A treatment of the problem of nuclearhyphen;spin entropy in symmetrical molecules is given. Symmetries of the acceptable nuclearhyphen;rotational states for molecules in the solid are calculated by applying the Pauli principle to the nuclear wavefunctions, and the numbers of such nuclear states are obtained from group theoretical considerations. Expressions for the nuclearhyphen;rotational partition functions and lowhyphen;temperature entropies are given for rapid conversion of nuclearhyphen;spin symmetry species as well as the more usual case of nonconversion. After applying the general theory to ethane and diborane, it is shown that the results obtained for the lowhyphen;temperature entropy depend sensitively upon the degeneracies and spacings of the librational states in the solid. When the symmetry of the potential field for librational motion in solid ethane and diborane is estimated from the known crystal structures, it is found that the librational degeneracies can exist which would give rise to sufficient randomness to cancel a calculated loss in nuclear entropy for the ethanes and to account for the experimental zerohyphen;point entropy in diborane. The implications of this work concerning the nature of molecular motion in these and similar solids at low temperature are briefly discussed.
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