C13 NMR spectroscopy

General introduction of Carbon-13 nuclear magnetic resonance spectroscopy.

1. C
13
NMR Spectroscopy
Introduction
Carbon-13 Nuclear Magnetic Resonance Spectroscopy
Technique is based on the fact that the net nuclear spin for C
13

isotope of carbon is half unlike C
12
isotope for which it is zero
(thus C
12
is non-magnetic and does not give any NMR signal).
Though C
13
has a low natural abundance of only 1.11% and is
inherently less sensitive than H
1
NMR (so in order to observe
the weak signals the spectra is scanned repeatedly), but it is
simpler to interpret.
Characteristics of C
13
NMR Spectra
1. Since it is impossible that a particular C
13
molecule will
have another C
13
nucleus as an immediate neighbour, thus
splitting of C
13
signal due to C
13
―C
13
coupling is
negligible and the signal is easy to interpret.
2. The proton decoupled C
13
NMR spectrum gives single
unsplit peak for each magnetically non-equivalent carbon
thus providing direct information about the carbon
skeleton.
3. In C
13
NMR, signals are spread over a chemical shift
range of 200 ppm as compared to H
1
NMR for which the
range is only 15 ppm, and thus only fewer peaks overlap.
4. In proton coupled spectra, signal for each carbon is split
by number of H
+
bonded directly to that carbon by (n+1)
rule, where ‘n’ is the number of H-atoms present on the
carbon.

2. Carbon Chemical Shift
Each carbon nucleus has its own electronic environment
different from the environment of other, non-equivalent
nuclei; it experiences a different magnetic field and absorbs at
different applied field strength. This causes chemical shift in
C
13
NMR spectrum.
C
13
chemical shift normally ranges between 0 -250 ppm and is
expressed in ppm downfield from TMS (Tetramethylsilane),
i.e., the reference compound.
The C
13
chemical shift follows the order:-
C=O (aldehyde and ketone) > C=O (acids, esters, amides) >
C=C, C≡N, aromatic carbons > C≡C > C−O (alcohols and
ethers) > C–X (X→ Cl, Br, N) > alkanes.


Figure 1. Range of C
13
Chemical Shifts

Important Features of C
13
Chemical Shift
 Alkanes absorb from -2 to 55 ppm. Methane absorbs at -
2.1 ppm upfield from TMS.
 In saturated acyclic alkanes, replacement of –H by CH3
group deshields C to which it is attached by 9.1 ppm (α-
effect), β- carbon is deshielded by 9.4 ppm (β- effect) and
γ- carbon is shielded by -2.5 ppm (γ- effect).

3.  Increasing alkylation moves the carbon resonance
downfield.
 In functionalized alkanes, when a H is replaced by a
heteroatom like >C=O, deshielding effect is caused due to
such electronegative substituents and downfield shift
occurs.
 The value of chemical shift indicates the type of
hybridization. The signal for sp
3
hybridized carbons occur
upfield in the range of -2 to 55 ppm. For sp
2
hybridized
carbons (in alkenes and arenes), signals appear in the
region of 80 to 170 ppm downfield from TMS. The triply
bonded hybridized carbons (sp hybridization) absorb in
the region between sp
3
and sp
2
carbons, i.e., 65 to 90 ppm.
 Carbon of carbonyl group absorbs far downfield (almost
220 ppm). This is due to:-
i) sp
2
hybridization
ii) The presence of electronegative O atom directly
bonded to the carbon.

Factors Affecting C
13
Chemical Shift
1. Deshielding effect of electronegative moiety:
Presence of electronegative groups like >C=O, ―OH, etc.
brings about a downfield shift in the resonance at α and β
positions and a small upfield at γ position.
For example, the spectra of butane and 1-butanol.
 In butane, H3 2 2 3, the spectrum shows
2 signals –
i) For ‘a’ type of 2 carbons at 13.2 ppm.

4. ii) For ‘b’ type of 2 carbons at 25.0 ppm.

 In 1-butanol, 3 2 2 2 O , the spectrum
shows 4 signals –
i) For ‘a’ type carbon, δ = 61.7 ppm (high downfield
shift, α-effect).
ii) For ‘b’ type carbon, δ = 35.3 ppm (downfield
shift, β-effect).
iii) For ‘c’ type carbon, δ = 19.4 ppm (upfield shift, γ-
effect).
iv) For ‘d’ type carbon, δ = 13.9 ppm.
2. Effect of Hybridization:
The signal for sp
3
hybridized carbons occur upfield in the
range of -2 to 55 ppm as compared to sp
2
hybridization for
which the range is 110 to 170 ppm; while for sp
hybridized carbons the range is 65 to 90 ppm which is an
intermediate to sp
3
and sp
2
.
For example, the values chemical shifts for ethane, ethene
and ethyne. For all the three, the spectra shows only 1
signal –
i) For CH3 3, δ = 5.9 ppm
ii) For CH2=CH2, δ = 122.8 ppm
iii) For ≡ , δ = 71.9 ppm
It is generally found that terminal =CH2 group absorbs
upfield than an internal =CH― group.
For example, the spectra of 1-butene and 2-butene.
 In 1-butene, CH3 2 2, δ value for ‘a’ type
of C = 112.8 ppm and for ‘b’ type of C = 140.2 ppm.

5.  In 2-butene, CH3 3, δ value for ‘a’ type of
C = 13.2 ppm and for ‘b’ type of C = 123.3 ppm.

3. Effect of Substituent:
Substituents like Cl, Br, N, etc. shift the signal downfield.
This signal is much more downfield as compared to the
corresponding in H
1
NMR spectra.
For example, the spectra of n-pentane and 1-
chloropentane.
 In n-pentane, 3 2 2 2 3, the proton
decoupled spectra will show 3 signals –
i) For ‘a’ type of 2 carbons, δ = 13.7 ppm
ii) For ‘b’ type of 2 carbons, δ = 22.6 ppm
iii) For ‘c’ type of 1 carbon, δ = 34.5 ppm
 In 1-chloropentane, 3 2 2 2 2 l,
the proton decoupled spectra show 5 signals –
i) For ‘a’ type of C-atom, δ = 44.3 ppm instead of
13.7 ppm because of α-C effect (large downfield
shift of almost 30.6 ppm).
ii) For ‘b’ type of C-atom, δ = 32.7 ppm instead of
22.6 ppm because of β-C effect (downfield shift of
almost 10.1ppm).
iii) For ‘c’ type of C-atom, δ = 29.4 ppm instead of
34.5 ppm because of γ-C effect (small upfield shift
of almost -5.3 ppm).
iv) For ‘d’ type of C-atom, δ = 22.1 ppm.
v) For ‘e’ type of C-atom, δ = 13.6 ppm.
Thus this effect is not propound after γ-carbon.

6. References
1. Spectroscopy; Kaur, H.
2. Spectroscopy of Organic Compounds; Kalsi, P.S.
3. Experiments and Techniques in Organic Chemistry; Pasto,
Daniel J., Johnson, Carl R. and Miller, Marvin J.


THANKS