C nucleus has I
= ½ like 1
H, and the 13
C-NMR spectrum of a
compound can be observed using a different radio frequency range (in the
same magnetic field) to that for 1
H. The 13
C spectrum will give peaks for
each different type of carbon atom in a molecule but the properties of the 13
nucleus give some important and
useful differences in the spectrum obtained:
Interpreting 13C-NMR spectra
- 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
- 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/
- The chemical shift range for 13C is greater (δC = 0-250 ppm) than for 1H
(δH = 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
- 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.
Normally you will be given two 13
C-NMR spectra (Fig. 29.11). The upper
spectrum, which is more complex (more lines) is called the off-resonance
decoupled spectrum and shows the 13
C–1H coupling to enable you to
determine which carbon signals are CH3
, 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 13
C–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 13
C atoms (δC
) vary in the same manner as those of
protons (Fig. 29.12):
- δC 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);
- for sp3 hybridized carbon: δC moves further downfield with the change
from CH3 to CH2 to CH to C;
- for sp2 hybridized carbon: aromatic carbons occur further downfield
(δC = 115-145 ppm) than alkene carbon atoms (δC = 100-140 ppm);
- bonding more electronegative atoms to carbon deshields the carbon
atom and moves the peaks downfield, e.g CH3C (δC ~ 6 ppm) and
CH3O (δC ~ 55 ppm), C=C (δC ~ 123 ppm) and C=O (δC ~ 205
|Fig. 29.12 Approximate chemical shift
positions in 13C-NMR.