Electronegativity and resonance parameters from the geometry of monosubstituted benzene rings

Reference values of the structural substituent parameters, SE and S-R, measuring the electronegativity and resonance effects, respectively, of functional groups (Campanelli et al. J Phys Chem A 107:6429-6440, 2003) have been determined from the benzene ring geometries of 100 Ph-X species, including different conformations of the same molecule. Geometries have been obtained by quantum chemical calculations at the HF/6-31G*, HF/6-311++G**, and B3LYP/6-311++G** levels of theory. The substituent parameters from HF/6-311++G** calculations are in close agreement with those determined at the HF/6-31G* level. Using the B3LYP density functional yields S-E and S-R values which-in general-correlate well with the corresponding HF values. Exceptions occur with some charged groups, and, in the case of S-E, with a few dipolar groups having very high or low electronegativities. SR values from B3LYP calculations are about 22% smaller than the corresponding HF values. The variations of the benzene ring geometry caused by electronegativity, resonance, and steric effects are illustrated in some detail.