The Variable Strength of the Sulfur-Sulfur Bond: 78 to 41 kcal-G3, CBS-Q, and DFT Bond Energies of Sulfur (S8) and Disulfanes XSSX (X = H, F, Cl, CH3,CN, NH2,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 S-8, reliable experimental data seem to lack entirely. To check experimental data and establish the strength of the sulfur-sulfur bond in S-8, 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, S-8, 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 center dot. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)