Transcript 13.14 C NMR Spectroscopy 13

13.14
13C
NMR Spectroscopy
1H
and 13C NMR compared:
both give us information about the number of
chemically nonequivalent nuclei
(nonequivalent hydrogens or nonequivalent
carbons)
both give us information about the
environment of the nuclei (hybridization state,
attached atoms, etc.)
it is convenient to use FT-NMR techniques for
1H; it is standard practice for 13C NMR
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
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
at the "natural abundance" level only 1.1% of
all the C atoms in a sample are 13C (most are
12C)
1H
and 13C NMR compared:
13C
signals are spread over a much wider
range than 1H signals making it easier to
identify and count individual nuclei
Figure 13.23 (a) shows the 1H NMR spectrum
of 1-chloropentane; Figure 13.23 (b) shows
the 13C spectrum. It is much easier to identify
the compound as 1-chloropentane by its 13C
spectrum than by its 1H spectrum.
1H
Figure 13.23(a) (page 572)
ClCH2CH2CH2CH2CH3
10.0
9.0
8.0
7.0
6.0
CH3
ClCH2
5.0
4.0
3.0
Chemical shift (, ppm)
2.0
1.0
0
13C
Figure 13.23(b) (page 572)
ClCH2CH2CH2CH2CH3
a separate, distinct
peak appears for
each of the 5
carbons
200
180
160
140
120
CDCl3
100
80
60
Chemical shift (, ppm)
40
20
0
13.15
13C
Chemical Shifts
are measured in ppm ()
from the carbons of TMS
13C
Chemical shifts are most affected by:
• electronegativity of groups attached to carbon
• hybridization state of carbon
Electronegativity Effects
Electronegativity has an even greater effect
on 13C chemical shifts than it does on 1H
chemical shifts.
Types of Carbons
Classification
CH4
Chemical shift, 
1H
13C
0.2
-2
CH3CH3
primary
0.9
8
CH3CH2CH3
secondary
1.3
16
(CH3)3CH
tertiary
1.7
25
(CH3)4C
quaternary
28
Replacing H by C (more electronegative) deshields
C to which it is attached.
Electronegativity effects on CH3
Chemical shift, 
1H
13C
CH4
0.2
-2
CH3NH2
2.5
27
CH3OH
3.4
50
CH3F
4.3
75
Electronegativity effects and chain length
Cl
Chemical
shift, 
CH2
CH2
CH2
CH2
CH3
45
33
29
22
14
Deshielding effect of Cl decreases as
number of bonds between Cl and C increases.
13C
Chemical shifts are most affected by:
• electronegativity of groups attached to carbon
• hybridization state of carbon
Hybridization effects
sp3 hybridized
carbon is more
shielded than sp2
sp hybridized
carbon is
more
shielded than
sp2, but less
shielded than
sp3
H
36
114
138 36 126-142
C
C
68
84
CH2
22
CH2
20
CH3
13
Carbonyl carbons are especially deshielded
O
127-134
CH2
C
41
171
O
CH2
CH3
61
14
Table 13.3 (p 573)
Type of carbon Chemical shift (), Type of carbon
ppm
Chemical shift (),
ppm
RCH3
0-35
RC
CR
65-90
R2CH2
15-40
R2C
CR2
100-150
R3CH
25-50
110-175
R4C
30-40
Table 13.3 (p 573)
Type of carbon Chemical shift (), Type of carbon
ppm
RCH2Br
RCH2Cl
20-40
25-50
RC
Chemical shift (),
ppm
N
110-125
RCOR
160-185
O
RCH2NH2
35-50
RCH2OH
50-65
O
RCH2OR
50-65
RCR
190-220
13.16
13C
NMR and Peak Intensities
Pulse-FT NMR distorts intensities of signals.
Therefore, peak heights and areas can be
deceptive.
Figure 13.24 (page 576)
CH3
7 carbons give 7
signals, but
intensities are not
equal
OH
200
180
160
140
120
100
80
60
Chemical shift (, ppm)
40
20
0
13.17
13C—H
Coupling
Peaks in a 13C NMR spectrum are typically
singlets
13C—13C
splitting is not seen because the
probability of two 13C nuclei being in the same
molecule is very small.
13C—1H
splitting is not seen because spectrum
is measured under conditions that suppress
this splitting (broadband decoupling).
13.18
Using DEPT to Count the Hydrogens
Attached to 13C
Distortionless Enhancement
of Polarization Transfer
Measuring a 13C NMR spectrum involves
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 13.26 (a) (page 578)
O
CCH2CH2CH2CH3
CH CH
CH2
CH2
CH
O
CH3
CH2
C
C
200
180
160
140
120
100
80
60
Chemical shift (, ppm)
40
20
0
Figure 13.23 (b) (page 578)
O
CCH2CH2CH2CH3
CH CH
CH3
CH
CH and CH3 unaffected
C and C=O nulled
CH2 inverted
200
180
160
140
120
100
CH2
80
60
Chemical shift (, ppm)
40
CH2
CH2
20
0
13.19
2D NMR:
COSY AND HETCOR
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
CH3CCH2CH2CH2CH3
1H
HETCOR
1H
and 13C spectra plotted separately on two
frequency axes
Coordinates of cross peak connect signal of carbon
to protons that are bonded to it.
1H-13C
HETCOR
O
13C
CH3CCH2CH2CH2CH3
1H