Nuclear magnetic resonance species

NMR Nuclear magnetic resonance [223, 224] Chemical shift of splitting of nuclear spin states in a magnetic field H [225], C [226, 227], N [228], F [229], 2 Xe [230] Other Techniques Chemical state diffusion of adsorbed species... 

These special features are explained by an interaction between the proton and one of the water molecules, which is not merely electrostatic but also covalent. This yields a new chemical species, the hydroxonium ion, HjO. The existence of such ions was demonstrated in the gas phase by mass spectrometry and in the solid phase by X-ray diffraction and nuclear magnetic resonance. The H -H20 bond has an energy of 712kJ/mol, which is almost two-thirds of the total proton hydration energy. 

The release of iron from intracellular ferritin stores is thought to involve the reduction of Fe to Fe " (Funk et al., 1985) and one would expect this reduction to be fecilitated by the low oxygen tension, increased levels of reducing species and the low pH shown by nuclear magnetic resonance (NM to be as low as 6.9 after only 6 h of cold storage (Fuller et al., 1988). Exogenous redox-active quinones such as adriamycin have been shown to catalyse lipid peroxidation in the presence of ferritin under hypoxic conditions (Vile and Winterbourne, 1988), and lipid peroxidation is stimulated in micro-somes in the presence of purified ferritin and flavin... 

Beryllium(II) is the smallest metal ion, r = 27 pm (2), and as a consequence forms predominantly tetrahedral complexes. Solution NMR (nuclear magnetic resonance) (59-61) and x-ray diffraction studies (62) show [Be(H20)4]2+ to be the solvated species in water. In the solid state, x-ray diffraction studies show [Be(H20)4]2+ to be tetrahedral (63), as do neutron diffraction (64), infrared, and Raman scattering spectroscopic studies (65). Beryllium(II) is the only tetrahedral metal ion for which a significant quantity of both solvent-exchange and ligand-substitution data are available, and accordingly it occupies a... 

The azoniaspirocycles described in this chapter have mostly been synthesised in situ, and thus were not isolated. As a result, complete characterization by nuclear magnetic resonance (NMR) spectroscopy is not always available. However, in many cases, the azoniaspiro species has been detected by H NMR analysis of the reaction mixture. In addition, the formation of the ammonium salts can sometimes lead to stable solids which can be kept for significant periods without decomposition. 

Although relaxation measurements have been widely used in nuclear magnetic resonance studies of solid catalysts and adsorbed molecules, they have not found such favor in similar ESR work. Relaxation phenomena, however, do play a very important role in any magnetic resonance experiment, whether or not this aspect of the problem is studied. In fact, the temperature at which most ESR experiments are conducted is dictated by the relaxation process. Furthermore, even qualitative data on relaxation times can be used as supporting evidence in the identification of a paramagnetic species. 

Laughlin et al. [122] analysed chloroform extracts of tributyltin dissolved in seawater using nuclear magnetic resonance spectroscopy. It was shown that an equilibrium mixture occurs which contains tributyltin chloride, tributyl tin hydroxide, the aquo complex, and a tributyltin carbonate species. Fluorometry has been used to determine triphenyltin compounds in seawater [123]. Triph-enyltin compounds in water at concentrations of 0.004-2 pmg/1 are readily extracted into toluene and can be determined by spectrofluorometric measurements of the triphenyltin-3-hydroxyflavone complex. 

Under commonly used reaction conditions, the intermediate species is indicated to be a quasiphosphonium ion. Isolation of the intermediates from reactions in which the initial substrate is sufficiently reactive not to require heating, thereby precluding the second step from occurring rapidly, has provided evidence (nuclear magnetic resonance [NMR] and x-ray) of a quasiphosphonium ion species rather than a phosphorane species.17-26... 

Borowitz, I.J., Yee, K.C., and Crouch, R.K., Determination of stereochemistry in vinyl phosphorylated species by nuclear magnetic resonance shift reagents. Revised mechanistic pathways for the Perkow reaction, /. Org. Chem., 38,1713, 1973. 

Using circular dichroism and nuclear magnetic resonance spectrometry, Veatch and coworkers1 established that four conformational species of gramicidin A exist in solution. Two were postulated to be helixes of opposite handedness. 

GC and GC-MS (see Chapter 2), are ideal for the separation and characterization of individual molecular species. Characterization generally relies on the principle of chemotaxonomy, where the presence of a specific compound or distribution of compounds in the ancient sample is matched with its presence in a contemporary authentic substance. The use of such 6molecular markers is not without its problems, since many compounds are widely distributed in a range of materials, and the composition of ancient samples may have been altered significantly during preparation, use and subsequent burial. Other spectroscopic techniques offer valuable complementary information. For example, infrared (IR) spectroscopy and 13C nuclear magnetic resonance (NMR) spectroscopy have also been applied. 

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