Free Enthalpy Difference Research Articles

Absorption of Ultrasonic Waves in Rotational Isomer Liquids of the Ring Type

The ultrasonic pulse technique has been extended to lower frequencies by the use of solid dielectric transducers and has been used here for the determination of the anomalous absorption of sound in several organic liquids over a frequency range of approximately 150 to 3500 kc/sec. The principal liquid studied, methylcyclohexane, has a large anomalous sound absorption which can be attributed to the irreversible perturbation of an equilibrium between two of its rotational isomers by the sound wave. An experimental evaluation of the temperature dependence of the anomalous absorption per wavelength and the ultrasonic relaxation frequency has yielded the free energy difference between the two isomers and the activation energy barrier between these two states. Values of 1.9 kcal/mole for the free enthalpy difference between the isomers, and 6.4: kcal/mole for the activation enthalpy of the higher energy state, indicate that a monomolecular reaction between the polar substituted and the equatorial substituted chair configurations of methylcyclohexane is responsible for the ultrasonic absorption anomaly. Values for the entropy differences between the low-energy, high-energy, and activated states yield insight into the nature and structure of the activated state. (This research was supported in part by the Office of Naval Research.)