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COMPARISION OF PROTON NMR WITH
13C NMR
Presented by:Sumit Kaushik
M.PHARM (Part-I)
Dept. of PHARMACEUTICS
KLE’S College of Pharmacy, BELGAUM.
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INTRODUCTION
PRINCIPLE OF NMR SPECTROSCOPY
IMPORTANCE
DIFFICULTIES ENCOUNTERED IN 13C NMR
13C CHEMICAL SHIFT
APPLICATIONS
REFERENCES
CONTENTS:
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INTRODUCTION13C NMR ( CMR) Proton NMR ( PMR)
It is study of spin changes of carbon
nuclei.
It is study of spin changes of proton
nuclei.
Chemical shift range is 0-240 ppm. Chemical shift range is 0-14 ppm.
Fourier transform Technique is used. Continuous wave method is used
Very fast process.
Gyromagnetic ratio is 1.4043
slow process.
Gyromagnetic ratio is 5.5854
Coupling constant range is 125-250Hz. Coupling constant range is 0-15Hz.
Solvent peak is observed. Solvent peak is not observed.
Area under the peak is not considered. Area under the peak is considered
TMS peak is quartet. TMS peak is singlet.
Effect of substitute on adjacent carbon
atom cannot varies chemical shift.
Effect of substituent on adjacent carbon
atom can varies chemical shift.
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Principle Any nucleus with odd mass number spins on its own axis
By the application of an external magnetic field (Ho), the nucleus
spins on its own axis and a magnetic moment is created,.
In this ground state the magnetic field caused by a spin of nuclei is
aligned with external magnetic field.
When the energy in the form of radio frequency is applied andwhen applied frequency is equal to processional frequency,
absorption of energy occurs and NMR signal is recorded.
Because of absorption of energy, the nucleus moves from ground
state to excited state, which results in spin reversal or anti-parallelorientation in which magnetic field caused by the spin of nucleus
opposes the external applied magnetic field.
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A spinning charge, such as the nucleus of 1H or13C, generates a magnetic field. The magneticfield generated by a nucleus of spin +1/2 isopposite in direction from that generated by anucleus of spin –1/2.
Nuclear Spin
+ +
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++
+
+
+
The distribution ofnuclear spins is
random in theabsence of anexternal magneticfield.
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++
+
+
+An external magnetic fieldcauses nuclear magnetic
moments parallel andantiparallel to appliedfield.
H 0
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no difference in absence of magnetic field
proportional to strength of external magnetic field
Energy Differences Between Nuclear Spin States
+
+
DE DE
'
increasing field strength
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Basic Features of NMR Spectrometer
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Radio Wave
Transceiver Radio Wave
Transceiver
A Modern NMR Instrument
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CMR is nondestructive and noninvasive method.
CMR of biological materials allows for theassessment of the metabolism of carbon.
CMR, chemical shift range is wider than PMR.
The low natural abundance of 13C nuclei (1.1%)can be made use of tagging the specific carbonposition by selective 13C enrichment.
13C nucleus is a stable isotope, hence notsubjected to dangers related to radiotracers.
Homo nuclear coupling of 13C provides novelbiochemical information.
IMPORTANCE OF 13C NMR
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The 13C nucleus is magnetically active and
which is similar to 1H nucleus.
Recording of CMR nucleus is difficult due tothe following reasons:
1. Natural abundance.2. Gyro magnetic ratio.
3. Coupling phenomenon.
DIFFICULTIES ENCOUNTERED IN 13C NMR
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The most abundant isotope of carbon12
C is notdetected by NMR, as it is magnetically inactive (I=0).
The low natural abundant isotope 13C is magnetically
active (I=1/2).
As a result of the natural abundance of 13C is 1.1% , the
sensitivity of 13C nuclei is only 1.6% that of 1H nuclei.
The availability of FT instrumentation enhances the
sensitivity of 13C nucleus.
1. Natural abundance:
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The gyro magnetic ratio of 13C is 1.4043 as compared
to 5.5854 of a proton.
13C nucleus resonance frequency is only 1/4th of PMR
at a given magnetic field.
Thus, CMR is less sensitive than PMR
Sensitivity of CMR can be increased by adopting FT
technique.
2. Gyro magnetic ratio:
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Both 13C and 1H have I =0, so that we expect coupling in the spectrum
between13
C -13
C and13
C -1
H.The probability of two 13C nuclei adjacent to each other in the same
molecule is extremely rare due to low natural abundance of 13C.
So that 13C- 13C coupling will not usually exist. However the 13C - 1H
coupling have observed in CMR spectrum.As a result of coupling makes the 13C spectrum extremely complex ,
consequently there is an overlap of multiplets.
These 13C - 1H coupling can be eliminated by adopting following
techniques.a) FT technique
b) Decoupling technique
c) Nuclear overhauser phenomenon for enrichment of the carbon
signal.
3. Coupling phenomenon:
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Earlier, the continuous wave method is used to record 13C spectra but it is
slow procedure, require large sample and for assessing takes long time .FTtechnique increases activity of 13C nuclei
FT technique permits simultaneous irradiation of all 13C nuclei .
In this method sample is irradiated with a strong pulse of radio frequencyradiation in desired range at once in a fixed magnetic field .
Advantages
1)The scanning takes place rapidlycompared to continuous wave NMR.
2)The sensitivity problems are eliminated in NMR, therefore whichhelps in
a) Analyses the sample at low conc.
b) NMR studies on nuclei with low naturalabundance and with low gyro magnetic ratio.
FT Technique:
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• Decoupling technique:
Generally the probability occurrence of 13C- 13C coupling is rare ,but the 13C - 1H coupling can usually observed . The problem of 13C
- 1H coupling can be eliminated by decoupling the proton from
carbon .
Types of decoupling in CMR1) Proton decoupling or noise decoupling .
2) Coherent and broadband decoupling .
3) Off resonance decoupling .
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The proton decoupled CMR spectrum can be recordedby irradiating the sample at two frequencies.
The first radio frequency signal is used to affect carbonmagnetic resonance, while simultaneous exposure to
second signal causes all the protons to be resonance atthe same time they spin or flip very fast.
As they flip so fast, there is no coupling and eachcarbon appears as a single unsplit peak at
corresponding chemical shift range.
Ex: Proton decoupled spectra of sec-butyl bromide.
Proton decoupling or noise decoupling:
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In this technique, all the proton resonance can be reduced and to
get sharp CMR spectral peaks, each directly reflecting a 13Cchemical shift.
The NMR spectrum of nucleus A is split by nucleus B, because A cansee B in different magnetic orientation.
Broadband decoupling
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1000-2000 Hz above the spectral region
In this primary carbon nuclei (bearing three protons) yield a
quartet of peaks, secondary carbons give three peaks, tertiary
carbon nuclei appear as doublets, and quaternary carbons exhibit
a single peak.
Off resonance decoupling
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This division gives a number independent
of the instrument used.
parts per
million
THE CHEMICAL SHIFT
The shifts from TMS in Hz are bigger in higher field instruments (300
MHz, 500 MHz) than they are in the lower field instruments (100 MHz,
60 MHz).
We can adjust the shift to a field-independent value , the “chemical
shift” in the following way:
A particular proton in a given molecule will always come at the same
chemical shift (constant value).
chemicalshift
= d =shift in Hz
spectrometer frequency in MHz= ppm
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01.02.03.04.05.06.07.08.09.010.0
Chemical shift (d, ppm)
measured relative to TMS
Upfield
Increased shielding
Downfield
Decreased shielding
(CH3)4Si (TMS)
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Electronegativity
Hybridization
Hydrogen bonding
Anisotropic
Factors affecting chemical shift
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Metabolic studies
Metabolic studies on human
1. Brain function
2. Glucose metabolism in liver
3. Glucose metabolism in muscle4. Determination of degree of unsaturation of
fatty acids in adipose tissue
5. Characteristic of body fluids and isolatedtissues
6. In diseased state
Industrial applications in solids
Applications of 13C NMR
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Morrison RT, Boyd RN. Organic chemistry. 6th edition.2001; P.no 604-629.
Sanders FRS, Jeremy KM, Hunters BK. Modern NMRSpectroscopy. 2nd edition. 1993; P.no 46.
Skoog, Holler, Nieman. Principles of Instrumentalanalysis. 5th edition 1991; P.no 480-484.
Kemp W. Organic Spectroscopy 3rd edition 1991; P.no110-130.
Silverstein RM, Webstar FX. SpectrometricIdentification of Organic Compounds 6th edition.1998; P.no 222.
REFERENCES
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