Spin-active nuclei

Tin has nine naturally occurring isotopes but only two have spin-active nuclei ( Sn, 8.7% abund., 1 = Vi-, Sn, 7.7% abund., I = Vr, remaining isotopes Sn, 83.6% abund., 1 = 0). Because Sn is slightly more abundant and has... 

Be able to identify simple organic molecules from their H NMR spectra through the interpretation of their integrals (the number of hydrogens in each environment), chemical shifts (the effect of functional groups on the electron density of the molecule) and coupling patterns (the number of spin active nuclei on adjacent atoms)... 

Definitive assignment of the 13C resonances has been possible only in few cases where 13C labeling studies have been carried out249 or when additional information is provided by coupling to other spin-active nuclei associated with the supporting metal-ligand fragments. In this respect, /(CP) values... 

Arsines (AsR3) and stibines (SbR3) are generally found to be more labile (sometimes an advantage) than phosphines, in part due to their reduced o-donor properties. Furthermore, arsenic and antimony lack conveniently measurable spin-active nuclei for NMR studies, in contrast to phosphines (31P / = Vi, 100% abundance), for which an enormous amount of informative data is available. Because of the direct bond between phosphorus and the metal centre, information is available about the electronic nature and stereochemistry of the metal centre, in addition to information about dynamic processes. This is further enhanced in the case of complexes involving metals with NMR-active nuclei, e.g. W, Rh, V, Pt, Hg and Ag, where /(M-31P) couplings may be observed. 

Explain why the F NMR spectrum of BFCI2 consists of a 1 1 1 1 quartet. What would you expect to observe in the F NMR spectrum of BF2CI Data for the spin-active nuclei in these compounds are given in Table 2.3. 

In Mc4Sn, all twelve protons are equivalent and one signal is expected. Sn has two NMR active nuclei " Sn (7.6%, 7 = i) and " Sn (8.6%, I = - The H nuclei couple to the Sn nucleus to give a doublet, and to the " Sn nucleus to give another doublet. The relative intensities of the lines in the signal reflect the abundances of the spin-active nuclei ... 

Ans. Take into account the % abundances of spin-active nuclei]... 

Although F, Cl, Br and I aU possess spin active nuclei, in practice only F (100%, / = 5) is used routinely. Fluorine-19 NMR spectroscopy is a valuable tool in the elucidation of structures and reaction mechanisms of F-containing compounds see case studies 1 and 5 and the discussion of stereochemicaUy non-rigid species in Section 2.11. 

Several of the metals have spin-active nuclei and this sometimes allows direct observation using NMR spectroscopy, e.g. has a shift range >1000 ppm and Y NMR spectroscopy is valuable for characterizing yttrium-containing compounds. In general, it is more convenient to make use of the coupling of metal nuclei to more easily observed nuclei such as h, or P. Some examples of nuclei with I = are... 

If we are to take full advantage of 3D and 4D experiments though, molecular biology techniques (Chapter 3) must be used to ensure that the protein of interest is effectively universally labelled with spin-active nuclei, in order to avoid the serious sensitivity problem created by the fact that spin-active nuclei of interest are normally found only in low natural abundance (a few per cent). As a general rule of thumb, where a protein is comprised of between 50... 

A one-bond coupling occurs when a single bond links two spin-active nuclei. The bonding electrons in a single bond are assumed to avoid each other such that when one electron is near nucleus A, the other is near nucleus B. According to the Pauli Principle, pairs of electrons in the same orbital have opposed spins therefore, the Dirac model predicts that the most stable condition in a bond is where both nuclei have opposed spins. FoUowing is an illustration of a H bond, where the nucleus of the atom (heavy black arrow) has a spin opposite to that of the hydrogen nucleus (heavy white arrow). 

Interaction between a static magnetizing field and the spin angular momentum of spin active nuclei processing with angular frequency of a sample placed inside a radiofrequency (rf ) coil H SS-NMR spectroscopy... 

The following case studies illustrate the use of NMR spectroscopy to study compounds containing spin-active nuclei other than H and C. 

Related spectra for the oxalate and terephthalate bridged Mo4-containing compounds supported by pivalate ligands are shown in Fig. 9. Here it is clearly evident that on the EPR time scale, 10 s , the oxalate cation involves complete delocalization of the positive charge over all four Mo atoms. The single electron in the HOMO sees equally all four Mo atoms. Here the presence of two spin active nuclei in the M center is more easily seen. In contrast, the EPR spectmm of the terephthalate radical cation is localized on one M02 center the mixed-valence ion is valence-trapped [34]. 

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