Ion contact distance

It is common to write a = for the ion contact distance. With this... 

Structural analyses of the triiodide ion, in crystals of this ion with various counterions, show that the I3 unit is always linear, or nearly so, and that there is considerable variety in the I I distances, which range from 2.67 A (covalent bond length) to 4.30 A (nonbonded contact distance). Further, there is strong correlation between the two I I distances in the I I I ion. Very similar correlations were obtained for the S-S S grouping in the thia-thiophthenes 37, for the O-H O groupings in a number of hydrogen-bonded... 

The ion at Ba(4) is associated with 8-ring oxygens. This position is located on the plane of an 8-ring but is off its center to enable more favorable approaches to framework oxide ions (Ba(4)-0(1) = 2.88 A, and Ba(4)-0(2) = 2.92 A). These Ba-0 contact distances are again shorter than those in hydrated Bag-A (see Table III). The final Eg value, 0.042, and the unusually featureless final difference Fourier function suggest that little or no water is present in this crystal. 

Crystallographic evidence of the contact ion pairing of sodium and chloride in the presence of the receptor 18 (Figure 18) clearly shows the two ions within contact distance (2.65 A) and interacting with the crown and the cleft respectively. 

Redox ions in solution are subject to chaotic Brownian movement. In principle, a certain range of tunneling distances between the metal and the redox species should be taken into account in a kinetic theory. The tunneling probability decays exponentially with increasing distance between the metal and the redox ion. Only redox ions nearest to the metal surface are, therefore, taken into account. Then, the inner solvation shell of the ion contacts the Helmholtz layer. There is no penetration of the reacting system into the electrochemical double layer (See Section 4.7.2). 

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