PROPERTIES OF TRANSITION SPECIES IN THE REACTION OF HYDROXYL WITH ETHANE FROM AB-INITIO CALCULATIONS AND FITS TO EXPERIMENTAL-DATA

Fully optimized geometries, harmonic vibrational frequencies, and zero-point energies for the reactants, transition state, and products of the reaction of ethane with hydroxyl radical were calculated at the MP2/6-31G(d,p) level of theory. The reaction path was followed in mass-weighted internal coordinates, using the intrinsic reaction coordinate method, at the MP2/6-31G(d,p) level. Energies were calculated using a modified version of Gaussian-2 theory. Transition-state parameters were also determined by a fit to experimental data and by the bond energy-bond order method. This study provides the most detailed description to date of the potential energy surface (PES) for this reaction. The transition state occurs early in the reaction. The PES is very flat for internal rotation about the C ... H ... O axis. The geometric mean of the five lowest frequency vibrational term values was found to be 352 +/- 7 cm(-1) from the fits to experimental data and 345 cm(-1) from the MP2/6-31G(d,p) calculations. The corresponding values of the barrier heights, corrected for zero-point energy, are 9.3 +/- 0.3 and 12.4 kJ mol(-1). The barrier widths at half the barrier heights are 0.74 +/- 0.11 and 0.88 Angstrom, respectively.