NMR silent

Of course, water molecules are not NMR silent and NMRI engineering has, indeed, advanced at a remarkable pace to provide extraordinary technical capabilities. These capabilities now enable studies of systems beyond those in the biomedical arena, systems that are, in many respects, far more technically challenging. This has led to the development of innovative and fascinating strategies and tactics to deal with NMRI-unfriendly samples and conditions. 

The authors note that only 50% of the initial rhenium is observed they presume conversion to NMR-silent paramagnetic compounds, but provide no further evidence. Addition of Me2S converts it immediately to 21 at -50°C the pyridine by-product displaces the DMSO. Warming the complex to 0°C causes formation of three new products. Phenoxy pyridine complex 23 is formed, presumably from carbon migration followed by trapping with pyridine. A phenoxy chloride is also present, rationalized by CT abstraction from solvent. Finally, a catecholate complex 24 is formed, presumably from subsequent oxidation of the initial phenoxy rhenium(V) cation the proportion of this species increases with increasing concentrations of the A-oxide. Identities of products 23 and 24 were confirmed by independent synthesis. This thus represents the first system where alkyl-to-oxo migration can be directly observed. 

Where the S nuclide is less than 100% abundant, breakthrough of signals from molecules containing NMR silent S nuclei is better suppressed by gradient... 

Many S nuclides have high natural abundance ( Yb, Rh, Ag, etc.) suppression of signals from molecules containing NMR silent 5 nuclei is not an issue. 

If the intensity of the desired signal is the limiting factor, then phase cycle selection is preferred. A phase cycle selection version of HMQC that uses a spin-lock to purge signals from molecules containing NMR silent 5 nuclei has been developed—poor man s gradient-heteronuclear... 

A BIRD filter (bilinear rotation decoupling) can be nsed to snppress signals from molecules containing NMR silent 5 nnclei when the insensitive nuclide S has /ownatmal abundance. 

The BIRD Filter. The BIRD filter is optional (see Experiment 3.4.1). If used, d3 is set so that the undesired magnetization from molecules containing NMR silent 5 is at the zero crossing point before proceeding to the spin-lock sequence. d2 = /lJ IS) (Figure 16). 

A nucleus with a spin 1= 0 has no NMR spectrum, it is NMR silent, and does not couple to other nuclei. Important examples are C, 0, and S. We are all familiar with the consequences from C-NMR. Here the predominant isotope is C, an NMR silent nucleus. The NMR active nucleus C has a natural abundance of only 1.1%. Coupling to H is observed and usually deactivated by broadband decoupling. Couphng to carbon is not observed, except in the case of C enrichment, due to probability constraints. 

NMR silent. N has a nnclear moment /= 1 and a sizeable qnadrnpolar moment that makes the NMR signals nsnally very broad and difficult to analyse. 

Attempts were also made to observe or isolate catalyticaUy relevant intermediates. Addition of one equivalent of PhCsCPh to ( PDI)Fe(N2)2 yielded a red paramagnetic solid identified as the iron Tj -alkyne complex, which was also characterized by X-ray diffraction. Treatment of the isolated product with 1 atm of H2 initially yielded stilbene, followed by 1,2-diphenylethane, establishing its catalytic competency (Scheme 4.8). Our laboratory has recently conducted experiments to elucidate the stability of related bis(imino)pyridine olefin complexes [87]. Treatment of C PDl)Fe(N2)2 vvith 1-hexene under a dinitrogen atmosphere resulted in slow conversion to n-hexane and the paramagnetic, NMR-silent intramolecular... 

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