carbon 13 spectroscopy
TRANSCRIPT
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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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