DIRECT EVALUATION OF EQUILIBRIUM MOLECULAR GEOMETRIES USING REAL-TIMEGAS ELECTRON-DIFFRACTION

A method is developed for the direct evaluation of equilibrium geometr ies of polyatomic molecules, based on real-time gas electron diffracti on 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 possib ility to use directly the available spectroscopic information in a way complementary to gas electron diffraction, and vice versa. Temperatur e-dependent molecular diffraction intensities provide an independent c heck of the accuracy of various anharmonic molecular force fields deri ved from high-resolution spectroscopy through the integral effect on t he molecular intensity function. In this way the temperature-invariant equilibrium molecular geometries for nonvibronically active systems g ive an independent test of the applicability of the anharmonic force f ield. The cumulant coefficients appear as sensitive functions of the a pproximations introduced in modeling the intramolecular motions. The m ethod is applied to the investigation of sulfur hexafluoride over the temperature range from 298 to 773 K. The effects of higher-order cumul ants, of multiple scattering, and of seven different spectroscopic anh armonic force fields have been tested. In O-h symmetry, the equilibriu m structure of SF6 obtained in this way yields r(e)(S-F) = 155.68(6) p m. The effects of intramolecular multiple scattering corrections on th e derived parameters are discussed.