1H
13C
and 13C NMR compared:
Both give information about the number of
chemically nonequivalent nuclei
(nonequivalent hydrogens or nonequivalent
carbons)
carbons)
NMR Spectroscopy
Both give information about the environment
of the nuclei (hybridization state, attached
atoms, etc.)
FT-NMR techniques are standard practice for
13C NMR
1H
and 13C NMR compared:
1H
and 13C NMR compared:
13C
requires FT-NMR because the signal for a
carbon atom is 10-4 times weaker than the
signal for a hydrogen atom
13C
A signal for a 13C nucleus is only about 1% as
intense as that for 1H because of the
magnetic properties of the nuclei, and
Figure #1 shows the 1H NMR spectrum of 1chloropentane;
chloropentane; Figure #2 shows the 13C
spectrum. It is much easier to identify the
compound as 1-chloropentane
1-chloropentane by its 13C
spectrum than by its 1H spectrum.
The "natural abundance" level is only 1.1% of
all the C atoms in a sample are 13C (most are
12C)
Figure #1
signals are spread over a much wider
range than 1H signals making it easier to
identify and count individual nuclei
Figure #2
1H
CH3
ClC
ClCH2
ClC
ClCH2CH2CH2CH2CH3
13 C
ClCH
ClCH2CH2CH2CH2CH3
a separate, distinct
peak appears for
each of the 5 noneqiuvalent carbons
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
Chemical shift (δ
(δ, ppm)
2.0
1.0
0
200
180
160
140
120
CDCl3
100
80
60
Chemical shift (δ
(δ, ppm)
40
20
0
Question
Question
• How many signals would you expect to
see in the 13C-NMR spectrum of mchloroanisole?
•
A) 4
•
B) 5
•
C) 6
•
D) 7
• Which compound has the most signals
in its 13C-NMR spectrum?
• A)
B)
• C)
D)
13C
13C
Chemical Shifts are Most Affected By:
• Electronegativity of groups attached to carbon
• Hybridization state of carbon
Chemical Shifts
Electronegativity has an even greater effect
on 13C chemical shifts than it does on 1H
chemical shifts.
are measured in ppm (δ
(δ)
from the carbons of TMS
Types of Carbons
Classification
Chemical shift, δ
1H
0.2
CH4
Electronegativity Effects on CH3
Chemical shift, δ
13C
-2
1H
13C
CH4
0.2
-2
CH3CH3
primary
0.9
8
CH3NH2
2.5
27
CH3CH2CH3
secondary
1.3
16
CH3OH
3.4
50
(CH3)3CH
tertiary
1.7
25
CH3F
4.3
75
(CH3)4C
quaternary
28
Replacing H with C (more electronegative) deshields
C to which it is attached.
Electronegativity Effects and Chain Length
Cl
Chemical
shift, δ
CH2
CH2
CH2
CH2
CH3
45
33
29
22
14
Deshielding effect of Cl decreases as
number of bonds between Cl and C increases.
Hybridization Effects
sp3 hybridized
carbon is more
shielded than sp2.
36
114
138 36 126-142
sp hybridized
carbon is
more
shielded than
sp2, but less
shielded than
sp3.
H
C
C
CH2
68
84
22
Table 1
Carbonyl Carbons are Especially Deshielded
Type of carbon Chemical shift (δ
(δ),
ppm
O
127-134
CH2
C
41
171
O
CH2
CH3
61
14
RCH3
0-35
R2CH2
15-40
R3CH
25-50
R 4C
30-40
Table 2
Table 3
Type of carbon Chemical shift (δ
(δ), Type of carbon
ppm
Chemical shift (δ
(δ),
ppm
Type of carbon Chemical shift (δ
(δ),
ppm
RCH3
0-35
RC
CR
65-90
RCH2Br
20-40
R2CH2
15-40
R 2C
CR2
100-150
RCH2Cl
25-50
R3CH
25-50
RCH2NH2
35-50
RCH2OH
50-65
RCH2OR
50-65
R 4C
30-40
110-175
CH2
20
CH3
13
Table 4
Type of carbon Chemical shift (δ
(δ), Type of carbon
ppm
Chemical shift (δ
(δ),
ppm
O
RCH2Br
20-40
RCH2Cl
25-50
RCH2NH2
35-50
O
RCH2OH
50-65
RCR
RCH2OR
50-65
RCOR
160-185
190-220
Question
• In the 13C-NMR spectrum 1,2,3,5tetramethylbenzene, how many peaks
are more shielded than δ 80 ppm?
•
•
•
•
A)
B)
C)
D)
1
2
3
4
13C
NMR and Peak Intensities
Pulse-FT NMR distorts intensities of signals.
Therefore, peak heights and areas can be
deceptive.
Figure
CH3
7 carbons give 7
signals, but
intensities are not
equal
OH
200
180
160
140
120
100
80
60
Chemical shift (δ
(δ, ppm)
40
20
0
13C—H
Coupling
Proton-Coupled 13C NMR of 2-Butanol
Peaks in a 13C NMR spectrum
13C—13C
splitting is not seen because the
probability of two 13C nuclei being in the same
molecule is very small.
13C—1H
splitting occurs but is not seen when
measured under conditions that suppress
this splitting (broadband decoupling)
decoupling).
Question
Proton-Decoupled 13C NMR of
2-Butanol
• Which compound has four signals in its 13CNMR spectrum (13C-1H coupled): a singlet, a
doublet, a triplet, and a quartet.
•
A)
B)
•
C)
D)
Measuring a 13C NMR spectrum involves
Using DEPT to Count the Hydrogens
Attached to 13C
Distortionless Enhancement
of Polarization Transfer
(De-coupling)
1. Equilibration of the nuclei between the lower
and higher spin states under the influence of
a magnetic field
2. Application of a radiofrequency pulse to give
an excess of nuclei in the higher spin state
3. Acquisition of free-induction decay data
during the time interval in which the equilibrium
distribution of nuclear spins is restored
4. Mathematical manipulation (Fourier transform)
of the data to plot a spectrum
Measuring a 13C NMR spectrum involves
Steps 2 and 3 can be repeated hundreds of times
to enhance the signal-noise ratio
2. Application of a radiofrequency pulse to give
an excess of nuclei in the higher spin state
3. Acquisition of free-induction decay data
during the time interval in which the equilibrium
distribution of nuclear spins is restored
Measuring a 13C NMR spectrum involves
In DEPT, a second transmitter irradiates 1H
during the sequence, which affects the appearance
of the 13C spectrum.
some 13C signals stay the same
some 13C signals disappear
some 13C signals are inverted
Figure #3
Figure #4
O
O
CCH 2CH2CH2CH 3
CCH 2CH2CH2CH 3
CH CH
CH
O
CH CH
CH2
CH2
CH2
CH3
CH
CH3
C
C
CH2
200
180
160
140
120
100
80
60
40
20
0
Chemical shift (δ
(δ, ppm)
CCH 2CH2CH2CH 3
CH CH
CH3
CH
CH2
100
80
60
160
140
120
100
80
60
40
20
0
DEPT 13C NMR distinguish among CH3, CH2, and CH
groups
O
Chemical shift (δ
(δ, ppm)
180
Chemical shift (δ
(δ, ppm)
Figure #5
CH and CH3
unaffected
C and C=O nulled
CH
inverted
200 2 180 160 140 120
200
CH2
CH2
40
CH2
CH2
20
0
Question
• Which isomer of C6H14 is consistent with the
following 13C-NMR spectrum data? δ 11.1
• (CH3); δ 18.4 (CH3); δ 29.1 (CH2); δ 36.4 (CH)
•
•
•
•
A)
B)
C)
D)
n-hexane
2,3-dimethylbutane
2-methylpentane
3-methylpentane
Question
• Identify the C4H10O isomer on the basis
of its 13C-NMR spectrum data: δ 31.2
(CH3, 3C); δ 68.9 (C, 1C)
•
•
•
•
A)
B)
C)
D)
2-butanol
2-methyl-2-propanol
1-butanol
CH3CH2-O-CH2CH3
2D NMR Methods:
COSY: Carbon Hydrogen Correlation Spectroscopy
2D NMR:
HETCOR: Heteronuclear Chemical Shift Correlation
COSY AND HETCOR
NOESY: Nuclear Overhauser Effects Spectoscopy
TROSY: Translation Relaxation Optimized Spectroscopy
2D NMR Terminology
1D NMR = 1 frequency axis
2D NMR = 2 frequency axes
COSY = Correlated Spectroscopy
1H-1H
COSY provides connectivity information
by allowing one to identify spin-coupled protons.
x,y-coordinates of cross peaks are spin-coupled
protons
1H-1H
COSY
O
1H
CH3CCH 2CH2CH 2CH3
1H
The COSY spectrum identifies protons that are coupled
COSY: Carbon Hydrogen Correlation Spectroscopy
COSY Spectrum of 1-Nitropropane
Cross peaks indicate pairs of protons that are coupled
HETCOR
and 13C spectra plotted separately on two
frequency axes.
1H-13 C
HETCOR
1H
Coordinates of cross peak connect signal of carbon
to protons that are bonded to it.
O
13C
CH3CCH 2CH2CH 2CH3
1H
The HETCOR spectrum of 2-methyl-3-pentanone
indicates coupling between protons and the carbon to
which they are attached
Other Current 2D-NMR
Methods
• NOESY: Nuclear Overhauser Effects
Spectoscopy
• TROSY: Translation Relaxation
Optimized Spectroscopy
HETCOR:: Heteronuclear Chemical Shift Correlation