Accuracy of G2 calculations for the reactions of hydroxyl radicals with alkanes

Ab initio calculations at the G2 and G2(MP2) levels have been used to investigate the reactions of hydroxyl with methane, ethane, propane (2 degrees position), and isobutane (3 degrees position). Transition states were located, and activation energies as well as reaction enthalpies were computed. In general, the two computational methods give similar results. In terms of reaction enthalpies, the calculations underestimate them by about 1 kcal/mel. As for activation energies, an average error of similar to 1 kcal/mol is observed; however, the ab initio results systematically overestimate the stabilizing effects of alpha-methyl substituents on the transition states. As a result, a fairly large error (3.2 kcal/mol) is found when comparing the barriers for methane and isobutane. The results suggest that G2 values be used with caution in these systems.