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Solid Sulfur Allotropes Table 22 Densities of solid sulfur allotropes 57 Allotrope S6 g-S7 d-S7 a-S8 b-S8 g-S8 a-S9 S10 S6 S10 S11 S12 S13 S14 a-S18 b-S18 S20 Density (g cm–3) 2.260 2.190 2.182 2.108/2.066 2.088/2.008 2.08 2.114 2.103 2.17 2.08 2.045 2.09 2.045 2.090 2.009 2.023 Temperature (K) 183 163 163 173/298 113/298 298 173 163 163 163 298 173 173 298 298 298 Reference [65] [20] [20] [76] [27, 81] [29] [33] [35] [35] [38] [100] [38] [46] [104] [103] [104] ic results. The comparison shows that S6 has the highest density and there- fore should be thermodynamically favored at high pressures. This is in fact the case; see later. The density of Crystex, the commercial polymeric sulfur, is 1.95 g cm3 [169]. 2.3.5 Photochemical Behavior All sulfur allotropes are light sensitive both in the solid state and in solution. If the energy of the absorbed photons is sufficiently high homolytic cleavage of the sulfur-sulfur bonds will occur and free radicals are formed. The ther- mal bond dissociation energy in liquid sulfur [170] is 150€2 kJ mol1 corre- sponding to a wavelength of 800 nm. However, the extinction coefficient of most sulfur molecules (rings and long chains) is very low at wavelengths >430 nm [171]. Therefore, in most photochemical experiments radiation of shorter wavelengths has been used. The following discussion will be restrict- ed to solid phases. The irradiation of solid sulfur allotropes or of solid solutions of sulfur rings produces not only free radicals but also new absorbing and unstable species the identity of which is however not always certain (for a discussion, see [61]). The properties of the radicals as well as their mode of generation are summarized in Table 23 [172, 173]. In addition to the formation of radi- cals and colored species the irradiation of sulfur induces also luminescence and enhanced electrical conductivity [174]. The three types of radicals observed in irradiated sulfur samples have been interpreted as follows: Radical b is a sulfur chain-end with the terminalPDF Image | Topics in Current Chemistry
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