Molecular structure and conformation of peroxydisulfuryl difluoride, S2O6F2, as determined by gas-phase electron diffraction and ab initio molecular orbital calculations

The structure and conformational composition of peroxydisulfuryl difluoride, S2O6F2, have been investigated by gas-phase electron diffraction. Some of the results from ab initio molecular orbital calculations (MP2/6-311 + G*) were used as constraints in the analysis. There is both experimental and theoretical evidence that only conformers with a gauche X-OO-X torsion angle are present in the gaseous sample. Of these, only two conformers of symmetry C-2 (G(+)G(+) and G(-)G(-)) and one of symmetry C-1 (G(+)G(-)), where G(+/-) designates positive/negative rotation around the O-S bonds at either end of the molecule) were found to be plausible components of the gaseous mixture. In experiments at 298 K, about equally good fits were found with two models, model A consisting of a ratio of G(-)G(-)/G(+)G(+) equal to 57/43 (2 sigma = 27), and model B consisting of a ratio of G(-)G(-)/G(+)G(-) equal to 38/62 (2 sigma = 14). The composition of model A corresponds to DeltaG(0) = G(0)(G(+)G(+)) = G(0)(G(-)G(-)) = 0.17 (sigma = 0.24) kcal/mol, and that of model B to DeltaG(0) = G(0)(G(+) G(-)) - G(o)(G(-)G(-)) = -0.29 (sigma = 0.12). A system composed of only C2 symmetry conformers, such as model A, is favored by IR and Raman data, but our electron-diffraction results do not allow a choice. However, the bond lengths and bond angles for all conformers are similar so that, except for the torsion angles, the parameter values for the G-G- form are also reasonably close to those in the other forms. These distances (r(g) (Angstrom)) and angles (angle (alpha) (degrees)) values with 2 sigma uncertainties are r(S=O)= 1.406(2) Angstrom, r(S-F)= 1.539(3) Angstrom, r(O-S) = 1.620(4) Angstrom, r(O-O)=1.453(14) Angstrom, (angle (O-S=O))= 1/2[(O-S=O-4)+ angle (O-S=O-5)] = 107.2(12)degrees Delta angle (O-S=O)= angle (O-S=O-4) - angle (O-S=O-9)= 8.10 (assumed), angle (O-S-F)= 98.2(18)degrees, angle (O-O-S)= 108.6(10)degrees angle (O=S=O)= 126.7(15)degrees, and angle (S-O-O-S)= 122.7(56)degrees. (C) 2001 Elsevier Science B.V. All rights reserved.