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) benz ene 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 su bstituent impacts on the benzene ring are generally in good agreement with the results reported in earlier experimental and lower level theoretical st udies. The impacts determined in the monosubstituted benzenes were used to estimate the ring distortions in the di- and trisubstituted derivatives. Th e estimated ring CC bond distances agree generally within 0.001 Angstrom an d the estimated CCC bond angles within 0.3 degree, with the optimized ones. The best agreement (deviations up to only 0.0003 Angstrom and 0.03 deg.) b etween the estimated and optimized geometrical parameters was obtained for the CH3 derivatives. Generally, the para-disubstituted derivatives showed t he best compliance with additivity, somewhat poorer agreement characterized the meta derivatives while the trisubstituted derivatives showed angular d istortions of up to about 0.4 degrees.