Ultrasonic studies of rotational isomerism in 2,3-dimethyl-butane and 2,2,4-trimethylpentane at elevated pressures

Abstract

Ultrasonic velocity and attenuation have been measured in 2,3-dimethyl-butane and 2,2,4-trimethylpentane from 213 to 243 K and pressures up to 100 MPa. The temperature- and pressure-dependences of the relaxation frequency permit the calculation of the activation enthalpy and the activation volume, respectively. Furthermore, equilibrium parameters are derived from the maximum of absorption per wavelength. Large and positive activation volumes of the order of 10 cm³ mol^–1 are observed, whereas the volume change associated with the rotational isomerism is negative, indicating a preference for the gauche states at elevated pressures.