Nuclear Relaxation of N14 by Quadrupole Interactions in Molecular Liquids

Abstract

Radio-frequency pulse methods have been used to measure the N14 spin-lattice relaxation time in 25 molecular liquids, with the N14 in several types of groups. For eight of the compounds, microwave or pure quadrupole resonance measurements of the N14 quadrupole coupling constant were combined with our T1 results to give values for the correlation time τq describing the molecular reorientations which govern T1. These values for τq at 25°C are about an order of magnitude shorter than reorientational correlation times calculated from the viscosity and molecular radius using the Debye—BPP approach. For the other compounds, τq was estimated and used with the observed T1 for N14 to predict values of the quadrupole coupling constant. The temperature dependence of T1 was observed for nine compounds, leading to activation energies for molecular reorientation of 1.4 to 3.2 kcal/mole and inverse frequency factors τq0 of 2×10—14 to 9×10—14 sec. The temperature dependence of the proton T1 was observed in CH3CN and it is compared with that of N14. It appears that the relaxation of some quadrupolar nuclei affords considerable promise for studying molecular reorientations in liquids and for separating diffusional and rotational processes. These possibilities, as well as that of estimating quadrupole coupling constants not otherwise readily accessible, are discussed.