Signals isotopic substitution

It is suggested that the nickel is bound to a low molecular weight factor, different from F430 of methanogens, and which may involve a corrinoid structure. The nickel is thought to cycle between Ni and Ni , with the CO bound to the metal. ESR data have been interpreted in terms of an Ni species with a bound radical derived from either CO or CO2. The involvement of a nickel-carbon bond has been unequivocally established by isotopic substitution (which is shown in the g = 2.08, 2.02 signals). About 40% of the total nickel is present as this species, suggesting that the Ni—C species is a viable intermediate in the catalytic conversion of CO to CO2. There are parallels with industrial and laboratory catalytic processes, but the involvement of Ni seems... 

The major advantages of the diaphragm-cell method are its low cost and its slightly superior accuracy (under ideal conditions). It is also possible to use the technique to look at diffusion in complex mixtures which are problematical by NMR. In order to obtain a valid spin-echo measurement for a particular species in a mixture, it is necessary to resolve a signal in the spectrum, with no underlying peaks. However, this may be overcome by isotopic substitution (that is, by deuteration) of a sample. Where this is not a problem, NMR is again the preferred technique, because the diffusion coefficient of all species in a mixture can potentially be obtained in one experiment. ... 

Finally, signal D is quite similar to that found upon adsorption of H2S on M0O3/AI2O3 samples [12], where it could be clearly ascribed, on the basis of isotopic substitution with and Mo, to symmetric S2 species bonded to... 

The interaction of CO with Ni -exchanged zeolites has been studied by i.r. spectroscopy. It was concluded that, under special preparative conditions, a fraction of Ni ions is reduced by CO to Ni with subsequent formation of Ni -CO complexes characterized by CO stretching frequency located at 2205 cm. Reduction of Ni ions in hydrated Y- and M-type zeolites occurs also when the catalyst is dosed with NO and the formation of Ni -NO complexes is observed (y = 1895 cm" and characteristic e.s.r. signal).On activated Ni M-zeolites, the CO interaction causes the formation of Ni (CO)2 dicarbonylic complexes, whose structure has been well documented by CO- CO isotopic substitution experiments. The effect of CO is to stabilize Ni ions formed by the reverse disproportionation reaction... 

Comparison of the signals from the reaction centers of such bacteria with those of bacteriochlorophyll radicals showed that the former exhibited a much reduced line width relative to the latter, although they displayed virtually identical g values. As the line widths were assumed, and later demonstrated by isotopic substitution, to arise from unresolved hyperfine interactions between the unpaired electron and the nuclei on the chlorophyll, a mechanism was sought to account for this reduction in hyperfine coupling. 

---