alpha- and beta-fluoro thyl radicals sinc e the experimental values are not available yet.