Valency angles from electron diffraction measurement

DIPOLE MOMENTS AND MOLECULAR STRUCTURE In a number of cases the magnitude of the dipole moment may be used to determine the geometrical configuration of the atoms in a molecule. Thus the first proof of the triangular structure of water arose from dipole moment measurements. Calculation may also be made of the valency angles from dipole moment data, but the values obtained are not as accurate as the more direct methods employing spectra, x-ray and electron diffraction measurements. However, for complex molecules when the application of more exact methods is not possible, the analysis of the dipole moment data may give valuable information. Thus the moment of thianthrene is 1-57, D,... 

In Fig. 23.12, the experimental UPS He I and He II valence band spectra of PPV prepared by the tetrahy-drothiophenium precursor route are compared with the DOVS as calculated from the VEH band structure [45J. XPS core-level spectroscopy (not shown) indicates a complete conversion of the precursor polymer. The corresponding band structure is shown in Fig. 23.12b. The energy scale is fixed relative to the experimental Fermi level. The PPV is taken to be planar in the VEH calculations used here, since neutron diffraction measurements on oriented PPV at room temperature have shown that the ring torsion angles, i.e., the twist of the phenyl rings out of the vinylene plane, are on the order of T 6° [71). Such small torsion angles result in negligible effects on the calculated electronic band structure compared with that for the fully coplanar conformation. 

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