Acyclic versus cyclic π-electron delocalization. How is the substituent effect related to π-electron delocalization?

Abstract

Substituent effect stabilization energies were estimated for sets of 27 para-substituted phenol derivatives, meta- and para-homodisubstituted benzene derivatives, trans-substituted ethenes, 4-substituted 1-hydroxy-1,3-cyclohexadienes and 1,4-homodisubstituted 1,3-cyclohexadienes based on the optimizations at the B3LYP/6-311+G** DFT level of theory. The following substituents were taken into account: C≡CH, C(CN)3, CF3, CH2NH2, CH3, CH=CH2, CHO, Cl, CN, COCH3, COCl, CONH2, COOCH3, COOH, F, NH2, NHCH3, N(CH3)2, NHOH, NO, NO2, OCH3, OH, Ph, H, SH, SO2CN. For hydroxyethenes and phenol derivatives the electron-acceptor substituents stabilize the systems, whereas the electron-donors lead to their destabilization. Both electron-acceptor and electron-donor substituents destabilize homodisubstituted ethene and meta- and para-homodisubstituted benzene species. The strongest destabilization is observed for derivatives of ethene, a weaker one for derivatives of cyclohexadiene and the weakest for benzene derivatives.