13.18 carbon-13 nmr. 12 c is not nmr-activei = 0 however…. 13 c does have spin, i = 1/2 (odd mass)...
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13.18 Carbon-13 NMR13.18 Carbon-13 NMR
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12C is not NMR-active I = 0
however…. 13C does have spin, I = 1/2 (odd mass)
1. Natural abundance of 13C is small (1.08% of all C)
2. Magnetic moment of 13C is small
13C signals are 6000 times weaker than 1H because:
SALIENT FACTS ABOUT SALIENT FACTS ABOUT 1313C NMRC NMR
PULSED FT-NMR IS REQUIRED
The chemical shift range is larger than for protons
0 - 220 ppm
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COUPLING TO ATTACHED PROTONS
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The effect of attached protons on 13C resonances
n+1 = 4 n+1 = 3 n+1 = 2 n+1 = 1
C13
3 protons 2 protons 1 proton 0 protons
H
H
H
C13
H
H
C13
H C13
Methylcarbon
Methylenecarbon
Methinecarbon
Quaternarycarbon
( n+1 rule applies )
COUPLING TO ATTACHED PROTONSCOUPLING TO ATTACHED PROTONS
(J’s are large ~ 100 - 200 Hz)
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ETHYL PHENYLACETATEETHYL PHENYLACETATE
13C coupledto the hydrogens
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DECOUPLED SPECTRA
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ETHYL PHENYLACETATEETHYL PHENYLACETATE
13C coupledto the protons
13C decoupledfrom the protons
in some casesthe peaks of the multiplets willoverlap
this is aneasier spectrumto interpret
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CHEMICAL SHIFTS OF 13C ATOMS
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AldehydesKetones
Acids AmidesEsters Anhydrides
Aromatic ringcarbons
Unsaturated carbon - sp2
Alkyne carbons - sp
Saturated carbon - sp3
electronegativity effects
Saturated carbon - sp3
no electronegativity effects
C=O
C=O
C=CC C
200 150 100 50 0
200 150 100 50 0
8 - 30
15 - 55
20 - 60
40 - 80
35 - 80
25 - 65
65 - 90
100 - 150
110 - 175
155 - 185
185 - 220
Correlation chart for 13C Chemical Shifts (ppm)
C-O
C-Cl
C-Br
R3CH R4C
R-CH2-R
R-CH3
RANGE
/
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Some useful information!
• sp3 carbons are upfield (shielded), 15 to 75
• When the carbon has electronegative groups, appear further downfield, 50 to 75
• sp2 carbons appear down field, 120 to 170
• C=O carbons appear furthest to the left, 170 to 220ppm
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SPECTRA
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WWU Chemistry
TolueneCH3
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Proton-decoupled 13C spectrum of 1-propanol (22.5 MHz)
200 150 100 50 0
1-Propanol1-Propanol
PROTONDECOUPLED
HO-CH2-CH2-CH3c b a
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CyclohexanolCyclohexanol
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CyclohexanoneCyclohexanone
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Cl
Cla
a
b
b
c
c
1,2-Dichlorobenzene1,2-Dichlorobenzene
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1,3-Dichlolrobenzene1,3-DichlolrobenzeneCl
Cl
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1,4-DichlorobenzeneCl
Cl
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Some Unknowns!
• The next three slides are carbon-13 spectra for:
• Cyclohexanol• Cyclohexanone• 2-Methylcyclopentanone
• Assign them! Hit the enter key to see• the answer.
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2-methylcyclopentanone
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cyclohexanone
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cyclohexanol
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The next slide shows some structures. Predict the number of carbon peaks you would observe in the 13C NMR.
Hit the enter key for answers.
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CH3
CH3
O
OH
BrO
O
6 peaks 8 peaks
4 peaks4 peaks 3 peaks
7 peaks
10 peaks
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The following four slides show carbon spectra for four isomers. They all have the formula C7H14O. They all showstrong peaks at 1715 cm-1 in the infraredspectrum. Determine the structuresfor each. Possible answers: 4-heptanone2-heptanone2,4-dimethyl-3-pentanone5-methyl-2-hexanone
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C7 H14O
Compound 1
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C7 H14O
Compound 2
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C7 H14O
Compound 3
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C7 H14O
Compound 4
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Answers to the previous four spectral questions:
1) 2,4-dimethyl-3-pentanone2) 4-heptanone3) 2-heptanone4) 5-methyl-2-pentanone