Natural bond orbital approach to the transmission of substituent effect through the fulvene and benzene ring systems

Abstract

Electronic structure of 22 monosubstituted derivatives of benzene and exocyclically substituted fulvene with substituents: B(OH)₂, BH₂, CCH, CF₃, CH₃, CHCH₂, CHO, Cl, CMe₃, CN, COCH₃, CONH₂, COOH, F, NH₂, NMe₂, NO, NO₂, OCH₃, OH, SiH₃, SiMe₃ were studied theoretically by means of Natural Bond Orbital analysis. It is shown, that sum of π-electron population of carbon atoms of the fulvene and benzene rings, pEDA(F) and pEDA(B), respectively correlate well with Hammett substituent constants $\sigma_{\rm{p}}^{+}$ and aromaticity index NICS. The substituent effect acting on pi-electron occupation at carbon atoms of the fulvene ring is significantly stronger than in the case of benzene. Electron occupations of ring carbon atoms (except C1) in fulvene plotted against each other give linear regressions with high correlation coefficients. The same is true for ortho- and para-carbon atoms in benzene. Positive slopes of the regressions indicate similar for fulvene and benzene kind of substituent effect – mostly resonance in nature. Only the regressions of occupation at the carbon atom in meta- position of benzene against ortho- and para-positions gives negative slopes and low correlation coefficients.