AB-INITIO STUDIES OF REACTIONS OF HYDROXYL RADICALS WITH FLUORINATED ETHANES

Geometries for transition states in the reactions of OH radicals with molecules in the series C2HnF6-n, n = 1-5, have been optimized at the: HF/6-31G(d) and MP2/6-31G(d,p) levels of theory. Vibrational frequency analyses were also performed at both levels of theory. Total energies have been calculated for the reactants, transition states, and products of the reactions at the MP2/6-311G(d,p)//HF/6-31G(d,p) level for all species, at the MP2/6-311G(d,p)//MP2/6-31G(d,p) level for all reactants and products, and transition states containing up to three fluorines, and at Gaussian-2 theory level for reactants and products with up to two fluorines and transition states with one fluorine. These calculations were performed for the abstraction of each inequivalent hydrogen from the most stable conformer of each molecule in the series. Activation entropies, classical barrier heights, with and without zero-point energy corrections at the MP2/6-31G(d,p) level, and enthalpies of reaction with zero-point and thermal energy corrections to 25 degrees C have been calculated. Energy values are affected by the degree and position of fluorine substitution. Trends throughout the series, and differences in the levels of theory, are discussed. In all cases an earlier transition state occurs when electron correlation is included in the geometry optimization. Evidence is seen for an intramolecular hydrogen bond between the hydroxyl hydrogen and a beta-fluorine positioned gauche tb the hydrogen being abstracted.