Substituent effects and the strengths of chemical bonds

Abstract

The dependence of bond strengths on substituent effects is studied with special reference to organic chemistry, on the basis of a theoretical definition of bond energies obtained from a molecular electronic state represented by one Slater determinant over bond orbitals. Two rules appear to hold: (1) the more negative the sum of the charges on the neighbours of an atom X, the more positive the charge on X, and vice versa; (2) the larger the electron populations of the atoms of a bond, the greater the strength of that bond. Values for the energies of a variety of bonds (CC, CO, CN, NN, NO, OO, etc.) in different environments have been derived using a semi-empirical version of the general definition, with the two-electron integrals evaluated by a population-dependent formula. The correlation with experimental bond energies is excellent. The possibility of evaluating bond energies in a rather simple way opens up the interesting opportunity of summarizing the theoretical description of a system of localized bonds by a charge and bond-energy diagram. Applications to bond cleavage and protonation problems are illustrated for ketones and polyamines, respectively.