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The 14N nuclear quadrupole resonance spectra and spin-lattice relaxation times of the amino acids histidine, methionine, cystine, cysteine, and tyrosine as well as of the nucleic bases uracil, thymine, cytosine, and guanine have been determined by a 14N-proton double resonance technique in the laboratory frame. The experiments were performed on polycrystalline samples at 77°K or above this temperature. A theoretical estimate of the sensitivity of this method for a variety of experimental conditions is as well presented. It is shown that in contrast to usual double resonance techniques the method works even in the case of a short proton spin-lattice relaxation time if only the nitrogen relaxation time is long.

Using proton–nitrogen double resonance and cross relaxation in the laboratory frame, the 14N NQR spectra of solid p-azoxyanisole (PAA), diheptyloxyazoxybenzene (HOAB), ethoxybenzilidene butylaniline (EBBA), and p-anisalazine have been measured. The room temperature 14N quadrupole coupling constant of p-anisalazine is e2qQ/h=4410 kHz whereas the asymmetry parameter is η=0.213. In EBBA, e2qQ/h=3990 kHz and η=0.186. In PAA and HOAB there are more than two chemically nonequivalent nitrogens in the unit cell. The 14N quadrupole coupling constants lie in the range between 4320–4370 kHz (PAA) and 4160–4400 kHz (HOAB) whereas the asymmetry parameters lie between 0.37 and 0.5. A comparison of the results for solid and nematic PAA shows that the largest principal axis of the 14N electric field gradient tensor is very nearly parallel to the N=N bond and that the central part of the PAA molecules containing the N=N bonds is rigid in the solid whereas it rotates around the long molecular axis in the nematic phase.

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The origin of the rf magnetic-field-induced coupling between spin systems is discussed. A new nuclear-double-resonance technique employing this coupling is proposed, which has particular value in measuring pure nuclear-quadrupole-resonance spectra of integer-spin nuclei by nuclear double resonance.