Ordering transitions in symmetrical molecules

The transitions occurring in the compounds C(CH/sub 2/X0/sub 4/; where X is OH, F, Cl, Br, or I; and in adamantane are studied in relation to their low temperature thermodynamic properties. The transitions which result from an orientation of the molecule within the crystal structure are described as being pseudo-rotational. Consequently, in the pseudo-rotational transitions, the entropy associated with the change occurring in the symmetry features of the structures, both molecular and crystallographic, should be in agreement with the entropy calculated statistically from the change in the number of symmetry features involved in the transitions. This hypothesis is discussed in terms of experimental results. A comparison of these transitions with the transitions in neopentane is also presented. (N.W.R.)