Does the interconversion of polysulfur compounds proceed via hypervalent intermediates? An ab initio MO study

Ab initio MO calculations at the CCSD(T)/6-311++G(2df,p)//MP2/6-311++G** level have been carried out to determine the reaction energies and Gibbs energies of the homolytic dissociation of the S-S bonds in the chainlike sulfanes H2Sn (n = 2-4). Good agreement with the experimental data is observed. At the same level of theory, the formation of the hypothetical sulfuranes H2S(SH)2, H2S(SSH)2, and S(SH)4 from H2S and the mentioned sulfanes has been studied. Species of this type had been proposed as intermediates in the interconversion reactions of polysulfur compounds (e.g., formation of S7 from S8 and vice versa). The three sulfuranes serve here as model compounds. On the basis of the Gibbs energies and activation energies at 298 K, it is shown that the formation of the three sulfuranes from sulfanes requires too much energy and activation energy to successfully compete with homolytic dissociation reactions. In addition, the formation of the methylsubstituted sulfurane S(SMe)4 from the sulfanes Me2S2 and Me2S3 was studied to elucidate the mechanism of the formal exchange of sulfur atoms between polysulfane molecules. However, both the reaction energy of 199 kJ mol(-1) and the activation energy of 287 kJ mol(-1), calculated at the MP2/6-31G* level, are much higher than the homolytic dissociation energy of the S-S bonds in chain- and ringlike polysulfur compounds, such as Me2S4 (140 kJ mol(-1)) and sulfur homocycles (150 kJ mol(-1)). Therefore, it is concluded that the observed interconversion reactions of sulfur rings and of chainlike polysulfanes do not proceed via sulfurane-type intermediates. Instead, these reactions will take place by a radical chain mechanism at high temperatures, while at temperatures below 100 degrees C they are most probably initiated either by traces of nucleophiles that are present as impurities or by the polar surface groups usually present on the walls of the vessels used.