13c+nmr 1085d
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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
4.5 Chemical Shift Equivalence CSE = Interchangeability by a symmetry operation or by rapid mechanism. Ex. The CH3 groups of t-Butyl alcohol are CSE due to rapid rotation
4.5 CSE continued 75.5 MHz C/C’ coincident
150.9 MHz C/C’ are resolved
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.
4.2.2-Chemical Shift Scale and Range For a given spectrometer, the resonance frequency for C13 is about ¼ that Of H1. Example on a 400 MHz instrument the carbon spectrum would be Obtained at 100 MHz. The chemical shift range for C13 is larger than for H1, thus overlap of signals Is less frequently encountered (it still happens though). To a first approximation, C13 chemical shifts parallel those of H1
1H
Proton Spectrum
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
Carbon Spectrum
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
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
13C
NMR and Peak Intensities
Pulse-FT NMR distorts intensities of signals. Therefore, peak heights and areas can be deceptive.
4.2.3-T1 Relaxation
T1 varies widely for different types of carbons. Generally, T1’s Decrease as the number of attached protons increases. T1’s are measured by the Inversion-Recovery Method. T1’s cover a range of several seconds for CH3 grps to over a min. for quat. C’s. A delay between pulses (5 x T1) is required for quantitative C13 spectra.
Attached Protons Affect T1 and Signal Intensity 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
4.4 Quantitative C13 Analysis Inverse-gated and Rd > T1
Inverse-gated decoupling With Rd < T1
Std. H1 decoupled Spectrum of DEP Rd < T1
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).
H1 Decoupling Techniques J values for C-H are typically 110-300 Hz (C-C-H and C-C-C-H are 0-60Hz) Thus a CH3 group would appear as a quartet, CH2-triplet CH-doublet etc. The H1 nuclei are irradiated with a broadband Rf to remove coupling to Carbon.
4.3 Interpretation of a Simple C13 Spectrum: Diethyl Phthalate
decoupled
coupled
Expansions: Additional splitting Is due to J2 and J3 couplings
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
Proton Decoupled Spectrum 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
DEPT Spectrum 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
4.6 Distortionless Enhancement by Polarization Transfer (DEPT) Pulse sequence developed to determine the number of protons Directly attached to a carbon.
DEPT 90 deg. CH only
DEPT 135 deg. CH &CH3 up, CH2 down Std H1 decoupled spectrum DEPT is a helpful to determine proton inventory, but it does not Record H’s on heteroatoms; must correlate with H1 spectrum.
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
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
4.5 Chemical Shift Equivalence CSE = Interchangeability by a symmetry operation or by rapid mechanism. Ex. The CH3 groups of t-Butyl alcohol are CSE due to rapid rotation
4.5 CSE continued 75.5 MHz C/C’ coincident
150.9 MHz C/C’ are resolved
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.
4.2.2-Chemical Shift Scale and Range For a given spectrometer, the resonance frequency for C13 is about ¼ that Of H1. Example on a 400 MHz instrument the carbon spectrum would be Obtained at 100 MHz. The chemical shift range for C13 is larger than for H1, thus overlap of signals Is less frequently encountered (it still happens though). To a first approximation, C13 chemical shifts parallel those of H1
1H
Proton Spectrum
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
Carbon Spectrum
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
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
13C
NMR and Peak Intensities
Pulse-FT NMR distorts intensities of signals. Therefore, peak heights and areas can be deceptive.
4.2.3-T1 Relaxation
T1 varies widely for different types of carbons. Generally, T1’s Decrease as the number of attached protons increases. T1’s are measured by the Inversion-Recovery Method. T1’s cover a range of several seconds for CH3 grps to over a min. for quat. C’s. A delay between pulses (5 x T1) is required for quantitative C13 spectra.
Attached Protons Affect T1 and Signal Intensity 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
4.4 Quantitative C13 Analysis Inverse-gated and Rd > T1
Inverse-gated decoupling With Rd < T1
Std. H1 decoupled Spectrum of DEP Rd < T1
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).
H1 Decoupling Techniques J values for C-H are typically 110-300 Hz (C-C-H and C-C-C-H are 0-60Hz) Thus a CH3 group would appear as a quartet, CH2-triplet CH-doublet etc. The H1 nuclei are irradiated with a broadband Rf to remove coupling to Carbon.
4.3 Interpretation of a Simple C13 Spectrum: Diethyl Phthalate
decoupled
coupled
Expansions: Additional splitting Is due to J2 and J3 couplings
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
Proton Decoupled Spectrum 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
DEPT Spectrum 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
4.6 Distortionless Enhancement by Polarization Transfer (DEPT) Pulse sequence developed to determine the number of protons Directly attached to a carbon.
DEPT 90 deg. CH only
DEPT 135 deg. CH &CH3 up, CH2 down Std H1 decoupled spectrum DEPT is a helpful to determine proton inventory, but it does not Record H’s on heteroatoms; must correlate with H1 spectrum.
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
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