SiH4 dipole moment

It is possible to investigate some more reactions, more likely to lead to HSiN [99]. They involve either the reactions of the silyl radical SiHa onto N" or the formation of a complex between N and the silyl cation SiH. We should mention that neither SiHa (a C-io species whose electronic dipole moment is computed to be 0.17 D at the MP2 level of calculation) nor SiH (a Vn, species having a vanishing electronic dipole moment for symmetry reasons) have been observed in the interstellar medium, while SiH4 is perfectly identified. 

In Tables 8.4 and 8.5 the dipole moments and Mulliken atomic charges found in the all-electron HF calculations are compared with those obtained with the use of a pseudosilicon atom and a hydrogen atom to saturate the bond. The disagreement between experimental and HF data is explained by the neglect of the correlation effects, and in the case of a SiHsI molecule the neglect of relativistic effects. The analysis of Table 8.4 allows us to conclude that the pseudosilicon potential generated for one molecule (SiH4) to represent the SiHs radical could be employed in calculations of other molecules (SiHsX, X = F, Cl, Br,I) with the same radical. 

In spherical tops the x, y and z axes are related to each other by a set of four 3-fold symmetry operations. Consequently, all three principal moments of inertia are equal, so there is only one rotation constant, B. This arises only in molecules with tetrahedral, octahedral, cubic or higher symmetry, such as SiH4 (7.1, symmetry) and SF5 (7.II, 0 symmetry). Spherical tops do not have a dipole moment, so they cannot be observed directly in a pure rotational spectrum. 

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