The infra-red spectra and hydrogen bonding of heterocyclic thioamides

Abstract

Hydrogen bonding of the heterocyclic thioamides thiazolidine-2-thione(I), benzthiazoline-2-thione(II) and 4-methylthiazoline-2-thione(III) is interpreted in terms of the formation of cyclic dimers, which favours co-operative proton transfer and resonance stabilization. The dimerization constants, β 20 have been determined for each of the three compounds in solution in carbon tetrachloride at 25 ± 1°C, [(I): β 20 = 88 ± 9 M −1; (II): β 20 = 570 ± 60 M −1, (III): β 20 = 5540 ± 500 M −1]. The strengths of the hydrogen bonds, which have been estimated from Δν = (ν NH free -ν NHṫS), increase in the order I < II < III. The strength of the hydrogen bond of III is approximately equal to that of thiopyridone.