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94 Ralf Steudel 3 Recent Results Since about 1970 it has been shown that almost all metastable sulfur homo- cycles between S6 and S20 can be prepared as pure solids by kinetically con- trolled syntheses at moderate temperatures [71]. The investigation of their physical properties and in particular of their spectra allowed the molecular composition of liquid sulfur to be determined using modern spectroscopic and chromatographic techniques. As a result of these investigations the pure sulfur rings S7, S12, S18, and S20 can now be prepared from quenched sulfur melts by fractional crystallization [71]. The analytical work will be described below while the preparation of solid sulfur allotropes is reported elsewhere [71]. 3.1 Analysis of Liquid Sulfur by Vibrational Spectroscopy The interpretation and assignment of the vibrational spectra of liquid sulfur and of quenched sulfur melts requires the knowledge of the spectra of the likely components of these mixtures. Therefore, the infrared and Raman spectra of all sulfur allotropes existing at ambient pressure had to be record- ed to collect the data needed to assign the spectra of the melt. It turned out that the vibrational spectra of the various sulfur rings and of polymeric chains are different enough to identify each species by this method in mix- tures, provided the particular concentration is not too low and the composi- tion of the mixture is not too complex. The differences in the spectra result from the increase in the vibrational degrees of freedom with increasing number of atoms in the molecule. But in addition, the molecular symmetry plays a major role since high-symmetry molecules like S6 (D3d), S8 (D4d), and S12 (D3d) give simpler spectra than molecules of similar size such as S7 (Cs) due to degeneracies of some of their vibrational modes. Most of the sulfur rings have either no or only small dipole moments ow- ing to the low or lacking polarity of the sulfur-sulfur bonds. Therefore, infra- red spectra of these species are of low intensity as a result of the selection rule for infrared absorption. In contrast, the Raman scattering intensity of S-S bonds is very strong and Raman spectra are therefore the best technique to study sulfur melts and samples prepared from the melt like p-sulfur and m-sulfur. In Fig. 1 a schematic comparison is made to demonstrate the differ- ences in the Raman spectra of the homocycles with between 6 and 12 atoms. Compounds containing S-S bonds are light sensitive and this holds partic- ular true for solid sulfur allotropes. Since Raman spectroscopy involves the irradiation with a high-intensity laser beam one has to take care that the sample composition is not affected by the irradiation. Therefore, red (kryp- ton ion) or infrared (Nd:YAG) laser lines are recommended and the solid samples should be cooled to at least 100 C whenever possible. The low sample temperature at the same time improves the spectral resolution sincePDF Image | Topics in Current Chemistry
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