Sumit Nmr Final

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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.



INTRODUCTION

PRINCIPLE OF NMR SPECTROSCOPY

IMPORTANCE

DIFFICULTIES ENCOUNTERED IN 13C NMR

13C CHEMICAL SHIFT

APPLICATIONS

REFERENCES

CONTENTS:



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.



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.



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

+ +



++

+

+

+

The distribution ofnuclear spins is

random in theabsence of anexternal magneticfield.



++

+

+

+An external magnetic fieldcauses nuclear magnetic

moments parallel andantiparallel to appliedfield.

H 0



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



Basic Features of NMR Spectrometer



Radio Wave

Transceiver Radio Wave

Transceiver

A Modern NMR Instrument



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



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



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:



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:



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:



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:



• 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 .



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:





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



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





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



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)



Electronegativity

Hybridization

Hydrogen bonding

Anisotropic

Factors affecting chemical shift



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



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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Sumit Nmr Final