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.