1H, 13C and 15N NMR study and molecular modelling of 2,3-di-substituted quinoxalines with sterically hindered aromatic and heteroaromatic substituents

Abstract

The 1H, 13C and 15N chemical shifts of a series of 2,3-di-substituted quinoxaline derivatives, substituted with heteroaromatic moieties in position 2, were measured and discussed with respect to π-electron distribution and stereodynamic behavior. The preferred conformations were calculated by semi-empirical (PM3 hamiltonian) and ab initio (HF/6–31G∗) calculations and compared with the experimental results. The observed NMR parameters are related to the calculated torsional twists. Steric interactions between the substituents in both the 3- and 3′-positions are the major cause for the non-planarity of the investigated compounds.