DIRECT EVALUATION OF EQUILIBRIUM MOLECULAR GEOMETRIES USING REAL-TIME GAS ELECTRON-DIFFRACTION

A method is developed for the direct evaluation of equilibrium geometries of polyatomic molecules, based on real-time gas electron diffraction and a cumulant expansion for molecular intensities of the scattered electrons. The experimental procedure utilizes multichannel detection for online data collection and reduction. The method opens the possibility to use directly the available spectroscopic information in a way complementary to gas electron diffraction, and vice versa. Temperature-dependent molecular diffraction intensities provide an independent check of the accuracy of various anharmonic molecular force fields derived from high-resolution spectroscopy through the integral effect on the molecular intensity function. In this way the temperature-invariant equilibrium molecular geometries for nonvibronically active systems give an independent test of the applicability of the anharmonic force field. The cumulant coefficients appear as sensitive functions of the approximations introduced in modeling the intramolecular motions. The method is applied to the investigation of sulfur hexafluoride over the temperature range from 298 to 773 K. The effects of higher-order cumulants, of multiple scattering, and of seven different spectroscopic anharmonic force fields have been tested. In O-h symmetry, the equilibrium structure of SF6 obtained in this way yields r(e)(S-F) = 155.68(6) pm. The effects of intramolecular multiple scattering corrections on the derived parameters are discussed.