Homolytic Bond Dissociation Enthalpies of the C−H Bonds Adjacent to Radical Centers

Homolytic bond dissociation enthalpies (BDEs) at 0 and 298 K of the C−H bonds adjacent to various radical centers have been obtained from ab initio CBS-4 (complete basis set) model calculations and experimental data available in the literature. The BDEs of the C−H bonds adjacent to the radical centers derived from 11 saturated hydrocarbons were found to be 33.5 ± 3 kcal/mol at 298 K. The BDEs of the C−H bonds adjacent to nine allylic and benzylic radical centers were found to be 48 ± 3 kcal/mol at 298 K, but the benzylic C−H BDE of the PhC H 2 CH 2 • radical was found to be only 29.7 and 30.5 kcal/mol at 0 and 298 K, respectively. The BDEs of the vinylic C−H bonds adjacent to four vinylic radical centers were found to be 35.5 ± 3.5 kcal/mol at 298 K. The BDEs of the vinylic C−H bonds adjacent to three allylic radical centers were found to be 56.5 ± 3 kcal/mol at 298 K. These results suggest that the radical centers weaken the adjacent C−H bond strengths by about 50−70 kcal/mol. The calculated BDEs agree within ±2 kcal/mol with most of the available experimental results. Isomerization enthalpies of butenes and pentenes have been obtained. Substituent effects on BDEs have also been examined.