| Abstract
| - The potential energy hypersurfaces (PESs) of heptasulfur (S7) and of [LiS7]+ have been investigated by ab initiomolecular orbital calculations at the G3X(MP2) level of theory. Besides the chairlike seven-membered ring (1a) asthe global minimum structure, eight local minimum structures and one transition state have been located on thePES of S7. The barrier for pseudorotation of 1a is only 5.6 kJ mol-1. The boatlike S7 ring (1b) is 12.1 kJ mol-1 lessstable than 1a, followed by three isomers of connectivity S6S and four open-chain isomers. On the basis ofmultireference calculations at the MRCI(4,4)+Q/6-311G(d) level, the most stable open-chain form of S7 is a tripletof 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 S6 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 variousisomers of S7 form stable complexes with Li+ with coordination numbers of 1−4 for the metal atom and bindingenergies in the range of −93.8 to −165.7 kJ mol-1. A total of 15 isomeric complexes have been located, with 13of them containing cyclic ligands. The global minimum structure (2a) is composed of 1a, with the Li+ cation linkedto the four negatively charged sulfur atoms (symmetry Cs). The corresponding complex 2c containing the ligand 1bis 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 lessstable than 2a. Even less stable are four complexes with the branched S6S ligand. SS bond activation by polarizationof the valence electrons takes place on coordination of Li+ to cyclo-S7 (1a).
- The S7 molecule, most stable as a chairlike ring, exists as many cyclic and chainlike isomers on the potential energy hypersurface. All of them react with lithium cations to complexes with coordination numbers of 1−4 for the metal atom and, in most cases, cluster-type structures.
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