Method for Separating the Inter- and Intramolecular Contributions to T1 in Liquids When the Pressure Varies. Rotational Correlation Time vs Pressure

Abstract

It is shown that to separate the intramolecular and intermolecular contributions to the spin—lattice relaxation time T₁ in liquids measured as a function of pressure and/or temperature, it is only necessary to perform this separation at a single pressure and temperature if the self‐diffusion coefficient D and density are also known as a function of pressure and/or temperature. T₁ may be separated by solution measurements in which the liquid under study is dissolved in a solvent identical to it but magnetically inert. The intramolecular part of T₁ and, hence, the intermolecular part also are obtained by extrapolation to infinite dilution. Such solution measurements would be cumbersome to perform as a function of pressure and/or temperature. It is argued that the results of such a separation may be extended to other pressures and temperatures if appropriate self‐diffusion and density data are available. To examine the usefulness of this idea, the pressure dependence of the rotational correlation time τ_R for benzene is derived from data already in the literature. Cyclohexane (assumed to be a rigid molecule) is similarly treated except that the separation data were not available and had to be determined. The behavior of τ_R and D as a function of pressure is compared to the behavior of the viscosity and also to the predictions based on the theories of Debye and Hill. The discrepancies which are observed are discussed in terms of molecular shape and internal reorientation.