An ab initio MO study of the gas-phase reactions 2 SF2 → FS-SF3 → S=SF4 - Molecular structures, reaction enthalpies and activation energies

Abstract

High level ab initio MO calculations at the G3(MP2) level of theory were employed to study the molecular structures of SF2, FSSF3, and SSF4, as well as the dimerization of gaseous SF2 to FSSF3 and the isomerization of FSSF3 to SSF4. The dimerization of SF2 was calculated to be an exothermic process (ΔH°298 = -77 kJ·mol-1) with an activation enthalpy (ΔH≠ 298) of 65 kJ·mol-1. The transition state of the dimerization reaction is characterized by a bridging fluorine atom that undergoes a 1,2-shift, and a weak sulfur-sulfur single bond. The calculated lowest-energy structure of FSSF3 (2a) is in excellent agreement with the experimentally derived structure, with the -SF group in an equatorial position of the distorted pseudo-trigonal-bipyramid at the central sulfur atom. A significantly less stable FSSF3 conformer 2b, 76 kJ·mol-1 higher in energy, has the -SF group in an axial position. Unimolecular rearrangement of FSSF3 (2a) to the trigonal-bipyramidal SSF4 (3), by a 1,2-fluorine shift (TS2), is endothermic by 37 kJ·mol-1 and is inhibited by a large activation barrier of 267 kJ·mol-1. SSF4 is predicted to be an observable species in the gas phase. Calculated infrared spectra of 2a and 3 are also reported.