Gas electron diffraction (GED) is one of the applications of electron diffraction techniques. The target of this method is the determination of the structure of gaseous molecules, i.e., the geometrical arrangement of the atoms from which a molecule is built up. GED is one of two experimental methods (besides microwave spectroscopy) to determine the structure of free molecules, undistorted by intermolecular forces, which are omnipresent in the solid and liquid state. The determination of accurate molecular structures by GED studies is fundamental for an understanding of structural chemistry. The total scattering intensity is composed of two parts: the atomic scattering intensity and the molecular scattering intensity. The former decreases monotonically and contains no information about the molecular structure. The latter has sinusoidal modulations as a result of the interference of the scattering spherical waves generated by the scattering from the atoms included in the target molecule. The interferences reflect the distributions of the atoms composing the molecules, so the molecular structure is determined from this part.

thumb|Figure 2: Diffraction pattern of gaseous benzene

Experiment

left|thumb|440x440px|Scheme 1: Schematic drawing of an electron diffraction apparatus

left|thumb|440x440px|Scheme 2: Data reduction process from the concentric scattering pattern to the molecular scattering intensity curve

Figure 1 shows a drawing and a photograph of an electron diffraction apparatus. Scheme 1 shows the schematic procedure of an electron diffraction experiment. A fast electron beam is generated in an electron gun, enters a diffraction chamber typically at a vacuum of 10<sup>−7</sup> mbar. The electron beam hits a perpendicular stream of a gaseous sample effusing from a nozzle of a small diameter (typically 0.2&nbsp;mm). At this point, the electrons are scattered. Most of the sample is immediately condensed and frozen onto the surface of a cold trap held at -196&nbsp;°C (liquid nitrogen). The scattered electrons are detected on the surface of a suitable detector in a well-defined distance to the point of scattering.

center|thumb|500x500px|Figure 1: Gas-diffraction apparatus at the University of Bielefeld, Germany

alt=Figure 3: Scheme of a rotating sector, placement of the rotating sector within a GED apparatus and two examples of diffraction pattrens recorded with and without rotating sector.|thumb|440x440px|Figure 3: Scheme of a rotating sector, placement of the rotating sector within a GED apparatus and two examples of diffraction pattrens recorded with and without rotating sector.

The scattering pattern consists of diffuse concentric rings (see Figure 2). The steep decent of intensity can be compensated for by passing the electrons through a fast rotation sector (Figure 3). This is cut in a way, that electrons with small scattering angles are more shadowed than those at wider scattering angles. The detector can be a photographic plate, an electron imaging plate (usual technique today) or other position sensitive devices such as hybrid pixel detectors (future technique).

The intensities generated from reading out the plates or processing intensity data from other detectors are then corrected for the sector effect. They are initially a function of distance between primary beam position and intensity, and then converted into a function of scattering angle. The so-called atomic intensity and the experimental background are subtracted to give the final experimental molecular scattering intensities as a function of s (the change of momentum).

These data are then processed by suitable fitting software like UNEX for refining a suitable model for the compound and to yield precise structural information in terms of bond lengths, angles and torsional angles.

Theory

left|thumb|440x440px|Scheme 2: Schematic scattering process of an electron passing a positively charged atomic nucleus

thumb|440x440px|Firgure 4. Electron wave scattered at a pair of atomic nuclei at different distances

GED can be described by scattering theory. The outcome if applied to gases with randomly oriented molecules is provided here in short:

  • Structure of the planar trisilylamine
  • Determinations of the structures of gaseous elemental phosphorus P<sub>4</sub> and of the binary P<sub>3</sub>As
  • Determination of the structure of C<sub>60</sub> and C<sub>70</sub>
  • Structure of tetranitromethane
  • Absence of local C<sub>3</sub> symmetry in the simplest phosphonium ylide H<sub>2</sub>C=PMe<sub>3</sub> and in amino-phosphanes like P(NMe<sub>2)3</sub> and ylides H<sub>2</sub>C=P(NMe<sub>2</sub>)<sub>3</sub>
  • Determination of intramolecular London dispersion interaction effects on gas-phase and solid-state structures of diamondoid dimers
  • http://molwiki.org/wiki/Main_Page—A free encyclopaedia, mainly focused on molecular structure and dynamics.
  • The story of gas-phase electron diffraction (GED) in Norway

References