CARBON13 NMR

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
0 - 200 ppm
3
SALIENT FACTS ABOUT 13C NMR
For a given field strength 13C has its resonance
at a different (lower) frequency than 1H.
Divide the hydrogen frequency by 4 (approximately)
1H
for carbon-13
1.41 T 60 MHz
2.35 T 100 MHz
13C
7.05 T 300 MHz
1.41 T 15.1 MHz
2.35 T 25.0 MHz
7.05 T 75.0 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 PROTONS
3 protons
2 protons
1 proton
0 protons
H
H
H
H
H
H
n1 4
n1 3
n1 2
n1 1
Methyl carbon
Methylene carbon
Methine carbon
Quaternary carbon
The effect of attached protons on 13C resonances
( n1 rule applies )
(Js are large 100 - 200 Hz)
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ETHYL PHENYLACETATE
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.
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ETHYL PHENYLACETATE
in some cases the peaks of the multiplets
will overlap
this is an easier spectrum to interpret
12
SOME INSTRUMENTS SHOW THE MULTIPLICITIES OF THE
PEAKS ON THE DECOUPLED SPECTRA
s singlet t triplet d doublet q quartet
CODE
This method gives the best of both worlds.
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CHEMICAL SHIFTS OF 13C ATOMS
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APPROXIMATE 13C CHEMICAL SHIFT RANGES FOR
SELECTED TYPES OF CARBON (ppm)
R-CH3 8 - 30
C C 65 - 90
R2CH2 15 - 55
CC 100 - 150
R3CH 20 - 60
C N 110 - 140
110 - 175
C-I 0 - 40
C-Br 25 - 65
O
O
C-Cl 35 - 80
R-C-OR
R-C-OH
155 - 185
O
C-N 30 - 65
R-C-NH2
155 - 185
O
O
C-O 40 - 80
R-C-H
R-C-R
185 - 220
15
200
150
100
50
0
RANGE
R-CH3
8 - 30
R-CH2-R
Saturated carbon - sp3
15 - 55
no electronegativity effects
20 - 60
R3CH
R4C
/
40 - 80
C-O
Saturated carbon - sp3
35 - 80
C-Cl
electronegativity effects
25 - 65
C-Br
Alkyne carbons - sp
65 - 90
C C
Unsaturated carbon - sp2
CC
100 - 150
Aromatic ring carbons
110 - 175
Acids Amides Esters Anhydrides
155 - 185
CO
Aldehydes Ketones
185 - 220
CO
Correlation chart for 13C Chemical Shifts (ppm)
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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
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SPECTRA
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1-PROPANOL
HO-CH2-CH2-CH3
c
b
a
PROTON DECOUPLED
200
150
100
50
0
Proton-decoupled 13C spectrum of 1-propanol (22.5
MHz)
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2,2-DIMETHYLBUTANE
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BROMOCYCLOHEXANE
21
CYCLOHEXANOL
22
TOLUENE
23
CYCLOHEXENE
24
CYCLOHEXANONE
25
1,2-DICHLOROBENZENE
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1,3-DICHLOROBENZENE
solvent
d
b
a
c
d
a