Nitrogen-14 quadrupole cross-relaxation spectroscopy

Cross-relaxation spectroscopy can be used as a sensitive method of detecting 14N quadrupole-resonance signals in hydrogen-containing solids. The 1H 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 1H Zeeman splitting matches one of the 14N quadrupole splittings. If the 14N spin–lattice relaxation time is much shorter than that of the 1H nuclei, a drastic loss of 1H polarization occurs that is measured by recording the residual 1H 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 1H spin–lattice relaxation times (T1) in high and low field, the 14N spin–lattice relaxation (T1Q) and cross-polarization times (TCP), 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 T1/T1Q is large. They have the advantage over other double-resonance techniques in that long proton T1 values are not necessary for the success of an experiment; it is also possible to select conditions in which the recovered 1H signal is directly proportional to the relative numbers of 14N 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 14N and 1H 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 14N spectra of compounds in which quadrupole-resonance signals have not previously been recorded.