13C Nuclear Magnetic Resonance

Inherent Difficulties

• Low abundance of 13C (1.1% vs, 99.9% for 1H) • Lower gyromagnetic ratio (1/4 that of 1H) - 13C: 67.28 vs. 1H 267.53 • Slow T1 relaxation times (10-100 sec vs. 0.1-1 sec for 1H) • 13C-1H coupling complicates spectrum; decreases peak intensity

ΔE

magnetic field (B0)

-1/2

+1/2

1,000,000 nuclei at 60 MHz

1,000,009 nuclei at 60 MHz

E

13C Nuclear Magnetic Resonance

Chemical Shift

see also Pavia Table 6.1; Appendices 7 & 8

13C Nuclear Magnetic Resonance

Chemical Shift

Factors the Influence Chemical Shift

1. hybridization 2. electronegativity of element bonded to carbon 3. anisotropy

OH O

**

*

**

δ 22.6 δ 139.0 δ 38.2

δ 62.3 δ 202.8

13C Nuclear Magnetic Resonance

Differences Between 1H and 13C NMR

1. range of 13C chemical shifts is very large compared to 1H 2. 13C chemical shifts are more sensitive to small changes in chemical environment 3. Influence of electronegative atoms on chemical shift greater in 13C NMR than 1H NMR 4. Homonuclear (13C / 13C) splitting is not observed

See Pavia, Appendix 8 for calculation of 13C chemical shifts

CH3 CH2 CH2 CH2 CH2 CH2 OH

0.90

1.32

1.32

1.32

1.56

3.62

CH3 CH2 CH2 CH2 CH2 CH2 OH

14.2

22.8

32.0

25.8

32.8

61.9

1H shifts 13C shifts

NMR Spectra of n-Butylbenzene

13C Nuclear Magnetic Resonance

Heteronuclear Spin-Spin Splitting (1H-13C)

CH2 CH3 CH2

3J = 0-3 Hz

13C Nuclear Magnetic Resonance

Heteronuclear Spin-Spin Splitting

1H coupled spectrum

1H decoupled spectrum

HOH H

HH

cholesterol

13C Nuclear Magnetic Resonance

Proton Decoupled 13C NMR

13C Nuclear Magnetic Resonance

Proton Decoupled 13C NMR 1H coupled spectrum

1H broadband decoupled spectrum

1H off resonance decoupled spectrum

CH3

CH3

13C Nuclear Magnetic Resonance

Nuclear Overhauser Effect & Cross Polarization of Spin

300 MHz

Signal Enhancement: CH3 > CH2 > CH >> C

B0

1H

decoupler

B0

13C

crosspolarization

Boltzmandistribution

NupperNlower

= 1,000,0001,000,048

1,000,0001,000,096

=

NupperNlower

= 1,000,0001,000,048

1,000,0001,000,000

=Boltzmandistribution

11

=

13C Nuclear Magnetic Resonance 13C Pulse Sequence

FID:

13C Nuclear Magnetic Resonance

Relaxation Times (T1)

CH3 C CH2 CH CH3

CH3

CH3

CH3

9.3 s

68 s

13 s23 s

9.8 s

T1 : C > CH > CH2 > CH3

in general

tumbling

HOH H

HH

T1 = 37 s T1 = 20 sT1 = 1-2 s for CH, CH2, CH3T1 = 4-6 s for quaternary C

13C Nuclear Magnetic Resonance

Molecular Symmetry

How Many Lines?

C-CH2-CH3CH3

CH3CH3

O H3C

H3C

CH3

OH

CH3H3C

A B C D

13C Nuclear Magnetic Resonance

Integration

A

B

C

D

2 peaks

1

2

3

4

13C Nuclear Magnetic Resonance

Distortionless Enhancement by Polarization Transfer (DEPT)

DEPT-45 DEPT-90 DEPT-135 C 0 0 0

CH + + +

CH2 + 0 -

CH3 + 0 +

13C Nuclear Magnetic Resonance

Distortionless Enhancement by Polarization Transfer (DEPT)

CCHCH2CH3

DEPT-450+++

DEPT-900+00

DEPT-1350+-+

DEPT-135

DEPT-90

DEPT-45

13C NMR 1 2 46 78 9 935

CH3

OH

H3C CH3

menthol

3

5

12

4 6

7

89 9

13C Nuclear Magnetic Resonance

DEPT-135 O

NH

CH3O H

OH

CH3O2C OH

OH

OH

O CH3

13C Nuclear Magnetic Resonance

Alkanes

2-methylpentane

CH-CH2-CH2-CH3CH3

CH3a

b dc e

a

e b

c

a

d

R CH3

R2 CH2

CR4

15-55 ppm

30-40 ppm

8-30 ppm

chemical shifts

CHR3 20-60 ppm

13C Nuclear Magnetic Resonance

Alkenes

cis-2-hexene

C C 110-150 ppm

chemical shifts

C CH

H3C CH2CH2CH3

H

ea

b

d

c

f

e b c

a

d 123.9 130.7

29.1 f

13C Nuclear Magnetic Resonance

Alkenes

1-hexene

C C 110-150 ppm

chemical shifts

b

a

114.2

139.2

e abd cf

CH3-CH2-CH2-CH2-CH CH2

13C Nuclear Magnetic Resonance

Alkenes

dihydropyran

C C 110-150 ppm

chemical shifts

b

a

100.7

144.3 O a

b

13C Nuclear Magnetic Resonance

Alkenes

4-methyl-2-penten-1-one

C C 110-150 ppm

chemical shifts

b

a 154.8 124.3 H3C

H3C

O

CH3

a

b

13C Nuclear Magnetic Resonance

Aromatics

nitrobenzene

C110-175 ppm

chemical shifts

b c

a

d

NO2ab

c

d

13C Nuclear Magnetic Resonance

Aromatics

p-cymene

CH3

C110-175 ppm

chemical shifts

13C Nuclear Magnetic Resonance

Aromatics

dichlorobenzenes

Cl

Cl

Cl

Cl

Cl

Cl

C110-175 ppm

chemical shifts

13C Nuclear Magnetic Resonance

Alkynes

2-heptyne

b

a

65-90 ppm

chemical shifts

C≡C

C C CH3CH3CH2CH2CH2ab

79.4

75.3

13C Nuclear Magnetic Resonance

Alkyl Halides

1-chloropentane

C I

25-65 ppm

35-80 ppm

-5-40 ppm

chemical shifts

C Br

C Cl

aCH3 CH2-CH2-CH2-CH2-Cl a

45.1

13C Nuclear Magnetic Resonance

Alcohols, Ethers & Amines

2-hexanol 40-60 ppm

50-80 ppm

chemical shifts

C-OR

C-NR2

CH3CH2CH2CH2 CHCH3OH

aa

68.0

13C Nuclear Magnetic Resonance

Alcohols, Ethers & Amines

1,3-dioxane 40-60 ppm

50-80 ppm

chemical shifts

C-OR

C-NR2

bc

O Oa

a

b

c

13C Nuclear Magnetic Resonance

Alcohols, Ethers & Amines

N-methylisobutylamine 40-60 ppm

50-80 ppm

chemical shifts

C-OR

C-NR2

d

b c

NH

a b d

c

a

13C Nuclear Magnetic Resonance

Carbonyl Compounds

aldehydes & ketones 185-220 ppm carboxylic acids, esters, amides, etc. 155-185 ppm nitriles 110-140 ppm

13C Nuclear Magnetic Resonance

Aldehydes & Ketones

2-hexanone 205-220 ppm

185-200 ppm

chemical shifts

R CO

H

R CO

R'

CH3 CO

CH2-CH2-CH2-CH3a

a

209.0

13C Nuclear Magnetic Resonance

Carboxylic Acid Derivatives

methacrylic acid

175-185 ppm

165-175 ppm

chemical shifts

R CO

OR'

R CO

OH

165-175 ppmR CO

NR'2

CH2CCH3

CO

HOa

a

173.5

13C Nuclear Magnetic Resonance

Carboxylic Acid Derivatives

heptyl acetate

175-185 ppm

165-175 ppm

chemical shifts

R CO

OR'

R CO

OH

165-175 ppmR CO

NR'2

CH3 COO-CH2CH2-CH2-CH2-CH2-CH2-CH3

a b

a 171.0

b 64.6

13C Nuclear Magnetic Resonance

Amides

N-methyl-6-hexanelactam

175-185 ppm

165-175 ppm

chemical shifts

R CO

OR'

R CO

OH

165-175 ppmR CO

NR'2

a

175.7 d

36.9

c

51.3 N

OCH3ab

c

d b

35.7

13C Nuclear Magnetic Resonance

Nitriles

hexanenitrile

119.9

110-140 ppm

chemical shifts

R C N

a

C NCH3CH2CH2CH2a

13C Nuclear Magnetic Resonance

Heteronuclear Coupling to Deuterium

multiplicity

relative intensity

multiplicity = 2nI + 1

where: n = # of nuclei I = spin

Cb

Da

Jab = 45 Hz

13C Nuclear Magnetic Resonance

Solvent Peaks

CDCl3 D3C(S=O)CD3

13C Nuclear Magnetic Resonance

Heteronuclear Coupling to 19F

fluorotribromomethane

1J = 370 Hz

C Cb

FCa

F

JFb = ~50 HzJFa = ~350 Hz

13C Nuclear Magnetic Resonance

Heteronuclear Coupling to 19F

2,2,2-trifluoroethanol (proton decoupled)

1J = 278 Hz

2J = 35 Hz

C Cb

FCa

F

JFb = ~50 HzJFa = ~350 Hz

13C Nuclear Magnetic Resonance

Heteronuclear Coupling to 19F

commonly observed systems

C Cb

FCa

F

JFb = ~50 HzJFa = ~350 Hz

F3C SO

OROF3C

C OR

O

trifluoroacetates trifluoromethanesulfonates(triflates)

13C Nuclear Magnetic Resonance

Heteronuclear Coupling to 31P

tetramethylphosphonium chloride (proton decoupled)

1J = 56 Hz

C Cb

PCa

P

JPa, JPb variable

13C Nuclear Magnetic Resonance

Heteronuclear Coupling to 31P

dimethyl methylphosphonate (proton decoupled)

2J = 6 Hz 1J = 144 Hz

C Cb

PCa

P

JPa, JPb variable

1H Nuclear Magnetic Resonance

1. Get an Overview • identify any reference solvents in proton decoupled spectrum 2. Symmetry: • count the number of carbons (remember 13C is not integrated) • verify carbon count based on the molecular formula • is there any symmetry?

- rarely see overlap of peaks in absence of symmetry - ok to see fewer carbons (if reasonable); not okay to have more!

3. Chemical Shift: • determine the chemical shift of each resonance in the spectrum

- what functional groups may be present - consider the presence of heteroatoms 4. If Needed Collect DEPT Data • determine how many Hs are on each carbon 5. Put it all Together: • often helpful to consider 1H NMR data simultaneously

How to Interpret a 13C NMR Spectrum

1H Nuclear Magnetic Resonance

Reporting 13C NMR Data (1H decoupled)

CH3CH2CH2CH2CH2C≡N

13C NMR (25 MHz, CDCl3) δ 119.9, 30.9, 25.2, 22.0. 17.1, 13.8.

119.9030.8725.2121.9617.1113.78

2619701000920814950

ppm Int

1H Nuclear Magnetic Resonance

Reporting 13C NMR Data (1H decoupled)

13C NMR (25 MHz, CDCl3) δ 119.9, 30.9, 25.2, 22.0. 17.1, 13.8.

frequency (1/4 1H frequency)

list chemical shift of each peak to 1 decimal place*

solvent

*if two peaks round to same value, can add second decimal place (e.g. 69.87, 69.89);

13C Nuclear Magnetic Resonance

Chemical Shift