Absorption of Ultrasonic Waves in Pure Methylcyclohexane

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Abstract
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 methylcyclohexane over a frequency range of approximately 150 to 3500 kc/sec. The 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. The theoretical relationship between anomalous ultrasonic absorption and irreversibly perturbed monomolecular equilibria is used in conjunction with an experimental evaluation of the temperature dependance of the anomalous absorption per wavelength and the ultrasonic relaxation frequency to obtain the free energy difference between the two isomers and the activation energy barrier between these two states. Values of 1.9 kcal per mole for the free enthalpy difference between the isomers, and 6.4 kcal per 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.

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