Valency angle tetrachloride

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. 

Table 13.1. Gaseous dichlorides of the Group 14 elements E-Cl bond distances, R mean bond energies, MBE and valence angles, ZCIECI. Gaseous tetrachlorides of the Group 14 elements E-Cl bond distances and mean bond energies.

The enthalpy of formation of crystalline imidazole is 14.6 0.8 kcal mole-1 its heat of sublimation is 16.0 1.0 kcal mole-1, and the heat of formation of gaseous imidazole is 30.6 1.8 kcal mole-1.291 From these data the resonance energy of imidazole has been calculated as 14.2 kcal mole-1.291,292 From an infrared examination of the association of 4-methylimidazole in carbon tetrachloride and in 1,1,2,2-tetrachloroethane,293 the concentration dependence of the maximum extinction coefficient of the free NH valency band allowed determination of the monomer content, the mean viscosity, and the equilibrium constants K12 and Kl >. From the temperature dependence of these values the mean heat of addition (8.2 0.5 kcal mole-1) and the heats of formation of the dimers and trimers (10.2 and 8.1 kcal mole-1, respectively) were obtained. Although the mean heat of addition was found to be about 3 kcal mole-1 higher than that of Zimmermann203 this is not contradictory to the assumption that the imidazoles form chainlike associations with angled structures. 

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