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Solid Sulfur Allotropes 59 torsion angle positive, while this angle is negative for radical b0. Radical a is the same as b or b0 but neighboring to another radical resulting in a broad absorption line lacking any fine structure or anisotropy due to spin-spin in- teraction. Radical a is therefore the least stable thermally since recombina- tion of the two chain ends can easily occur. This species is obviously the pri- mary product of the homolytic dissociation if the two chain ends are trapped by the cage effect of the surrounding molecules [61]. On warming the concentration of radical a steadily decreases and practically disappears at 210 K. The signals due to radicals b and b0 could be observed up to much higher temperatures and even in irradiated samples stored for some days at 298 K [61]. Radicals b and b0 have also been observed in quenched liquid sulfur (without irradiation) [170d]. Bombardment of thin sulfur films, cooled to low temperatures, by He+ ions of MeV energy (sputtering) results in erosion and loss of material by sublimation [175]. These results are of relevance to the surface properties of Jupiters moon Io large parts of which are covered by elemental sulfur [176] which is constantly bombarded by fast particles from the magnetosphere of nearby Jupiter. 2.4 Analysis Qualitatively, solid sulfur allotropes can best be characterized by Raman spectroscopy which is an extremely sensitive method. Depending on the number of atoms in the molecule and of the molecular symmetry quite dif- ferent spectra are obtained [151]. A quantitative analysis of even complex mixtures of sulfur rings can be achieved by reversed-phase HPLC analysis after dissolution in carbon disulfide and using a UV absorbance detector [49]. This method is also extremely sensitive and only minute amounts of sample are needed [49]. The polymer content is simply determined by weighing of the extraction residue after the solvent has been removed in a vacuum. 3 High-Pressure Allotropes 3.1 Introduction Application of high pressure in the megabar range (up to a few hundred GPa) has become a valuable tool in solid state physics as well as in solid state chemistry. Molecular systems under high pressure are experiencing extreme forces which cause the intermolecular distances to approach values of the in- tramolecular bond distances. As a result, there is an increased overlap be- tween adjacent molecular orbitals which gives rise to a number of conse- quences such as structural changes (towards close packing), electronic tran-PDF Image | Topics in Current Chemistry
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