Does the Interconversion of Polysulfur Compounds Proceed Via Hypervalent Intermediates?—An Ab Initio MO Study

Abstract

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.

Tetraamidosulfurane and its Decomposition Products — an ab initio MO Study

“…Their structures are in agreement with the expectations from VSEPR theory placing the lone pair at sulfur in an equatorial position of the trigonal bipyramids. The two axial substituents are bent towards the equatorial ones with N ax ϪSϪN ax bond angles of 162.7°(5), 165.4°(7) and 171.1° (6). The most symmetrical structure 5 seems to be stabilized by four N···H hydrogen bonds of length 215.8 pm between the axial nitrogens and the hydrogen atoms of the equatorial amido groups.…”

“…[4] Tetrachlorosulfurane (SCl 4 ) is thermodynamically unstable with respect to SCl 2 and Cl 2 . [2] Therefore, SCl 4 is still unknown and the same holds for SMe 4 , [5] S(SH) 4 [6] and SH 4 . [7] However, in favorable cases, sulfuranes with substituents of low electronegativity can be prepared, as the successful synthesis of bis(2,2Ј-biphenylylene)sulfurane has demonstrated.…”

Power Law Distribution Concerning Absolute Free Energies of Linear Sulfur Chains, Polythiazyls, Polyisoprenes, Linear trans-Polyenes, and Polyynes

“…Recently, many quantum-chemical calculations concerning sulfur-rich compounds have been published. − …”

Cell‐Penetrating Dynamic‐Covalent Benzopolysulfane Networks

“…Compared to other COCs, benzopolysulfanes offer different reactivity, culminating in ring contraction and expansion from trisulfides to nonasulfides, i.e., 1 – 3 , 16 – 21 , reminiscent of elemental sulfur S n , with pentasulfides 1 – 3 being clearly preferred, followed by trisulfides 16 (Figure a,e and Supporting Information, Figure S38) . The mechanism of these reversible interconversions remains under debate, with transient ring opening by traces of nucleophilic impurities the most likely explanation . BPS chemistry further includes sulfur replacement, nucleophilic displacement and oxidation, radicals, metal coordination, and photochemistry, presumably much influenced, if not determined by the strings of electrophilic σ holes next to nucleophilic lone pairs on the lined‐up sulfur atoms .…”