Carbon spectrum acquisition

Because of the inherently poor sensitivity, the acquisition of a carbon spectrum should be considered optional. Deciding whether to collect carbon data depends on the expected sensitivity based on the proton spectrum, the anticipated sample stability over the duration of the experiment, the required degree of carbon resolution necessary, and the importance of directly detecting quaternary carbons. Alternatively, the carbon resonances can be... 

Typically, H and NMR spectroscopy have been apphed when studying the structure of hb polymers. Although the proton spectrum is the easier to acquire, it is often less informative than the carbon spectrum. In the case of monomers based on amines, sihcon, or phosphorus, valuable information has been obtained from [78, 79], Si [76, 80-88], and NMR [89] spectra. The acquisition of a NMR spectrum should be considered if fluorine is present in the subunits [56,... 

Figure 1. Background (no sample) spectrum at different scale expansions illustrating system wavelength response, absorption attributable to typical amounts of residual water and carbon dioxide, and high signal-to-noise. The spectrum required 12 minutes of data acquisition at 8-cm optical retardation.

The order in which various NMR data are acquired is largely one of user preference. Acquisition of the proton reference spectrum will invariably be undertaken first. Whether a user next seeks to establish homo- or heteronuclear shift correlations is where individual preferences come into play. Many spectro-scopists proceed from the proton reference spectrum to either a COSY or a TOCS Y spectrum next, while others may prefer to establish direct proton-carbon chemical shift correlations. This author s preference is for the latter approach. From a multiplicity-edited HSQC spectrum you obtain not only the carbon chemical shifts, which give an indication of the location of heteroatoms, the degree of unsaturation and the like, but also the number of directly attached protons, which eliminates the need for the acquisition of a DEPT spectrum [51, 52]. The statement in the prior sentence presupposes, of course, that there the sensitivity losses associated with the acquisition of multiplicity-edited HSQC data are tolerable. 

Figure 5.3 GHMBC spectrum of a colored impurity formed during the synthesis of Tipranavir. The long-range delay in the experiment was optimized for 10 Hz the data were acquired in 12.5 h. Chemical shift labels show the chemical shift of the carbon to which a given proton is long-range coupled. As can be seen by simple inspection, there was considerable degradation of the sample during the course of the data acquisition as there are peaks in the contour plot corresponding to responses that were not observed in the proton spectrum taken at the outset of data acquisition, which is plotted above the contour plot.

Figure 4. 31.94 MHz 13C NMR data for intact lime cutin (bottom) and the solid residue of a depolymerization treatment (top). Both spectra were obtained with a 1H-13C contact time of 1.0 ms, repetition rate of 1.0 s, spinning rate of 3.0 kHz, a H decoupling field of 60 kHz, and a line broadening of 20 Hz. (For the chosen contact time, peak intensities within each spectrum reflect the approximate numbers of each carbon type.) Only the intact cutin spectrum retained signal intensity near 30 ppm when decoupling was delayed before acquisition (13,14).

Fig. 3. Long range and one-bond carbon-13 satellite spectrum of a 5% w/w solution of ethanediol in D2O at 94°C. 16 transients were measured on a Varian Associates Unity 500 spectrometer using the sequence of fig. 1, with 2.5 s presaturation, a t value of 100 ms, spin lock pulses of 450 ps, no homospoil pulse, and no homodecoupling during acquisition.

Fig. 4. (a) 300 MHz proton spectrum and (b)-(e) selective reverse INEPT spectra of 28% menthone (Aldrich) in acetone-ds, measured using a 5 mm sample in the 10 mm broadband probe of a Varian Associates XL300 spectrometer using the sequence of fig. 1. The sample contains substantial quantities of isomenthone, seen clearly in the methyl region of trace (a). Spectra (b) to (e) used selective excitation of carbon sites 6, 7, 2 and 8, respectively, with delays 2r of 3.85, 3.85, 1.92 and 1.54 ms. 32 transients were used for each trace no spin lock pulses or 180° pulses were used. Traces (b) to (e) have a vertical scale lOOOx that of trace (a). No homodecoupling was used during acquisition. 

Fyfe et al. (355) were able to produce a very informative 13C CP/MAS NMR spectrum of the triphenylmethyl carbonium ion by using the tetra-fluoroborate counterion and by employing simultaneous 19F and H decoupling during spectral acquisition (see Fig. 81). The nonequivalence of the ortho and meta carbons is readily seen in the spectrum. 

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