introduction to c-13 nmr
DESCRIPTION
Introduction to C-13 NMR. The 13 C nucleus is present in only 1.08% natural abundance. Therefore, acquisition of a spectrum usually takes much longer than in 1 H NMR. The magnetogyric ratio of the 13 C nucleus is about 1/4 that of the 1 H nucleus. - PowerPoint PPT PresentationTRANSCRIPT
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• The 13C nucleus is present in only 1.08% natural abundance. Therefore, acquisition of a spectrum usually takes much longer than in 1H NMR. • The magnetogyric ratio of the 13C nucleus is about 1/4 that of the 1H nucleus. Therefore, the resonance frequency in 13C NMR is much lower than in 1H NMR. (75 MHz for 13C as opposed to 300 MHz for 1H in a 7.04 Tesla field). • At these lower frequencies, the excess population of nuclei in the lower spin state is reduced, which, in turn, reduces the sensitivity of NMR detection.
• Unlike 1H NMR, the area of a peak is not proportional to the number of carbons giving rise to the signal. Therefore, integrations are usually not done.
• Each unique carbon in a molecule gives rise to a 13C NMR signal. Therefore, if there are fewer signals in the spectrum than carbon atoms in the compound, the molecule must possess symmetry.
• When running a spectrum, the protons are usually decoupled from their respective carbons to give a singlet for each carbon atom. This is called a proton-decoupled spectrum.
Introduction to C-13 NMR
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http://www.chemistry.ccsu.edu/glagovich/teaching/316/nmr/images/fig15.gif
Carbon-13 Chemical Shift Table
CC triple bonds
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Alkane: 2-methylpentane
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Alcohol: 2-hexanol
OH
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Br
Alkyl Halide: 3-bromopentane
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Alkene: 1-hexene
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Aromatic Ring: eugenol
HO
O
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Carboxylic Acid: pentanoic acid
CO2H
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Ester: ethyl valerate
O
O
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Amide: pentanamide
O
NH2
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Ketone: 3-methyl-2-pentanone
O
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Aldehyde: 2-methylpentanal
O
H
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Symmetry in C-13 NMREach unique carbon in a molecule gives rise to a 13C NMR signal. Therefore,if there are fewer signals in the spectrum than carbon atoms in the compound,the molecule must possess symmetry. Examples:
CH2CH3CH3CH2 OH
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Enantiotopic vs Diastereotopic CH3’s
*
O
O
CH3
enantiotopic
*
*
* *
diastereotopic
OH
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Determine the number of signals in the proton-decoupledC-13 NMR spectrum of each of the following compounds:
H3C
CH3
O
CH3
OH
CH3
NH
OCH3OCCH3
O
HO
OH
H3C CH3
CH3
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ppm Carbon #139.07 1 131.62 2 124.07 3 123.71 4 59.16 5 39.64 6 26.51 7 25.66 8 17.66 9 16.24 10
8
9
Carbon-13 NMR Spectrum of Geraniol
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T1 and NOE Effects in C-13 NMRBecause of unequal T1 and NOE effects, peaks heights vary widely in C-13 NMR.This is why C-13 spectra are normally not integrated.
Carbon T1 (sec) NOE
CH3 16 0.61
1 89 0.56
2 24 1.6
3 24 1.7
4 17 1.6
CH31
2
34
1
2
3
4
CH3
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Carbon-13 Proton-Coupled Patterns
http://www.chemistry.ccsu.edu/glagovich/teaching/316/nmr/13ccoupled.html
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Carbon-13 Proton-Coupled Spectrum of Ethyl Phenylacetate
Typical coupling constants for 13C-1H one-bond couplings are between 100 to 250 Hz.
http://www.chemistry.ccsu.edu/glagovich/teaching/316/nmr/13ccoupled.html
O
O
Difficult to interpret
C=O
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DEPT Spectra
normal C-13 spectrum
DEPT-45
DEPT-90
DEPT-135
C
CH CH2 CH3
Quaternary carbons (C) do not show up in DEPT.
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O
O
Simulated DEPT Spectra of Ethyl Phenylacetate
Normal C-13 spectrum
DEPT-45
DEPT-90
DEPT-135O
O
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DEPT Spectra of Codeine
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Predict the normal C-13, DEPT-90, and DEPT-135 spectra of ipsenol, whose structure appears below.
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www.lasalle.edu/~price/DEPT%20and%20COSY%20Spectra.ppt
DEPT Spectra of Ipsenol
Normal C-13 spectrum
CDCl3
DEPT-135
DEPT-90
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Determine the number and appearance of the signals in the DEPT-45, DEPT 90, and DEPT 135 NMR spectrum of each of the following compounds:
H3C
CH3
O
CH3
OH
CH3
NH
OCH3OCCH3
O
HO
OH
H3C CH3
CH3