¹³C-NMR spectra

The ¹³C nucleus has I = ½ like ¹H, and the ¹³C-NMR spectrum of a compound can be observed using a different radio frequency range (in the same magnetic field) to that for ¹H. The ¹³C spectrum will give peaks for each different type of carbon atom in a molecule but the properties of the ¹³C nucleus give some important and useful differences in the spectrum obtained:

• The natural abundance of ¹³C is only 1.1% compared with 98.9% for ¹²C - in any molecule no two adjacent atoms are likely to be ¹³C and therefore coupling between ¹³C nuclei will not be seen, giving a very simple spectrum.
• In a sample of a compound, which contains many molecules, the ¹³C isotope is randomly distributed and all the different carbon atoms in a sample of a compound will be seen in the ¹³C-NMR spectrum.
• The sensitivity of the ¹³C nucleus is low and this, together with its low natural abundance, means that FT -NMR is the only practical system to produce a spectrum by accumulation of spectra by repetitions. Larger sample size in bigger NMR tubes also assist in solving the sensitivity/ abundance problem.
• The chemical shift range for ¹³C is greater (δ_C = 0-250 ppm) than for ¹H (δ_H = 0-15 ppm) giving greater spectral dispersion, i.e. the peaks for carbons with very slight differences in chemical shifts are separated and do not overlap.
• ¹³C nuclei will couple with the ¹H nuclei to which they are directly bonded, e.g. CH₃ will appear as a quartet, CH₂ as a triplet, CH as a doublet, but C with no hydrogen atoms attached will appear as a singlet. This introduction of complexity in the ¹³C-NMR spectrum is removed by broadband decoupling.
• The peak areas of the different carbon atoms are not related to the number of carbon atoms having the same chemical shift, as in the case for ¹H-NMR spectra.

Interpreting ¹³C-NMR spectra
Normally you will be given two ¹³C-NMR spectra (Fig. 29.11). The upper spectrum, which is more complex (more lines) is called the off-resonance decoupled spectrum and shows the ¹³C–1H coupling to enable you to determine which carbon signals are CH₃, CH₂, CH and C. Then overlapping of peaks may make the identification of different carbon atoms difficult. The lower spectrum is a broadband decoupled spectrum in which the molecule is irradiated with a second radio frequency range for the protons in the molecule and effectively removes all the ¹³C–1Hcouplings from the spectrum. The resulting simplicity of the spectrum makes identification of the different types of carbon in the molecule relatively easy.

Fig. 29.11 13C–NMR spectra of 1-methoxypropanone: (i) off-resonance decoupled; (ii) broadband decoupled.

The chemical shifts of ¹³C atoms (δ_C) vary in the same manner as those of protons (Fig. 29.12):

1. δ_C moves downfield as the hybridization of the carbon atom changes from sp³ (0-50 ppm) to sp (75-105 ppm) to sp²(100-140 ppm);
2. for sp³ hybridized carbon: δ_C moves further downfield with the change from CH₃ to CH₂ to CH to C;
3. for sp² hybridized carbon: aromatic carbons occur further downfield (δ_C = 115-145 ppm) than alkene carbon atoms (δ_C = 100-140 ppm);
4. bonding more electronegative atoms to carbon deshields the carbon atom and moves the peaks downfield, e.g CH₃C (δ_C ~ 6 ppm) and CH₃O (δ_C ~ 55 ppm), C=C (δ_C ~ 123 ppm) and C=O (δ_C ~ 205 ppm).

Fig. 29.12 Approximate chemical shift positions in 13C-NMR.