carbon 13 spectroscopy

8/11/2019 Carbon 13 Spectroscopy

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CARBON-13 NMR SPECTROSCOPY

The most significant nucleous other than the proton is

Carbon 13 which has a net nuclear spin equal to half. It has a

low natural abundance(1.11%)and is inharentily less

sensitive than the the proton because of its lower magnatogyric ratio( )

Carbon 13 NMR is used routinely to compliment protonNMR specteroscopy .Carbon 13 NMR is perticulerly

important in the analysis of large, biochemicaly singnificant

molecules since the Carbon 13 NMR spectra can be much

simpler than the corresponding proton spectra


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Comparison of Carbon 13 NMR and proton NMR Spectroscopy

1) PMR Spectroscopy gives indirect information about

carbon skeleton of an organic molecule because most of

the carbon atoms have atlest one attached hydrogen where

as carbon 13 NMR spectra display signals arising from allthe carbon atoms and thus give direct information about

the carbon skeleton

2) if carbon 13 Spectroscopy signal are spread over a

chemical shift range of 200 ppm ,compared with a range

less than 20ppm for proton spectra


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3) carbon 13 spectra are generally much simplerthan the corresponding 1H spectra because of

the very low natural abundance of 13C.

It is improbable that a particular 13C nucleus in a

molecule will have a second 13C-nucleus as an

immediate neighbor .therefore, splitting of a

13

Cresonance by coupling with a neighboring 13c-

nucleus is unlikely


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Chemical shifts in 13C NMR- and factors affecting them:-

Chemical shifts of 13C Nuclei, like the PMR, are expressed in ppm

downfield from TMS. However , the range of 13C-chemical shifts is

much greater (200 ppm) compared to 20 ppm range of proton

chemical shifts.

Because of this wide spread of signals, it is unlikely that the two 13C

nuclei will have identical chemical shifts unless they are equivalentor enantiotopic.For eg, every individual carbon atom can be

observed in 13 C NMR spectrum of secondary butyl bromide.


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Br

2-bromobutane

a

c d b

d

Proton coupled 13C NMR spectrum of sec-Butyl bromide


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Br

2-bromobutane

a

cd b

Proton decoupled 13C NMR spectrum of sec-Butyl bromide


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All the four carbon atoms in the molecule are

different(non equivalent) and consequently the

spectrum displays four signals, one for each

carbon atom

The 13C chemical shifts for various types of compounds are in the following order:

C=O (aldehydes and ketones) > C=O (carboxylicacids, esters and amides ) > C=C, C= N and

aromatic carbon > C=C > C-O (alcohols and

ethers ) > C-X (X=Cl,Br,N,) > alkanes

The 13C chemical shifts for various types of

compounds are in the following order:


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Important points regarding the chemical shift of carbon nuclei

1. Alkanes generally absorb from -2 to 55 ppm

2. Increasing alkylation generally moves the carbon resonance

downfield.This can be observed in the behaviour both of sp3

hybridized carbon (alkanes) and sp2 hybridised carbon (Alkene)

3. The values of the chemical shifts indicate the type of

hybridisation ( sp3,sp2,or sp) at each carbon

4. Carbon of both benzene ring and alkene absorb in the same region

.This makes PMR useful to distinguish between two types of

compounds.

5. Carbon of carbonium group absorbs far downfield (

200 ppm )


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6. In proton decoupled 13 C NMR spectra, the

number of signals exhibit how many different

carbons or different sets of equivalent carbons arepresent in the molecule.

7. In 13 Cproton off resonance decoupled NMR

spectrum, the splitting of the signal indicates the

number of hydrogen atoms attached to the carbon

giving rise the signal

8. In 13C NMR spectrum, the peak areas are not

necessarily proportional to the number of identical

13C nuclei under conditions normally used to run

the spectrum.


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Factors affecting the 13C chemical shifts

Effect of hybridisation

The signals for sp3 hybridised carbon occur upfield in the

range from 2-55 ppm, whereas for sp2hybridised carbons, the

signal appear over 100 ppm downfield from them. ie occur in

the range from 110170 ppm.For eg,the 13C NMR spectrum

of 1-octene displays the following shift values

14.1 22.9 32.1 29.3 29.1 34.1 139 114

CH3 - CH2 - CH2 - CH2 - CH2 - CH2 - CH = CH2


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The carbon atoms of the aromatic ring are sp2

hybridised and absorb downfield similar toalkene carbons.

The spectrum of ethyl benzene exhibit two

widely separated sets of signals; an upfield set

pertaining sp3hybridized carbons in the side

chain and a downfield set, consisting of sp2

hybridized carbons of the benzene ring, absorbsover 100 ppm downfield from sp3hybridized

carbons.


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13C-Spectra ofEthyl benzene


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H2C CH3

29.1 15.6

144.2

127.9

128.4

125.7

Chemical shifts for various Carbons in ethyl benzene


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The triply bonded hybridized carbon in acetylenes absorb in

the region between sp3 and sp2 hybridized carbons ie, in therange from 65-90 ppm, for eg,1-hexyne


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13.7 22.1 30.9 18.3 84.5 68.4

CH3 - CH2 - CH2 - CH2 - C CH


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Effect of substituents:

The substituents on the carbon atom shift the signal much more

downfield ascompared to the corresponding shift in PMR spectra.

Effects of Chlorin substitution;- The effects of Chlorine

substitution on the chemical shifts exhibited by various carbon of

saturated chain, may be explained by comparing the 13Cspectral

data of n-pentane and 1-Chloropentane.The shift values forvarious carbons in the two compounds are given below

13.7 22.6 34.5 22.6 13.7

CH3 - CH2 - CH2 - CH2 - CH2-H

13.6 22.1 29.2 32.7 44.3

CH3 - CH2 - CH2 - CH2 - CH2-Cl


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The effect exerted by various substituents attached to C-1

of pentane are as follows;

F Br Cl NH2 OH NO270.1 19.3 30.6 29.7 48.3 64.5


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-effect

Chlorine causes for the -carbon a large downfield shift

from22.6 to 32.7 ( a differance of +10.1ppm) as shown below:

(formula with value)

C C C Cl+10.1


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-effect

The effect of Chlorine on C3 is the upfield

shift from 34.5 to 29.4 ( a differance of -

5.3ppm) as illustrated below (atomic skelitonvalue)

C C C

Cl

-5.3


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In genaral the various substituents follow the

same pattern of substituent effects as those for

chlorine , on the absorption by sp3 hybridised

carbons: and -effect downfield .with

greater than , and -effects,though smaller

but upfield


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Effects of Alkyl substitution

Alkyl groups exerts smaller effects compared with other

substituents . This is illustrated by the 13 C spectral data

of n-pentane and hexane as given below (formulas and

valued)

13.7 22.6 34.5 22.6 13.7

CH3 - CH2 - CH2 - CH2 - CH2-Hn-pentane

13.9 22.9 32.0 32.0 22.9 13.9

CH3 - CH2 - CH2 - CH2 - CH2 - CH3n-Hexane


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Now considering n-hexane as n-pentane with a methyl

substituent on C1- the following substitueent effect of

methyl group may be calculated: (skeleton with value)

These effects are typical of alkyl groups and -effect downfield

with greater than , and -effects,though smaller but upfield. The

13C upfield shift due to - carbon has been attributed to the steric

compression of a gausche interaction but has no counter part in

PMR spectra.

-2.5 +9.4 +9.2

C C C CH3

S b i ff h i l hif i l fi i


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Substituent effects on chemical shifts in olefinic system

The presence of Carbon-Carbon double bond in a molecule , due

to geometrical isomerism , exerts significant effects on the

absorption exhibited by sp3 hybrid carbons, as illustrated

belowformulas & values)

Thus the absorption by methyl carbon in propylene is affectedby

substitution of one or the other of the vinylic hydrogens by a

methyl group; the -effects are upfield (-7.3ppm for cis isomer

and -1.9ppm for the trans,ie. -effects for the cis isomer is

stronger by 5.4ppm)

CC

H

HH

H3C 115

136

18.7

CC

CH3

HH

H3C

124

124

11.4 11.4

CC

CH3H

H3C125

125

16.8

H

16.8


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Aldehydes

Ketones

Acids AmidesEsters 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

/


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INTEGRATION(13Cpeak area)

Peak area measurments are not usually obtained in routiene 13C-

spectra. The loss of corrilation between the number of carbon

nuclei comprising a peak and the integrated peak area is due

mainely to

The possible differantial saturation effects from variable spin

lattice relaxationtimes and

Variable NOE

Simplification of 13C spectra:


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Simplification of 13C spectra:

The low natural abundance of 13C minimize theprobability of finding 13Cnuclie adjacent to each other

in the same molecule. Therefore spin-spin coupling

between carbon nuclei will not be observed.

However there is substantial coupling between the

carbon and their attached hydrogens. and with more

distant hydrogen in many cases. consequently the

proton coupled 13C spectra of organic molecules are

quite complex.

Broadband or noise decoupling


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Broadband or noise decoupling

C-H Coupling is removed by a technique called

broadband decoupling.

In this method,as the carbon spectrum is obtained,the

sampe is simultaneously irradiated with a band of

radiofrequency radiations that excites all of thehydrogens. This causes each of the hydrogens to flip

rapidly between its spin states,so its two magnetic

orientations average to zero.

As a result no coupling occurs with the carbon and each

peak appears as a singlet at the position corresponding to

its chemical shift.


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Off-resonance decoupling.

While the broadband decoupling simplifies thespectra but with the loss of coupling in

formations and the c-c coupling are fairly rare in

routine spectra, the C-H coupling would give asubstantial amount of information regarding the

number of hydrogen atoms directly bonded to a

given carbon.

These coupling however can be very complex

and seldom produce simple first order spectra.


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Off resonance decoupling techniques produces a simplifiedspectrum with the retention of residual 13C-H coupling

information.

This techniques involeve offsetting the central frequency of

the broadband proton decoupler by about 1000-2000Hz

upfield or 2000-3000 Hz downfield from the proton frequency

of TMS.


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This results in the residual coupling from proton directly

bonded to the 13carbon atoms whereas long rangecoupling is usually lost. The observed residual coupling is

usually smaller than the true coupling .Thus the

multiplicity of the 13C band can readily be observed .

ie a methyl group appears as quartet, a metylene group as

triplet etc. and a quartinary carbon as singlet.


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13C Off-resonance decoupled

spectrum


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13C Off-resonance & Broadband

decoupled spectra

Broadband

Off-resonance


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13C NMR n-Hexane

Broadband


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13C NMR Acetone

Broadband


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Broadband

1H & 13C NMR: 1,1,2-trichloropropane


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1H & 13C NMR: 2-methyl-1-butene

Broadband


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13C NMR 6-methyl-5-hepten-2-ol

DEPT 90Only CH carbons


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13C NMR 6-methyl-5-hepten-2-ol

DEPT 135Methyl and CH positive

Methylene negative


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carbon 13 spectroscopy

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