The Variable Strength of the Sulfur–Sulfur Bond: 78 to 41 kcal – G3, CBS‐Q, and DFT Bond Energies of Sulfur (S 8 ) and Disulfanes XSSX (X = H, F, Cl, CH 3 , CN, NH 2 , OH, SH)

Experimental values for the strength of the sulfur–sulfur bond scatter over a wide range and are frequently in disagreement with each other. For S8, reliable experimental data seem to lack entirely. To check experimental data and establish the strength of the sulfur–sulfur bond in S8, high-precision thermochemical calculations (G3, CBS-Q) and DFT methods were employed. The calculations confirm a stunning range for the strength of the sulfur–sulfur bond with energies between 77.7 kcal for FSSF and only 41.8 kcal for tetrasulfane, HSS–SSH. For the pivotal bond energy of elemental sulfur, S8, the bond energy is 40.5 kcal but can be narrowed to 38.0–39.2 kcal mol–1 by including CBS-QB3 data. While older DFT methods are not well suited to accurately reproduce the S–S bond energies, excellent data can be obtained with the recently introduced Boese–Martin (BMK) hybrid DFT method. Atoms in molecules (AIM) calculations reveal significant multiple bonding and spin delocalization in the sulfur radicals XS·. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)