Intraatomic distance

The strain in four- and three-membered rings (contrary to five-membered systems) hinders the silicon and donor atoms to approach each other when they occupy the P or a position in a linear chain. The observed distortions of tetrahedral silicon environment, however, and intraatomic distances shorter than the sums of van der Waals radii point to the possibility of intramolecular Si<-D coordination in these systems (Table 9). 

Let us briefly explain the rehabihty of such an approach. As the calculations demonstrated the values of Pg-parameters equal numerically (in the range of 2%) the total energy of valence elections U) by the atom statistic model. Using the known correlation between the electron density (P) and intraatomic potential by the atom statistic model [12], we can obtain the direct dependence of Pg-parameter on the electron density at the distance r. from the nucleus. 

Graph 16.8 Variations of intraatomic ( self ) energies with respect to the X...N distance (the origin corresponding to the self energies inside the isolated optimized fragment) for the CI3COX... NH3 complexes... 

In the field-induced ferromagnetically saturated phase at 4.2 K and in an applied field Ha = 120 kOe, 6 =-12.45 0.03. Application of pressure up to 19.6 kbar causes 6 to become less negative at a rate d6/dp = (9 2)x10" mm-s kbar" compared with d6/dp = (9.5 2.0) X10" and (11 2) x 10" mm-s" kbar" in EuS and EuTe, respectively. The volume coefficient changes less remarkably from d5/dlnV = -4.8 mm/s in EuSe to -4.4 mm/s in EuTe. The pressure-induced increase of the electron density at the Eu nuclei was ascribed to an increased intraatomic 4f-5d exchange interaction brought about by the decrease of the interatomic distances, Zaker [14, pp. 41,45, 62], Sauer et al. [11,15]. A remarkable change in d6/dp and a nearly constant volume coefficient were also observed for the Eu monochalcoge-nides in a zero field at 4.2 K and p<87 kbar. However, the effect of pressure on 6 has been ascribed primarily to the compression of the closed inner shells, especially 5s, of the Eu ", Klein et al. [16]. Some details have been reported in the chapter on EuS, Rare Earth Elements C7, 1983, p. 427. 

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