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13.18 Carbon13.18 Carbon--13 NMR13 NMR
12C is not NMR-active I = 0
however…. 13C does have spin, I = 1/2 (odd mass)
1. Natural abundance of 13C is small (1.08% of all C)
2. Magnetic moment of 13C is small
13C signals are 6000 times weaker than 1H because:
SALIENT FACTS ABOUT SALIENT FACTS ABOUT 1313C NMRC NMR
PULSED FT-NMR IS REQUIRED
The chemical shift range is larger than for protons
0 - 220 ppm
COUPLING TO ATTACHED PROTONS
The effect of attached protons on 13C resonances
n+1 = 4 n+1 = 3 n+1 = 2 n+1 = 1
C13
3 protons 2 protons 1 proton 0 protons
H
H
H
C13
H
H
C13
H C13
Methyl
carbon
Methylene
carbon
Methine
carbon
Quaternary
carbon
( n+1 rule applies )
COUPLING TO ATTACHED PROTONSCOUPLING TO ATTACHED PROTONS
(J’s are large ~ 100 - 200 Hz)
ETHYL PHENYLACETATEETHYL PHENYLACETATE
13C coupled
to the hydrogens
DECOUPLED SPECTRA
ETHYL PHENYLACETATEETHYL PHENYLACETATE
13C coupled
to the protons
13C decoupled
from the protons
in some cases
the peaks of the
multiplets will
overlap
this is an
easier spectrum
to interpret
CHEMICAL SHIFTS OF 13C ATOMS
Aldehydes
Ketones
Acids Amides
Esters Anhydrides
Aromatic ring
carbons
Unsaturated
carbon - sp2
Alkyne
carbons - sp
Saturated carbon - sp3
electronegativity effects
Saturated carbon - sp3
no electronegativity effects
C=O
C=O
C=C
C C
200 150 100 50 0
200 150 100 50 0
8 - 30
15 - 55
20 - 60
40 - 80
35 - 80
25 - 65
65 - 90
100 - 150
110 - 175
155 - 185
185 - 220
Correlation chart for 13C Chemical Shifts (ppm)
C-O
C-Cl
C-Br
R3CH R4C
R-CH2-R
R-CH3
RANGE
/
SPECTRA
WWU Chemistry
TolueneCH3
Proton-decoupled 13C spectrum of 1-propanol (22.5 MHz)
200 150 100 50 0
11--PropanolPropanol
PROTON
DECOUPLED
HO-CH2-CH2-CH3c b a
CyclohexanolCyclohexanol
CyclohexanoneCyclohexanone
Cl
Cla
a
b
b
c
c
1,21,2--DichlorobenzeneDichlorobenzene
1,31,3--DichlolrobenzeneDichlolrobenzene
Cl
Cl
1,4-Dichlorobenzene
Cl
Cl
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