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 τ_q describing the molecular reorientations which govern T₁. These values for τ_q 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, τ_q 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 τ_q⁰ 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.