Polysulfide dianions and radical anions play a crucial role in biological chemistry, geochemical processes, alkali metal–sulfur batteries, organic syntheses, coordination chemistry, and materials sciences.

A detailed reaction mechanism is proposed for the formation of crystalline elemental sulfur from aqueous sulfide by oxidation with transition-metal ions like VV, FeIII, CuII, etc. The first step is the formation of HS• radicals by one-electron oxidation of HS- ions. These radicals exist at pH values near 7 mainly as S•-. Their spontaneous decay results in the formation of the disulfide ion S2 2-. The further oxidation of disulfide either by S•- radicals or by the transition-metal ions yields higher polysulfide ions from which the homocyclic sulfur molecules S6, S7, and S8 are formed. In water these hydrophobic molecules form clusters which grow to droplets of liquid sulfur (sulfur sol). Depending on the composition of the aqueous phase, crystallization of the liquid sulfur as either α- or β-S8 is rapid or delayed. Surfactants delay this solidification, while certain cations promote it. All these reactions are proposed to take place in desulfurization plants working by the Stretford, Sulfolin, Lo-Cat, SulFerox, or Bio-SR processes. In addition, the sulfur produced from sulfide by oxidizing sulfur bacteria is formed by the same mechanism, which now explains many observations made previously (including the formation of the byproducts thiosulfate, polythionates, and sulfate).

No abstract

SS bonds are extraordinarily flexible and have properties that are observed only on isolated occasions for other homonuclear bonds: the bond lengths very between 1.8 and 3.0Å, the bond angles between 90 and 180° and the dihedral angles between 0 and 180°; the bond energies amount to up to 430 kJ/mol. The SS stretching frequencies can appear over the range 177–820 cm−1 and force constants of 1.4 to 6.3 mdyne/Å have been calculated. This variability is illustrated with examples containing isolated and cumulated SS bonds.

No abstract

No abstract

Polythionate ions containing up to 22 sulfur atoms have been separated by ion‐pair chromatography. The ions are formed in mixtures of thiosulfate and chlorosulfanes and upon oxidation of tetra‐ or pentathionate with Thiobacillus ferrooxidans. The sulfur globules formed as intermediates were also investigated. Their core consists mainly of S8 with polythionate ions on the hydrophilic surface. The eluent consisted of a mixture of acetonitrile and water with a linearly decreasing concentration of (NBu4)H2PO4 and Na2CO3.

The sodium-sulfur (NAS) battery is a candidate for energy storage and load leveling in power systems, by using the reversible reduction of elemental sulfur by sodium metal to give a liquid mixture of polysulfides (Na(2)S(n)) at approximately 320°C. We investigated a large number of reactions possibly occurring in such sodium polysulfide melts by using density functional calculations at the G3X(MP2)/B3LYP/6-31+G(2df,p) level of theory including polarizable continuum model (PCM) corrections for two polarizable phases, to obtain geometric and, for the first time, thermodynamic data for the liquid sodium-sulfur system. Novel reaction sequences for the electrochemical reduction of elemental sulfur are proposed on the basis of their Gibbs reaction energies. We suggest that the primary reduction product of S(8) is the radical anion S(8)(˙-), which decomposes at the operating temperature of NAS batteries exergonically to the radicals S(2)(˙-) and S(3)(˙-) together with the neutral species S(6) and S(5), respectively. In addition, S(8)(˙-) is predicted to disproportionate exergonically to S(8) and S(8)(2-) followed by the dissociation of the latter into two S(4)(˙-) radical ions. By recombination reactions of these radicals various polysulfide dianions can in principle be formed. However, polysulfide dianions larger than S(4)(2-) are thermally unstable at 320°C and smaller dianions as well as radical monoanions dominate in Na(2)S(n) (n=2-5) melts instead. The reverse reactions are predicted to take place when the NAS battery is charged. We show that ion pairs of the types NaS(2)˙, NaS(n)(-), and Na(2)S(n) can be expected at least for n=2 and 3 in NAS batteries, but are unlikely in aqueous sodium polysulfide except at high concentrations. The structures of such radicals and anions with up to nine sulfur atoms are reported, because they are predicted to play a key role in the electrochemical reduction process. A large number of isomerization, disproportionation, and sulfurization reactions of polysulfide mono- and dianions have been investigated in the gas phase and in a polarizable continuum, and numerous reaction enthalpies as well as Gibbs energies are reported.