1. CARBON-13 NMR

2. SALIENT FACTS ABOUT 13C NMR
12C is not NMR-active I = 0
however….
13C does have spin, I = 1/2 (odd mass)
13C signals are 6000 times weaker than 1H because:
1. Natural abundance of 13C is small (1.08% of all C)
2. Magnetic moment of 13C is small
PULSED FT-NMR IS REQUIRED
The chemical shift range is larger than for protons
ppm

3. 1H 13C SALIENT FACTS ABOUT 13C NMR
For a given field strength 13C has its resonance at a
different (lower) frequency than 1H. 1H
Divide the hydrogen
frequency by 4 (approximately)
for carbon-13
1.41 T MHz
2.35 T MHz
13C
7.05 T MHz
1.41 T MHz
2.35 T MHz
7.05 T MHz

4. SALIENT FACTS ABOUT 13C NMR
(cont)
Because of its low natural abundance (0.0108) there
is a low probability of finding two 13C atoms next to
each other in a single molecule.
not probable
13C - 13C
coupling
NO!
Spectra are determined by many molecules contributing
to the spectrum, each having only one 13C atom.
However, 13C does couple to hydrogen atoms (I = 1/2)
very common
13C - 1H
coupling
YES!

5. COUPLING TO ATTACHED PROTONS

6. COUPLING TO ATTACHED PROTONS13 C 13 C 13 C 13 H H H H
n+1 = 4
n+1 = 3
n+1 = 2
n+1 = 1
Methyl
carbon
Methylene
carbon
Methine
carbon
Quaternary
carbon
The effect of attached protons on 13C resonances
( n+1 rule applies )
(J’s are large ~ Hz)

7. ETHYL PHENYLACETATE
13C coupled
to the hydrogens

8. DECOUPLED SPECTRA

9. DECOUPLING THE PROTON SPINS
PROTON-DECOUPLED SPECTRA
A common method used in determining a carbon-13
NMR spectrum is to irradiate all of the hydrogen
nuclei in the molecule at the same time the carbon
resonances are being measured.
This requires a second radiofrequency (RF) source
(the decoupler) tuned to the frequency of the hydrogen
nuclei, while the primary RF source is tuned to the 13C
frequency.
RF source 1
RF source 2
1H-13C
pulse tuned to
carbon-13
“the decoupler”
continuously
saturates
hydrogens
13C signal (FID) measured

10. In this method the hydrogen nuclei are “saturated”,
a situation where there are as many downward as
there are upward transitions, all occuring rapidly.
During the time the carbon-13 spectrum is being
determined, the hydrogen nuclei cycle rapidly between
their two spin states (+1/2 and -1/2) and the carbon nuclei
see an average coupling (i.e., zero) to the hydrogens.
The hydrogens are said to be decoupled from the
carbon-13 nuclei.
You no longer see multiplets for the 13C resonances.
Each carbon gives a singlet, and the spectrum is
easier to interpret.

11. ETHYL PHENYLACETATE 13C coupled to the hydrogens 13C decoupled
in some cases
the peaks of the
multiplets will
overlap
13C coupled
to the hydrogens
this is an
easier spectrum
to interpret
13C decoupled
from the hydrogens

12. All types of 13C spectra

13. SOME INSTRUMENTS SHOW THE MULTIPLICITIES
OF THE PEAKS ON THE DECOUPLED SPECTRA
s = singlet t = triplet
d = doublet q = quartet
CODE : q t s d
This method gives the best of both worlds.

16. Decrease NOE
Enhancement H
CH3
Irradiation

17. CHEMICAL SHIFTS OF 13C ATOMS

18. APPROXIMATE 13C CHEMICAL SHIFT RANGES FOR
SELECTED TYPES OF CARBON (ppm)
R-CH
C C
R2CH
C=C
R3CH
C N
C-I
C-Br O O
C-Cl
R-C-OR
R-C-OH O
C-N
R-C-NH2 O O
C-O
R-C-H
R-C-R

19. Correlation chart for 13C Chemical Shifts (ppm)
200
150
100 50
RANGE
R-CH3
8 - 30
Saturated carbon - sp3
R-CH2-R
no electronegativity effects
R3CH /
R4C
C-O
Saturated carbon - sp3
C-Cl
electronegativity effects
C-Br
Alkyne
carbons - sp
C C
Unsaturated
carbon - sp2
C=C
Aromatic ring
carbons
Acids Amides
Esters Anhydrides
C=O
C=O
Aldehydes
Ketones
200
150
100 50
Correlation chart for 13C Chemical Shifts (ppm)

20. 13C Correlation Chart for Carbonyl and Nitrile Functional Groups
nitriles
acid anhydrides
acid chlorides
amides
esters
carboxylic acids
aldehydes
a,b-unsaturated ketones
ketones
220
200
180
160
140
120
100
ppm
13C Correlation Chart for Carbonyl and Nitrile Functional Groups

21. C sp
C sp
C-X
C sp
C=O acid, amide, ester
C=O aldehyde, ketone

22. SPECTRA Shielding and deshielding
Intensity 1.NOE and relaxation time 2. Equal carbons

23. 1-PROPANOL HO-CH2-CH2-CH3 c b a
PROTON
DECOUPLED
200
150
100 50
Proton-decoupled 13C spectrum of 1-propanol (22.5 MHz)

24. 2,2-DIMETHYLBUTANE

25. BROMOCYCLOHEXANE

26. CYCLOHEXANOL

27. TOLUENE

29. CYCLOHEXENE

30. CYCLOHEXANONE

31. 1,2-DICHLOROBENZENE a b c

32. 1,3-DICHLOROBENZENE
solvent d b a c d a

34. Solvent Splitting
Carbon Spectrum
-CD3
-CHD2
Hydrogen Spectrum
-CHD2