The effect of substituents upon proton coupling constants in substituted pyrimidines and benzenes

Abstract

The n.m.r. spectra for carbon tetrachloride and trifluoroacetic acid solutions of 4-pyenylpyrimidine are reported. The azine-ring coupling-constants for the neutral and protonated molecules are compared, and the differences interpreted in terms of delocalisation of the nitrogen lone-pair. Similar arguments involving delocalisation of the C–X bond molecular-orbitals are used to explain the effect of substituents X on the coupling constants in monosubstituted benzenes C₆H₅X.