Se NMR Parameters

The most important Se NMR parameter is the chemical shift 5(/ Sc) reflecting the chemical environment around the selenium atom. Chemical shift tensors can be obtained from the rotational side-band pattern in solid-state Se NMR spectra and offer more detailed information. Reports on coupling constants involving Se are comparably rare although at least /( Se, H) and 7( Se, C) can often be read easily from the Se-satellites in the ll and 

In the early days there was a sensitivity problem when using CW techniques which has been overcome - at least in part - by indirect multiple-resonance experiments, but with the advent of pulse Fourier Transform spectrometers (ca. 1970) sensitivity is no longer an obstacle. 

Glass et al., were the first to mention the enormous sensitivity gain achievable by H-detected Se correlation spectroscopy They showed for a number of representative organoselenium compound that the theoretical SIN ratio enhancement factor of 72, as compared to direct Se detection, can nearly be achieved so that this technique should be applied for recording biomolecules in low concentration, e.g., in selenoproteins. If inverse detection is combined with isotopic enrichment of Se, the factor may be increased up to 800. Selenomethionine, selenocysteine, and related compounds have been recorded using Se, H HETCOR experiments.  

Interestingly, even small Se, C coupling constants via two to four bonds (13.7-1.7 Hz) could be identified benzoselenadiazoles by ID C-detected Se HMBC spectroscopy.  

Various medium effects - such as temperature, concentration, and pH - play 

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The Se CP MAS NMR spectra of a number of selenium compounds have been reported by Collins et al. (1986). A selection of Se NMR parameters are given in Table 9.6. 

The Se NMR parameters of 1,2- (159) and 1,3-selenazoles (e.g., 160) have been mentioned before (Scheme 29). Due to its large electronegativity, the nitrogen atom deshields the Se nucleus to a great extent. Recently, the synthesis of further 1,3-selenazole derivatives, such as 16 P and 162 have been reported confirming this trend. Acylation of the amino group of 162... 

A pre-requisite for the successful extraction of key NMR parameters from an experimental spectrum is the way it is processed after acquisition. The success criteria are low noise levels, good resolution and flat baseline. Clearly, there are also experimental expedients that can further these aims, but these are not the subject of this review per se. In choosing window functions prior to FT, the criteria of low noise levels and good resolution run counter to one another and the optimum is just that. Zero filling the free induction decay (FID) to the sum of the number acquired in both the u and v spectra (in quadrature detection) allow the most information to be extracted. 

The contrast parameters relevant to material characterization through NMR imaging are the intrinsic NMR parameters of the sample. They are referred to as the contrast parameters per se. They can be divided into chemical and physical parameters, and into molecular, mesoscopic, microscopic, and macroscopic parameters. A list of NMR parameters for contrast in NMR imaging is compiled in Table 7.1.1. 

This section on NMR studies of silver has focused on the role of lO /i Ag NMR parameters, because of their unique importance. A great deal of additional information can be obtained by monitoring other nuclei, such as H, C, F, P and Se, present in the hgands bound to silver. We will not present such studies in detail, because the strategies are similar to those described in the section on NMR studies of gold compounds, for which Au NMR is not a practical tool. This section will close with three recent examples of silver NMR studies of interesting systems. 

Effects in NMR Chiroptical Spectroscopy, Orientated Molecules and Anisotropic Systems Heteronuclear NMR Applications (Ge, Sn, Pb) Heteronuclear NMR Applications (O, S, Se, Te) Liquid Crystals and Liquid Crystal Solutions Studied By NMR Magnetic Resonance, Historical Perspective NMR Data Processing NMR in Anisotropic Systems, Theory NMR Relaxation Rates NMR Spectroscopy of Alkali Metal Nuclei in Solution P NMR Parameters in NMR Spectroscopy, Theory of Product Operator Formalism in NMR Relaxometers Xenon NMR Spectroscopy. 

In the heterocycle 16, P and Se high-resolution solid-state NMR spectroscopy was used to study structural properties. Both P CP/MAS and Se CP/ MAS experiments revealed that the asymmetric unit consists of two independent molecules with a different geometry around phosphorus and selenium centres. The established values of anisotropy and asymmetry parameters reflected the distortion of the phosphorus environment, and correlated with X-ray diffraction data. P NMR spectroscopy has been used to follow a new type of P-decomposi-tion in diphosphorylated amines and to characterise new organophosphorus compounds with -N-P(0)-N- linkages. A series of dioxaphosphocin-6-oxides (17), of varying substituent X, have been characterised by using P NMR and... 

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