Unperturbed dimensions of polycatenasulphur and polycatenaselenium
The evidence in support of a large barrier to internal rotation about the sulphur-sulphur bond is summarized. Each bond in an unperturbed chain of polycatenasulphur may be considered to be in one of two rotational isomeric states symmetrically disposed about the trans state at ϕ≅± 90°. Following analysis of the chain structure, it is proposed that bond rotational states are approximately mutually independent. On this basis, the unperturbed dimensions of polycatenasulphur are calculated to be close to those expected for a freely rotating chain; so that, with a bond angle supplement θ, the characteristic ratio 〈r2〉0/nl2≅(1+cos θ)/(1 – cos θ). The experimental entropy of formation of cyclooctasulphur in liquid sulphur is shown to be compatible with the calculated chain dimensions. An analogous treatment of polycatenaselenium is presented; unperturbed selenium chains should be even more highly coiled than those of sulphur. The dipole moments of some n-alkyl polysulphides are calculated using the rotational isomeric state model of polymeric sulphur, and the stable conformations of catenated sulphur and selenium derivatives, and small sulphur rings, are discussed.