13.18 Carbon-13 NMR
SALIENT FACTS ABOUT 13C NMR
12C
is not NMR-active
however….
13C
13C
I= 0
does have spin, I = 1/2 (odd mass)
signals are 6000 times weaker than 1H because:
1. Natural abundance of
2. Magnetic moment of
13C
13C
is small (1.08% of all C)
is small
PULSED FT-NMR IS REQUIRED
The chemical shift range is larger than for protons
0 - 220 ppm
COUPLING TO ATTACHED PROTONS
COUPLING TO ATTACHED PROTONS
3 protons
2 protons
H
13
C
0 protons
H
H
H
n+1 = 4
Methyl
carbon
1 proton
13
C
H
n+1 = 3
Methylene
carbon
13
C
H
n+1 = 2
Methine
carbon
13
C
n+1 = 1
Quaternary
carbon
The effect of attached protons on 13C resonances
( n+1 rule applies ) (J’s are large ~ 100 - 200 Hz)
ETHYL PHENYLACETATE
13C
coupled
to the hydrogens
DECOUPLED SPECTRA
ETHYL 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
200
150
100
50
0
RANGE
R-CH3
Saturated carbon - sp3
no electronegativity effects
8 - 30
R-CH2-R
15 - 55
R3CH / R4C
20 - 60
40 - 80
C-O
Saturated carbon - sp3
35 - 80
C-Cl
electronegativity effects
C-Br
C
Unsaturated
carbon - sp2
C=C
C
25 - 65
Alkyne
carbons - sp
65 - 90
100 - 150
Aromatic ring
carbons
C=O
C=O
200
110 - 175
Acids Amides
Esters Anhydrides
Aldehydes
Ketones
150
100
155 - 185
185 - 220
50
0
Correlation chart for 13C Chemical Shifts (ppm)
Some useful information!
• sp3 carbons are upfield (shielded), 15 to 75
• When the carbon has electronegative
groups, appear further downfield, 50 to 75
• sp2 carbons appear down field, 120 to 170
• C=O carbons appear furthest to the left, 170
to 220ppm
SPECTRA
Toluene
CH3
WWU Chemistry
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)
0
Cyclohexanol
Cyclohexanone
1,2-Dichlorobenzene
b
a
a
b
c Cl
c Cl
1,3-Dichlolrobenzene
Cl
Cl
1,4-Dichlorobenzene
Cl
Cl
Some Unknowns!
• The next three slides are carbon-13
spectra for:
• Cyclohexanol
• Cyclohexanone
• 2-Methylcyclopentanone
• Assign them! Hit the enter key to see
• the answer.
2-methylcyclopentanone
cyclohexanone
cyclohexanol
The next slide shows some structures.
Predict the number of carbon peaks
you would observe in the 13C NMR.
Hit the enter key for answers.
O
Br
8 peaks
6 peaks
O
CH 3
OH
CH 3
4 peaks
4 peaks
3 peaks
O
10 peaks
7 peaks
The following four slides show carbon
spectra for four isomers. They all have
the formula C7H14O. They all show
strong peaks at 1715 cm-1 in the infrared
spectrum. Determine the structures
for each.
Possible answers:
4-heptanone
2-heptanone
2,4-dimethyl-3-pentanone
5-methyl-2-hexanone
C7 H14O
Compound 1
C7 H14O
Compound 2
C7 H14O
Compound 3
C7 H14O
Compound 4
Answers to the previous four
spectral questions:
1) 2,4-dimethyl-3-pentanone
2) 4-heptanone
3) 2-heptanone
4) 5-methyl-2-pentanone