The document discusses the principles and applications of 13-carbon nuclear magnetic resonance (13C NMR), highlighting its ability to determine the structure of organic compounds through chemical shifts and the number of carbon signals. It outlines the techniques used for signal analysis, including decoupling methods and the significance of nuclear overhauser enhancement (NOE) in improving signal clarity. Additionally, it emphasizes the practical applications of 13C NMR in biological analysis and drug purity assessment.

1. MODERN PHARMACEUTICAL ANALYSIS
SEMINAR ON
13 CARBON NUCLEAR MAGNETIC
RESONANCE
Presented By :- Tejaswini C
1st Mpharm

2. 13Carbon Nuclear magnetic Resonance
The 13C NMR is directly about the carbon skeleton not just the proton attached to it. a. The number
of signals tell us about how many different carbons or set of equivalent carbons . This helps in
easier determination of the structure.

4. THEORY

5. 𝑽 =
𝜸
𝟐𝝅
𝑩
Calculation of radio frequency
Ex: if B= 1.41 T or 14,100 G
γ= 67.28 Radians/sec
𝑉 =
67.28
2 × 3.14
1.41 = 15.1 𝑀𝐻𝑧

6. Chemical Shift
The frequency at which a nucleus will resonate is dependent on
the magnetic field strength.
Because this can vary from instrument to instrument, frequency
is expressed relative to magnetic field strength, “chemical shift”
Chemical Shift = frequency of resonance (Hz)
frequency of instrument(MHz)
units = parts per million = ppm

7. CHEMICAL SHIFT IN C13 NMR
The chemical shift in absolute terms is defined by the frequency of the resonance expressed
with reference to a standard compound which is defined to be at 0 ppm.
 The scale is made more manageable by expressing it in parts per million (ppm) and
is independent of the spectrometer frequency.
Chemical shift ranges from 0 to 220ppm
It is approximately 20 times greater than proton NMR
TMS is taken as reference in c13 NMR which has delta value 0
Upfield
shielded
Down field
deshielded

8. CHEMICAL SHIFTS are mostly
affected by
 Hybridisation state of
carbon
 Electronegativity of groups
attached to carbon

12. No of sigals in 13C NMR
 No of signals gives the no of equivalent set of carbon present in the
sample
 Each signal in c13 nmr represent the equivalent set of carbon atoms
 Carbon atoms present in the same environment gives the single signal

13. • CH3CH2OCH2CH3
2*CH3 equivalent 2*CH2 equivalent

14. • The probability of finding two 13C atoms adjacent to each other in the same
molecule is even lower and its interaction, homo nuclear spin-spin splitting
patterns is rare
• the spins of proton attached directly to 13C atoms do interact with the spin of
carbon and cause the carbon signal to be split according to the n+1 rule.
• The effect of protons directly attached to a 13C atom. The n+1 rule predicts the
degree of splitting .The resonance of a 13C atom with three attached protons,
for instance, is split into a quartet.
Proton – coupled 13C spectra or 13C-NMR spin-spin splitting

16. Complex coupling patterns
 The neighbouring protons are often not equivalent to each other
 Different types of neighbours interact with different coupling
constants.
 In these cases, the "n+1" rule has to be refined so that each type of
neighbour causes n+ 1 line.
 For example for a proton with two types of neighbour, then the
number of lines, l = (n1 + 1) (n2 + 1).
 However, in many cases the lines overlap with each other and the
result is further distortion from the "ideal" pattern.

17. DECOUPLING:
The process of removing the spin spins splitting between the spin off
protons is called decoupling. Most 13C spectra are recorded using hetero
nuclear decoupling.
Decoupling techniques:
 Noise decoupling/multiplicity and proton decoupling.
 Coherent and broad band decoupling.
 Off resonance decoupling
 Selective proton decoupling
 Deuterium substitution

18. NOISE DECOUPLING /MULTIPLICITYAND PROTON DECOUPLING
 In this method of sample analysis all the protons present in the sample are
decoupled from the carbons.
This is done by irradiation of the sample with a noise decoupler at the 1H
frequency, while observing the spectrum at the 13C frequency.
Due to the strong radiation in the range of all the proton frequencies in the
sample, the protons change their spin states too rapidly and are effectively
decoupled from the carbons.
The proton-noise decoupling simplifies the 13C spectrum and increases the
intensities of signals.

21. COHERENT AND BROAD BAND DECOUPLING
Most widely used decoupling technique, which involves
simply broadband decoupling of all proton resonance to reduce
the 13C spectrum to a set of sharp peaks each directly, reflecting a
13C chemical shift.
Requirements for 13C broadband decoupling
 Sufficient strong decoupling field strength.
 Method of modulation that will spread decoupling field over
the ranger of proton chemical shift.

24. OFF RESONANCE DECOUPLING
The off resonance decoupling can be achieved by off setting the high power proton
decoupler by about 1000-2000 HZ upfield or about 2000-3000 downfield from the
frequency of TMS without using noise generator.
Mechanism
 In the off resonance decoupling is done while recording the CMR spectrum the sample
is irradiated at a frequency close to the resonance frequency of proton.
 So, consequently the multiplets become narrow and not removed together as in fully
decoupled spectra. This also results in residual coupling from protons directly bonded
to 13C atoms but long-range coupling is usually lost.
Example : decoupling of 2-butanol

26. SELECTIVE PROTON DECOUPLING:
 When a specific proton is irradiated at its exact frequency at a lower
power level than is used for off resonance decoupling, the
absorption of the directly bonded 13C becomes a singlet, while the
other 13C absorption shows residual coupling.
 This technique, known as selective proton decoupling is used for
peak assignment, but satisfactory results are difficult to achieve.
Example : decoupling of propyl nitrite

27. DEUTERIUM SUBSTITUTION:
 Deuterium has a spin no of 1 and magnetic moment 15% that of H, it split
the 13C absorption into 3 lines ratio 1:1:1, Deuterium can also be used to
replace most of the normal hydrogen in a large biomolecule like a protein.
 If one replaces most of the normal hydrogen with deuterium then this
relaxation effect is decreased and the observed normal hydrogen peaks are
sharper. T for 13C-D is longer than that for C-H because of decreased
dipole dipole relaxation. Finally NOE is lost, since there is no irradiation
of deuterium.
Eg: decoupling of 4-hydroxy propiophenone

29. CDCl3

30. NUCLEAR OVERHAUSER ENHANCEMENT:
 Carbons atoms with H atoms directly attach are enhanced the most, and
enhancement increases as more hydrogen is attached.
 The effect is known a nuclear over Hauser effect and the degree of increase in the
signal is called NOE.
The maximum enhancement can be observed is
Irr is magnetic ratio of the nucleus being irradiated and abs is that absorbed)
NOE is enhancement of signal and it must be added to the original signal strength.
Total predicted intensity= 1+NOE

31. DEPT( Distortionless enhancement by polarisation transfer)
It requires FT pulsed spectrometer
Though it is complicated than off resonance , it gives same information
more reliably and more clearly
 In DEPT technique the sample irradiated with a complex sequence of
pulses in both the 13c and h channels
This results in representation of carbon atoms in molecule in different
phases depending on the number of hydrogens attached to each carbon

33. Applications of 13 C NMR:
 used in repetitive In-vivo analysis of the sample without harming the tissues .
 CMR of biological materials allows for the assessment of the metabolism of carbon,
which is so elementary to life on earth
 CMR, chemical shift range(0-240 ppm) is wider compared to H- NMR(0-14 ppm),
which permits easy seperation and identification of chemically closely related
metabolites.
 C-13 enrichment, which the signal intensities and helps in tracing the cellular
metabolism.
 used for quantification of drug purity to determination of the composition of high
molecular weight synthetic polymers

34. INTERPRETATION OF SPECTRA:

38. REFERENCES
1. Organic spectroscopy ,William Kemp,3rd Edition page no 177-193
2. Sharma BK. Spectroscopy. Goel publishing house. Meerut. 17th ed.
2005.
3. Pavia DL, Lampman GM, Kriz GS, Vyvyan JA. Introduction to
spectroscopy, 5th edition Cengage learning; 2014.
4. http://bionmr-c1.unl.edu