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124 Ralf Steudel et al. of D2h symmetry (“S2 dimers”) may also be a minimum of comparable ener- gy as was predicted by the density functional calculations published by Jones et al. [45, 46]; see Table 3. In fact, matrix infrared spectra of quenched sulfur vapor originally pro- vided some evidence for two planar chain structures of S4 as well as for di- mers of S2 [42] but a subsequent reinvestigation of the spectra by Hassan- zadeh and Andrews yielded a different assignment [43]. The latter authors identified two S4 isomers: The cis-planar structure (absorbing green light near 520 nm) and the branched three-membered ring S3=S (absorbing red light in the 560–660 nm region). By proper illumination of the matrix-sam- ples these two isomers could even be reversibly converted into each other [43]! However, Picquenard et al. assigned certain lines observed in the reso- nance Raman spectra of hot sulfur vapor also to two different isomers of S4. Saturated and unsaturated vapors were investigated and a strong depen- dence of the spectra on the laser power was observed and explained by a photothermic effect. In other words, the vapor was not always in the thermal equilibrium. On the basis of isotopic shifts and force constant calculations the species absorbing at 530 nm was now identified as the trans-planar structure of S4 and the species absorbing at 620 nm was proposed to have the branched three-membered ring structure [23]; only the latter assignment is in agreement with the suggestions by Hassanzadeh and Andrews [43]. Despite much effort, the full set of the six fundamental vibrations of any S4 isomer has not been observed yet. Most recent high-level G3X(MP2) and CCSD(T)/aug-cc-pVTZ [53] calculations confirmed the singlet planar C2v structure as the global minimum with the C2h isomer significantly higher in energy (see Table 3). The rectangular D2h geometry is not a stable equilibri- um structure at all. It corresponds to a transition state which interconverts two cis-planar forms. All calculations agree that the branched three-mem- bered ring is considerably higher than the global minimum. The calculated order of stability of the various S4 isomers is: C2vPDF Image | Topics in Current Chemistry
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