Theoretical Investigation of the Additivity of Structural Substituent Effects in Benzene Derivatives

The additivity of substituent effects in 1,3- and 1,4-disubstituted C6H4X2, and 1,3,5-trisubstituted C6H3X3 (X=F, Cl, CN, NO2, CH3, CF3, NH2, OH) benzene derivatives on the ring geometry has been investigated. The analysis is based on ab initio calculations at the MP2/6-31G** level of theory. The substituent impacts on the benzene ring are generally in good agreement with the results reported in earlier experimental and lower level theoretical studies. The impacts determined in the monosubstituted benzenes were used to estimate the ring distortions in the di- and trisubstituted derivatives. The estimated ring CC bond distances agree generally within 0.001 Å and the estimated CCC bond angles within 0.3 degree, with the optimized ones. The best agreement (deviations up to only 0.0003 Å and 0.03 deg.) between the estimated and optimized geometrical parameters was obtained for the CH3 derivatives. Generally, the para-disubstituted derivatives showed the best compliance with additivity, somewhat poorer agreement characterized the meta derivatives while the trisubstituted derivatives showed angular distortions of up to about 0.4°.