Nitrogen-15 NMR of pyridine in high magnetic fields

Abstract

The 15N NMR of pyridine was studied in the liquid and solid state. In the liquid the spin-lattice relaxation time T1 was studied from −60 to +55°C at 14 and 30 MHz. At low temperature, the two most important relaxation mechanisms are relaxation due to anisotropic chemical shift and intermolecular dipole-dipole interaction. The small contribution of intermolecular dipole-dipole interaction of 15N with protons was determined by making use of correlation times τc obtained from 13C relaxation rates, which were also measured. At higher temperatures, relaxation by spin-rotation interaction becomes important. Analysis of the relaxation data shows that the anisotropy of the motion in liquid pyridine is rather small. The principal elements of the chemical shift tensor were obtained from powder spectra at −105°C by FT NMR: σxx = −313 ± 10 ppm, σyy = −94 ± 10 ppm, σzz =+469 ± 10 ppm relative to liquid pyridine. From these values the following spin-rotation components were calculated: Cxx = 16.5 ± 1 kHz, Cyy = 11.5 ± 1 kHz, Czz = −0.6 ± 1 kHz. In both cases the z-axis is perpendicular to the molecular plane.