CARBON-13 NMRCARBON-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 - 200 ppm

SALIENT FACTS ABOUT SALIENT FACTS ABOUT 1313C NMRC NMR

For a given field strength 13C has its resonance at adifferent (lower) frequency than 1H.

1H

13C1.41 T 60 MHz

2.35 T 100 MHz7.05 T 300 MHz

1.41 T 15.1 MHz2.35 T 25.0 MHz7.05 T 75.0 MHz

Divide the hydrogenfrequency by 4 (approximately)

for carbon-13

Because of its low natural abundance (0.0108) thereis a low probability of finding two 13C atoms next toeach other in a single molecule.

However, 13C does couple to hydrogen atoms (I = 1/2)

13C - 13C coupling NO!

13C - 1H coupling YES!

Spectra are determined by many molecules contributingto the spectrum, each having only one 13C atom.

SALIENT FACTS ABOUT SALIENT FACTS ABOUT 1313C NMRC NMR (cont)

not probable

very common

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

DECOUPLING THE PROTON SPINSDECOUPLING THE PROTON SPINSPROTON-DECOUPLED SPECTRA

A common method used in determining a carbon-13NMR spectrum is to irradiate all of the hydrogen nuclei in the molecule at the same time the carbonresonances are being measured.

This requires a second radiofrequency (RF) source (the decoupler) tuned to the frequency of the hydrogen nuclei, while the primary RF source is tuned to the 13C frequency.

1H-13CRF source 2 RF source 1

continuouslysaturateshydrogens

pulse tuned tocarbon-13

13C signal (FID) measured

“the decoupler”

In this method the hydrogen nuclei are “saturated”,a situation where there are as many downward asthere are upward transitions, all occuring rapidly.

During the time the carbon-13 spectrum is beingdetermined, the hydrogen nuclei cycle rapidly betweentheir two spin states (+1/2 and -1/2) and the carbon nucleisee an average coupling (i.e., zero) to the hydrogens.

The hydrogens are said to be decoupled from thecarbon-13 nuclei.

You no longer see multiplets for the 13C resonances.Each carbon gives a singlet, and the spectrum is easier to interpret.

ETHYL PHENYLACETATEETHYL PHENYLACETATE

13C coupledto the hydrogens

13C decoupledfrom the hydrogens

in some casesthe peaks of the multiplets willoverlap

this is aneasier spectrumto interpret

q

tt

s s

d d

d

SOME INSTRUMENTS SHOW THE MULTIPLICITIES SOME INSTRUMENTS SHOW THE MULTIPLICITIES OF THE PEAKS ON THE DECOUPLED SPECTRAOF THE PEAKS ON THE DECOUPLED SPECTRA

s = singlet t = tripletd = doublet q = quartetCODE :

This method gives the best of both worlds.

CHEMICAL SHIFTS OF 13C ATOMS

R-CH3 8 - 30

R2CH215 - 55

R3CH 20 - 60

C-I 0 - 40

C-Br 25 - 65

C-N 30 - 65

C-Cl 35 - 80

C-O 40 - 80

C C 65 - 90

C=C 100 - 150

C N 110 - 140

110 - 175

R-C-OR

O

R-C-OH

O

155 - 185

R-C-NH2

O

155 - 185

R-C-H

O

R-C-R

O

185 - 220

APPROXIMATE APPROXIMATE 1313C CHEMICAL SHIFT RANGES FOR C CHEMICAL SHIFT RANGES FOR SELECTED TYPES OF CARBON (ppm)SELECTED TYPES OF CARBON (ppm)

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

/

nitriles

acid anhydrides

acid chlorides

amides

esters

carboxylic acids

aldehydes

,-unsaturated ketones

ketones

220 200 180 160 140 120 100 ppm

13C Correlation Chart for Carbonyl and Nitrile Functional Groups

SPECTRA

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

2,2-DIMETHYLBUTANE2,2-DIMETHYLBUTANE

BROMOCYCLOHEXANEBROMOCYCLOHEXANE

CYCLOHEXANOLCYCLOHEXANOL

TOLUENETOLUENE

CYCLOHEXENECYCLOHEXENE

CYCLOHEXANONECYCLOHEXANONE

Cl

Cla

a

b

b

c

c

1,2-DICHLOROBENZENE1,2-DICHLOROBENZENE

solvent

1,3-DICHLOROBENZENE1,3-DICHLOROBENZENE

Cl

Cl

ac

abd

d