Nuclear magnetic resonance spectroscopy experiments

Requirements for a Successful Nuclear Magnetic Resonance Spectroscopy Experiment... 

The mechanism outlined above is supported by experimental findings. An intermediate 5 has been isolated, " and has been identified by and N-nuclear magnetic resonance spectroscopy. Side-products have been isolated, which are likely to be formed from intermediate 4. N-isotope labeling experiments have shown that only the nitrogen remote from the phenyl group is eliminated as ammonia. 

Nuclear magnetic resonance (NMR) experiments are used to study the exchange kinetics of chemical systems in equilibrium.28,68,69 As is the case for fluorescence correlation spectroscopy no perturbation of the chemical system in equilibrium is required to obtain kinetic information from NMR experiments. However, NMR is not very sensitive to concentration changes. 

Three different isomers are formed consecutively on reacting [RuCl2(PPh3)3] with 4,6 -bis(pyrazol-l-yl)pyrimidine, bpzpm the final product is cis,trons-[RuCl2(PPh3)2(bpzpm)] (160). Nuclear magnetic resonance spectroscopy (NMR) shows facile cis < - trans interconversion for [H(dtbp)Ru(p-Cl)2Ru(dtbp)H], where dtbp = bis[di(t-butyl)phosphano-methane or -ethane. Crossover experiments implicate mononuclear intermediates in these isomerizations (161). 

Hyphenated analytical techniques such as LC-MS, which combines liquid chromatography and mass spectrometry, are well-developed laboratory tools that are widely used in the pharmaceutical industry. Eor some compounds, mass spectrometry alone is insufficient for complete structural elucidation of unknown compounds nuclear magnetic resonance spectroscopy (NMR) can help elucidate the structure of these compounds (see Chapter 20). Traditionally, NMR experiments are performed on more or less pure samples, in which the signals of a single component dominate. Therefore, the structural analysis of individual components of complex mixtures is normally time-consuming and less cost-effective. The... 

The above authors rationalized formation of the more hindered i omer by assuming the equilibrium to be displaced in its favor by virtue of the lower solubility of this isomn relative to the other. House and Ho,885 on the other hand, showed by deuteration experiments and nuclear magnetic resonance spectroscopy that the epimeriza-tion observed by Washerman and oo-workera1Wfi IH0 proceeds by way of an unueual oxide anion in which negative charge resides on carbon as ehovra in Eq. (SS). 

Stott K, Stonehouse J, Keeler J, Hwang T-L, Shaka AJ. Excitation sculpting in high-resolution nuclear magnetic resonance spectroscopy application to selective NOE experiments. J. Am. Chem. Soc. 1995 117 4199-4200. 

To determine the nature of the silicon moieties in a polymer, clearly the easiest method would be a technique that provides a direct observation of the silicon atom and meaningful, interpretable information on the atom. Nuclear magnetic resonance spectroscopy tuned to the Si isotope ( Si NMR) is a tool of this nature it can directly probe the state of the silicon atom, and with it one can often readily determine the extent to which Si-O-Si crosslinks (fi-om silanol condensation), have formed. One can observe spectra of silicon-containing compounds either dissolved in a solvent or in the solid state. Liquid-state Si NMR, while the most sensitive, cannot be used quantitatively on heterogeneous systems such a latex formulations. Therefore, one must separate the liquid and solid portions of the latex (without heat, which would promote hydrolysis and condensation) and use the solid residue for the Si NMR experiments. 

Figure 5-2 A stack plot for the inversion recovery experiment of the carbon-13 resonances of chlorobenzene at 25 MHz. The time t in the pulse sequence 180°-t-90° is given in seconds at the right. (From R. K. Harris, Nuclear Magnetic Resonance Spectroscopy Pitman Publishing, Ltd., London, 1983, p. 82. Reproduced with permission of Addison Wesley Longman, Ltd.)...

M. Hohwy and N. C. Nielsen, Systematic design and evaluation of multiple-pulse experiments in nuclear magnetic resonance spectroscopy using a semi-continuous Baker-Campell-Hausdorff expansion. J. Chem. Phys., 1998, 109, 3780-3791. 

The addition of a chemical species with a large dielectric constant to induce desired microwave effects in matrices devoid of such substances, or lacking substances with significantly different dielectric constants, can be compared, on a conceptual basis, to cross-polarisation experiments carried out in nuclear magnetic resonance spectroscopy (see Chapter 6). In that case, a nucleus that relaxes relatively rapidly is excited selectively and allowed to transfer that excitation energy to neighbouring nuclei with low or relatively lower relaxation rate (e.g., nuclei being cross-polarised to nuclei). 

Nuclear magnetic resonance spectroscopy is such a powerful tool for stracture determination because protons in different environments experience different degrees of shielding and have different chemical shifts. In compounds of the type CH3X, for example, the shielding of the methyl protons increases as X becomes less electronegative. Inas-... 

Experiments in which specifically labeled deuteriotoluene was passed through the RF. discharge afforded additional experimental data which supported the importance of radical intermediates leading to condensable products. The materials formed from the labeled toluene were collected, separated by chromatographic techniques, and the distribution of the deuterium label determined by infrared and nuclear magnetic resonance spectroscopy and mass spectrometry. 

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