Pressure dependencies of rotational, translational, and viscous friction coefficients in water-d2, acetonitrile-d3, acetonitrile, chloroform, and benzene

Molecular rotational friction coefficients (ζ) were determined for neat water-d2, neat acetonitrile-d3, neat acetonitrile, a 15% solution of chloroform-d1 in chloroform, and a 3% solution of benzene-d6 in benzene by measuring 2H and 14N nuclear magnetic resonance spin–lattice relaxation times as a function of pressure (0.1–300 MPa). The pressure dependencies of the rotational ζ values were obtained from the single-body rotational correlation times for deuterated molecules in each liquid. The pressure dependencies were compared with those of the translational and viscous ζ values derived, respectively, from the known self-diffusion coefficients and viscosities. In such simple molecular liquids as chloroform and benzene, the translational and viscous ζ values had almost the same pressure coefficient or activation volume, whereas the rotational ζ values had considerably smaller pressure coefficients. The fractional viscosity (η) exponent α in the phenomenological linear relation between ζ and ηα was 0.9 for the translational ζ in acetonitrile and 0.4–0.6 for the rotational ζ in acetonitrile (tumbling motion), chloroform, and benzene. Water was found to be exceptional because the pressure dependence of ζ depended more strongly on the modes of molecular motions. The deviation of the viscosity exponent from unity clearly indicates a breakdown of the Stokes–Einstein–Debye law with respect to pressure variations. The viscosity exponent is not universal, but specific to intermolecular interactions and therefore dependent on the liquid structure.