Studies in the field of aromatic heterocycles

Abstract

An analysis of the PMR spectra of benzo- 2,1,3-selenadiazoles and also of compounds containing 2,1,3-oxadiazole and -thiadiazole nuclei has been carried out. It has been shown that the key atoms O, S, and Se ensure the possibility of the establishment of a ring current in the corresponding heterocycles. “Aromaticity” in the sense of a strong ring current in these compounds is somewhat weaker than in naphthalene. The estimates were obtained by calculating the differenceϒ₀δ_ab of the chemical shifts of the protons of the benzene ring linked to the heterocycle from the experimental spectra (A₂B₂ system) and the subsequent comparison of these data with the theoretical dependence ofϒ₀δ_ABon the distance separating the proton under study and the center of the contour of the ring current. In the case of the benzo-2, 1, 3-X-diazoles (X = O, S, Se), this contour is the heterocyclic ring. Direct measurements of the magnetic susceptibility of these compounds have confirmed the considerable equalization of the bonds in the heterocycle, i.e., the participation of the key atoms O, S, and Se in the formation of the ring system of delocalized electrons. It has been shown that the proposed structural scheme permits the prediction of the magnitudes of the chemical shifts and the PMR spectra of such compounds. The question has been raised of the evaluation of the real acceptor properties (electronegativity) of the key atoms completing the heterocycle.