AN AB-INITIO MO STUDY OF SELENIUM SULFIDE HETEROCYCLES SENS8-N

The structures and relative stabilities of SenS8-n ring molecules have been studied by the use of ab initio molecular orbital techniques usi ng MIDI-4 basis sets for atomic orbitals. Full geometry optimizations have been carried out for all 30 isomers at HF level of theory, and t he fundamental vibrations have been calculated to ascertain the nature of the stationary point. Each molecule lies at a local minimum and is a crown-shaped eight-membered ring like Ss. The calculated bond param eters indicate single bonds and agree with experimental information wh ere available. The relative stabilities of the different isomers have been calculated at the MP2 level of theory including the correction fo r the zero-point vibrational energy. The total binding energies of the molecular species decrease with increasing selenium content in the mo leules. By contrast, the energies of the different isomers with the sa me chemical compositions are virtually identical. All Se,Ss, species s how very similar valence electronic structures. The energies of the 16 highest occupied molecular orbitals that represent the chalcogen-chal cogen bonding orbitals and the p lone-pair orbitals of the chalcogen a toms are affected very little by the nature of the chalcogen atoms in the molecule. On the other hand, the energies of the next eight molecu lar orbitals that represent the s lone-pair orbitals of the chalcogen atoms increase with the selenium content of the molecule. A selection of interconversion reactions between the different SenS8-n rings demon strates that the energy change in the transformation of one S-S and on e Se-Se bond into two Se-S bonds is very small (ca. 1 kJ mol(-1)) in a greement with the experimental evidence. This small energy difference together with the similarities in the valence electronic structures of the S-S, Se-S and Se-Se bonds is consistent with the observation that most preparative routes lead to a complicated molecular mixture of di fferent selenium sulfides. (C) 1998 Elsevier Science B.V. All rights r eserved.