An ab Initio Study on Reactivity of Fluoroethane with Hydroxyl Radical: Application of G2 Theory

Abstract

The reaction of fluoroethane with hydroxyl radical has been investigated by ab initio molecular orbital theory, and two possible reaction pathways were examined, α and β abstractions. Optimized geometries and harmonic vibrational frequencies have been calculated for reactants, products, and transition states up to the (U)MP2/6-31G(d,p) level of theory. Accurate energies of reactants, products, and transition states are obtained using G2 theory. The calculated barrier height and reaction enthalpy of α abstraction are 1.35 and −18.36 kcal mol-1, respectively. A very good agreement is found with available experimental data. Two rotamers were found as the transition-state structures involved in the β abstraction reaction. This reaction has higher barrier heights, 2.66 and 3.73 kcal mol-1, respectively, and it is less exothermic than α abstraction, with reaction enthalpy of −14.42 kcal mol-1. One rotamer is stabilized by the delocalization of electrons from a fluorine atom into the O−H bond. The transition-state interactions between reacting units have been analyzed in terms of the natural bond orbital (NBO) method. The main delocalization process for all transition-state structures seems to be between the oxygen nonbonding orbitals and the NBOs of the reactive C(1)−H(3) bond. The results on fluoroethane reactivity with hydroxyl radical have been compared with previous results on chloroethane and ethane. The influence of halogen substitution on the barrier heights and reaction enthalpies has been rationalized in terms of three different effects: the electron density 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)//UMP2/6-31G(d,p) levels of theory as well as the G2 enthalpies of formation of α- and β-fluoroethyl radicals since the experimental values are not available yet.