A New Look on Larger Sulfur and Selenium Rings – Dispersion Forces and Shapes of Larger Cycles

In this paper we discuss the shape of sulfur and selenium rings S 6 to S 20 and Se 6 to Se 20 . The sizes and conformations of the sulfur rings are known by X‐ray data. A comparison between the results of B3LYP/cc‐pVTZ, B3LYP‐D3/cc‐pVTZ, and B3LYP‐D3BJ/cc‐pVTZ calculations reveals structural relaxation dispersion energy values of 3.0 kJ/mol (S 10 ) and 19.4 kJ/mol (S 18 ) for the sulfur allotropes. In the case of selenium rings, we investigated rings with the same size. We found that at low temperatures the selenium rings Se 13 , Se 14 , Se 18 and Se 20 should be more stable than Se 8 if the empirical dispersion corrections D3 and D3BJ are taken into account. All larger rings reveal structural relaxation dispersion between 6.0 kJ/mol (Se 10 ) and 34.7 kJ/mol (Se 18 ). Furthermore, the calculations reveal that the consideration of the dispersion energy using the correction terms D3 and D3BJ leads in some cases to a distortion of the molecular structure accompanied by a degradation of the molecular symmetry. In these cases, the transannular distances strongly depend on the method used and the difference sometimes amounts to more than 1 Å. For the structural relaxation dispersion energy, the following applies: A few short noncovalent bonds are better than many medium‐sized ones. The structural distortion is also found in the X‐ray data which stresses the importance of the usage of the dispersion correction terms for the structural investigation of these cycles.