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 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, 113-126°. 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 π donor or a π acceptor. The second effect is associated with π-charge alternation on the ring carbons. It involves all the internal ring angles, and depends on the substituent being a π donor or a π 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.

A universal scale of aromaticity for π‐organic compounds

“…66 The major changes in the geometry of 74 monosubstituted benzenes concern the ring angles and they are related to the electronegativity of substituents. 67 Only substituents CH 2 1 and CH 2 2 cause substantial bond length changes and HOMA decreases to $ 0.7. 68 Moreover, all substituents induce a loss of aromaticity independently of their electron donating or accepting character.…”

How far the substituent effects in disubstituted cyclohexa-1,3-diene derivatives differ from those in bicyclo[2.2.2]octane and benzene?

“…One of molecular parameters sometimes used to characterize the effect of the substituent is a valence angle at the ipso carbon, particularly well documented for monosubstituted benzene derivatives [38]. Its interpretation has been rationalized by electronegativity of the substituent [39,40]. We present an application of these ideas for 1-4 disubstituted CHD derivatives.…”

“…The obtained linear regressions have good determination coefficients (R 2 > 0.923) indicating good interrelationships between φ(1) and charges at C1 and C4 active regions. As abovementioned, the ipso angle φ is related to group electronegativity [39,40]. Therefore, it allows to assume that interactions between X and Y change electronegativities of both interacting X and Y [41].…”

Studies on bis(1′-ortho-carboranyl)benzenes and bis(1′-ortho-carboranyl)biphenyls

“…2 Icosahedral carboranyl residues as substituents have larger cone angles and occupy slightly larger volumes than fully rotating phenyl groups, [3][4][5][6] as illustrated by B3LYP/6-31G* optimized geometries of 1-phenyl-ortho-carborane 1, 7,8 biphenyl 7,9 and tert-butylbenzene, 9 the ranges of H...H nonbonded intramolecular interactions in which are shown in Fig. 1.…”