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Speciation and Thermodynamics of Sulfur Vapor 129 Fig. 1 Calculated structures of the lowest-energy branched ring molecules Sn of size n=4–10 [53] reaction enthalpy (154 kJ mol1) while the transformation of the cis-planar S4 structure into a triplet chain requires the lowest enthalpy (50 kJ mol1). It is, however, evident that all of these enthalpy values are too large for triplet mo- lecules to substantially contribute to the molecular composition of equilibri- um sulfur vapor except as reaction intermediates present in trace amounts. As mentioned above, branched sulfur rings with one exocyclic atom are also potential isomers which may be present in hot sulfur vapor. Their rela- tive energies and enthalpies with respect to the ground state structures have also been examined by the G3X(MP2) theory [53]. For the species with be- tween 4 and 10 atoms these isomers are shown in Fig. 1, and their relative energies are given in Table 7. It follows that the ratio S8(D4d):S8(C2) in sulfur vapor at the boiling point is 1:7103. For all other molecules, this ratio is much larger. Branched rings other than S8 (C2) will therefore hardly be de- tected. There remains the unresolved problem of the isomeric S4 structures (see Scheme 1 above). To provide a more definitive assignment of the various ob- served S4 isomers, we have calculated their electronic and vibrational spec- tra. Table 8 summarizes the three strongest absorptions in the UV-Vis region and their transition probabilities obtained at the CIS/6-311+G(2df) level. As demonstrated by the computed transition energies, only the C2v and C2h iso- mers have UV absorptions at wavelengths above 400 nm. The branched ring S3=S is predicted to absorb at 240 nm. This is in agreement with the ob- served UV spectrum of F2S=S, which is characterized by a peak at 244 nmPDF Image | Topics in Current Chemistry
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