SiOSi angle

The deformation density peaks in the Si—O bonds of coesite are located away from the midpoint of the bond towards the oxygen atom, and also tend to be displaced towards the interior of the SiOSi angle. Downs reports a bending of the... 

Si—O bonds of 9° and 14° relative to the 137.22° SiOSi angle, based on the peaks in the deformation density. This would indicate a bending of the Si—O bonds due to O O nonbonded repulsion, like the bending of the bonds in small ring compounds described in the following chapter. However, according to the... 

The experimental deformation density features, including the displacement of the bond peaks from the bond axes, are well reproduced in theoretical maps on the molecule H6Si207, which contains two linked Si04 tetrahedra in a doubly eclipsed configuration. The results for different values of the central SiOSi angle are shown in Fig. 11.4. 

Si(Pc)(OSiMe3)2 was analyzed by NMR and the calculated SiOSi angle of 162° in solution is in close agreement with the solid state angle 157° determined by X-ray analysis. The short Si—O distance (1.68 A) reflects the strength of this bond. 

Tables I and II summarize the results found for the silicate ring clusters. Table I,A presents the computed total energies per SiO(OH)2 unit. Bond lengths and bond angles in the rings were optimized within the constraint of Cnv symmetry. As can be seen in Table I,A the energy difference between the 3-ring and the other rings is considerable. However, the energy differences per unit SiO(OH)2 of the 4-, 5— and 6-rings are computed to be within 1 kJ. Computed SiO distances and SiOSi angles are presented in Table II.

Structural parameters of the majority of molecules containing Si—O and Si—N bonds differ greatly from those of the carbon analogs. For silicon compounds enhanced values of the SiOSi angle (130-180°) and the tendency to a planar configuration of nitrogen bonds in the NSij grouping are typical Silicon bonds with electro-... 

In cyclic siloxanes, the SiOSi angle is determined by the size of the ring system. 

The decrease in the SiOSi angle in trimeric (OSiMe2)3 from its optimum value of about 145°, as well as the nearly planar geometry of the cycle, probably are related to the strained character of the ring. Similar effects operate in the case of phosphazenes (see above). 

The POP angles from yield an average value of 140° for the PF2OPF2 molecule, which is less than the SiOSi angle in (SiF3)20,156°. Polycyclic molecules of phosphorus oxides, which can be regarded as systems of fused 6-membered cycles, yield POP angles of about 124—126°. 

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