Ab initio study of the additivity concept applied for the effects of one substituent within cyclic compounds

A comparative analysis of the electron density distributions derived from a b initio calculations at the B3LYP/6-31G(**)//B3LYP/6-31G(**) and RHF/6-31G (**)//RHF/3-21G(**) levels in the framework of the Mulliken population appr oach has been carried out for a number of carbon core conjugated compounds (benzene, cyclopentadienyl anion, cycloheptatrienyl cation, butadiene, cycl opentadiene, hexatriene, pentadienyl anion,heptatrienyl cation, and their m onosubstituted derivatives RX, where X is NH2, OH, F, Me, CHO, CN, NO, or N O2), changes in the effective atomic charges due to substitution in the mol ecules being examined. The response of cyclic compounds to substitution has been explained in the framework of the additivity concept applied for two effects of one substituent within cyclic molecules, namely, a substituent e ffect on a non-ipso atom of a monosubstituted cyclic compound has been trea ted as a sum of two substituent effects arriving at the atom from the subst ituent via two opposite sides of the cyclic chain. Both ab initio methods, the B3LYP and RHF ones, at the same (6-31G(**)) basis set level represent t he additivity of effects of one substituent within cyclic compounds alike. The ab initio study of the influence of electron correlation on the electro n density distributions of unsaturated compounds has shown that at the same (6-31G(**)) basis set level in the framework of the Mulliken population ap proach the changes in the pi, sigma, or total electron density distribution s due to substitution in unsaturated compounds for the B3LYP and RHF calcul ations are the same (with an accuracy of the proportionality coefficients). The inclusion of electron correlation by using the B3LYP method was found to lead to greater variations of the pi substituent charge changes than of the sigma ones as compared with the RHF data. (C) 2001 Elsevier Science B.V . All rights reserved.