Molecular orbital studies of small molecules containing sulfur and selenium

Abstract

The bonding and allotropy of chalcogen species, and molecular orbital methods are briefly discussed. The molecular valence method is applied to small sulfur clusters S2 to S5 in order to test the applicability of the method. The double zeta basis set augmented with polarization functions (DZP) is needed to reproduce the structural parameters calculated by ab initio all-electron MO methods or found by experiments. For a given molecular species the energy differences between isomers agree well with those obtained by the use of HF all-electron calculations at the corresponding level of theory. Pseudorotation in cyclo-S6 and cyclo-S7 is studied using the ab initio all-electron MO method. The energetics is corrected for electron correlation at the MP2 level of theory. Hexasulfur is rigid, but heptasulfur undergoes a facile pseudorotation. Results are in agreement with the experimental findings. The energetics of the various suggested chalcogen interconversion pathways is studied by ab initio MO techniques using suitable model reactions involving hypervalent chalcogen hydrides H2EE and H2E(EH)2 as well as the radicals HE' and HEE' (E = S or Se). The formation of hypervalent interconversion intermediates is seen as an energetically plausible alternative to the homolytic cleavage of the chalcogen-chalcogen bonds. The present calculations provide support for the previous observations that in the case of heterocyclic selenium sulfides the interconversion proceeds with a selenium-atom transfer rather than with a sulfur-atom transfer. Geometries, energies and fundamental frequencies of selenium-sulfur nitride six-membered ring molecules SenS4-nN2 (n = 0 - 4) and eight-membered ring molecules SenS4-nN4 (n = 0 - 4) are calculated, and the bonding and the relative stabilities of the molecules are discussed. Calculated energies show that it should be possible to synthetize all members of the both series. Calculated fundamental frequencies are in fair agreement with the observed experimental vibrational spectra where these are available. It seems reasonable to use calculated frequencies to assist the experimental characterization of those molecules that are not yet fully investigated.