TRANSANNULAR INTERACTIONS IN S-8(2-8(2+) - REALITY OR ARTIFACT() AND SE)

Electronic structure calculations, carried out at the HF/6-311G, MP2/ 6-311G, and BLYP/G-311G* levels of theory, reveal an unexpectedly lar ge influence of electron correlation on bonding in the octasulfur dica tion S-8(2+). Whereas a structure with a strong transannular bond link ing two sulfur atoms with localized positive charges is predicted with in the :Hartree-Fock (HF) approximation, second-order Moller-Plesset ( MP2) and Becke-Lee-Yang-Parr density functional (BLYP) calculations pr oduce delocalized dications in which the transannular interactions are completely missing. Rigorous analysis of the computed electronic wave functions explains this phenomenon, which is also observed in the HF/ TZVP and MP2/TZVP optimized geometries of Se-8(2+). In agreement with the experimental data, the endo-exo conformers are found to possess th e lowest energies for the octachalcogen dications, in contrast to the corresponding neutral species in which steric interactions determine t he relative energies, favoring the exo-exo ring conformations. None of the three theoretical approaches is capable of reproducing the experi mental solid-state geometries of S-8(2+) and Se-8(2+), whichare almost certainly distorted to a large degree by the crystal packing and coun terion effects. The results of the present study underscore the need f or a concerted experimental and theoretical effort to provide the defi nitive answer to the question of transannular interactions in chalcoge n compounds. (C) 1997 John Wiley & Sons, Inc.