Jean-Paul Gouesnard

15N Chemical Shifts

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.

Sodium nitrite reactivity. Part 3. Reaction with levamisole

The reaction of aqueous sodium nitrite with levamisole has been studied and the products obtained have been identified by spectroscopic methods, in particular natural abundance 13C and 15N n.m.r. spectroscopy. (E)-1-Nitroso-2-oxo-3-(2-nitrosothioethyl)-5-phenylimidazolidine (3) is initially formed and this then dimerizes to the disulphide compound by loss of the nitrosyl group on the sulphur atom. Derivatives of levamisole have also been prepared for the determination of the reaction mechanism and identification of the products.

Medium Effects in 15N Spectroscopy

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.

nJ15N ~ X Coupling Constants

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.

Nuclear magnetic resonance investigations of iminium ion intermediates. Part 9. Multinuclear study of the reaction between Lewis acids and vinylogous amides

When treated with Lewis acids the vinylogous amides have analogous synthetic behaviour to that of amides. The structure and the electronic properties of the intermediate complexes formed in the course of the reactions between COCl2 or POCl3 and enaminoaldehydes, enaminoketones, or p-dimethylaminobenzaldehyde have been investigated. These complexes are identified and the mechanism is compared with that of the Vilsmeier reaction. The stereochemistry of the products is elucidated and examined in the light of the possible exchange processes. Electron delocalization in both the neutral compounds and cations of the type (CH3)2N C C C A are discussed on the basis of nitrogen chemical shifts and rotational barriers.

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

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.

Reference for 15N Chemical Shifts

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!

Experimental Techniques in 15N Spectroscopy

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.

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

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 * .