Electron Diffraction Theory and Methods

Abstract

Gas-phase electron diffraction is one of the principal techniques of molecular structure determination. A beam of fast electrons is scattered on a molecular beam in the experiment, and the resulting interference pattern is determined by the charge distribution of the molecule. The molecular component of the total electron scattering intensities is extracted from them and, aided by model building, the molecular parameters are determined from the molecular intensities. Fourier transformation of the molecular intensities leads to the so-called radial distribution, which is the probability density distribution of the internuclear distances in the molecule. The results of the structure determination appear as average internuclear distances or, in terms of internal coordinates, as bond lengths, bond angles, and angles of torsion. Gas-phase electron diffraction is an efficient tool of conformational analysis, yielding the geometries of the conformers and their relative abundances. In the best cases, the gas-phase electron diffraction technique yields precisions of a few tenths of a picometer for bond lengths, a few tenths of a degree for bond angles, and a few degrees for angles of torsion.