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After the proton and carbon, nitrogen is, with oxygen, the most impor­ tant 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 iso­ tope, 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 1 (~ 1T) using a 15 mm o.d. cell in the field sweep mode (~ 0.16 min- ). 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 dis­ advantage for 15 spectroscopy (Table 1.1). However, nitrogen-15 has, N probably more than any other nucleus, benefited from the advances of NMR technology, i.e. Fourier transformation, multinuclear facilities, wide-bore super conducting 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 pecu­ liar to 15N may be necessary in order to adjust the pulse sequences and the decoupler duty cycle correctly (Section 2).

Inhaltsverzeichnis

Frontmatter

1. Introduction

Abstract
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).
Gérard J. Martin, Maryvonne L. Martin, Jean-Paul Gouesnard

2. Relaxation Phenomena and Nuclear Overhauser Effects. Molecular Dynamics and Observation of the 15N Signals

Abstract
A knowledge of the relaxation processes which govern nitrogen relaxation is.especially useful, not only in order to obtain information on molecular dynamics, but also in order to select the best conditions for the observation of the 15N signals. Owing to the lack of sensitivity of 15N-NMR, the latter problem is indeed of prime importance. It is therefore helpful to be able to anticipate the behaviour of T1 as a function — of the reorientation rate of the compound — of the molecular structure — of the spectrometer frequency and — of the medium properties and temperature. Determination of the pulse sequence is, in fact, critically conditioned by the values of T1. Moreover, as the value of the transverse relaxation time T 2 * governs the signal width (∆υ1/2 = 1/πT 2 * ) it is also important to appreciate the influence of the various experimental parameters upon T 2 * .
Gérard J. Martin, Maryvonne L. Martin, Jean-Paul Gouesnard

3. Experimental Techniques in 15N Spectroscopy

Abstract
Although the vast majority of 15N spectra is now directly recorded by pulse FT spectroscopy, indirect detection may still be of interest. Indeed, a lot of 15N parameters have been obtained in the past through CW double resonance experiments. In this area, the various techniques of 1H{15N}double irradiation can be exploited and information about the 15N resonances is then obtained via proton responses to perturbations applied, more or less selectively, to the 15N transitions (M 31d). The INDOR method, in particular, is well suited to indirect detection of the 15N spectrum (M 28) (M 33). In this method a proton line corresponding to a given 15N~H scalar coupling is continuously monitored while the double irradiation field \( \overrightarrow {{B_2}} \) is swept with an amplitude *B2 ≃ ∆υ1/2 (HZ) through the 15N transitions. Responses are then obtained in the 1H spectrum each time that the double irradiation frequency encounters a 15N transition connected with the considered proton line. This technique allows a determination of the 15N coupled spectrum which benefits from the sensitivity of the corresponding proton resonances. However, it requires the existence of detectable 15N~H coupling constants and lacks general applicability.
Gérard J. Martin, Maryvonne L. Martin, Jean-Paul Gouesnard

4. Reference for 15N Chemical Shifts

Abstract
Chemical shifts are relative parameters which can be measured with a high degree of reproductibility providing that the same reference substance is used in the same experimental conditions. Unfortunately, this goal was not achieved in the last decade, which saw the sudden rise of 15N spectroscopy, since at least thirteen different molecules in a variety of solutions were used as standards for 15N chemical shifts. Indeed, this is not really surprising if we bear in mind that about fifteen years were required for the TMS δ-scale to be universally accepted in 1H spectroscopy!
Gérard J. Martin, Maryvonne L. Martin, Jean-Paul Gouesnard

5. Medium Effects in 15N Spectroscopy

Abstract
Due to the fact that a nitrogen atom incorporated in a molecule, exhibits a Lewis basicity which depends on the degree of lone-pair delocalisation, solvent effects are expected to be greater in 15N spectroscopy than in carbon NMR. Examination of Table 5.1 shows that amines, amides (or ureas), nitriles and nitro derivatives are characterized by different values of polarity parameters indicatory of their electron-donating or -accepting abilities.
Gérard J. Martin, Maryvonne L. Martin, Jean-Paul Gouesnard

6. 15N Chemical Shifts

Abstract
Compilation until 1979 of the literature on 15N parameters, namely chemical shifts and coupling constants, has provided a large amount of data. Examination of Fig. 1.1 shows that most 15N resonances lie in a 500 ppm range, although some compounds, such as nitroso derivatives, extend the field of δ 15N to 800 ppm.
Gérard J. Martin, Maryvonne L. Martin, Jean-Paul Gouesnard

7. nJ15N ~ X Coupling Constants

Abstract
A number of coupling constants between 15N and other nuclei, e.g. X = 1H, 19F, 31P, can be obtained using CW spectroscopy and 15N enriched samples. However, with the availability of commercial FT spectrometers, the direct determination of nJ15N ~ X coupling constants from X-undecoupled 15N spectra, at the natural abundance level of the 15N isotope. No transformation of J 14N~X coupling constants into J 15N~X coupling constants has been attempted. Moreover we shall not discuss the theoretical analysis of coupling here since this aspect has been treated elsewhere (W 29), and, rather than successively examining the coupling constants in given series of compounds, whe shall present the results in terms of the more or less quantitative correlations which have been discussed.
Gérard J. Martin, Maryvonne L. Martin, Jean-Paul Gouesnard

8. Application of 15N Spectroscopy to the Study of Dynamic Processes and Reaction Mechanisms

Abstract
When dynamic processes are investigated by NMR, the reactions are usually considered as fast or slow according to whether the lifetimes, τ of the species concerned are short or long with respect to the NMR time scale. In fact, information about molecular dynamics corresponding to very fast motions, is obtained by relaxation time determinations, and this topic has already been discussed in Sect. 2. Here we shall be concerned with chemical exchange processes involving species at equilibrium or with relatively slow reactions in non-equilibrated systems. In terms of lifetimes, such chemical processes may be characterized by τ values down to about 10−4 s which correspond to rate constants up to 104 s−1 for first-order reactions. This limits depends, to a certain extent, on the nucleus used as a probe, moreover, it can be displaced towards higher rate constants for second-order reactions investigated in very diluted solutions. Here, we shall consider the exploitation of line-shape modification experiments and then the investigation of equilibration processes by the determination of line intensity variations.
Gérard J. Martin, Maryvonne L. Martin, Jean-Paul Gouesnard

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