Kinetic barriers of H-atom transfer reactions in alkyl, allylic, and oxoallylic radicals as calculated by composite ab initio methods.

Composite ab initio and density functional theory (DFT) methods were used to explore internal hydrogen-atom transfers in a variety of primary, secondary, and tertiary alkyl and functionalized radicals with implications for combustion environments. The composite ab initio method G3MP2B3 was found to achieve the most reasonable balance between accuracy and economy in modeling the energetics of these reactions. Increased alkyl substitution reduced barriers to isomerization by about 10 and 20 kJ mol(-1) for secondary and tertiary radical formation, respectively, relative to primary radical reactions and was relatively insensitive to the transition-state ring size (extent of H-atom internal shift). Reactions involving alkenyl and alkanoyl radicals were also explored. Hydrogen-atom transfers involving allylic radical formation demonstrated barrier heights that were 15-20 kJ mol(-1) lower than those in corresponding alkyl radicals, whereas those involving oxoallylic species (alpha-site radicals of aldehydes and ketones) were 20-40 kJ mol(-1) lower. In the cases of the alkyl radicals, enthalpies of activation were seen to scale with enthalpies of reaction. This correlation was not seen, however, in the cases of the allylic and oxoallylic radicals; this fact has significant implications in combustion chemistry and mechanism development, considering that such Evans-Polanyi correlations are widely used in estimating barrier heights for rate expressions.