Nuclear Magnetic
Resonance
Spectroscopy
D. M. DHABARDE
KNCP, BUTIBORI

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Nuclear magnetic resonance is the study of molecules by measuring the interaction of
radiofrequency electromagnetic radiations in the region 4 to 900 MHz with the nuclei of
molecules placed in a strong magnetic field.
It is used to characterize organic molecules by identifying carbon-hydrogen frameworks within
the molecules and also used in quality control and research for determining the content and
purity of a sample as well as its molecular structure. The protons present in the molecule will
behave differently depending on the surrounding chemical environment, making it possible to
elucidate their structure.

Types of NMR-
Two types of NMR spectroscopy are used
1. 1 H NMR- used to determine the type and number of H atoms in a molecule.
2. 13 C NMR- used to determine the type of carbon atoms in a molecule
TheoryofNMR
The rules for determining the net spin of a nucleus are as follows;
If the number of neutrons and the number of protons are both even, then the nucleus has NO spin.
If the number of neutrons plus the number of protons is odd, then the nucleus has a half-integer spin
(i.e. 1/2, 3/2, 5/2)

Spinquantumnumber(I) isrelatedtotheatomicandmassnumberofthenucleus
I Z A Eg;
Half integer Odd Odd 1H (1/2)
Half integer Odd Even 13C (1/2)
Integer Even Odd 2H(1)
Zero Even Even 12C (0)
Elementswitheitheroddmassoroddatomicnumberhavethepropertyofnuclear“spin”
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• If anexternalmagneticfieldis applied,thenumberofpossibleorientationscalculatedby(2I+1).
Eg.:-HydrogenhasspinquantumnumberI=1/2andpossibleorientationis
(2*1/2+1=2)two,ie,+1/2and-1/2.
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PRINCIPLE OF NMR
•The principleis basedonthe-
spinning of nucleus and
generating amagnetic field.
•Withoutexternal magnetic(Bo)
– field nuclear spinarerandom
in direction.
•With Bo ,nuclei align
themselves eitherwith oragainst
field of external
magnetic field
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• If anexternalmagneticfieldis applied,anenergytransfer(ΔE) is possiblebetweengroundstatetoexcited
state.
• Whenthespinreturnstoitsgroundstatelevel,theabsorbedradiofrequencyenergyisemittedatthesamefrequency
level.
• TheemittedradiofrequencysignalthatgivetheNMRspectrumoftheconcernednucleus.andis
directlyproportionaltothestrengthoftheappliedfield
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NMR SPECTRUM
NMRspectrumis aplotofintensityofNMRsignalsvs magneticfield(frequency)inreference
toTMS
ReasonsfortakingTMS asreferenceStandard
1. Chemicallyinert,magneticallyisotopic,volatile&solubleinmostorganicsolvents.
2. TMSgivesanintensesignal.
3. TMScanbeeasilyremoved.
4. Electronegativity is low.
5. Itdoesn’tmakeanyintermolecularassociationwithsample
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CHEMICAL SHIFT
• Is theresonancefrequencyofanucleusrelativetoastandardinamagneticfield.
• TMSis themostcommonreferencecompoundinNMR,itis setat𝛿 = 0𝑝𝑝𝑚
𝛿=ν𝑠𝑎𝑚𝑝𝑙𝑒 − 𝜈𝑟𝑒𝑓
𝜈𝑟𝑒𝑓
Shieldingofprotons:-Highelectrondensityaroundanucleusshieldsthenucleusfrom
theexternalmagneticfieldandthesignalsareupfieldintheNMRspectrum
Deshieldingofprotons:-Lowerelectrondensityaroundanucleusdeshieldsthenucleus
fromtheexternalmagneticfieldandthesignalsaredownfieldintheNMRspectrum
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FACTORSAFFECTING CHEMICAL SHIFT
• Electronegativegroups/Inductiveeffect
• Magneticanisotropyofπ-systems/diamagneticeffectofpibond
• Hydrogenbonding
• VanderWaal'sdeshielding
• Effectoftemperature&Effectofsolvent
Electronegativegroups:-ElectronegativegroupsattachedtotheC-H systemdecreasethe
electrondensityaroundtheprotons,andthereislessshielding(i.e.deshielding)andchemical
shiftincreases Compound Chemical shift
CH3I 2.16
CH3Br 2.65
CH3Cl 3.10
CH3F 4.26
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MAGNETICANISOTROPY
 Somagneticanisotropymeansthatthereis a"non-uniformmagneticfield".
 Electronsin π systems(e.g.aromatics,alkenes,alkynes,carbonylsetc.)interactwiththeappliedfield
whichinducesamagneticfieldthatcausestheanisotropy.
 Itcausesbothshieldingandde-shieldingofprotons.
Example:-Benzene
Vander Waal'sdeshielding
 Theelectroncloudofabulkiergroupwill tendtorepeltheelectroncloudsurroundingtheproton.
 suchaprotonwill bedeshielded&will resonateatslightlyhighervalueofδ thanexpectedinthe
absenceofthiseffect.
Effectof temperature
Resonancepositionofmostsignalsis littleaffectedbytemperature.
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CHEMICAL SHIFT CHANGEWHENTHESOLVENT CHANGEDFROMCCL4 TOCDCL3 IS0.1
PPM.BUTCHANGETO MOREPOLAR SOLVENTS LIKEMETHANOL,THECHANGEIS 0.3PPM.
Effect of Solvent
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SOLVENTS USEDIN NMR
 CCl4, CS2, CDCl3, C6D6, D2O
 Hydrogen bonding
 Protons thatareinvolvedin hydrogenbondingaretypically changethechemicalshift
values.
 Themorehydrogenbonding,themoreproton is deshieldedandchemicalshift valueis
higher.

PROTON NMR
Themostcommon spectrum forNMRis basedonthehydrogen-1(1H),nucleusorproton.It
cangive
informationaboutthestructureofanymoleculecontaininghydrogenatoms.
Eg,ethanol
3typesof;-CH2,CH3,OH
Interpretationof1HNMR spectra:-
Numberofsignals
Positionofsignals
Relativeintensityofsignals
Splittingofsignals(spinspincoupling)
- Indicateshowmany"differentkinds"ofprotonsarepresent.
- Indicatessomethingabout(chemicalshift) magnetic
(electronic)environmentofprotons
- Proportionaltonumberofprotonspresentsignals
- Indicatesthenumberofnearbynuclei usually protons13

N+1 RULE
• Themultiplicity ofsignalis calculatedbyusingn+1rule.
• This is oneoftheruletopredictthesplittingofprotonsignals.This is consideredbythe
nearbyhydrogennuclei.
Therefore,n=Numberofprotonsinnearbynuclei
ZeroH atomasneighbourn+1=0+1=1(singlet)
OneH atomasneighbourn+1=1+1= 2(doublet)
TwoH atomasneighbourn+1=2+1=3(triplet)
Spin-spincoupling(splitting)
Theinteractionbetweenthespinsofneighbouringnucleiinamoleculemaycausethe
splittingofNMR spectrum.Thesplittingpatternis relatedtothenumberofequivalentH-
atomatthenearbynuclei. Eg.,Ethylacetate
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SPIN-SPIN COUPLING (SPLITTING) CONSTANT
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RULES FOR SPIN-SPIN COUPLING
• Chemicallyequivalentprotonsdonotshowspin-spincoupling &Onlynonequivalent protonscouple.
X
X Hb Hc Hd H
Ha C C C C H
HbcoupleswithHc
Hb&Hadonotcouplebecausetheyareequivalent
Hc &Hd donotcouplebecausetheyareequivalent
H H H H
• Protonsonadjacentcarbonsnormallywill couple.
• Protonsseparatedbyfourormorebondswill notcouple.
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ORIGINS OF SIGNAL
SPLITTING
Theoriginsofsignalsplittingpatternsin
which,eacharrowrepresentsanHbnuclear
spinorientation.
COUPLING CONSTANT (J-Hz)
• Measurementofsplittingeffectis basedon
thedistancebetweenthepeakinagiven
multiplet.
• Usefulin1HNMRofcomplexstructure 17

INSTRUMENTATI
ON
1. Sampletube/sampleholder
2. Permanentmagnet
3. Magnetcoil
4. Sweepgenerator
5. Radiofrequency transmitter
6. Radiofrequency reviever
7. Readoutsystem
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• Sampletube/sampleholder
Itshouldbechemicallyinert, durable&transparenttoNMRradiation.
Generallyabout8.5cmlong&approximately0.3cmindiameterisemployed.
• Sampleprobe
It’sthedevicethatholdsampletubeinposition&is providedwithanairdriventurbine
forrotatingthesampletubealmost100revolutionspermin.
• PermanentMagnet
Itprovidehomogenousmagneticfieldat60-100MHz
• Magneticcoil
Itinducemagneticfieldwhencurrentflowthroughthem
• Sweepgenerator
Toproduceequalamountofmagneticfieldpassthroughthesample
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• Radiofrequencytransmitter
transmitterisfedontoapairofcoilsmountedonrightangles tothepathoffield.
60MHzcapacity isnormallyused.
• Radiofrequencyreceiver
detectradiofrequenciesemittedas nuclei relaxatlowerenergylevel
• Signal detector& recording system
Theelectricalsignalgeneratedis amplifiedbymeans
ofamplifier&thenrecorded.
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APPLICATION
………
• 1HNMRusedforstructuralelucidationoforganicandinorganicsolids
• determinesthephysicalandchemicalpropertiesofatoms
• Applicationinmedicine
• Anatomical imaging
• Measuringphysiological function
• Flowmeasurementandangiography
• Tissueperfusionstudies
• Tumours
• MRI
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• Distinguishinggreymatter&whitematter
• Imagingposteriorfossae, brainstem,spinalcord
• Detectdemyelinatinglesions,tumour
,haemorrhage,infarctions
ABDOMEN
• Metabolicliverdisease
• Focalareasofinflammationinchronicactivehepatisis
KIDNEY
• Distinguishingrenalcorta&medulla
• Toevaluatetransplantedkidney
PELVIS
• Differentiatebetweenbenignprostatichyperplasia&prostaticcarcinoma
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REFERENCES
• Organicspectroscopy,WillaimKemp
• Instrumental methodofchemical analysis
• Instrumental methodofchemical analysis. Wilard
• https://www.slideshare.net/solairajananant/nmr-spectroscopy-13887430
• http://sydney.edu.au/science/chemistry/facilities/nmr/nmr-applications.shtml
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