C-13 NMR and DEPT

8/13/2019 C-13 NMR and DEPT

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CARBON-13

NMR SPECTROSCOPY

Viju Kumar V G

Assistant Professor

Department of Chemistry

University college, Thiruvananthapuram


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After hydrogen, the most useful atom providing information is carbon-13.

Natural carbon contains about 1% of this isotope so the instruments

for its detection need to be sensitive and spectra will take longer to record.

Only the chemical shift is important as each spectrum gives only single lines for each

chemically equivalent carbon.

Environment Chemical shift / dC- C (alkanes) 10 - 35

C- C=O 10 - 35

C- Cl or C- Br 30 - 70

C- N (amines) 35 - 65

C - OH 50 - 65

C= C (alkenes) 115 - 140aromatic Cs(benzene rings) 125 - 150

C=O (esters, acids, amides) 160 - 185

C=O (aldehydes, ketones) 190220

Carbon-13 nmr has wide applications in the study of natural

products, biological molecules and polymers

CARBON-13 NMR SPECTROSCOPY


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1H: natural abundance 99.9844relative sensitivity 1

chemical shift range 10 ppm

1H-1H-spin-spin coupling chemical environmentchemical structure, regiochemistry, stereochemistry,

conformation

13C: natural abundance 1.108%

relative sensitivity 1.5910-2

Chemical shift range 250 ppm

long relaxation times

sensitive to subtle changes in the near electronic

environmentbut insensitive for long-range inter-

actions (solvent effects, diamagnetic anisotropy ofneighbouring groups)

no homonuclearcoupling

Separate resonance for every C in a molecule

Comparison of HYDROGEN-1 and CARBON-13


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Isomers

of C3H7Br

3 peaks 2 peaks

all three carbons are different the two outer carbons are similar

CARBON-13 NMR SPECTRA

HCCCBrH

H

H

H

H

H

HCCCH

H

H

Br

H

H

H


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Isomers

of C3H7Br

3 peaks 2 peaks

all three carbons are different the two outer carbons are similar

Ethanol

C2H5OH


HCCCBrH

H

H

H

H

H

HCCCH

H

H

Br

H

H

H

This is where the

proton nmrspectrum of ethanol

would be on the

same scale.

H CCOHH

H H

H

HCH

H

COHH

H


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The carbon-13 spectrum of 2-methylbutane (CH3)2CHCH2CH3

Other isomers of C5H12

pentane CH3CH2CH2CH2CH3 3 peaks

2,3-dimethylpropane (CH3)4C 2 peaks


There are four

chemically different

carbon atoms in the

molecule so there

are four peaks in the

C-13 nmr spectrum.

NO SPLITTING WITH C-13

ONLY ONE PEAK FOR

EACH CARBON

chemically

equivalent

carbon atoms

H CCCCHH

H

CH3

H

H

H H

H


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How many peaks would you expect there to be in the carbon-13 spectrum of

butane CH3CH2CH2CH3

2-methylpropane CH3CH(CH3)CH3

butanal CH3CH2CH2CHO

butanone CH3COCH2CH3

pentan-2-one CH3COCH2CH2CH3

pentan-3-one CH3CH2COCH2CH3

cyclohexane C6H12

CARBON-13 NMR SPECTRA - QUESTIONS


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How many peaks would you expect there to be in the carbon-13 spectrum of

butane CH3CH2CH2CH3 2 2-methylpropane CH3CH(CH3)CH3 2

butanal CH3CH2CH2CHO 4

butanone CH3COCH2CH3 4

pentan-2-one CH3COCH2CH2CH3 5

pentan-3-one CH3CH2COCH2CH3 3

cyclohexane C6H12 1


19


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Identify the isomers of C4H8O



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Identify the isomers of C4H8O


A butanal

B butanone

C 2-methylpropanal


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Identify the isomers of C6H12



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Identify the isomers of C6H12


X hex-1-ene or hex-2-ene or

2-methylpent-1-ene or

3-methylpent-1-ene or2-methylpent-2-ene or

3-methylpent-2-ene or

Y cyclohexane

Z 2,3-dimethylbut-2-ene


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The proton (lower) and carbon (upper) spectra have full width of the proton

spectrum is 10 ppm, while the width of the 13C spectrum is 200 ppm (20 times as

large). For example, the peak for the aldehyde proton is at 9.5 in the protonspectrum, so we expect the peak for the aldehyde carbon to appear at a chemical

shift between 15 and 20 times as large (between 144 and 192) in the carbon

spectrum. The actual position is at 180.


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The triplet at 77 in the l3C NMR spectrum is the carbon signal for

deuterated chloroform (CDCI3), split into three equal-sized peaks by

coupling with the deuterium atom. Chloroform-d (CDCI3) is a

common solvent for I3C NMR because the spectrometer can "lock"

onto the signal from deuterium at a different frequency fromcarbon. The CDCl3 solvent signal is a common feature of carbon

NMR spectra.


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1H NMR and 13C NMR spectra of 1,2,2-trichloropropane.

chemical shift effects are larger in 13C NMR, an electron-withdrawing group has a

substantial effect on the chemical shift of a carbon atom beta (one carbon removed)

to the group. The methyl (CH3) carbon absorbs at 33 ppm downfield from TMS

because the two chlorine atoms on the adjacent - CCl2- carbon have a substantial

effect on the methyl carbon. The chemical shift of this methyl carbon is about 15

times that of its attached protons (2.1 ).


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Differences between Proton and Carbon techniques


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Spin-spin Splitting

Only 1 % of the carbon atoms in the 13C NMR sample are magnetic, so there is only a

small probability that an observed 13C nucleus is adjacent to another 13C nucleus.Therefore, C-C splitting can be ignored. But Carbon-Hydrogen coupling is common.

Extensive C-H coupling produces complicated splitting patterns.

Proton Spin Decoupling : To simplify 13C NMR spectra, they are commonly recorded

using proton spin decoupling, where the protons are continuously irradiated with a

broadband ("noise") proton transmitter. As a result, all the protons are continuously inresonance, and they rapidly flip their spins. The carbon nuclei see an average of the

possible combinations of proton spin states. Each carbon signal appears as a single,

unsplit peak because any carbon-hydrogen splitting has been eliminated.

Off- Resonance Decoupling : Proton spin decoupling produces spectra that are very

simple, but some valuable information is lost in the process. Off-resonance decouplingsimplifies the spectrum but allows some of the splitting information to be retained.

With off-resonance decoupling, the 13C nuclei are split only by the protons directly

bonded to them. The N + 1 rule applies, so a carbon atom with one proton appears as

a doublet, a carbon with two attached protons gives a triplet, and a methyl carbon

gives quartet. Off resonance decoupled spectra are recognized by the appearance of

TMS as a quartet at 0 ppm, split by the three protons of each methyl group.


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The best procedure for obtaining a 13C NMR spectrum is to run the spectrum twice: The

singlets in the broadband-decoupled spectrum indicate the number of nonequivalent

types of carbon atoms and their chemical shifts. The multiplicities of the signals in the off-

resonance-decoupled spectrum indicate the number of hydrogen atoms bonded to each

carbon atom. 13C spectra are often given with two traces, one broadband decoupled and

the other off-resonance decoupled. If just one trace is given, it is usually broadband

decoupled.


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Try this


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Decoupling of heteronuclear spin coupling causes theNUCLEAR OVERHAUSER EFFECT (NOE)

Decoupling 1H-13C saturates 1H and changes the 13C-spin population

excess13

C in the lower level comparedwith the equilibrium distribution

more energy is absorbed

DE = 1 + (gH/2gC)

NOE depends on the specific resonance makes quantification difficult

better S/N

The NOE is sensitive to the distance NOESY experiment


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DEPT


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DEPT: Distortionless Enhancement by

Polarisation Transfer

Differentiation between CH, CH2and CH3by

positive (CH, CH3) or negative (CH2) signal

amplitudes, using improved sensitivity of

polarisation transfer.

DEPT


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DEPT and APT (Attached Proton Test) experiments gives us information on

the "multiplicity of the carbon atom (quaternary, methine, methylene,

methyl) which can be carried out very quickly.

DEPT find its origin in the spin-echo sequence, devised by Hahn in 1952

and used for the determination of relaxation times.

The important feature of the experiment is that the carbon signals appear

to have been simply broad-band decoupled, but that according to themultiplicity they appear either in positive (normal) phase or in negative

phase.

DEPT


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Each 13C nucleus is magnetically coupled to the protons bonded to it. Under the

right circumstances, this magnetic coupling allows the transfer of polarization

from the protons to the carbon nucleus. The number of protons bonded to the

13C nucleus determines how this polarization transfer occurs. A DEPT

experiment usually includes three spectral scans:

1. The normal decoupled scan, in which each type of 13C nucleus appears as a

singlet.

2. The DEPT-90 scan, in which only the CH (methine) carbons bonded to exactly one

proton appear.

3. The DEPT- 135 scan, in which the CH3(methyl) groups and CH (methine)

groups appear normally, and the CH2groups give negative peaks. Carbons that

are bonded to no protons do not appear.

DEPT


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DEPT


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The carbonyl carbon (Cb, no protons) appears only in

the regular spectrum.

Cc, with 1 proton, appears normally in all the spectra.

Cd, with two protons, appears as a negative peak in

the DEPT- 135 spectrum.

Ca, the methyl carbon with three protons, vanishes in

the DEPT-90 spectrum but appears as a normal peakin the DEPT- 135 spectrum.

DEPT


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DEPT

Now Your turn


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DEPT

The spectrum in detail.


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DEPT


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DEPT


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The J-modulated spin-echo and the more frequently used DEPT

experiment are pulse sequences, which transform the information

of the CH signal multiplicity and of spin-spin coupling into phase

relationships (positive and negative amplitudes) of the 13C signals in

the proton decoupled 13C NMR spectra. The DEPT experiment

benefits from a 1H - 13C polarisation transfer which increases the

sensitivity by up to a factor of 4. For this reason, this technique

provides the quickest way of determining the 13C1H multiplicities.

DEPT


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DEPT


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DEPTA & B


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DEPT

(A)Pulse sequence for the DEPT experiment.


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DEPT

(B)Effect of the pulse sequence on 1H and13C magnetization vectors. 13C magnetization

can be recorded either as multiplets or, if

broad-band decoupling is applied during the

d l

C-13 NMR and DEPT

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