Introduction to C-13 NMR
• The 13C
nucleus is present in only 1.08% natural abundance. Therefore,
acquisition of a spectrum usually takes much longer than in 1H NMR.
• The magnetogyric ratio of the 13C nucleus is about 1/4 that of the 1H nucleus.
Therefore, the resonance frequency in 13C NMR is much lower than in 1H NMR.
(75 MHz for 13C as opposed to 300 MHz for 1H in a 7.04 Tesla field).
• At these lower frequencies, the excess population of nuclei in the lower spin state
is reduced, which, in turn, reduces the sensitivity of NMR detection.
• Unlike 1H NMR, the area of a peak is not proportional to the number of
carbons giving rise to the signal. Therefore, integrations are usually not done.
• Each unique carbon in a molecule gives rise to a 13C NMR signal. Therefore,
if there are fewer signals in the spectrum than carbon atoms in the compound,
the molecule must possess symmetry.
• When running a spectrum, the protons are usually decoupled from their respective
carbons to give a singlet for each carbon atom. This is called a proton-decoupled
spectrum.
Carbon-13 Chemical Shift Table
CC triple bonds
http://www.chemistry.ccsu.edu/glagovich/teaching/316/nmr/images/fig15.gif
Alkane: 2-methylpentane
Alcohol: 2-hexanol
OH
Alkyl Halide: 3-bromopentane
Br
Alkene: 1-hexene
Aromatic Ring: eugenol
O
HO
Carboxylic Acid: pentanoic acid
CO2H
Ester: ethyl valerate
O
O
Amide: pentanamide
NH2
O
Ketone: 3-methyl-2-pentanone
O
Aldehyde: 2-methylpentanal
O
H
Symmetry in C-13 NMR
Each unique carbon in a molecule gives rise to a 13C NMR signal. Therefore,
if there are fewer signals in the spectrum than carbon atoms in the compound,
the molecule must possess symmetry. Examples:
CH3CH2
CH2CH3
OH
Enantiotopic vs Diastereotopic CH3’s
O
enantiotopic
OH
CH3
O
diastereotopic
*
*
*
**
Determine the number of signals in the proton-decoupled
C-13 NMR spectrum of each of the following compounds:
O
O
H3C
CH3
CH3
OCCH3
HO
OCH3
N
H
CH3
OH
OH
CH3
H3C
CH3
Carbon-13 NMR Spectrum of Geraniol
ppm
139.07
131.62
124.07
123.71
59.16
39.64
26.51
25.66
17.66
16.24
Carbon #
1
2
3
4
5
6
7
8
9
10
8
9
T1 and NOE Effects in C-13 NMR
Because of unequal T1 and NOE effects, peaks heights vary widely in C-13 NMR.
This is why C-13 spectra are normally not integrated.
CH3
1
3
2
2
3
4
Carbon
T1 (sec)
NOE
CH3
16
0.61
1
89
0.56
2
24
1.6
3
24
1.7
4
17
1.6
4
1
CH3
Carbon-13 Proton-Coupled Patterns
http://www.chemistry.ccsu.edu/glagovich/teaching/316/nmr/13ccoupled.html
Carbon-13 Proton-Coupled Spectrum of Ethyl Phenylacetate
Difficult to interpret
O
O
C=O
Typical coupling constants for 13C-1H onebond couplings are between 100 to 250 Hz.
http://www.chemistry.ccsu.edu/glagovich/teaching/316/nmr/13ccoupled.html
DEPT Spectra
DEPT-135
DEPT-90
DEPT-45
CH
C
CH2
CH3
normal C-13 spectrum
Quaternary carbons (C) do not show up in DEPT.
Simulated DEPT Spectra of Ethyl Phenylacetate
O
O
DEPT-135
O
DEPT-90
O
DEPT-45
Normal C-13 spectrum
DEPT Spectra of Codeine
Predict the normal C-13, DEPT-90, and DEPT-135
spectra of ipsenol, whose structure appears below.
DEPT Spectra of Ipsenol
DEPT-90
DEPT-135
CDCl3
Normal C-13 spectrum
www.lasalle.edu/~price/DEPT%20and%20COSY%20Spectra.ppt
Determine the number and appearance of the signals in the DEPT-45, DEPT 90,
and DEPT 135 NMR spectrum of each of the following compounds:
O
O
H3C
CH3
CH3
OCCH3
HO
OCH3
N
H
CH3
OH
OH
CH3
H3C
CH3