13C-Signals

Most 13C spectra are run on Fourier-transform NMR (FT-NMR) spectrometers using broadband decoupling of proton spins so that each chemically distinct carbon shows a single unsplit resonance line. As with NMR, the chemical shift of each 13C signal provides information about a carbon s chemical environment in the sample. In addition, the number of protons attached to each carbon can be determined using the DEPT-NMR technique. 

The 13C spectrum of normal sterigmatocystin can thus be compared with the spectra of the molecules resulting from separate feeding experiments with 13C enriched (o) 13CH3C02H and (b) CH313C02H, respectively. Those carbon atoms, in each case, which now show enhanced 13C signals can thereby be identified ... 

Other good news comes in the shape of the 13C nucleus having a spin quantum number of /2. This means that 13C signals are generally sharp as there are no line-broadening quadrupolar relaxation issues to worry about and we don t have to deal with any strange multiplicities. 

Fig. 11 Plot of the 13C signal intensity as a function of contact time for two distinct methyl resonances of two polymorphic forms of a developmental drug substance.

Broadband XH decoupling, in which the entire proton spectral window is irradiated, collapses all of the 13C multiplets to singlets, vastly simplifying the 13C spectrum. An added benefit of broadband proton decoupling is NOE enhancement of protonated 13C signals by as much as a factor of three. 

Fig. 10.12. Pulse sequence for amplitude modulated 2D J-resolved spectroscopy. The experiment is effectively a spin echo, with the 13C signal amplitude modulated by the heteronuclear coupling constant(s) during the second half of the evolution period when the decoupler is gated off. Fourier transformation of the 2D-data matrix displays 13C chemical shift information along the F2 axis of the processed data and heteronuclear coupling constant information, scaled by J/2, in the F1 dimension.

Fig. 10.13. 2D J-resolved NMR spectrum of santonin (4). The data were acquired using the pulse sequence shown in Fig. 10.12. Chemical shifts are sorted along the F2 axis with heteronuclear coupling constant information displayed orthogonally in F . Coupling constants are scaled as J/2, since they evolve only during the second half of the evolution period, t /2. 13C signals are amplitude modulated during the evolution period as opposed to being phase modulated as in other 13C-detected heteronuclear shift correlation experiments.

Carbon-13 NMR was utilized to study different aspects of the reactivity of the metal complexes as a function of certain structural features in the selected oxocyano complexes of Mo(IV), W(IV), Tc(V), Re(V), and Os(VI) as depicted in Scheme 1 and illustrated in Figs. 1-4. The NMR spectral properties were similar to those obtained from 13C NMR in general, i.e., very sharp lines indicative of fairly long relaxation times in the order of a few seconds. The large quadrupolar moment ofTc-99 (7 = 9/2, 100% abundance) led to a very broad bound 13C signal (Fig. 5), thus excluding the quantitative study of the cyanide exchange by 13C NMR. However, 16N NMR was successfully used instead. 

The values of a series of a- and /3-shifts have been determined for mono-O-methylglucoses and mono-O-methylmannoses, in order to aid in assignments of 13C signals of oligosaccharides and polysaccha-... 

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