Nuclei other than hydrogen

Some less frequently encountered nuclides, such as 57Fe and 59Co, have chemical shift ranges of more than 10,000 ppm, hence encompass a very large frequency range in spite of smaller magnetogyric ratios. The chemical shift of monatomic 129Xe gas is very sensitive to its environment, and hyperpolarized xenon (Section 2.5) can be used to probe adsorption sites in solids. 

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A special type of substituent effect which has proved veiy valuable in the study of reaction mechanisms is the replacement of an atom by one of its isotopes. Isotopic substitution most often involves replacing protium by deuterium (or tritium) but is applicable to nuclei other than hydrogen. The quantitative differences are largest, however, for hydrogen, because its isotopes have the largest relative mass differences. Isotopic substitution usually has no effect on the qualitative chemical reactivity of the substrate, but often has an easily measured effect on the rate at which reaction occurs. Let us consider how this modification of the rate arises. Initially, the discussion will concern primary kinetic isotope effects, those in which a bond to the isotopically substituted atom is broken in the rate-determining step. We will use C—H bonds as the specific topic of discussion, but the same concepts apply for other elements. 

For nuclei other than hydrogen, with a much larger range of chemical shifts, CTrfA is not very different from the free-atom term, terms are relatively small and often neglected, and chemical shift relationships are... 

For other chiral reagents and application of this method to other classes of compounds, such as a-amino esters, a-hydroxy esters, amino alcohols, chiral a-deuterated benzylamine, phosphorus compounds, and the use of nuclei other than hydrogen (l3C, 19F), see reference 186 and Section D.4.1. 

The treatment of Saika and Slichter has served as model for several subsequent calculations of chemical shifts of nuclei other than hydrogen. The most successful of these calculations have been those of Griffith and Orgel (39) and Freeman et al (35) on the shifts of complexed Co69(III). Shifts for Co59(III) in a variety of octahedrally coordinated complexes are shown in Table I. The range of 14,000 ppm in chemical shifts for these rather similarly constituted complexes is such as to suggest that a para-... 

Separations Based on Molecular Size, Shape, and 140. 2. Nuclei Other Than Hydrogen... 

For nuclei other than hydrogen, such as 13C, the paramagnetic shielding term [Pg.110]

Many substances that are vapors at room temperature and atmospheric pressure may be used as NMR solvents in sealed tubes or at reduced temperature. For example, S02 has a vapor pressure of about 3 atm at room temperature and can be easily contained in sealed thin-walled, 5 mm diameter NMR sample tubes. Supercritical fluids are also used as NMR solvents in specialized sample tubes. For NMR studies of nuclei other than hydrogen and carbon, suitable solvents that do not contain the nucleus being studied are usually readily available. Frequently, the use of two or more solvents can provide valuable information on molecular structure, as indicated in Chapter 4. 

While complete X-ray analysis will establish the structure in the solid state, it is useful to have NMR data on the solution state that illustrate the increase of the coordination number of silicon. It would seem that NMR spectroscopy of nuclei participating directly in donor-acceptor interaction is especially important in investigating silicon compounds with an expanded coordination sphere. This requires the use of Si NMR spectroscopy since the electron shell of the silicon atom, the bond angles and lenghts are strongly affected upon complexation. Valuable information could also be obtained with by " N, N, 0, F NMR data since these elements act as donors. Chemical shifts of nuclei other than hydrogen are determined by various factors and not yet understood well anough to provide easily applied correlations of other physical properties of the molecules. 

We have now discussed three situations in which the n -f-1 Rule fails (1) when the coupling involves nuclei other than hydrogen that do not have spin (e.g., deuterium. Section 4.13), (2) when there is nonequivalence in a set of protons attached to the same carbon, and (3) when (sometimes unexpectedly) the chemical shift difference between two sets of protons is small compared to the coupling constant linking them. Section 5.10 wiU discuss this situation. 

Nuclei other than hydrogen The point dipolar method to estimate distances is difficult to apply for atoms carrying appreciable spin density. This applies to the centre atom of a radical or a transition metal ion complex, but often also to atoms or ligands other than hydrogen. This is for instance the case for in the fluorocarbon anions discussed in Chapter 5. Theoretically computed values are in these cases often sufficiently accurate to support or reject an assignment. In favourable cases modern DFT or ab initio methods can even be of predictive value to reproduce experimental spectra as exemplified in Chapter 5. 

A special type of substituent effect that has proved very valuable in the study of reaction mechanisms is the replacement of an atom by one of its isotopes. Isotopic substitution has most often involved replacing protium by deuterium (or, less often, tritium), but the principle is applicable to nuclei other than hydrogen. The quantita-... 

Once the potential energy surface is obtained, the reaction can be studied by solving for the nuclear motion along the reaction coordinate. Most nuclei other than hydrogen are sufficiently massive that classical mechanics is thought to be an adequate approximation, but quantum calculations are also carried out. Neither the classical nor quantum equations can be solved in closed form, and the motions are numerically simulated using computer programs. If classical mechanics is used, the calculation is carried out for a number of different trajectories. The fraction of the trajectories that... 

R44. Use of the nuclear magnetic resonance of nuclei other than hydrogen in tumor studies Granger, P. J. Biophys. Med. Nucl. 1981, 5, 137-140. A short review with 17 references, including oxygen, sodium, phosphorus, and potassium resonance. 

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