Nuclear Spin Relaxation in Liquid CHFCl2

Abstract

Nuclear spin relaxation times have been measured for liquid CHFCl₂, the values of T_1H and T_1F between 132° and 298°K at 27, 20 and 17 Mc, and T_2H and T_2F over the same temperature range at 20 Mc. The results of these measurements are discussed, and the following relaxation mechanisms are shown to be important: (a) intermolecular dipole—dipole interactions, including their effect upon the electronic, scalar coupling of the proton and fluorine nucleus, (b) the quadrupolar relaxation of the chlorine nuclei which are coupled to the proton and the fluorine nucleus by scalar couplings, and (c) the spin‐rotation interaction between the fluorine nucleus and the reorientation of the molecule. It is noted that relaxation of the same type as mechanism (b) accounts for the relatively large natural linewidths and poorer resolution often found in spectra of heavier nuclei such as F and P, compared to spectra of protons in the same liquid compound. The statistical assumptions of rotational Brownian motion of the molecule, in the liquid, when applied to the spin‐rotation interaction, are found to predict the wrong temperature dependence of T_1F at high temperatures. A transient rotation model is proposed in which the molecules ``jump'' from one orientation to another at random times; the spin‐rotation interaction is assumed to operate during these ``jumps'' when the molecule is actually rotating. The statistical properties of such a model are calculated, and it is shown that T_1F is predicted to have the correct temperature dependence. The model is compared with that developed by Johnson and Waugh for nuclear relaxation by the spin‐rotation interaction in gases. The dipole—dipole and quadrupole interactions are discussed in detail, and a treatment of intermolecular dipole—dipole relaxation by Redfield's method is given, with results indicating that the electronic, scalar coupling of nuclei contributes to the inequality T₂<T₁ often found in liquids.