A PRACTICAL METHOD FOR DIFFRACTION ANALYSIS OF EQUILIBRIUM GEOMETRIESOF MOLECULES WITHOUT REFINED FORCE-FIELDS

An approximate method for diffraction analysis of equilibrium geometri es of molecules which extensively uses dynamical information from comp utational sources is developed. The method obviates refinement of the harmonic force field of a molecule by using the eigenvector matrix obt ained from MOPAC semiempirical calculations of the PM-3 type and spect roscopically observed or scaled MOPAC vibrational frequencies. The fre quency scale factors were adjusted in the course of the analysis of di ffraction data. A simple account for anharmonicity was also made by th e aid of the Morse oscillator potential function. The method was check ed against analysis of diffraction data for the sample molecules CO2, SO2 and SF6. (C) 1997 Elsevier Science B.V.