Structures and Thermodynamics of the Sulfuranes SF 3 CN and SF 2 (CN) 2 as well as of the Persulfurane SF 4 (CN) 2 − An ab initio MO Study by the G3(MP2) Method

At the G3(MP2) level of theory the trans isomer 1a of the hypothetical molecule SF 4 (CN) 2 is more stable than the cis isomer 1b by 8 kJ·mol −1 . The isomerization of 1a to 1b requires an activation enthalpy of 319 kJ·mol −1 at 298 K. The decomposition of trans ‐SF 4 (CN) 2 to SF 2 (CN) 2 and F 2 is endothermic (Δ H o 298 = 395 kJ·mol −1 ) but the elimination of FCN from trans ‐SF 4 (CN) 2 is exothermic by −7 kJ·mol −1 . The elimination of (CN) 2 from cis ‐SF 4 (CN) 2 is exothermic by −137 kJ·mol −1 . The activation enthalpies for the latter two reactions were calculated as 251 and 311 kJ·mol −1 , respectively. Thus, SF 4 (CN) 2 should be a thermally stable compound. In the sulfuranes SF 3 CN and SF 2 (CN) 2 the CN ligands prefer the equatorial positions; mutual exchange of an axial F atom by an equatorial CN group requires a reaction enthalpy of 51 kJ·mol −1 [SF 3 CN] or 58 kJ·mol −1 [SF 2 (CN) 2 ]. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)