C Isotopic Enrichment

Photolysis of ketones in micelles with simultaneous application of an external magnetic field permits a C isotope enrichment. (Cf. Section 6.1.5.5.) This is the case because C nuclei have a magnetic moment and thus accelerate the spin inversion by the hyperfine interaction mechanism. (Cf. Example 4.9.) Due to the more efficient recombination of radicals containing C, the initial product formed after photolysis in a back reaction is C enriched (TUrro et al., 1980b). 

Figure 4 The three main components in the IDMS procedure (a) natural standard, (b) sample and (c) isotopically enriched spike...

These studies show that the lack of hydrogen content does not prevent the use of solid-state NMR to achieve important local information on carbon materials. Direct excitation methods, in some favorable cases combined with C isotope enrichment, are a promising alternative for the recording of informative C NMR spectra for many important new carbon materials, containing little or no hydrogen. 

Agarwal et al. have also shown that H, C correlation spectra in uniformly H, C isotopically enriched peptides and proteins can be recorded in MAS solid-state NMR with site specific resolution. A resolved deuterium dimension is obtained by evolving double-quantum coherences. Experimental linewidths are obtained that are as small as 16 Hz (0.17 ppm at 600 MHz) in the double-quantum dimension. The unprecedented resolution in the deuterium dimension obtained for proteins opens new perspectives for correlation experiments and, in particular, for the characterization of dynamics of proteins in the solid-state. 

Clough, R., Belt, S. T., Evans, E. H., Sutton, R, Eairman, B., and Catterick, T. (2003) Uncertainty contributions to species specific isotope dilution analysis. Part 1 Characterisation and stability of and C isotopically enriched methylmercury by H NMR. J. Anal. At. Spectrom., 18, 1033—8. 

Tieszen 1991). Water stress, for instance, lowers photo synthetic discrimination against C, resulting in isotopic enrichment. C4 plants are not affected isotopically by environmental parameters although their distributions are (Ehleringer et al. 1997). Under conditions of increasing aridity, the climate variable of most likely importance in African sites, C3 plants should become slightly more enriched on average. Therefore one should be cautious about use... 

Fe Q-band ENDOR study of the isotopically enriched Ni-C state of D. gigas and D. desulfuricans hydrogenases and Ni-B state of D. desulfuricans revealed a weak coupling between the Fe and the nickel atoms when the enzyme was in the Ni-A forms while no coupling was observed for the Ni-B form (186). A careful analysis of linewidth of Ni-A and Ni-B EPR signals detected in Fe enriched and nonenriched hydrogenase samples indicated that hyperfine interactions are lost in the spectral linewidth and, hence, nonde-tectable. 

C is present in 1.1% natural abundance and so approximately n% of a M (CO) complex is isotopically labelled. General infrared instrumental improvements have made studies on such natural-abundance species perfectly feasable, for most i>(CO) infrared peaks are rather narrow in hydrocarbon solvents. The commercial availability of 13 CO and C180 (substitution by either of which has very similar frequency perturbation consequences) has led to studies of isotopically enriched species. Perhaps because of the emphasis on studies of v(CO) vibrations, little work has been reported... 

Fig. 5.10 The mass dependence of the critical temperature of the superconducting/resistive transition in isotopically enriched samples of mercury (Triangles Reynolds, C. A., et al. Phys. Rev. 78, 487 (1950). Circles Maxwell, E., Phys. Rev. 78, 477 (1950))...

Figure 3. Lithium isotope data for a range of commercially-available synthetic concentration standards (Qi et al. 1997a). The inset expands the c. 60%o range of reported natural samples. Although most anthropogenically-processed Li retains a broadly terrestrial value, nearly 20% of the samples examined show enormous isotopic enrichment in the heavy isotope.

Example Isotopic enrichment is a standard means to enhance the response of an analyte in nuclear magnetic resonance (NMR). Such measures gain importance if extremely low solubility is combined with a large number of carbons, as is often the case with [60]fullerene compounds. [19] The molecular ion signals, IVT, of Qo with natural isotopic abundance and of C-enriched Cgo are shown below (Fig. 3.11 for EI-MS of [60]fullerenes cf. Refs. [20-22]). From these mass spectra, the enrichment can be determined by use of Eq. 3.1. For Qo of natural isotopic abundance we obtain Mrceo = 60 x 12.0108 u = 720.65 u. Applying Eq. 

The solution structures of the reduced and oxidized forms of a biologically fully active 10.5 kDa fragment of the Paracoccus denitrificans cytochrome c-552 enriched with and N isotopes were obtained. No significant... 

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