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It was at one time rather widely believed that certain ions and molecules could rotate virtually freely in the lattice containing them. It now appears that such almost free rotation, in the literal sense, is probably rare. The solids which have been most thoroughly investigated are ammonium chloride, bromide, and iodide. It has been established beyond doubt that in the simple cubic forms of these salts the ammonium ions do not rotate, being prevented from doing so by energy barriers of 6410, 4645 and 4100 callg-ion, respec­ tively-. It is to be noted, however, that there is a considerable decrease in this energy barrier as the lattice dimensions increase, and the possibility must still be considered that in ammonium salts where the anion is large, symmetrical, and not too highly charged, the barrier is small enough for the ammonium ion to undergo something approaching free rotation at room temperature. Another circumstance which will be favourable to free rotation is if the disposition of the anions about the ammonium ion is such as to give only a slight energetic preference for one orientation rather than another. This happens, for example, in the high-temperature face-centred cubic form of ammonium iodide, where the tetrahedral cation is surrounded octahedrally by six anions. Each ammonium ion directs one N-H bond towards one of the anions. The barrier to one-dimensional rotation about this bond is, however, then so small ( < 100 cal/g-icn) that the ammonium ions must at room temperature undergo almost free rotation about oneN-H bond>. With these considerations in mind we have studied the motion of the ammonium ion in several selected ammonium salts by attempting to estimate the contribution made to the heat capacity C p of the salts by the rotational or torsional movements of the ammonium ions. All the salts investigated have isomorphous rubidium salts, and the lattice dimensions of the ammo­ nium and rubidium salts differ by less than 1 per cent. It is therefore reasonable to supposethat,for a given pair ofsuch salts, the quantity(Cp -Cv) and the contribution to C p from the torsional oscillations of the anion will be almost equal. Consequently, at temperatures high enough for the lattice vibrations to make effectively their maximum and hence identical contri­ butions to Cp , the quantity ilCp = C p , N H 4 salt - Cp,Rbsalt - Cint should give the contribution made by the torsional oscillations (or rotations) of the ammonium ions (Ci nt is the relatively small contribution from the internal vibrations of the ammonium ion). We may distinguish, for convenience, between three possibilities for ilC p (expressed per g-ion) as a function of temperature.

Ionic and molecular rotation in ammonium salts and clathrate compounds