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Solid Sulfur Allotropes 21 Fig. 3 Structure of the S7 molecules in g-S7 and numbering of atoms; bond lengths in pm The neighboring bonds are shortened in accordance to the common in- terpretation in terms of the bond-alternation concept (see [4, 75], and later). In consequence, the bond lengths of the S7 molecules of the g- and d-al- lotropes can be divided into four sets according to the Cs mirror plane of the heptamer: 205 pm, 210 pm, 200 pm, and one very large bond length of 218 pm (see Fig. 3 and Table 4). This is in sharp contrast to the structures of homocyclic sulfur molecules of higher symmetry and regular motifs such as S6, S8, and S12 which contain bonds of lengths close to 205 pm as well as al- most equal bond angles and torsion angles. The mean strain energy of the S7 molecule is approximately of the same amount as of the S6 molecule [66]. However, the unusual bond S6-S7 is re- sponsible for the very low stability of S7 and, finally, for its high reactivity. 2.2.1.3 Allotropes of S8 The only stable form of sulfur at STP conditions is the well known ortho- rhombic a-S8 modification which was already known in antiquity. No won- der that this allotrope is by far the best studied. Although there is a consid- erable amount of knowledge on the structural and physical as well as chemi- cal properties of a-S8, from the experimental and theoretical point of view there are also ambiguities. For example, the thermal volume expansion be- low 300 K was reported contradictorily [76, 77]. At about 369 K a-S8 transforms to monoclinic b-S8 which is stable up to the melting temperature of about 393 K. The transformation is reversible. One third of the molecules of b-S8 shows a twofold orientational disorder. Other allotropic forms of cyclo-octasulfur have been reported in the past. Most of these, however, are probably mixtures of well known forms or tran- sient species (see [3, 8, 78]). Just one solid allotrope, g-S8, which is quite sta- ble, could be obtained as single crystals allowing its structure to be deter- mined.PDF Image | Topics in Current Chemistry
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