Novel species for the sulfur zoo:: isomers of S8

Ab initio calculations at the G3X(MP2) level have been employed to examine the structures and energies of S-8 and H2S6 isomers. A cluster structure with a spiraling chain of 8 atoms (C-2 symmetry) lies close in energy (33 U mol(-1)) to the eight-membered crown-S-8 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 pi*-pi* bond between the two pi* orbitals of the chain-end groups. A low-energy cluster-type structure is also predicted for hexasulfane. (C) 2002 Elsevier Science B.V. All rights reserved.