Theoretical calculation of ionization potentials for disubstituted benzenes: Additivity vs non-additivity of substituent effects

The ionization potentials of 55 para- and 55 meta-disubstituted benzenes, c onsisting of all binary combinations of electron-withdrawing groups (-NO2, -CF3, -CHO, -COOH) and electron-donating groups (-Cl, -CH3, -OH, -OCH3, -NH 2, and -N(CH3)(2)) have been calculated using density functional theory wit h the B3LYP functional and a 6-31G(d) basis set. Relative ionization potent ials (Delta IP), referred to benzene, are compared with experimental values and shown to be in good agreement. The disubstituted data were correlated with monosubstituted Delta IP data and shown to require quadratic terms in order to achieve a good fit; the validity of this conclusion was possible d ue to the low scatter in the calculated data. A simple MO analysis gives a semiquantitative interpretation of the observed trends in substitutent effe cts, including a discussion of combinations of substituents for which nonad ditivity should be expected.