Least-Squares Determination of Structures of Gas Molecules Directly from Electron-Diffraction Intensities

A new least-squares procedure has been devised for interpreting electron-diffraction patterns by a comparison of experimental total intensities with calculated intensities. It differs from all procedures published previously in its direct use of observed quantities in place of the customary ``reduced intensities'' derived subjectively from manually smoothed data. The computer is programmed to select the atomic background function needed to establish the molecular intensity and to refine the background as it refines the desired structural parameters. Operating characteristics of the new procedure are discussed. Results of its application in the structure determination of gaseous B(CH3)3 are presented and compared with results determined by the radial distribution method. Although the new procedure requires somewhat more accurate trial values of molecular parameters for convergence than does the radial distribution analysis, it is intrinsically a faster, and, in some ways, a more powerful tool for deducing optimum molecular parameters.