Electronegativity, resonance, and steric effects and the structure of monosubstituted benzene rings: An ab initio MO study

The deformation of the carbon skeleton of the benzene ring under substituent impact has been analyzed from the structures of 74 monosubstituted derivatives, as determined by ab initio MO calculations. The geometry of the substituted ring is shown to contain valuable information on the electronegativity, resonance, and steric effects of the substituent, and also on other, more subtle effects, affecting primarily the length of the C-ipso-C-ortho bonds. The results obtained substantially augment previous knowledge from the analysis of experimental geometries (Domenicano, A.; Murray-Rust, P.; Vaciago, A. Acta Crystallogr., Sect. B 1983, 39, 457). Varying the electronegativity of the substituent causes a concerted change of the ring angles at the ipso, ortho, and para positions, coupled with a change in the C-ipso-C-ortho. bond length. The values of the ipso angle span a remarkably wide range, 113-126degrees. Enhancing the resonance interaction between a substituent and the ring causes a complex pattern of angular distortions, arising from the superposition of two separate effects. The first originates from the decreased length of the C-X bond, and consists primarily in a concerted change of the ipso and ortho angles. It occurs irrespective of whether the substituent is a pi donor or a pi acceptor. The second effect is associated with pi-charge alternation on the ring carbons. It involves all the internal ring angles, and depends on the substituent being a pi donor or a pi acceptor. These angular changes are generally accompanied by changes in all C-C bond lengths, as expected from an enhanced contribution of polar canonical forms to the electronic structure of the molecule. By using symmetry coordinates, we have derived two orthogonal linear combinations of the internal ring angles, S-E and S-R, measuring the electronegativity and resonance effects of a substituent, respectively, as seen from their impact on the ring geometry. S-E and S-R values are affected in a typical way by steric effects.