Isomers of cyclo-heptasulfur and their coordination to Li+: An ab initio molecular orbital study

Abstract

The potential energy hypersurfaces (PESs) of heptasulfur (S7) and of [LiS7]+ have been investigated by ab initio molecular orbital calculations at the G3X(MP2) level of theory. Besides the chairlike seven-membered ring (1a) as the global minimum structure, eight local minimum structures and one transition state have been located on the PES of S7. The barrier for pseudorotation of 1 a is only 5.6 kJ mol -1. The boatlike S7 ring (1b) is 12.1 kJ mol-1 less stable than 1a, followed by three isomers of connectivity S6=S and four open-chain isomers. On the basis of multireference calculations at the MRCI(4,4)+Q/6-311G(d) level, the most stable open-chain form of S7 is a triplet of relative energy 133.1 kJ mol-1. Thus, the reaction energy (ΔE0) for the ring opening of 1a is 133.1 kJ mol -1, halfway between those of the highly symmetrical rings S 6 and S8. Because of their strong multireference characters, the stabilities of the biradicalic singlet chains are significantly overestimated by the single-reference-based G3X-(MP2) method. The calculated vibrational spectrum of 1a is in good agreement with experimental data. The various isomers of S7 form stable complexes with Li+ with coordination numbers of 1-4 for the metal atom and binding energies in the range of -93.8 to -165.7 kJ mol-1. A total of 15 isomeric complexes have been located, with 13 of them containing cyclic ligands. The global minimum structure (2a) is composed of 1a, with the Li+ cation linked to the four negatively charged sulfur atoms (symmetry Cs). The corresponding complex 2c containing the ligand 1b is by 23.4 kJ mol-1 less stable than 2a, and a bicyclic crown-shaped LiS7 cation (2e) is by 34.9 kJ mol-1 less stable than 2a. Even less stable are four complexes with the branched S6=S ligand. SS bond activation by polarization of the valence electrons takes place on coordination of Li+ to cycto-S 7 (1a). © 2005 American Chemical Society.