Ultrasonic Relaxation Due to trans—gauche Rotational Isomerism in the Liquid Normal Paraffins

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Abstract
The ultrasonic absorption has been measured in liquid normal butane, pentane, hexane, and heptane at frequencies of 5, 15, and 45 Mc/sec over the temperature range from their freezing to boiling points at atmospheric pressure. The absorption was also measured in propane and two mixtures of pentane with propane at a frequency of 45 Mc/sec for temperatures in the range −130° to −50°C. The measurements indicate internal rotation about internal C–C bonds to be the relaxation mechanism and a barrier for the conversion from a gauche to a trans configuration of 3.4±0.4 kcal/mole for n‐butane, 4.2±0.4 kcal/mole for pentane, 3.1±0.4 kcal/mole for hexane, and 3.8±0.4 kcal/mole for heptane. They also indicate a difference between the molar volume of gauche and trans isomers of butane which is less than 0.3% and a value for the ratio of the compressional to shear viscosities caused by molecular rearrangement of 1.1 for propane, 6.3 for pentane, 6.2 for hexane, and 5.7 for heptane.