https://www.youtube.com/channel/UCTWTCRDLBs2M3EZdhIxGpyw

1. By Prashant J Patel
(Department of pharmaceutical technology)
Indukaka Ipcowala College of Pharmacy
New vidhyanagar,anand

2. 
Why C13-NMR is required though we
have H1-NMR?

3.  1H nmr spectroscopy - The powerful and useful tool
a tool for structural analysis.
Useless when portions of a molecule lack C-H
bonds, no information is forthcoming.
Ex: polychlorinated compounds such as
chlordane, polycarbonyl compounds such as
croconic acid, and compounds incorporating triple
bonds (structures below, orange colored carbons).

4.  Even when numerous C-H groups are present, an
unambiguous interpretation of a proton nmr spectrum
may not be possible.
The following three pairs of isomers (A & B) which
display similar proton nmr spectra. Although a careful
determination of chemical shifts should permit the first
pair of compounds (blue box) to be distinguished, the
second and third cases (red & green boxes) might be
difficult to identify by proton nmr alone.

5. Natural Abundance

 Since the major isotope of carbon (12C) has no
spin, this option seems unrealistic.
 Fortunately, 1.1% of elemental carbon is the 13C
isotope, which has a spin I = 1/2, so possible to
conduct a carbon nmr experiment.
 It is worth noting here, that if much higher
abundances of 13C were naturally present in all
carbon compounds, proton nmr would become
much more complicated due to large one-bond
coupling of 13C and 1H.

6. Many obstacles needed to be overcome before
carbon nmr emerged as a routine tool :

 As noted, the abundance of 13C in a sample is very low
(1.1%), so higher sample concentrations are needed.
 The 13C nucleus is over fifty times less sensitive than a
proton in the nmr experiment, adding to the previous
difficulty.
 Hydrogen atoms bonded to a 13C atom split its nmr signal
by 130 to 270 Hz,
 1H-13C splitting is overcome by using an instrumental
technique that decouples the proton-carbon interactions, so
that every peak in a 13C NMR spectrum appears as a singlet.

7.  The two features of a 13C NMR spectrum that
provide the most structural information are the
number of signals observed and the chemical
shifts of those signals.

8. 13C NMR—Number of Signals
The number of signals in a 13C spectrum gives the number
of different types of carbon atoms in a molecule.
Because 13C NMR signals are not split, the number of
signals equals the number of lines in the 13C spectrum.
In contrast to the 1H NMR situation, peak intensity is not
proportional to the number of absorbing carbons, so 13C
NMR signals are not integrated.

9. 13C NMR—Position of Signals
In contrast to the small range of chemical shifts in 1H
NMR (1-10 ppm usually), 13C NMR absorptions occur over a
much broader range (0-220 ppm).
The chemical shifts of carbon atoms in 13C NMR depend
on the same effects as the chemical shifts of protons in 1H
NMR.

11. 13C Chemical shifts are mainly most affected
by:

 Electronegativity of groups attached to the Carbon
 Hybridization state of Carbon
sp3 hybridized carbon is more shielded than sp2
sp hybridized carbon is more shielded than sp2, but
less shielded than sp3
 Anisotropy
 All affect 13C Chemical shifts in nearly same fashion
as they affect 1H chemical shift

12. Types of Carbons
Classification Chemical shift,
1H 13C
CH4 0.2 -2
CH3CH3 primary 0.9 8
CH3CH2CH3 secondary 1.3 16
(CH3)3CH tertiary 1.7 25
(CH3)4C quaternary 28
Replacing H by C (more electronegative) deshields
C to which it is attached.

13. Electronegativity effects on CH3
Chemical shift,
1H 13C
CH4 0.2 -2
CH3NH2 2.5 27
CH3OH 3.4 50
CH3F 4.3 75

14. Electronegativity effects and chain length
Cl CH2 CH2 CH2 CH2 CH3
Chemical 45 33 29 22 14
shift,
Deshielding effect of Cl decreases as
number of bonds between Cl and C increases.

15. Corrrelation chart for C13-NMR chemical
shift(ppm)

16. Spin-Spin Splitting

 Homonuclear spin-spin splitting:
Because of its low natural abundance there is a
low probability of finding two C13 atoms next to each
other in a single molecule.
C13-C13 coupling negligible.
 Hetronucler spin-spin splitting:
C13 will magnetically couple with attached
protons and adjacent protons. N+1 rule is obeyed.

18. Off-Resonance Decoupling

 13C nuclei are split only by the protons attached
directly to them.
 The N + 1 rule applies: a carbon with N number of
protons gives a signal with
N + 1 peaks.

19. 13C Off-resonance decoupled spectrum


20. Proton-decoupled spectra

 A common method used in determining a carbon-C13
NMR spectrum is to irradiate all of the hydrogen
nuclei in the molecules at the same time the carbon
resonances are being measured.
 Thins required a second radiofrequency(RF) source
(the decoupler) tuned to the frequency of the
hydrogen nuclei, while the primary RF source is
tuned to the C13 frequency.

21.  In this method the hydrogen nuclei are “saturated”, a
situation where there are as many downward as there are
upward transition, all occurring rapidly.
 During time the C-13 spectrum is being determined, the
hydrogen nuclei cycle rapidly between their two spin state
(+1/2 and -1/2) and the carbon nuclei see an average
coupling (i.e. zero) to the hydrogen.
 The hydrogen are said to be coupled from the carbon-13
nuclei.
 You no longer see multiples for the c13 resonances. Each
carbon gives a singlet, and the spectrum is easier to
interpret.

22. Nuclear Over Hauser enhancement effect
 When we obtain a proton-decoupled c13 spectrum, the
intensities of many of the carbon resonances increase
significantly above those observed on a proton-coupled
experiments.
 Carbon atoms with hydrogen atoms directly attached are
enhanced the most, and the enhancement increases as more
hydrogen are attached. This efface is called the Nuclear Over
Hauser enhancement (NOE).
 Shown when two different type of atoms are irradiated
while NMR spectroscopy of other type is determined.
 The effect can be either positive or negative, depending on
which atom types are involved.
 In case od c-13 interacting with H-1 the effect is positive.so,
Intensities of signals increases.

23. Magnetogyric
ratio of nucleus
being irradiated
Magnetogyric
ratio of nucleus
being observed

24. 1H & 13C NMR: 1,1,2-trichloropropane


26. DEPT spectra (Distortionless Enhancement by
Polarization Transfer)
 Useful method for determining the presence of
primary, secondary and tertiary carbon atoms.
 The DEPT experiment differentiates between CH, CH2 and
CH3 groups by variation of the selection angle parameter (the
tip angle of the final 1H pulse.
 45° angle gives all carbons with attached protons (regardless
of number) in phase
 90° angle gives only CH groups, the others being suppressed
 135° angle gives all CH and CH3 in a phase opposite to CH2
 Signals from quaternary carbons and other carbons with no
attached protons are always absent (due to the lack of
attached protons.

27. DEPT Spectrum
O
CCH2CH2CH2CH3
CH CH
CH3
CH
CH and CH3 unaffected
C and C=O nulled
CH2 inverted
CH2 CH2
CH2
200 180 160 140 120 100 80 60 40 20 0
Chemical shift ( , ppm)

28.  blue box- cyclohexane and2,3-dimethyl-2-butene
 single sharp resonance signal in the proton nmr spectrum
(the former at δ 1.43 ppm and the latter at 1.64 ppm).
 carbon nmr spectrum :- cyclohexane displays a single signal
at δ 27.1 ppm, generated by the equivalent ring carbon
atoms (colored blue) and isomeric alkene shows two signals
1) at δ 20.4 ppm from the methyl carbons (colored brown)
(2)at 123.5 ppm (typical of the green colored sp2 hybrid
carbon atoms)

29.  The C8H10 isomers in the center (red) box have pairs of
homotopic carbons and hydrogens, so symmetry should
simplify their nmr spectra. The fulvene (isomer A) has five
structurally different groups of carbon atoms (colored
brown, magenta, orange, blue and green respectively) and
should display five 13C nmr signals (one near 20 ppm and
the other four greater than 100 ppm).

30. ortho-xylene (isomer B) will have a proton nmr very similar to isomer
A, it should only display four 13C nmr signals, originating from the four
different groups of carbon atoms (colored brown, blue, orange and green).
The methyl carbon signal will appear at high field (near 20 ppm), and the
aromatic ring carbons will all give signals having δ > 100 ppm.
Finally, the last isomeric pair, quinones A & B in the green box, are easily
distinguished by carbon nmr. Isomer A displays only four carbon nmr
signals (δ 15.4, 133.4, 145.8 & 187.9 ppm); whereas, isomer B displays five
signals (δ 15.9, 133.3, 145.8, 187.5 & 188.1 ppm), the additional signal
coming from the non-identity of the two carbonyl carbon atoms (one
colored orange and the other magenta).