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High-level ab initio molecular orbital calculations were employed to explore the potential energy hypersurface of hexasulfur, S6. Twelve isomeric structures of S6 have been identified: two unbranched rings (chair and boat), one trigonal prism of D3h symmetry, two singly branched rings (S5==S), three triplet chains, one singlet chain, and three doubly branched rings (S==S4==S). The prism structure is essentially a cluster of three S2 molecules connected via a six-center π*-π*-π* interaction. It is by 51 kJ mol-1 less stable than the lowest-energy chair form. The reactions to generate the boat, the prism, and the singly branched isomers from the chair form are predicted to have lower barriers than the ring opening reaction of cyclo-S6, which requires an activation energy of 149 kJ mol-1. The prism and singly branched isomers are found to be more reactive species than the chair form and they are potential sources of S2 in chemical reactions involving elemental sulfur. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ab initio calculations at the G3X(MP2) level have been employed to examine the structures and energies of S8 and H2S6 isomers. A cluster structure with a spiraling chain of 8 atoms (C2 symmetry) lies close in energy (33 kJ mol−1) to the eight-membered crown-S8 global energy minimum. This cluster species is characterized by two three-coordinate atoms and a rectangular arrangement of four sulfur atoms at the formal chain-ends. This unusual cluster geometry can be rationalized in terms of a weak π*–π* bond between the two π* orbitals of the chain-end groups. A low-energy cluster-type structure is also predicted for hexasulfane. [Copyright &y& Elsevier]