13.18 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 13C 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

C 13

3 protons 2 protons 1 proton 0 protons

H

H

H

C 13

H

H

C 13

H C 13

Methyl

carbon

Methylene

carbon

Methine

carbon

Quaternary

carbon

( n+1 rule applies )

COUPLING TO ATTACHED PROTONS

(J’s are large ~ 100 - 200 Hz)

ETHYL PHENYLACETATE

13C coupled

to the hydrogens

DECOUPLED SPECTRA

ETHYL PHENYLACETATE

13C coupled

to the protons

13C decoupled

from the protons

in some cases

the peaks of the

multiplets will

overlap

this is an

easier spectrum

to interpret

CHEMICAL SHIFTS OF 13C ATOMS

Aldehydes

Ketones

Acids Amides

Esters Anhydrides

Aromatic ring

carbons

Unsaturated

carbon - sp2

Alkyne

carbons - sp

Saturated carbon - sp3

electronegativity effects

Saturated carbon - sp3

no electronegativity effects

C=O

C=O

C=C

C 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

/

SPECTRA

WWU Chemistry

Toluene CH3

Proton-decoupled 13C spectrum of 1-propanol (22.5 MHz)

200 150 100 50 0

1-Propanol

PROTON

DECOUPLED

HO-CH2-CH2-CH3 c b a

Cyclohexanol

Cyclohexanone

Cl

Cla

a

b

b

c

c

1,2-Dichlorobenzene

1,3-Dichlolrobenzene

Cl

Cl

1,4-Dichlorobenzene

Cl

Cl

Exercício

Composto A Composto B Composto C

Fórmula

Molecular

C7H14O C6H14O C5H6N2

Deslocamento

químico

(ppm),

multiplicidade

e quantidade

23, q (2) 11, q (2) 16, t (2)

28, d 23, t (2) 22, t

29, q 44, d 119, s

33, t 65, t

42, t

206, s