13.18 Carbon-13 NMR13.18 Carbon-13 NMR

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

COUPLING TO ATTACHED PROTONS

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)

ETHYL PHENYLACETATEETHYL PHENYLACETATE

13C coupledto the hydrogens

DECOUPLED SPECTRA

ETHYL PHENYLACETATEETHYL PHENYLACETATE

13C coupledto the protons

13C decoupledfrom the protons

in some casesthe peaks of the multiplets willoverlap

this is aneasier spectrumto interpret

CHEMICAL SHIFTS OF 13C ATOMS

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

/

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

SPECTRA

WWU Chemistry

TolueneCH3

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

CyclohexanolCyclohexanol

CyclohexanoneCyclohexanone

Cl

Cla

a

b

b

c

c

1,2-Dichlorobenzene1,2-Dichlorobenzene

1,3-Dichlolrobenzene1,3-DichlolrobenzeneCl

Cl

1,4-DichlorobenzeneCl

Cl

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.

2-methylcyclopentanone

cyclohexanone

cyclohexanol

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.

CH3

CH3

O

OH

BrO

O

6 peaks 8 peaks

4 peaks4 peaks 3 peaks

7 peaks

10 peaks

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

C7 H14O

Compound 1

C7 H14O

Compound 2

C7 H14O

Compound 3

C7 H14O

Compound 4

Answers to the previous four spectral questions:

1) 2,4-dimethyl-3-pentanone2) 4-heptanone3) 2-heptanone4) 5-methyl-2-pentanone