Homolytic bond dissociation energies associated with acyl radicals and electron demands of acyl groups

Composite ab initio G3 and CBS-4M methods were used to calculate the homolytic bond dissociation energies (BDE) associated with various acyl radicals (R-C((.))=O). The structures and the vibration properties of the related acyl compounds and radicals were also obtained and discussed. It was found that the BDEs and the structures of the acyl compounds are affected by the electron demand (i.e. electron withdrawing or donating ability) of the acyl groups. According to the Hammett analysis, the electron withdrawing ability of the acyl groups and radicals decreases in the order CO-CN > CO-Cl > CO-F > C((.))=O > CO-H > CO-OH > CO-SCH3 > CO-CH3 > CO-NH2. This order is not consistent with the electronegativity of each group. Therefore, it is the electron demand, not the electronegativity, that determines the substituent effects on the BDEs associated with the acyl radicals.