Ultrasonic Studies of Saturated Hydrocarbons at Low Temperatures

Ultrasonic propagation properties of 2-methylbutane, 3-methylpentane, n-pentane, 2,3-dimethylbutane, and 2,2-dimethylbutane were studied using a pulse method in the range from 1 mc to 35 mc and from 130°K to 280°K. These studies revealed the existence of an attenuation peak in 2-methylbutane, in 2,3-dimethylbutane, and in 3-methylpentane. These peaks were found to be consistent with a single relaxation process. Activation energies for the relaxation process in 2-methylbutane and 3-methylpentane were found to be about 4.7 kcal/M, and about 3.3 kcal/M in 2,3-dimethylbutane. No absorption was observed in n-pentane and 2,2-dimethylbutane. In a mixture of 3-methylpentane and n-pentane, measurements at 5 mc showed an absorption peak at the same temperature as in 3-methylpentane, reduced in amplitude in proportion to the reduction in the number of molecules of 3-methylpentane. These observations indicate that the mechanism of absorption is explainable in terms of a hindered rotation (about the middle C–C bond in butanes and about the 2nd and 3rd C–C bonds in 3-methylpentane), leading to potential minima of unequal values for the different rotational isomeric states. Such asymmetry in the different rotational isomers appears to be an essential requirement for ultrasonic absorption in these molecules.