77Se NMR spectroscopic study of the molecular composition of sulfur–selenium melts

Abstract

A series of six sulfur–selenium mixtures containing 0.4, 5, 15, 25, 35, and 45 mol% of selenium were heated in evacuated ampoules at 430 °C for 4 h and, keeping the material molten at 135 °C, their natural-abundance 77Se NMR spectra were recorded. The equilibrium melt in each case was found to contain both heterocyclic SenS8−n species and polymeric material. The most abundant eight-membered selenium sulfide rings were SeS7, which was the main cyclic component in all melts studied in this work, and 1,2-Se2S6, the quantity of which increased with increasing selenium content of the melt. In addition the melts contained smaller amounts of 1,3, 1,4, and 1,5 isomers of Se2S6, 1,2,3, 1,2,4, and 1,2,5 isomers of Se3S5, 1,2,3,4, 1,2,3,5, and 1,2,5,6 isomers of Se4S4, 1,2,3,4,5-Se5S3, 1,2,3,4,5,6-Se6S2, and Se8. The relative distribution of the cyclic products as a function of the selenium content in the melt was virtually identical with that obtained earlier in the CS2 solutions of the quenched melts, implying that the extracted quenched melts actually represent the original melt composition as far as the SenS8−n species are concerned. The resonances due to the polymeric material in the melts could best be interpreted by the statistical random distribution of selenium and sulfur in the polymeric chains, the probability of occurrence of the different fragments being governed by the overall composition of the melt. As the selenium content increases, the relative amount of selenium bound in the polymer seems to increase at the expense of heterocyclic selenium sulfides. Keywords: selenium sulfides, sulfur–selenium melt, 77Se NMR spectroscopy.