Ions, central deformability

We can draw conclusions regarding deformation of ions from observations of the diamagnetic susceptibility just as from those of the mole refraction. Thus in the series C03 , N03 and PO, SO4" the experimental values of / show successively greater deviations from the theoretical ones (assuming undeformed 0= ions) with increasing electrical charge of the central ion. ... 

In practical applications of this equation, one must pick values for constant a. To a first approximation it can be regarded as equal to the sum of the radii of two solvated ions. It is not clear, however, whether the solvation sheaths of approaching ions would not be deformed. Moreover, in deriving Eq. (7.43) it was assnmed without sufficient reasoning that the constant a for a given central ion will be the same for different ions present in the ionic atmosphere. 

The radius ratio is considered important because the central ion must be prevented from rattling around in a cavity (see Orgel, 1966). However, the radius ratio is not a rigorous prognosticator, since the concept applies to hard spheres. It has already been noted that ions may be polarizable and deformable, sometimes with a tendency to directional covalent bond formation. These properties affect models based on hard spheres and the extent to which the radius ratio determines the coordination number of a particular ligand. Thus, .. . we can accept the radius ratio rule as a useful, if imperfect, tool in our arsenal for predicting and understanding the behavior of ionic compounds. (Huheey, 1983). 

The IR spectrum of the unsubstituted cydopropenylium ion exhibits only four bands at 3105, 1276, 908 and 736 cm , as expected from the Djij-symmetry. The band at 1276 cm is due to an E ring deformation vibration of the central three-membered ring, which is generally observed as a strong band in the range of 1400 to 1490 cm for the aryl- and alkyl-substituted derivatives (Table 2). The frequency of this band for the trimethyl-, tri-zc-rt-butyl, triphcnyl, and unsubstituted cydopropenylium ions exhibits a linear correlation with the index of the cation s thermodynamic stability such as For heteroatom-substituted derivatives... 

Another contribution has been added by Onsager and Fuoss to their original expression for the relaxation field. Since the ionic atmosphere is deformed by the displacement of the central ion, there are more anions (cations) behind a central cation (anion) than in front of it. Hence collisions from behind a central cation (anion) are more frequent than in front of it. This will result in an increased velocity of the central ion. The calculated effect has a minor effect on the conductance, contributing only terms linear in the concentration. Valleau has cast doubts on the reality of this effect. 

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