Isotopic enrichment, nuclear magnetic

Interactions between one or more nuclei and the unpaired electron yield a wealth of information concerning molecular structure. In addition, they have proven invaluable in the identification of paramagnetic species. As indicated in Table II many of the common elements have isotopes with nuclear magnetic spins which distinguish them from the other elements. If the isotopes of interest are not sufficiently abundant in the natural form, enriched samples may be purchased. The quantity used in surface studies is usually quite small, so relatively expensive isotopes such as 170 can be studied. In fact, it is possible to recover most of the isotope following an experiment, should the cost require it. 

Stable isotope ratio analysis (SIRA, GC-IRMS [Gas Chromatography-Isotope Ratio Mass Spectrometry], and Site Specific Isotope Fractionation — Nuclear Magnetic Resonance [SNIF-NMR]) have proven useful in many adulteration situations. In nature, and exist at relative proportions of 1.11 98.89 [27]. The photosynthetic process selectively enriches the plant in dependent upon the type of photosynthetic process used by the plant. Plants using the Hatch-Slack pathway (e.g., com, sugar cane, millet, and lemon grass) give 8 C values the closest to the standard, i.e., 5 C values of ca. -10. The 5 C value is calculated as ... 

Probably, one of the most valuable advances in this field has dealt with the first chemoenzymatic synthesis of the stable isotope-enriched heparin from a uniformly double labelled 13C, 15N /V-acetylheparosan from E. coli K5. Heteronuclear, multidimensional nuclear magnetic resonance spectroscopy was employed to analyze the chemical composition and solution conformation of N-acety 1 hcparosan, the precursors, and heparin. Isotopic enrichment was found to provide well-resolved 13C spectra with the high sensitivity required for conformational studies of these biomolecules. Stable isotope-labelled heparin was indistinguishable from heparin derived from animal tissues and might be employed as a novel tool for studying the interaction of heparin with different receptors.30... 

Example Isotopic enrichment is a standard means to enhance the response of an analyte in nuclear magnetic resonance (NMR). Such measures gain importance if extremely low solubility is combined with a large number of carbons, as is often the case with [60]fullerene compounds. [19] The molecular ion signals, IVT, of Qo with natural isotopic abundance and of C-enriched Cgo are shown below (Fig. 3.11 for EI-MS of [60]fullerenes cf. Refs. [20-22]). From these mass spectra, the enrichment can be determined by use of Eq. 3.1. For Qo of natural isotopic abundance we obtain Mrceo = 60 x 12.0108 u = 720.65 u. Applying Eq. 

All of the heteroatoms possess at least one naturally occurring isotope with a magnetic moment (Table 15). The nuclei 14N, 170 and 33S also possess an electric quadrupole moment which interacts with the electric field gradient at the nucleus, providing a very efficient mechanism for relaxing the nuclear spin. The consequence of this facilitation of relaxation is a broadening of the NMR signals so that line widths may be 50-1000 Hz or even wider. To some extent this problem is offset by the more extensive chemical shifts that are observed. The low natural abundances and/or sensitivities have necessitated the use of accumulation techniques for all of these heteroatoms. The relative availability of 170 and 15N enriched... 

As the glucose is metabolized, the metabolic derivatives become enriched with the isotopic carbon label and are detected using advanced technologies such as mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy. Importantly, incorporation of isotopes into molecules can alter reaction mechanisms including the rate of a reaction-this is termed isotopic substitution and forms the basis of the kinetic isotope effect. 

Incorporation studies of C-labeled precursors require extensive nuclear magnetic resonance (NMR) measurements. The signals of C-NMR-spectra of the analyzed metabolites must be assigned fully. The C-NMR spectra of the reference compound of natural abundance (1.1%) and of the labeled metabolite must be recorded in the same conditions. A comparison between the relative signal intensities of the two spectra indicates that the carbon atom is labeled and indicates the magnitude of the isotope enrichment. 

Remarkable effects of nuclear spin on the photochemistry of dibenzyl ketone and its derivatives (see Special Topic 6.11) have been reported and substantial isotopic enrichment of 13C in specific products has been achieved under favourable conditions.434 The influence of magnetic fields and of microscopic reactors (micelles, zeolites, etc.) on these reactions was investigated in detail.435... 

CP spectra of silica surfaces in experiments that normally require isotopic enrichment because the small nuclear magnetic moment (about 14% of that of 1H) and quadrupolar broadening exacerbate both sensitivity and... 

Solvents for nuclear magnetic resonance (NMR) spectroscopy, enriched in deuterium to an extent of >99%, are commercially available. The separation of deuterium from naturally occurring hydrogen is achieved electrol5fiically with the isotope in the form of D2O. When an aqueous solution of NaOH (natural isotopic abundances) is electrolysed (equation 2.36) using an Ni electrode, the separation factor defined in equation 2.37 is The choice of electrode is critical to the... 

Of all the isotopes, only Mo possess nuclear magnetic moments (7 = 5/2 for both see Table 2). Therefore, these two isotopes only contribute to 25% of the total spectrum with the remaining 75% not showing any hyperfine interaction. To further illustrate this, Figure 9 shows the X-band spectrumof Mo " " in Very Rapid xanthine oxidase using natural abundance Mo (top) or Mo-enriched enzyme (bottom). For Mo the hyperfine... 

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