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

Abstract

Electronic structure calculations, carried out at the HFr6-311GU , MP2r6-311G U , and BLYPr6-311G U levels of theory, reveal an unexpectedly large influence of electron correlation on bonding in the octasulfur dication S 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.

The Nature of Transannular Interactions in E4N4 and E82+ (E = S, Se)

“…19 As discussed in the introduction, the HF method underestimates the E⋯E contacts in E4N4 and particularly in E8 2+ , whereas MP2 predicts virtually no interaction between the two chalcogen centers in either system. 25,29,30 The data in Figure 2 shows that the established trends persist also when using these methods together with modern DFT is formally a single determinant method and hence it is natural to discuss the results obtained with different density functionals in the current context. The data in Figure 2 shows a clear trend towards shorter E⋯E contacts when the amount of exact exchange in the functional is increased.…”

“…29 In such a case, the HF determinant is both quantitatively and qualitatively a poor approximation of the true many-electron wave function, which bodes ill for any perturbative treatment of correlation effects. In contrast, the coupled cluster and quadratic configuration interaction are known to give accurate results for many computationally difficult cases provided that at least approximate treatment of triple excitations is employed, i.e.…”

“…29 Moreover, any attempt to include electron correlation effects in either case by means of second order perturbation theory (MP2) leads to significant overestimation of the E⋯E interaction. 25,29 Even density functional theory (DFT) has difficulties in providing a consistent picture of bonding in these systems and the results are highly dependent of the chosen functional and the amount of exact exchange employed. For example, the B3LYP hybrid functional predicts the cross-ring S⋯S distances in dithiatetrazocines rather accurately, 21 but for no obvious reasons it overestimates those in S4N4 and E8 2+ .…”

“…For example, the B3LYP hybrid functional predicts the cross-ring S⋯S distances in dithiatetrazocines rather accurately, 21 but for no obvious reasons it overestimates those in S4N4 and E8 2+ . 25,40 On the other hand, the non-hybrid version of the same functional, namely BLYP, has considerable difficulties in reproducing the transannular interactions in E8 2+ , 29 whereas the MPW91PW91 functional is known to provide highly accurate bond lengths even when combined with very small basis sets. 19 Based on the current theoretical data for 14, it is safe to conclude that a solid physical explanation of their weak cross-ring bonds is still lacking.…”

“…To the best of our knowledge, the only detailed, albeit preliminary, post-HF wave function based investigation of the nature of transannular bonding in any of 14 has been presented by Cioslowski and Gao in 1997. 29 Their analysis of the localized MP2 orbitals of Se8 2+ revealed significant fractional occupancies for the formally doubly filled and empty frontier MOs. Thus, as the authors conclude, the actual wave functions of E8 2+ dications are "expected to possess considerable multiconfigurational character", which implies that their ground states would have a sizable singlet diradical component.…”