Sulfur chains, energy levels

Meyer and Spitzer (52) have calculated the electronic properties of rings and chains of sulfur atoms. An EH procedure was employed with parameters chosen to fit the energy levels of S8. For S4 it was found that the tetrahedron was only weakly stable, with branched chains and zigzag chains more stable. The calculations predict that above S5 the gas-phase species are cyclic. In addition to these geometry effects, a long-wavelength shift was observed for light absorption by the chains. 

Figure 11. Energy leveb of (a) sulfur rings, (b) sulfur chains, (c) isomers and conformers of Sj, and (d) sulfanes. AU energy levels were computed with an...

The concentration of this species in liquid sulfur was estimated from the calculated Gibbs energy of formation as ca. 1% of all Ss species at the boihng point [35]. In this context it is interesting to note that the structurally related homocyclic sulfur oxide Sy=0 is known as a pure compound and has been characterized by X-ray crystallography and vibrational spectroscopy [48, 49]. Similarly, branched long chains of the type -S-S-S(=S)-S-S- must be components of the polymeric S o present in liquid sulfur at higher temperatures since the model compound H-S-S-S(=S)-S-S-H was calculated to be by only 53 kJ mol less stable at the G3X(MP2) level than the unbranched helical isomer of HySs [35]. 

By ab initio MO and density functional theoretical (DPT) calculations it has been shown that the branched isomers of the sulfanes are local minima on the particular potential energy hypersurface. In the case of disulfane the thiosulfoxide isomer H2S=S of Cg symmetry is by 138 kj mol less stable than the chain-like molecule of C2 symmetry at the QCISD(T)/6-31+G // MP2/6-31G level of theory at 0 K [49]. At the MP2/6-311G //MP2/6-3110 level the energy difference is 143 kJ mol" and the activation energy for the isomerization is 210 kJ mol at 0 K [50]. Somewhat smaller values (117/195 kJ mor ) have been calculated with the more elaborate CCSD(T)/ ANO-L method [50]. The high barrier of ca. 80 kJ mol" for the isomerization of the pyramidal H2S=S back to the screw-like disulfane structure means that the thiosulfoxide, once it has been formed, will not decompose in an unimolecular reaction at low temperature, e.g., in a matrix-isolation experiment. The transition state structure is characterized by a hydrogen atom bridging the two sulfur atoms. 

Coenzyme Q passes electrons through iron-sulfur complexes to cytochromes b and ch which transfer the electrons to cytochrome c. In the ferric Fe3+ state, the heme iron can accept one electron and be reduced to the ferrous state Fe2+. Since the cytochromes carry one electron at a time, two molecules on each cytochrome complex are reduced for every molecule of NADH that is oxidized. The electron transfer from coenzyme Q to cytochrome c produces energy, which pumps protons across the inner mitochondrial membrane. The proton gradient produces one ATP for every coenzyme Q-hydrogen that transfers two electrons to cytochrome c. Electrons from FADH2, produced by reactions such as the oxidation of succinate to fumarate, enter the electron transfer chain at the coenzyme Q level. 

The occupied states are derived from energy distribution curves of the photoelectrons, excited by synchrotron illumination. We present XPS spectra at resonant excitation energies at the Cls (285 eV), and at the S2p (165 eV) ionisation thresholds. For the Cls threshold, we in addition show the spectrum taken off resonance. These valence band spectra are dominated by two broad features at -7 eV and -11.5 eV which are due to carbon-derived (//c) and sulfur-derived (//s) HOMOs of the thiophene monomer. These levels are pronounced in all data, as marked by the dashed lines. The weaker emission of the highest band at -3.1 eV is attributed to emission from electrons out of the 71-band, which is no longer assigned to individual monomers but is delocalised along the polymeric chain see also [33]. 

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