Effect of Environment on the Orientation of the Nitrate Ions in Crystals

Abstract

A ROTATIONAL or orientational disorder at room temperature in the arrangement of the nitrate ions has been observed in the crystalline lattices of a number of co-ordination compounds^1–4, although no one has so far offered any explanation for this phenomenon. The factors which are known to influence the arrangement of any ionic group in a crystal are the symmetry, the size, and the moment of inertia of the group in question, as well as the nature of packing of various cations and anions. In some cases the charge on the complex cation is completely shielded by neutral groups which surround it, and so the electrostatic interaction with the anionic group is very weak. The net result is that the anion exhibits free rotation. An example of this is provided by the crystal structure of silver perchlorate dioxane complex⁵ in which the perchlorate ion exhibits free rotation.