After the proton and carbon, nitrogen is, with oxygen, the most important atom in organic and especially bioorganic molecules. However, the development of nitrogen spectroscopy is indeed very recent. This is due to the fact that nitrogen-14, which is the naturally abundant isotope, suffers, for structural studies, from the disadvantages inherent in nuclei with a quadrupolar moment (Table 1.1). Actually, indirect 15N measurements were reported in the early days of double resonance spectroscopy and the first direct detection of 15N resonance signals at the natural abundance level was realized in 1964 (R 17) at 4.33 MHz (~ 1T) using a 15 mm o.d. cell in the field sweep mode (≃ 0.16 min−1”). Signal-to-noise ratios only of 3–4 were obtained for neat liquids and this low sensitivity of the 15N resonance still remains the main disadvantage for 15N spectroscopy (Table 1.1). However, nitrogen-15 has, probably more than any other nucleus, benefited from the advances of NMR technology, i.e. Fourier transformation, multinuclear facilities, wide-bore superconducting solenoids, and, with the new generation of spectrometers, 15N-NMR is entering the field of routine investigation. Nevertheless, in spite of these spectacular improvements, obtaining 15N spectra of diluted species or large biochemical molecules is often not very easy and a good knowledge of the relaxation properties peculiar to 15N may be necessary in order to adjust the pulse sequences and the decoupler duty cycle correctly (Section 2).
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