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104 Ralf Steudel Fig. 4 Structures of nine isomeric molecules of composition S8 with internuclear dis- tances (in pm) as optimized at the B3LYP/6–31G(2df) level of theory. The relative Gibbs energies DGo298 are given in parentheses [109] could be a candidate since ab initio MO calculations at the very high G3X(MP2) level of theory place the lowest energy isomer of S8 at only DGo298=28.3 kJ mol1 above the eight-membered ring [109]. This isomer is of C2 symmetry [110] and has the structure of a partly opened ring or a clus- ter-like spiral (species 2 in Fig. 4); its concentration in liquid sulfur was esti- mated from the calculated Gibbs energy of formation as ca. 1% of all S8 spe- cies at the boiling point. The structurally related homocyclic sulfur oxide S7=O is known as a pure compound and has been well characterized by X-ray crystallography and vibrational spectroscopy [111]. In addition, S8 ring conformations of lower symmetry than D4d are likely candidates in liquid sulfur. For example, the exo-endo isomer 3 (Fig. 4) is by only DGo298=27.8 kJ mol1 less stable than the ground state conformation and therefore its relative concentration in liquid sulfur at the boiling point will be also ca. 1% of all S8 species [109]. The HOMO/LUMO gaps of the isomeric sulfur molecules shown in Fig. 4 (generally of branched rings and chains) are considerably smaller than for the crown-shaped S8 ring [109]. Therefore, the UV-Vis spectra of these spe- cies will exhibit absorption bands at longer wavelengths than the ground state species S8. In addition, these species possess a dipole moment in con- trast to the S8 ring of D4d symmetry. All of these species can be expected to be present in liquid sulfur at high temperatures, and from the Gibbs energiesPDF Image | Topics in Current Chemistry
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