High-field NMR spectrometer

The recent development of high-field NMR spectrometers with increased resolution, dynamic range and... 

If one wishes to obtain a fluorine NMR spectrum, one must of course first have access to a spectrometer with a probe that will allow observation of fluorine nuclei. Fortunately, most modern high field NMR spectrometers that are available in industrial and academic research laboratories today have this capability. Probably the most common NMR spectrometers in use today for taking routine NMR spectra are 300 MHz instruments, which measure proton spectra at 300 MHz, carbon spectra at 75.5 MHz and fluorine spectra at 282 MHz. Before obtaining and attempting to interpret fluorine NMR spectra, it would be advisable to become familiar with some of the fundamental concepts related to fluorine chemical shifts and spin-spin coupling constants that are presented in this book. There is also a very nice introduction to fluorine NMR by W. S. and M. L. Brey in the Encyclopedia of Nuclear Magnetic Resonance.1... 

With advances in multidimensional NMR technology and the development of methods for uniform labeling of proteins with 15N and 13C, direct NMR studies on unfolded states of full-length proteins, rather than protein fragments, became possible. These studies were aided by the advent of high-field NMR spectrometers, which provide the necessary dispersion and sensitivity. High sensitivity is a critical factor in the study of unfolded and partly folded proteins, since NMR experiments must often be performed at very low concentrations to prevent aggregation. 

This research was supported by a grant from the National Institutes of Health (GM-22350). The high field NMR spectrometers... 

Traditionally, the principal tools for the study of vanadate speciation in aqueous solution were UV/vis and electrochemistry. Unfortunately, the complex chemistry associated with vanadate has rendered much, but certainly not all, of the earlier work obsolete. The reaction solutions often contained numerous products that, a priori, could not be specified. Properly describing the chemistry was somewhat like doing a jigsaw puzzle without knowing what the pieces looked like or how many there were. Only with the advent of 51V NMR spectroscopy in high field NMR spectrometers was there a tool in place that allowed a coherent picture of V(V) chemistry to be fully developed. The combination of potentiometry with NMR spectroscopy has proven a certain winner. Additionally, x-ray diffraction studies have provided an invaluable source of information, but it is information that, in all cases, must be used with extreme caution when attempting to describe the chemistry in solution. 

A well-established area in the field of NMR is the use of Pulsed Field Gradients, or PFG NMR. It is ironic to consider that so much effort has been expended over so many years to provide a homogenous or stable magnetic field. Today, most modern high field NMR spectrometers are routinely equipped with hardware (coils)... 

With these features, 33S is one of the most difficult nuclei to detect by NMR. However, the increasing availability of high-field NMR spectrometers and the development of hardware and probe technology can partly facilitate its detection. 

Most of the betalain pigments described in the 1960s have not been characterized by mass or NMR spectra. With FAB MS now at hand, the molecular ions of underivatized betalains can be easily determined, and the sensitivity of high-field NMR spectrometers allows the complete structural assignment, even of small samples. For the structural determination of oligosaccharide moieties, modem two-dimensional (2D) NMR techniques are now the method of choice. 

As a nucleus, is far from ideal for NMR observation thus a high-power and high-field NMR spectrometer is necessary for its study in solution. Isothiazole absorbs at +53.7 ppm and 3-methylisothiazole at +51 ppm, while a considerable difference in chemical shift is seen for the 2-methylthiazole isomer (—72 ppm) <2002J(P2)225>. 

NMR spectroscopy has grown to become the most powerful and widely used technique for structure elucidation and identification in small organic molecules, and is finding more and more applications in structural biology with the development of very high-field NMR spectrometers. NMR spectroscopy is so useful because of the high-resolution spectra that can be acquired, and interpretation of spectra gives very detailed and accurate information about how atoms are connected and on the local environments of individual nuclei. 

Disadvantages are that absolute configurations cannot generally be determined without reference to a known sample, and that both enantiomers must be available to insure peak separation. An additional disadvantage has developed with the advent of high-field NMR spectrometers the technique is not as effective at high fields, as explained below. 

COMMENT. Magnetic fields above 20 T have not yet been obtained for use in NMR spectrometers. As discussed in the solution to Exercise 15.4(b), it is the field, not the frequency, that is fixed in high-field NMR spectrometers. Thus an NMR spectrometer that is called a 300 MHz spectrometer refers to the resonance frequency for protons and has a magnetic field fixed at 7,05 T. 

Development of new instrumentation in DNP. The new design of instruments for DNP should be able to couple conveniently with the current high field NMR spectrometers. 

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