PENNING IONIZATION ELECTRON-SPECTROSCOPY OF ORGANIC-MOLECULES - STEREOCHEMISTRY OF MOLECULAR-ORBITALS

In Penning ionization electron spectroscopy, the kinetic energy of electrons ejected by collisions between targets (gas or solid) and raetastable atoms is analyzed. In a gas phase spectrum, an orbital exposed outside the repulsive molecular surface gives a stronger band than does an inner orbital localized inside the surface. Thus, the relative band intensity of the spectrum depends on the spatial electron distribution of individual molecular orbitals. Furthermore, with the introduction of a bulky group some molecular orbitals can be protected from the impact of metastable atoms and become less reactive to the metastables. Therefore, the analysis of the relative band intensity also provides information on the steric shielding effect on the molecular orbital. When Penning ionization electron spectroscopy is applied to the solid, the electron distribution of individual molecular orbitals exposed outside the outermost surface layer is selectively probed. In this case, we can see the shape of the molecular orbital from various directions, if we can control the orientation of surface molecules. The characteristics of Penning ionization electron spectroscopy stated above have been applied to the study of stereochemical properties of various organic molecules. © 1990, IUPAC.