Nitrogen-14 quadrupole cross-relaxation spectroscopy

Cross-relaxation spectroscopy can be used as a sensitive method of detecting 14 N quadrupole-resonance signals in hydrogen-containing solids. The 1 H spin system is polarized in a high magnetic field that is then reduced adiabatically to a much lower value satisfying the level­-crossing condition, when the 1 H Zeeman splitting matches one of the 14 N quadrupole splittings. If the 14 N spin–lattice relaxation time is much shorter than that of the 1 H nuclei, a drastic loss of 1 H polarization occurs that is measured by recording the residual 1 H magnetic resonance signal after the sample has been returned to the higher field. The experimental cycle can be run in several different ways according to the relative values of the 1 H spin–lattice relaxation times ( T 1 ) in high and low field, the 14 N spin–lattice relaxation ( T 1Q ) and cross-polarization times ( T CP ), all of which can markedly influence the spectra. The line shapes are broadened by the presence of the magnetic field and Zeeman shifts of the peak frequencies also occur, for which simple corrections may be derived. The methods used have high sensitivity, particularly if the ratio T 1 / T 1Q is large. They have the advantage over other double-resonance techniques in that long proton T 1 values are not necessary for the success of an experiment; it is also possible to select conditions in which the recovered 1 H signal is directly proportional to the relative numbers of 14 N nuclei present and the magnitude of the cross-relaxation field. Multi-proton relaxation jumps also give rise to signals at subharmonics of the fundamental, whose relative intensities reflect the extent to which the 14 N and 1 H relaxation is coupled via their dipole–dipole interactions, which are not completely quenched in the finite magnetic fields necessary in cross-relaxation spectroscopy. These conclusions are illustrated in a number of 14 N spectra of compounds in which quadrupole-resonance signals have not previously been recorded.