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52 Ralf Steudel · Bodo Eckert of the molecule while, on the other hand, in monoclinic d-S7, orthorhombic S11, monoclinic S13 and monoclinic exo-S18 (b-S18) the asymmetric unit is much larger containing two molecules [73]. With the exception of S18 all rings Sn which form different solid allotropes (S7, S8, S9, S6, S10, S6·S10, S12, S12·CS2) show up with the same molecular conformation. The same is true for the polymeric modifications at STP conditions although one has to con- sider that the polymeric solids may, at least partly, consist of a random coil network. The variety of structures and symmetries both of the molecular and crys- talline forms of sulfur gives rise to significantly different vibrational spectra which, therefore, can serve as a fingerprint for an actual allotropic form [151]. 2.3 Physical Properties 2.3.1 Melting Points None of the sulfur allotropes melts without (some) decomposition [55]. Therefore, the melting points measured depend on the heating rate. As lower the heating rate as lower the melting point observed since the decomposi- tion products lower the melting temperature. Only in the case of S8 a con- stant triple point temperature (115 C) can be measured since the composi- tion of the melt reaches an equilibrium after some time (usually 10 h). In the case of the other cyclic allotropes the melting process yields S8 and S1 as the primary decomposition products from which other ring molecules are then formed. Nevertheless, S7 and S8 show reversible melting tempera- tures if the heating is done quickly enough and if the melt is cooled down immediately before the last crystal has disappeared. With the exception of S8 all sulfur allotropes decompose already at temperatures slightly below the melting temperature if kept at such a temperature for some time. Therefore, the following “melting points” should be taken with caution (for more de- tails see under “Thermal Behavior”): S6 88 C [55] S7 39 C [16, 55] a-S8 115 C [152] (112 C [153], 116 C [55]) b-S8 120 C [152] (119 C [153, 154], 120 C [155], 121 C [55]; triple point: 115 C [48]) g-S8 109 C [152] S9 63 C [33] S10 92 C [55] S6·S10 92 C [35] S11 74 C [36]PDF Image | Topics in Current Chemistry
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