Nuclear magnetic resonance spectroscopy anisotropy

This anisotropy also poses a challenge to the analysis of crystals by nuclear magnetic resonance spectroscopy, which is covered in Section 5.5. Each change in orientation of a crystal in the spectrometer corresponds to a different angle between the external magnetic field and the chemical bond axes, and a different spectrum. The rapid and random motions of molecules in a liquid average over these differences to yield narrow lines in the spectrum. To duplicate the same effect in crystals, the crystal is powdered and the sample is then rotated at rates of 10 kHz or faster. 

Grimmei A.-R., and Zanni, H. 1998. Si NMR study of chemical shift tensor anisotropy of tricalcium silicate. In Nuclear magnetic resonance spectroscopy of cement-based materials, Colombet, R, A.-R. Grimmei H. Zanni and P. Sozzani (eds.). Berlin Springer-Verlag, 57-68. 

Lipari G. and Szabo A. (1980) Effect of Vibrational Motion on Fluorescence Depolarization and Nuclear Magnetic Resonance Relaxation in Macromolecules and Membranes, Biophys. J. 30, 489—506. Steiner R. F. (1991) Fluorescence Anisotropy Theory and Applications, in Lakowicz J. R. (Ed.), Topics in Fluorescence Spectroscopy, Vol. 2, Principles, Plenum Press, New York, pp. 127-176. 

Phosphorous nuclear magnetic resonance ( P-NMR) No isotope labeling is required for P-NMR spectroscopy. The chemical shielding anisotropy, Aa, in P-NMR is comparable to the deuterium quadrupole splitting in H-NMR and can be determined from the edges of the spectrum. 

There are also pulse EPR methods that probe the chemical or rather magnetic environment. These are pulse electron nuclear double resonance (ENDOR) and hyperfine sublevel correlation (HYSCORE) spectroscopy, which allow measuring hyperfine couplings from the unpaired electron spin to surrounding magnetically active nuclei ([20] in Fig. 3 this is a P nucleus). As these experiments are performed in frozen solution (e.g., in all examples of this chapter) or in solids, from the anisotropy and orientation dependence of the hyperfine coupling one can obtain valuable information on the structure up to 1 nm. 

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