Nuclear magnetic resonance high-resolution spectra

Fig. 3.73 Decoupled spectrum of crotonaldehyde. Data reproduced from W. McFarlane and R. F. M. White (1972). Techniques of High Resolution Nuclear Magnetic Resonance Spectroscopy. London Butterworths, p. 28.

Physicists have long been aware of the power and usefulness of high-resolution solid-state NMR. Even prior to the development of cross-polarization, in the early to mid-seventies, it was clear that, by recording the nuclear magnetic resonance spectrum when the sample was spun at the magic angle (5 M1) to the magnetic field, chemical shifts could be readily identified... 

FIGURE 7-17 High resolution H spectrum of ethanol (redrawn from the data of L. M. Jackman and S. Stemhell, Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry, Peigamon Press, 1969). 

The nuclear magnetic resonance (NMR) spectrometer has become in recent years one of the most popular and useful tools available to the chemist for structural studies. In the usual high-resolution NMR spectrum of a liquid the scalar parameter called the chemical shift is readily obtained and may be interpreted in an approximate and qualitative fashion to establish features of the overall structure. The task of placing the interpretation of the chemical shift on a more quantitative and rigorous basis is not an easy one but considerable progress has been made in this direction. The potential rewards of improved understanding in this area seem most attractive. 

N. Bhacca, L. F. Johnson, and J. N. Shoolery, High Resolution Nuclear Magnetic Resonance Spectra Catalog, Varian Associates, Palo Alto, Calif., 1962, Vol. 1. N. Bhacca, D. Hollis, L. F. Johnson, and E. A. Pier, ibid., 1963, Vol. 2. For the spectrum of tribenzyl trithioorthoformate, see Vol. 2, Spectrum 688. 

We present a solid-state nuclear magnetic resonance (NMR) experiment that allows the observation of a high-resolution two-dimensional heteronuclear correlation (2D HETCOR) spectrum between aluminum and phosphorous in aluminophosphate molecular sieve VPI-5. The experiment uses multiple quantum magic angle spinning (MQMAS) spectroscopy to remove the second order quadrupolar broadening in Al nuclei. The magnetization is then transferred to spin-1/2 nuclei of P via cross polarization (CP) to produce for the first time isotropic resolution in both dimensions. 

Then, with the help of a colleague, Asher Mandelbaum, at the Technion in Haifa, we got a high-resolution mass spectrum, which indicated that the molecule contains a nitrogen, certainly not a common feature in fatty acids. However, the structure was now close at hand. Some more mass spectra and a better nuclear magnetic resonance (NMR) led to a final formulation of the ligand as arachidonoyl ethanol amide (Devane, Hanus, et al., 1992). 

Nuclear magnetic resonance spectroscopy is likely the most versatile method for the study of biomolecules. The power of NMR originates from the fact that practically every atom with a magnetic nucleus gives rise to an individual signal in the NMR spectrum that carries spatial and temporal information of the local chemical environment of that atom. Solution conditions in high resolution NMR experiments mimic the natural biological environment and results relate to functional assays. 

In a nuclear magnetic resonance measurement in solution, the direct dipolar interaction Xo actually disappears because, due to rapid molecular motion, the interspin (internuclear) vectors are rapidly space-averaged within the time-scale of a measurement. Hence, the terms Xs and Xj are detectable as sharp lines or splittings in a high-resolution spectrum of H or and they can be related to detailed molecular structure or conformation of the substance investigated. In a solid, on the contrary, the directions of the internuclear vectors are stationary even if they are distributed randomly in space. Then, X gives a very wide linewidth to the zeroth-order absorption line and completely masks all lines due to Xs and Xj. 

---