Low field, NMR spectroscopy

Pore Structure Characterization of Catalyst Supports via Low-Field NMR Spectroscopy... 

Valentin JL, L6pez D, Hem dez R, Mijangos C, Saalwachter K (2009) Structure of poly (vinyl alcohol) cryo-hydrogels as studied by proton low-field NMR spectroscopy. Macromolecules 42 263... 

Dalitz, R, Cudaj, M., Maiwald, M., and Guthausen, G. 2012. Process and reaction monitoring by low-field NMR spectroscopy. Prog. Nucl. Magn. Reson. Spectrosc. 60 52-70. 

In the context of improving the efficiency of use of natural resources and of profit optimization, the question arises whether a chemical process can explicitly be monitored in real time by low field NMR spectroscopy. Typical requirements are A resolution of about 0.1 ppm which is often sufficient to detect and quantify concentrations of reactants, a measurement time short enough for temporal resolution of the reaction (several 10 s). Questions like pre-magnetization and accuracy have to be addressed. The most obvious way to follow the reaction by NMR is via a bypass line. Representative sampling is here a crucial point, which was addressed in a recent publication. The esterification reaction of... 

International Standard Organization ISO 10565 1998L Oil and moisture in seeds by low field NMR spectroscopy. 

The value of -NMR and 13C-NMR spectroscopy in characterizing transition metal carbene complexes was noted in Section III,B,2. The carbene carbon resonance is invariably found at low field (200-400 ppm) in the 13C-NMR spectrum, while protons attached to Ca in 18-electron primary and secondary carbene complexes also resonate at low fields. NMR data for some Ru, Os, and Ir alkylidene complexes and related compounds are given in Table V. 

Metabolomic approaches use analytical techniques such as high-field NMR spectroscopy and MS to measure populations of low-molecular-weight metabolites in biological samples. Advanced statistical and bioinformatics tools are then employed to maximize the recovery of information and interpret the large data sets generated. 

If the compoimd of interest has a very low reaction rate (<1% of the activity of a known standard substrate), further evaluations with concentrated enzyme are recommended. There are examples of traces of reactive substrates remaining at these low levels in inactive compoimds. Since 0.1% conversion of a reactant to product can easily be detected in radiochemical glycosyltransferase assays, confirmation that the low rate of conversion is not due to the presence of contaminants is essential. This level of contamination in modified synthetic substrates is sometimes difficult to detect even by high-field NMR spectroscopy. Therefore before the actual preparative synthesis is performed, 10% conversion of substrate to product should be verified by assay to ensure that the low rate of reaction is not due to traces of substrate in a largely inactive compound. 

Despite considerable progress in the field, NMR spectroscopy still has two significant limitations the intrinsically low sensitivity, due to the low Boltzmann polarization of nuclear spins in thermal equilibrium, and the low dispersion of observed frequencies, due to small differences in nuclear shielding by surrounding electrons for nuclei of the same kind. The first problem is continuously... 

In contrast, the preparation of stable alkane complexes remains elusive, with only a limited number of transient species having been observed using matrix isolation techniques, time-resolved spectroscopic methods, or low-temperature NMR spectroscopy or probed indirectly measuring isotope effects (see also Section 1.03.3.6). The field of C-H bond activation is nevertheless one of the most challenging and vibrant areas of research in modern organometallic chemistry.For a review of the main features of C-H bond activation chemistry, see Chapter... 

In principle, all kinds of spectroscopic techniques lend themselves to on-line measurements. Only a very few are practical. Although low-field NMR has been used to measure various material properties by applying empirical relationships, NMR is still not a realistic proposition for on-line measurements. Ironically, ETIR spectroscopy suffers from too much sensitivity. Typically, good spectra can be obtained only from very thin polymeric films (or solutions). Attenuated total reflection (ATR) probes, in which only a fraction of the IR light penetrates a very short distance into the sample, reduce the problem of excessive sensitivity. However, they aggravate the problems of variations in the baseline and nonlinear response. The latter problem also obstructs the use of UV spectrometry for monitoring polymerization reactions. Of the remaining options, near-infrared (NIR) and Raman spectroscopy are the most attractive. 

Alkenyl hydrogens (and carbons) are deshielded and give rise to relatively low-field NMR signals compared with those in saturated alkanes (Section 11-4). In contrast, alkynyl hydrogens have chemical shifts at relatively high field, mnch closer to those in alkanes. Similarly, the sp-hybridized carbons absorb in a range between that recorded for alkenes and alkanes. Alkynes, especially terminal ones, are also readily identified by IR spectroscopy. Finally, mass spectrometry can be a useful tool for identification and structure elucidation of alkynes. 

Analytical and Test Methods. For a review of detection, deterrnination, and identification of ketenes see Reference 67. Typical properties are the strong ir absorption bands at 2151 cm (C—O) and at 1120 cm as weU as a very low field signal of the j hybridi2ed carbon at approximately 194 to 206 ppm and a very high field signal of the s hybridi2ed carbon at approximately 2.5 to 27 ppm in C-nmr spectroscopy. 

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