Ultrasonic relaxation in the study of rotational isomers

The ultrasonic absorption in a series of unsaturated aldehyde and ketone liquids has been measured over the frequency range 5 to 200 Mc/s. The observed relaxational behaviour is attributed to perturbation by the sound wave of an equilibrium between rotational isomers. In a number of cases the measurements are sufficiently extensive to permit the evaluation of the potential energy barrier restricting internal rotation and of the energy difference between the equilibrium configurations of the molecule in question. In all cases the results can be explained in terms of a single relaxation time. Relaxational behaviour has also been observed in some saturated aldehydes: in such a liquid the relaxation time, $\tau $, is less than in a corresponding unsaturated compound since in the latter conjugation across a single C$\chembond{1,0} $C bond restricts the freedom of rotation about that bond. The relaxational behaviour of solutions of crotonaldehyde in a non-polar solvent (n-hexane) and in a polar solvent (acetone) has been investigated and the characteristic frequency has been found to be approximately independent of concentration. Further, the maximum absorption per wavelength is linearly dependent on the molar concentration. These results confirm the intra-molecular nature of the reaction: the potential barrier to rotation is predominantly determined by forces internal to the molecule and is relatively independent of the force fields due to surrounding solvent molecules. Incipient relaxation has been observed in butadiene; both this and the relaxations found in a number of vinyl ethers are attributed to the existence of rotational isomers.