Nuclear relaxation and molecular properties Part I. 14N nuclear quadrupolar relaxation and 1H line shapes in nitrogen-containing heterocycles

The origin of the broadening of the N.M.R. signal of the proton(s) in the α position to the nitrogen atom in nitrogen-containing heterocycles (pyridine, thiazole, isothiazole, isoxazole) is examined. The contributions of the various nuclear relaxation mechanisms to the spin-lattice T 1 and spin-spin T 2 relaxation times of H α are considered. It is concluded that the observed broadening arises from incomplete washing out of an N-H α spin-spin coupling by 14 N quadrupolar relaxation. The calculated line-broadenings agree well with the observed line-widths. The effect of temperature changes, solvents, protonation and quaternization is discussed in terms of the 14 N quadrupolar relaxation time. Various 15 N-H couplings over two or three bonds are reported.