Chemical Shift Scale and Range

As with H NMR, the 13C NMR frequency axis is converted to a unitless scale this scale is in 8 units (ppm). The chemical shifts in routine 13C spectra range over about 220 ppm from TMS—about 20 times that of routine H spectra ( 10 ppm). As a result of the large range and the sharpness of the decoupled peaks, coincidences of 13C chemical shifts are uncommon, and impurities are readily detected. Often, even mixtures provide useful information. (See Appendix B for an extensive list of common impurities.) For example, stereoisomers that are difficult to analyze by means of H spectrometry usually show discrete, 3C peaks. 

The fundamental NMR equation [v = (y/27r)B0] is used to calculate the resonance frequency for the l3C nucleus at a given magnetic field strength. For example, a 600 MHz instrument (for H) is used at 150.9 MHz to produce a l3C spectrum—i.e., the ratio is about 4 1. The 

FIGURE 4.2 (a) Standard l3C pulse sequence with proton decoupling. Rd is relaxation delay, 0 is a variable pulse angle, t2 is acquisition time, (b) FID sinusoidal display of decoupled cholesterol at 150.9 MHz in CDCI3. (c) Expanded small section of FID. 

Unlike H NMR spectra, whose integration of signals represents the ratio of protons, integration of 13C peaks do not correlate with the ratio of carbon atoms in routine spectra. There are two major factors that account for the problem of peak intensities in 13C spectra  

As discussed in Section 3.2.3, 7) and T2 relaxation times are short for protons resulting in intensities that 

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