Transannular interactions in S82+ and Se82+: Reality or artifact?

Abstract

Electronic structure calculations, carried out at the HF/6-311G*, MP2/6-311G*, and BLYP/6-311G* levels of theory, reveal an unexpectedly large influence of electron correlation on bonding in the octasulfur dication S82+. Whereas a structure with a strong transannular bond linking two sulfur atoms with localized positive charges is predicted within the Hartree–Fock (HF) approximation, second-order Møller-Plesset (MP2) and Becke-Lee-Yang-Parr density functional (BLYP) calculations produce 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 Se82+. In agreement with the experimental data, the endo–exo conformers are found to possess the lowest energies for the octachalcogen dications, in contrast to the corresponding neutral species in which steric interactions determine the relative energies, favoring the exo–exo ring conformations. None of the three theoretical approaches is capable of reproducing the experimental solid-state geometries of S82+ and Se82+, which are almost certainly distorted to a large degree by the crystal packing and counterion effects. The results of the present study underscore the need for a concerted experimental and theoretical effort to provide the definitive answer to the question of transannular interactions in chalcogen compounds. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 609–616, 1997