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 t heoretical evidence that only conformers with a gauche X-OO-X torsion angle are present in the gaseous sample. Of these, only two conformers of symmet ry 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 e nd of the molecule) were found to be plausible components of the gaseous mi xture. 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(-) equa l 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 t hat of model B to DeltaG(0) = G(0)(G(+) G(-)) - G(o)(G(-)G(-)) = -0.29 (sig ma = 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 d o not allow a choice. However, the bond lengths and bond angles for all con formers are similar so that, except for the torsion angles, the parameter v alues for the G-G- form are also reasonably close to those in the other for ms. 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, (an gle (O-S=O))= 1/2[(O-S=O-4)+ angle (O-S=O-5)] = 107.2(12)degrees Delta angl e (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.