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.

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Methylzinc alkoxide complexes are precursors for the preparation of nanosized zinc oxide particles, which in turn are catalysts or reagents in important industrial processes such as methanol synthesis and rubber vulcanization. We report for the first time the structures, energies, atomic charges, dipole moments, and vibrational spectra of more than 20 species of the type [(MeZnOR')n] with R' = H, Me, tBu and n = 1-6, calculated by density functional theory methods, mostly at the B3LYP/6-31+G* level of theory. For R' = Me, the global minimum structure of the tetramer (n = 4) is a highly symmetrical heterocubane but a ladder-type isomer is by only 70.9 kJ mol(-1) less stable. The corresponding trimer is most stable as a rooflike structure; a planar six-membered ring of relative energy 13.5 kJ mol(-1) corresponds to a saddle point connecting two equivalent rooflike trimer structures. All dimers form planar four-membered Zn2O2 rings whereas the monomer has a planar CZnOC backbone. A hexameric drumlike structure represents the global minimum on the potential energy hypersurface of [(MeZnOMe)6]. The enthalpies and Gibbs energies of the related dissociation reactions hexamer --> tetramer --> trimer --> dimer --> monomer as well as of a number of isomerization reactions have been calculated. The complexes [(MeZnOMe)n] (n = 1-3) form adducts with Lewis bases such as tetrahydrofuran (thf) and pyridine (py). The binding energy of py to the zinc atoms is about 65% larger than that of thf but is not large enough to break up the larger clusters. The bimolecular disproportionation of [(MeZnOMe)4] with formation of the dicubane [Zn{(MeZn)3(OMe)4}2] and Me2Zn is less endothermic than any isomerization or dissociation reaction of the heterocubane, but for steric reasons this reaction is not possible if R' = tBu. A novel reaction mechanism for the reported interconversion, disproportionation and ligand exchange reactions of zinc alkoxide complexes is proposed.

The homolytic dissociation of the important vulcanization accelerator tetramethylthiuram disulfide (TMTD) has been studied by ab initio calculations according to the G3X(MP2) and G3X(MP2)-RAD theories. Homolytic cleavage of the SS bond requires a low enthalpy of 150.0 kJ mol-1, whereas 268.0 kJ mol-1 is needed for the dissociation of one of the C-S single bonds. To cleave one of the SS bonds of the corresponding trisulfide (TMTT) requires 191.1 kJ mol-1. Me2NCS2* is a particularly stable sulfur radical as reflected in the low S-H bond dissociation enthalpy of the corresponding acid Me2NC(=S)SH (301.7 kJ mol-1). Me2NCS2* (2B2) is a sigma radical characterized by the unpaired spin density shared equally between the two sulfur atoms and by a 4-center (NCS2) delocalized pi system. The ESR g-tensors of the radicals Me2NCSn* (n = 1-3) have been calculated. Both TMTD and the mentioned radicals form stable chelate complexes with a Li+ cation, which here serves as a model for the zinc ions used in accelerated rubber vulcanization. Although the binding energy of the complex [Li(TMTD)]+ is larger than that of the isomeric species [Li(S2CNMe2)2]+ (12), the dissociation enthalpy of TMTD as a ligand is smaller (125.5 kJ mol-1) than that of free TMTD. In other words, the homolytic dissociation of the SS bonds of TMTD is facilitated by the presence of Li+ ions. The sulfurization of TMTD in the presence of Li+ to give the paramagnetic complex [Li(S3CNMe2)2]+ is strongly exothermic. These results suggest that TMTD reacts with naked zinc ions as well as with the surface atoms of solid zinc oxide particles in an analogous manner producing highly reactive complexes, which probably initiate the crosslinking process during vulcanization reactions of natural or synthetic rubber accelerated by TMTD/ZnO.

The vulcanization of rubber by sulfur is a large-scale industrial process that is only poorly understood, especially the role of zinc oxide, which is added as an activator. We used the highly symmetrical cluster Zn(4)O(4) (T(d)) as a model species to study the thermodynamics of the initial interaction of various vulcanization-related molecules with ZnO by DFT methods, mostly at the B3LYP/6-31+G* level. The interaction energy of Lewis bases with Zn(4)O(4) increases in the following order: COCSSH.

High-level ab initio molecular orbital calculations were employed to explore the potential energy hypersurface of hexasulfur, S(6). Twelve isomeric structures of S(6) have been identified: two unbranched rings (chair and boat), one trigonal prism of D(3h) symmetry, two singly branched rings (S(5)double bondS), three triplet chains, one singlet chain, and three doubly branched rings (Sdouble bondS(4)double bondS). The prism structure is essentially a cluster of three S(2) molecules connected via a six-center pi(*)-pi(*)-pi(*) interaction. It is by 51 kJ mol(-1) less stable than the lowest-energy chair form. The reactions to generate the boat, the prism, and the singly branched isomers from the chair form are predicted to have lower barriers than the ring opening reaction of cyclo-S(6), which requires an activation energy of 149 kJ mol(-1). The prism and singly branched isomers are found to be more reactive species than the chair form and they are potential sources of S(2) in chemical reactions involving elemental sulfur.

Ab initio MO calculations at the G3(MP2) level of theory show that sulfur tetraamide S(NH 2 ) 4 is a local minimum energy structure but thermodynamically unstable with respect to both sulfur diamide S(NH 2 ) 2 plus hydrazine and sulfurimi-