Self-Diffusion Coefficients and Rotational Correlation Times in Polar Liquids

Self‐diffusion coefficients and rotational correlation times have been measured in several polar liquids by pulsed nuclear magnetic resonance techniques. Diffusion coefficients were measured for HCCl₃, ClC₆H₅, HF, and HCl and relaxation times for HF. The self‐diffusion coefficient of a liquid is related to the rotational correlation time ${\tau }_2$ by means of the quasilattice model of liquids. The rotational diffusion equation and the Stokes–Einstein expression for the rotational diffusion coefficient are inadequate and are respectively replaced by the results of the large‐step random‐walk theory of rotational diffusion and an expression for ${\tau }_2$ based on the quasilattice model. An expression for ${\tau }_2$ , obtained from the dynamical rotational coherence theory, is shown not to apply in the case of the spherical‐top molecule CCl₄ which would be anticipated to have a small friction constant. The correspondence between calculated values of ${\tau }_2$ based on the quasilattice model and the observed rotational correlation times is reasonably good except for liquid HCl for for which the rotational correlation times are anomalously short. The possibility that for short times the molecular rotation in HCl is dynamically coherent is examined and this effect is quantitatively taken into account. Water is also exceptional in that the activation energies for the rotational correlation times of deuterium and oxygen‐17 are larger than the activation energy for proton self‐diffusion. The Stokes–Einstein expression for ${\tau }_2$ is in approximate agreement with the observed values for water and liquid hydrogen fluoride.