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Ultrasonic velocity and attenuation have been measured in 2‐methylbutane from 203 to 253 K and pressures up to 100 MPa. P,V,T data are determined in the temperature range from 210 to 290 K up to pressure of 300 MPa. Both activation and equilibrium parameters are derived from the temperature and pressure dependences of the relaxation frequency and the maximum of absorption. P,V,T data enable us to treat the rotational isomerism under isochoric conditions. Activation volumes of the order 4–8 cm3 mol−1 are observed, increasing with increasing temperature. The isochoric activation energy is of the order of 15–18 kJ mol−1, decreasing with increasing density. This finding is in reasonable accordance with the negative volume change associated with the rotational isomerism.

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