1. Interpreting Carbon NMR Spectra
1H and 13C NMR compared:
Both spectra give us information about the number of chemically nonequivalent nuclei (nonequivalent hydrogens or nonequivalent carbons)
Both spectra 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 to use FT-NMR for 13C NMR
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MC 13.3 Spectroscopy, Pt III

2. Interpreting Carbon NMR Spectra (cont)
1H and 13C NMR compared (cont):
13C requires FT-NMR because the signal for a carbon atom is 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
In addition, at the "natural abundance" level only 1.1% of all the C atoms in a sample are 13C (most are 12C)
13C signals are spread over a much wider range than 1H signals making it easier to identify and count individual nuclei
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MC 13.3 Spectroscopy, Pt III

3. Interpreting Carbon NMR Spectra (cont)
1H and 13C NMR compared (cont):
13C requires FT-NMR because the signal for a carbon atom is 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
In addition, at the "natural abundance" level only 1.1% of all the C atoms in a sample are 13C (most are 12C)
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MC 13.3 Spectroscopy, Pt III

4. Interpreting Carbon NMR Spectra (cont)
Cl CH2 CH2 CH2 CH2 CH3
1H NMR Spectrum:
CH2 CH3
Cl CH2 CH2 2 4 3
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Chemical shift (, ppm)
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5. Interpreting Carbon NMR Spectra (cont)
Cl CH2 CH2 CH2 CH2 CH3
13C NMR Spectrum:
Chemical shift (, ppm) 20 40 60 80
100
120
140
160
180
200
A separate, distinct peak appears for each of the 5 carbons
CDCl3
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MC 13.3 Spectroscopy, Pt III

6. Interpreting Carbon NMR Spectra (cont)
13C Chemical Shifts:
Just as in 1H NMR spectroscopy, chemical shifts in 13C NMR depend on the electron density around the carbon nucleus
Decreased electron density causes the signal to move downfield (deshielding)
Increased electron density causes the signal to move upfield (shielding)
Because of the wide range of chemical shifts, it is rare to have two 13C peaks coincidentally overlap
A group of 3 peaks at d 77 comes from the common NMR solvent deuteriochloroform and can be ignored
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MC 13.3 Spectroscopy, Pt III

7. Interpreting Carbon NMR Spectra (cont)
13C Chemical Shifts (cont):
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MC 13.3 Spectroscopy, Pt III

8. Interpreting Carbon NMR Spectra (cont)
DEPT 13C NMR:
DEPT (distortionless enhanced polarization transfer) spectra are created by mathematically combining several individual spectra taken under special conditions
The final DEPT spectra explicitly show C, CH, CH2 , and CH3 carbons
To simplify the presentation of DEPT data, the broadband decoupled spectrum is annotated with the results of the DEPT experiments using the labels C, CH, CH2 and CH3 above the appropriate peaks
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MC 13.3 Spectroscopy, Pt III

9. Interpreting Carbon NMR Spectra (cont)
Cl CH2 CH CH3 OH
DEPT 13C NMR:
(a) The 13C spectrum and (b) a set of DEPT spectra showing the separate CH, CH2, and CH3 signals
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10. Interpreting Infrared Spectroscopy
Infrared spectroscopy gives information about the functional groups in a molecule
The region of infrared that is most useful lies between m ( cm-1)
The infrared absorption depends on transitions between vibrational energy states
Stretching
Bending
MC 13.3 Spectroscopy, Pt III
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11. Interpreting Infrared Spectroscopy (cont)
Stretching Vibrations of a CH2 Group:
Symmetric
Antisymmetric
MC 13.3 Spectroscopy, Pt III
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12. Interpreting Infrared Spectroscopy (cont)
Bending Vibrations of a CH2 Group:
In plane
In plane
MC 13.3 Spectroscopy, Pt III
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13. Interpreting Infrared Spectroscopy (cont)
Bending Vibrations of a CH2 Group (cont):
Out of plane
Out of plane
MC 13.3 Spectroscopy, Pt III
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14. Interpreting Infrared Spectroscopy (cont)
CH3CH2CH2CH2CH2CH3
Infrared Spectrum of Hexane:
bending
C—H stretching
bending
bending
CH3CH2CH2CH2CH2CH3
2000
3500
3000
2500
1000
1500
500
Wave number, cm-1
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15. Interpreting Infrared Spectroscopy (cont)
H2C=CHCH2CH2CH2CH3
Infrared Spectrum of 1-Hexene:
C=C
H C
C=C H
H2C=C
H2C=CHCH2CH2CH2CH3
2000
3500
3000
2500
1000
1500
500
Wave number, cm-1
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16. Interpreting Infrared Spectroscopy (cont)
Infrared Absorption Frequencies:
Structural unit Frequency, cm-1
Stretching Vibrations (single bonds):
sp C — H
sp2 C — H
sp3 C — H
sp2 C — O 1200
sp3 C — O
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MC 13.3 Spectroscopy, Pt III

17. Interpreting Infrared Spectroscopy (cont)
Infrared Absorption Frequencies (cont):
Structural unit Frequency, cm-1
Stretching Vibrations (multiple bonds): C
— C
C —
— C N
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MC 13.3 Spectroscopy, Pt III

18. Interpreting Infrared Spectroscopy (cont)
Infrared Absorption Frequencies (cont):
Structural unit Frequency, cm-1
Stretching Vibrations (carbonyl groups):
Aldehydes and ketones
Carboxylic acids
Acid anhydrides and
Esters
Amides C O
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MC 13.3 Spectroscopy, Pt III

19. Interpreting Infrared Spectroscopy (cont)
Infrared Absorption Frequencies (cont):
Structural unit Frequency, cm-1
Bending Vibrations of Alkenes:
CH2
RCH
CH2
R2C
890
CHR'
cis-RCH
CHR'
trans-RCH
CHR'
R2C
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20. Interpreting Infrared Spectroscopy (cont)
Infrared Absorption Frequencies (cont):
Structural Unit Frequency, cm-1
Bending Vibrations of Derivatives of Benzene:
Monosubstituted and
ortho-Disubstituted
meta-Disubstituted and
para-disubstituted
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MC 13.3 Spectroscopy, Pt III

21. Interpreting Infrared Spectroscopy (cont)
C6H5C(CH3)3
Infrared Spectrum of tert-butylbenzene:
Ar H
Aromatic Double Bond
H C
Monsubstituted Benzene
C6H5C(CH3)3
2000
3500
3000
2500
1000
1500
500
Wave number, cm-1
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22. Interpreting Infrared Spectroscopy (cont)
Infrared Absorption Frequencies (cont):
Structural Unit Frequency, cm-1
Stretching Vibrations (single bonds):
O — H (alcohols)
O — H (carboxylic acids)
N — H
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MC 13.3 Spectroscopy, Pt III

23. Interpreting Infrared Spectroscopy (cont)
Infrared Spectrum of 2-Hexanol:
H C
O H OH
CH3CH2CH2CH2CHCH3
2000
3500
3000
2500
1000
1500
500
Wave number, cm-1
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MC 13.3 Spectroscopy, Pt III

24. Interpreting Infrared Spectroscopy (cont)
Infrared Spectrum of 2-Hexanone: O
CH3CH2CH2CH2CCH3
H C
C = O
2000
3500
3000
2500
1000
1500
500
Wave number, cm-1
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MC 13.3 Spectroscopy, Pt III