Dipole valency angles from

Tabu XCVJI. Valency Angles from Dipole Moment Data... 

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,... 

Figure Bl.6.12 Ionization-energy spectrum of carbonyl sulphide obtained by dipole (e, 2e) spectroscopy [18], The incident-electron energy was 3.5 keV, the scattered incident electron was detected in the forward direction and the ejected (ionized) electron detected in coincidence at 54.7° (angular anisotropies cancel at this magic angle ). The energy of the two outgoing electrons was scaimed keeping the net energy loss fixed at 40 eV so that the spectrum is essentially identical to the 40 eV photoabsorption spectrum. Peaks are identified with ionization of valence electrons from the indicated molecular orbitals.

SH, SGH3, GOOR, CHO. Fuchs and WoLF have calculated from the dipole moment data the values of the valency angles Table XCVII),... 

The force constant of the Si—Si bond has been calculated as 1.3 x 10 dynes cm however, it is believed that this value is probably in error (f04). The dipole moments of several aromatic disilanes have been reported (2) which allowed one to calculate an aryl—Si—aryl valence angle of 115°. The dipole moment of 1,2-dichlorotetramethyldisilane was found (105) to be 1.75 debye in carbon tetrachloride and 1.35 debye in benzene. On the basis of the dipole moments, the infrared and the Raman spectra (in the gas, liquid, and solid state), information on the rotation about the Si—Si axis in 1,2-dichlorotetramethyldisilane was obtained. In the solid state, the chlorine atoms assume the tram position, whereas in the liquid and gas state the molecule exerts torsional oscillations about the Si—Si axis to a certain extent. The phase transformations of hexamethyldisilane were studied by NMR (80) and thermodynamically by means of differential thermal analysis (25). From such studies it appears that at higher temperatures rotations about both the Si—Si and Si—CH3 axes occur in combination with the overall molecular rotation about the molecular axis, whereas at lower temperatures all movements are hindered except for the Si—CH3 axial rotation. 

The dipole method has proved particularly useful, apart from valence angle determination, in investigating cis- rans-isomerism, because it allows of a simple and satisfactory differentiation between the various modifications. As shown in Table 15, the moments of the rans-dihalogenated ethylenes are practically always nil, while the ctVeompounds are definitely polar. Similar conditions hold for other isomerisms of analogous nature. 

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