The structure of amorphous sulfur

The lambda -transition of elemental sulfur occurring at about 159 degreesC has long been associated with the conversion of cyclic S-8 rings (c-S-8) to amorphous polymer (a-S) via a ring opening polymerization, Here we demonst rate, with the use of both density functional and Hartree-Fock quantum mech anical calculations, the existence of an energetically accessible, locally stable, hypervalent state of S that can form branch sites in the polymer. T he significance of this finding is that the lambda -transition is best desc ribed as a gelation transition, The geometry of the tetravalent S atom is t rigonal bipyramidal, with a lone pair occupying one of the three equatorial sites; it lies in a local energy minimum about 31 kcal/mol above the norma l divalent state, and so is accessible both thermally and photochemically. Because the branched structure is formed endothermically, Le Chatelier's pr inciple confirms that a percolation network can form on heating the element . The reactions that form branched structures are reversible, implying that the gel is fluxional. It decomposes at higher temperatures as chain scissi on competes with branching. The hypervalent structure provides an essential insight into the chemistry of elemental sulfur.