Homolytic bond dissociation energies of 4‐substituted bicyclo[2.2.2]octanyl compounds: inductive/field effects on the stabilities of organic radicals

Abstract

AbstractThe inductive/field effects on homolytic bond dissociation energies (BDEs) were studied for the first time using substituted 4‐X‐bicyclo[2.2.2]octane‐Y—Z systems. A variety of very different chemical bonds (i.e. Z—Y: Z = CH2, NH, O, SiH2, PH, S; Y = H, F, Li) were considered, and popular substituents including H, CH3, F, OH, NH2, SH, CN and NO2 were utilized. High‐quality BDE values were obtained for the first time for many bicyclo[2.2.2]octane systems from carefully calibrated G3B3/B3LYP calculations. Significant effects of the substituents at the 4‐position of bicyclooctane were found for the Z—Y BDEs of bicyclooctanyl‐Z—Y systems. Nice Hammett‐type correlations were obtained for these substituent effects using the inductive/field F constants. It was found that the reaction constants (i.e. ρ values) of the Hammett correlations varied dramatically from −1.96 to +23.01 kJ mol−1 for different Z—Y systems: The ρ values for the Z—H BDEs were about ∼1.0–5.0 kJ mol−1; the ρ values for the Z—F BDEs were about −2.0 to −1.0 kJ mol−1; the ρ values for the Z—Li BDEs were ∼13.0–23.0 kJ mol−1. The substituent effects on both the stability of the parent molecules before homolysis and the stability of the radical products after homolysis were demonstrated to be important for the BDEs. It was shown that the inductive/field substituent effects on BDEs could not be explained by the electronegativity or bond polarity theories. Nevertheless, we developed a theoretical model on the basis of the classic electrostatic theories for the inductive/field effects. This model successfully explained the intriguing inductive/field substituent effects on BDEs. Copyright © 2004 John Wiley & Sons, Ltd.