The application of the Hammett equation to 13C N.M.R. spectrometry. VI. Remote ring effects in azobenzenes and stilbenes

Abstract

The 13C n.m.r. chemical shifts for the non-substituted rings of azobenzene, stilbene, 26 monosubstituted azobenzenes and 28 monosubstituted stilbenes are reported. The effects of substituents in one ring on the chemical shifts of the other have been interpreted in terms of their inductive and resonance effects by means of a dual substituent parameter analysis. The results show that inductive effects are transmitted to the remote ring from the meta and para positions with equal efficiencies and polarize the phenyl group independently of the rest of the molecule. Resonance interaction is greatest when the substituent is para to the azo or vinylene linking group and is relayed with greatest efficiency to the ortho and para carbons of the remote ring. The results of the study are compared with previously unreported data for the corresponding ring carbons of the ethyl arylazo- and arylethenyl-cinnamates, and with literature data on para-substituted biphenyls and terphenyls.