Magic-angle spinning labeling

Although limited by sensitivity, chemical reaction monitoring via less sensitive nuclei (such as 13C) has also been reported. In 1987 Albert et al. monitored the electrochemical reaction of 2,4,6-tri-t-butylphenol by continuous flow 13C NMR [4]. More recently, Hunger and Horvath studied the conversion of vapor propan-2-ol (13C labeled) on zeolites using 1H and 13C in situ magic angle spinning (MAS) NMR spectroscopy under continuous-flow conditions [15]. 

It was reasoned that the polymer must consist of a, a -coupled pyrrole units, with fi coupling less important, because of the fact that oc-substituted pyrroles do not polymerise whereas -substituted species do, and on the basis of magic angle spinning 13C nmr and IR techniques. The method of labelling the monomer ring positions is also shown in Scheme 3.9. 

In the presence of excess ammonia, the surface complex coexists with its ammonia adduct ](=SiO)2Ta(=NH)(NH2)(NH3)]. NMR studies on the fuUy N-labeled samples have led to unambiguous discrimination between imido, amido and amino resonances of the surface complex and its NH3 adduct, and [=Si- NH2] through the combination of soHd-state magic-angle spinning(MAS), heteronuclear correlation (HETCOR), 2D proton double-quantum (DQ), singlequantum (SQ) correlation, and 2D proton triple quantum (TQ) single quantum... 

The 2,2,6,6-tetramethylpiperidinoxyl radical (TEMPO) was first prepared in 1960 by Lebedev and Kazarnovskii by oxidation of its piperidine precursor.18 The steric hindrance of the NO bond in TEMPO makes it a highly stable radical species, resistant to air and moisture. Paramagnetic TEMPO radicals can be employed as powerful spin probes for elucidating the structure and dynamics of both synthetic and biopolymers (e.g., proteins and DNA) by ESR spectroscopy.19 Unlike solid-phase 1H-NMR where magic angle spinning is required in order to reduce the anisotropic effects in the solid-phase environment, solid-phase ESR spectroscopy can be conducted without specialized equipment. Thus, we conducted comparative ESR studies of various polymers with persistent radical labels, and we also determined rotational correlation times as a function of... 

By use of 13C CP MAS NMR (crosspolarization/magic angle spinning) and X-ray diffraction the structure of 3(5)-methyl-4-nitropyrazole in the solid state was examined [252], Desmotropy in azoles has been revealed for the first time. This means that the two tautomers, 3-methyl-4-nitro- and 5-methyl-4-nitropyrazole, are crystallized in two different crystals [252, 256], The problem of proton transfer in solid-state heterocycles, including nitropyrazoles, was studied by CP MAS NMR and X-ray structural analysis [253, 257], The kinetics of degenerate intermolecular triple proton and deuteron transfers in the cyclic trimer of polycrystalline 4-nitrop-yrazole (15N-labeled) has been studied as a function of temperature and is compared to the kinetic of triple proton transfer in bulk solid 3,5-dimethylpyrazole. The results show that the transfer kinetics in the new trimer is much faster than that in 3,5-dimethylpyrazole. Nevertheless, the kinetics of HHH/HHD/HDD/DDD isotope effects of 4-nitropyrazole is similar to that of 3,5-dimethyl derivative [257]. 

Peptides. The correlation of deuterium quadrupolar tensors by spin diffusion under slow magic-angle-spinning conditions can provide accurate measurements of their relative orientation. This work showed the technique applied to the cyclo-P-peptide cyclo[(5)- 3-homoalanyl-(R)-P-homoalanyl-(S)-P-homoalanyl-(i )-P-homoalanyl] with its amide hydrogens labeled by deu-terons. From the 2D spin-diffusion spectrum, the mutual orientation of the amide deuteron quadrupolar coupling tensors were found. Eight conformations that are all consistent with the NMR measurement were determined. 

The ability of NMR spectrometers to operate in the cross-polarization/ magic angle spinning mode is a powerful tool for structural elucidation of insoluble materials (556). Natural abundance solid-phase C-NMR spectra could be obtained for synthetic L-dopa eumelanin. The inordinate number of resonance signals, however, prevented definitive assignments of the peaks to specific carbons. Subsequently Chedekel et al. (357) used this technique to study the conversion of specifically labeled L-dopa and... 

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