Dissociation energies of labile C—H bonds in adducts of thiyl radicals with quinoids.

Dissociation energies (DC—H) of labile C—H bonds in radical adducts formed by coordination of thiyl radicals across the cyclohexadiene rings of 1,4-benzoquinone, N-phenyl-1,4-benzoquinone monoimine, and N,N′-diphenyl-1,4-benzoquinone diimine have been determined using composite methods of the Gaussian family and data on the enthalpies of formal homodesmotic reactions involving the adducts and simpler reference compounds. The DC—H values were found to be in the range of 140–170 kJ mol−1. These values are typical of the strength of bonds between atomic hydrogen and olefins. The weakness of the C—H bonds in both types of compounds is due to the compensation of bond breaking by the stabilization of the reaction products, viz., an olefin molecule or the cyclohexadiene ring of a quinoid. The proximity of the DC—H values in these adducts and the significant difference (∼50 kJ mol−1) between these values and the strength of the C—H bond in the adduct of atomic hydrogen with a benzene molecule confirms the fact that quinones and quinone imines are not aromatic compounds. The obtained DC—H values are required for the quantitative interpretation of the kinetic regularities found for the chain reactions of thiols with quinoids, as well as the catalysis of these reactions at the stage of chain propagation under the action of compounds with X—H groups (X = O, S, etc.). [ABSTRACT FROM AUTHOR]