High-resolution proton magnetic resonance spectra of polynuclear heterocycles-II : Benzo [ b] thiophen and dibenzothiophen

Abstract

Full iterative analyses have been carried out of 60 MHz spectra of benzo [ b] thiophen (I) and of 60 and 100 MHz spectra of dibenzothiophen (II) in acetone and carbon tetrachloride solutions at several concentrations. From the 33 peaks in the spectrum of I, analysed as a six-spin system with appreciable inter-ring coupling between H(3) and H(7) and between H(2) and H(6), 15 parameters were determined and 190 transitions assigned. For II, inter-ring coupling is absent but H(2) and H(3) of the ABCD system have closely similar shifts. Deshielding of protons of I and II in acetone solution relative to carbon tetrachloride is attributed to reaction-field effects; some ordering of the polar solute molecules is suggested. For the thiophen rings in I and II, currents calculated from chemical shifts remote from the CS bonds are comparable with those in benzene. In terms of partial bond fixation, the J 4,5, J 5,6 coupling constants observed in I lead to an aromaticity comparable with that in naphthalene, and J 2,3 of II implies a β-bond equivalence approaching that in fluorene.