Electronegativity, Resonance, and Steric Effects and the Structure of Monosubstituted Benzene Rings:  An ab Initio MO Study

Abstract

The deformation of the carbon skeleton of the benzene ring under substituent impact has been analyzed from\nthe structures of 74 monosubstituted derivatives, as determined by ab initio MO calculations. The geometry\nof the substituted ring is shown to contain valuable information on the electronegativity, resonance, and\nsteric effects of the substituent, and also on other, more subtle effects, affecting primarily the length of the\nCipso-Cortho bonds. The results obtained substantially augment previous knowledge from the analysis of\nexperimental geometries (Domenicano, A.; Murray-Rust, P.; Vaciago, A. Acta Crystallogr., Sect. B 1983,\n39, 457). Varying the electronegativity of the substituent causes a concerted change of the ring angles at the\nipso, ortho, and para positions, coupled with a change in the Cipso-Cortho bond length. The values of the ipso\nangle span a remarkably wide range, 113-126°. Enhancing the resonance interaction between a substituent\nand the ring causes a complex pattern of angular distortions, arising from the superposition of two separate\neffects. The first originates from the decreased length of the C-X bond, and consists primarily in a concerted\nchange of the ipso and ortho angles. It occurs irrespective of whether the substituent is a π donor or a π acceptor. The second effect is associated with π-charge alternation on the ring carbons. It involves all the\ninternal ring angles, and depends on the substituent being a π donor or a π acceptor. These angular changes\nare generally accompanied by changes in all C-C bond lengths, as expected from an enhanced contribution\nof polar canonical forms to the electronic structure of the molecule. By using symmetry coordinates, we have\nderived two orthogonal linear combinations of the internal ring angles, SE and SR, measuring the electronegativity\nand resonance effects of a substituent, respectively, as seen from their impact on the ring geometry. SE and\nSR values are affected in a typical way by steric effects.