NMR LEC spectroscopy and application and use

This PPT decode the nmr spectroscopy and it's applicable use and work machenics

1. UNIT-II
1
HNMRSPECTROSCOPY
Proton NMR Spectroscopy
Dr. NIKITA MISHRA
M.Sc., B.Ed., Ph.D.
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2. ➢Introduction,PrinciplesandInstrumentation.
➢Nuclearspinandmagneticnuclei,nuclearmagneticmoment,
behaviorofabarmagnetinamagneticfield,theNMRtransition
measuringthechemicalshift,shieldinganddeshieldingof
magneticnucleus.
➢Chemicalshiftsinaliphaticandaromaticcompounds.
➢Factorsaffectingchemicalshift.
➢Spin-spinsplitting:effectofspin-spinsplittingonthespectrum,
mechanismofspin-spinsplitting.
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3. !
•NMRisatechniquethatisusefulfor
determiningthestructureofmolecules
basedontheenergyrequiredfornuclear
spin.
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4. Felix Bloch, Stanford University and Edward Purcell, Harvard University
The Nobel Prize in Physics 1952
"for the development of new methods for nuclear magnetic precision measurements
and discoveries”.
•ThefirstNMRspectrawasfirstpublishedintheissueofthePhysicalReviewin
January1946.
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5. ▪Nuclearmagneticresonancespectroscopyisapowerful
analyticaltechniqueusedtocharacterizeorganic/inorganic
molecules.
▪Notonlycanitprovideinformationaboutthestructureof
themolecule,butitcanalsodeterminethecontentand
purityofthesample.
▪MainlytwocommontypesofNMRspectroscopyareused
tocharacterizeorganicmolecules;
1
HNMRand
13
CNMR
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6. ▪
1
HNMRgivestheinformationaboutthetypeandnumber
ofHatomsinamoleculewhereas
13
CNMRisusedto
determinethecarbonframeworkofthemolecule.
▪Theprotonspresentinthemoleculewillbehavedifferently
dependingonthesurroundingchemicalenvironment,
makingitpossibletoelucidatetheirstructure.
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7. Nuclear spin and magnetic nuclei, nuclear magnetic moment, behavior
of a bar magnet in a magnetic field
▪Shieldedproton : whenmoreofelectrondensityis
surroundingit.
▪Deshieldedproton: whenlessofelectrondensityis
surroundingit.
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8. It will be focused on signal splitting
✓Thenumberofsignalsdependsonthenumberofequivalentprotons
(hydrogens).
✓Positionofsignals(chemicalshift)-Thisdealswithwherethepeaksappearon
anNMRspectrum(aretheprotons“shielded”or“deshielded”?)
✓Relativeintensityofsignalswillgivetheratioofequivalentprotontypes-This
isknownasintegrationofhydrogensinmoleculedividedbythesumof
integrationhydrogenperintegrationunit.
✓Signalsplitting(spin-spincoupling)-Thisdealswithhowmanylines/peaksa
particularprotonwillsplit.Thisdependsontheprotonneighboursofthe
protonofinterest.
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9. NMR Spectroscopy Principle
Principle:Manynucleihavespin,andallnucleiareelectricallycharged.Anenergy
transferfromthebaseenergytoahigherenergylevelisachievablewhenanexternal
magneticfieldissupplied.
▪Allnucleiareelectricallycharged,andmanyhavespin.
▪Transferofenergyispossiblefrombaseenergytohigherenergylevelswhenan
externalmagneticfieldisapplied.
▪Thetransferofenergyoccursatawavelengththatcoincideswiththeradio
frequency.
▪Therefore,bymeasuringthesignalwhichmatchesthistransfertheprocessingof
theNMRspectrumfortheconcernednucleusisyield.
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10. NMR Spectroscopy Working
•Place the sample in a magnetic field. Excite the nuclei sample into nuclear magnetic
resonance with the help of radio waves to produce NMR signals.
•These NMR signals are detected with sensitive radio receivers.
•The resonance frequency of an atom in a molecule is changed by the intramolecular
magnetic field surrounding it.
•This gives details of a molecule’s individualfunctional groupsand its electronic
structure.
•Nuclear magnetic resonance spectroscopy is a conclusive method of identifying
monomolecularorganic compounds.
•This method provides details of the reaction state, structure, chemical environment
and dynamics of a molecule.
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11. NMR Spectroscopy Instrumentation
Thisinstrumentconsistsofninemajorparts.Theyarediscussedbelow:
•Sampleholder–Itisaglasstubewhichis8.5cmlongand0.3cmindiameter.
•Magneticcoils–Magneticcoilgeneratesmagneticfieldwhenevercurrentflowsthroughit.
•Permanentmagnet–Ithelpsinprovidingahomogenousmagneticfieldat60–100MHZ
•Sweepgenerator–Modifiesthestrengthofthemagneticfieldwhichisalreadyapplied.
•Radiofrequencytransmitter–Itproducesapowerfulbutshortpulseoftheradiowaves.
•Radiofrequency–Ithelpsindetectingreceiverradiofrequencies.
•RFdetector–Ithelpsindeterminingunabsorbedradiofrequencies.
•Recorder–ItrecordstheNMRsignalswhicharereceivedbytheRFdetector.
•Readoutsystem–Acomputerthatrecordsthedata.
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12. BRIEF DIAGRAM
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13. Position of Signals (Chemical Shift)
•Aspinningchargegeneratesamagneticfieldthatresultsinamagneticmoment
proportionaltothespin.Inthepresenceofanexternalmagneticfield,twospinstates
exist;onespinupandonespindown,whereonealignswiththemagneticfieldand
theotheropposesit.
•Chemicalshiftischaracterizedasthedifferencebetweentheresonantfrequencyof
thespinningprotonsandthesignalofthereferencemolecule.
•Nuclearmagneticresonancechemicalchangeisoneofthemostimportant
propertiesusableformolecularstructuredetermination.
•TherearealsodifferentnucleithatcanbedetectedbyNMRspectroscopy,
1
H
(proton),
13
C(carbon13),
15
N(nitrogen15),
19
F(fluorine19),amongmanymore.
1
H
and
13
Carethemostwidelyused.
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14. ChemicalShift
•Itisameasureofhowfarthesignalproducedfromtheprotonisfromthe
referencecompoundsignal,anditusuallymeasuredusingtheδ(delta)scale.
•TheTMSorreferencecompoundisatthezeropositionontheveryrightof
thespectrum,andasitmovestowardtheleft,theppmvaluesbecomelarger.
•Ppmstandsforpartspermillion,anditistheunitusedtomeasurechemical
shift.Theprotonchemicalshiftsrangefrom0ppmto15ppm.
•Thechemicalshiftisidenticalforaspecificprotonregardlessofthe
spectrometerused.
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15. Tetramethylsilaneisthemostconvenientreferenceandhasthefollowingcharacteristics:
(i)Itismisciblewithalmostallorganicsubstances.
(ii)Itishighlyvolatile(lowboiling)andisreadilyremovedfromthesystem.
(iii)Itdoesnottakepartinintermolecularassociationswiththesample.
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16. Greaterthedeshieldingofprotons,largerwillbethevalueof.
•In the other commonly used unit, a value of 10.00 is assigned to TMS peak. This unit expresses chemical shifts in
tau values as τ
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20. Factors
Affecting
Chemical
Shift
Electronegativity
and Inductive
Effect
Hydrogen
Bonding
Vanderwaal’s
Dishielding
Anisotropic
Effect
(Space
Effect)
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21. Electronegativityeffects
✓Aprotonissaidtobedeshieldedifitisattachedtotheelectronegativeatomorgroup.Greateristhe
electronegativityoftheatom,greateristhedeshieldingcausedtotheproton.Largeristhedeshieldingofaproton,
largeristhechemicalshiftvalue(δ).
✓ThechemicalshiftvaluesofCH
4,CH
3F,CH
3Cl,CH
3BrandCH
3Iprotonsare0.33,4.26,3.05,2.68and2.16
respectively.Since‘F’isthemostelectronegativeatomandhencethechemicalshiftofCH
3Fprotonsisveryhigh,
whereastheIodinebeingleastelectronegativeamongthehalogens,showthelowervalueofchemicalshift.
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22. Asthenumberofelectronegativeatomsorgroupsattachedtoaparticularcarboncontainingthe
protonincreases,thechemicalshiftvaluealsoincreasesbecauseoftheincreasingdeshieldingof
theconcernedproton.ThechemicalshiftvaluesofCH
4,CH
3Cl,CH
2Cl
2,CHCl
3protonsare
0.33,3.05,5.28and7.23respectively.
Theinductiveeffectweakenswithincreasingthedistance.Thechemicalshiftvaluesofmethyl
protonsinCH
3ClandCH
3CH
2Clare3.05and1.48respectively.
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23. Stericeffects
Inovercrowdedmolecules,itispossiblethatsomeprotonsmayoccupystericallyhindered
positionresultinginvanderWaal’srepulsion.Insuchmoleculestheelectroncloudofa
bulkygroup(hinderinggroup)willtendtorepeltheelectroncloudsurroundingtheproton.
Thussuchaprotonwillbedeshieldedandwillresonateatslightlyhighervalueofchemicalshift
thantheexpectedintheabsenceofthiseffect
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24. HydrogenBonding
Ingeneralhydrogenbondedprotonsshowhighervalueofchemicalshiftthanthenon-hydrogen
bondedprotons.
Greateristhedegreeofhydrogenbondingofproton,greaterthedownfieldshiftofaproton.
ThisisduetohydrogenbondingthatthechemicalshiftsofOH,NHandSHprotonsvaryovera
widerange.
Hydrogenbondingdependsonconcentration,temperature,solventandcompoundstructure.
TheOHsignalstendtomovedownfieldathighercompoundconcentrationbecauseofincreased
hydrogenbonding.
Alsoonincreasingtemperature,hydrogenbondingdecreasesandhenceOHsignalsmoves
slightlyupfield.
BothOHandNHsignalsmovedownfieldinH-bondingsolventslikeDMSOoracetone.
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25. Theintermolecularandintramolecularhydrogenbondingcanbedistinguishedbytheuseof
1
H
NMRspectroscopy.
Theintermolecularhydrogenbondingdependsontheconcentrationofthesampleanddecreases
ondilutionwithanon-polarsolventandalsowithincreasingtemperature.Thepureethanolshow
signalsnear5ppmbutondilutionwithnonhydrogen-bondedsolvents(CCl
4,CDCl
3,C
6D
6),the
OHsignalshiftupfieldduetothebreakingofintermolecularhydrogenbonds.
ThephenolicOHpeakat
differentpercentconcentrations
ofphenolinCDCl
3areshownin
thefollowingdiagram(C-H
signalsarenotshown).
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26. Theintramolecularhydrogenbondingdoesnotdependontheconcentrationofthesampleand
henceshownochangeintheirabsorptionpositionondilution.
Thesalicylates,enolsofβ-diketonesandcarboxylicacidsaresomeclassicexamplesof
intramolecularhydrogenbonding.
Thehydroxylprotonofenolformof2,4-pentanedione(4-hydroxypent-3-ene-2-one)showsa
peaksignificantlydownfield(10-16ppm)comparedtotheOHsignalinotherfunctiongroups
duetostrongintramolecularhydrogenbonding.
Similarly,thehydroxylprotonofcarboxylic
aciddisplaysaverydownfieldpeakat
about10-13ppmwhichmaybeattributed
tothefavoredhydrogen-bondeddimeric
association.
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27. SolventEffects
Chloroform(CDCl
3)isthemostcommonsolventforNMRmeasurements.Otherdeuterium
labelledcompounds,suchasdeuteriumoxide(D
2O),benzene-d6(C
6D
6),acetone-d
6
(CD
3COCD
3)andDMSO-d
6(CD
3SOCD
3)arealsousedasNMRsolvents.
Allthesesolventsvaryconsiderablyintheirpolarityandmagneticsusceptibility.
HencetheNMRspectrumrecordedinonesolventmaybeslightlydifferentfromthatrecordedin
othersolvent.
Henceitisimportanttomentionthesolventused.Thechangesinchemicalshiftsduetosolvents
changesareminorbeingontheorderof±0.1ppm.
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28. ShieldinganddeshieldinginAromaticProtons
shieldinginAromaticProtons
Electrondonatingsubstituentscauseshieldingofthearomaticprotonsastheyincrease
electrondensityonthebenzenering.Theorderofshieldingisortho>para>metabecause
theelectrondensityatorthoandparapositionsarethehigherduetothe+Meffect.Onthe
otherhand,theelectronwithdrawingsubstituentscausedeshieldingofallthearomatic
protons.Theorderofdeshieldingisintheorderofortho>para>metabecausetheelectron
withdrawingeffectismaximumatorthoandparapositionsduetothe+Meffect.
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30. Anisotropic effect (space effect)
Insomecasessuchasalkenes,alkynes,carbonylsandaromaticcompoundsthechemicalshiftcannotbeexplained
onlyonthebasisoftheabovefacts.
Anisotropymeansunevenmagneticfieldinspacei.e.differentregionsofspacearecharacterizedbydifferent
magneticfieldstrengths.Theanisotropiceffectdependsupontheorientationofthemoleculewithrespecttothe
appliedfield.
Thecarbonatominacetyleneismoreelectronegative(sp
hybridized)thanthecarboninethylene(sp
2
hybridized)butthe
chemicalshiftvalueforactylenicprotonsareless(more
shielded)thantheethylenicprotons.Suchanomalousbehaviour
canbeexplainedbytheanisotropiceffectproducedby
circulationofπ-electronsundertheinfluenceofapplied
magneticfield.
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31. AlkeneProtons
Analkenemoleculeinanexternalmagneticfieldissoorientedthattheplaneofthedoublebond
isperpendiculartotheappliedmagneticfield.Thusthecirculationofπ-electronsis
perpendiculartothecarbon-hydrogenframeworkofthemolecule.Asaresulttheinduced
magneticfieldcausedbythecirculationofπ-electronsisintheoppositedirectionofapplied
magneticfield(diamagnetic)aroundthecarbonatomsandinthedirectionofappliedmagnetic
field(paramagnetic)intheregionofolefinicprotons.
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32. Alkyneprotons
Analkynemoleculewhenplacedinanexternalmagneticfieldissoorientedthattheplaneofthetriplebondlies
paralleltothedirectionofappliedmagneticfield.Theinducedmagneticfieldcausedbythecirculationofπ-
electronsofthetriplebondisintheoppositedirectionofappliedfield(diamagnetic)intheregionofacetylenic
protons.
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33. Carbonylprotons
Carbonylcompoundwhenplacedinanexternal
magneticfieldissoorientedthattheplaneofthe
carbon-oxygendoublebondisperpendiculartothe
appliedfield.Thecirculationofπ-electrons
generatesaninducedmagneticfieldwhichisinthe
directionofappliedmagneticfieldatthealdehydic
protonsTherefore,aldehydicprotonsexperience
greaterfieldstrength(diamagneticeffect)and
consequentlyresonateatlargervalueofchemical
shift(deshielding).
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34. Aromaticprotons
Inbenzene,theπ-electronsaredelocalizedcylindricallyoverthearomaticring(aboveandbelow
theplaneofthering).Whenamagneticfieldisappliedperpendiculartotheplaneofthe
aromaticring,thecirculationofπ-electronsproducesaringcurrentwhichinducesamagnetic
fieldperpendiculartotheplaneofthering.Theinducedmagneticfieldisintheopposite
direction(diamagneticeffect)astheappliedfieldatthecentreofthering,butoutsidetheringit
isinthesamedirection(paramagneticeffect)oftheappliedfield
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36. Alkanes
•Thesigmabondingelectronsalsoproduceanisotropiceffectsbutitislesspronouncedthan
thatproducedbycirculatingπ-electrons.TheaxisoftheC-Cbondistheaxisofthe
deshielding.ThesinglebondanisotropycanbeobservedoccursinthealkanesoftypeR-
CH
3,R
2CH
2,R
3CHaswellastheequatorialversusaxialprotonsinafixedcyclohexanering
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37. The methyl protons (CH
3) appear near 0.9 ppm, a methylene protons (CH
2) shows signal near 1.2
ppm, and a methine proton (CH) shows near 1.7 ppm. The high frequency position of methine
protons (R
3CH) compared with methylene protons (R
2CH
2) and that of compared with methyl
protons R-CH
3) may be attributed to the anisotropy of the additional C-C bonds. The tertiary
proton falls in the deshielding region of three C-C bonds, secondary protons in the deshielding
region of two C-C bonds and the primary proton falls in the deshielding region of only one C-C
bond.
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39. Summary
Thefrequencyatwhichaparticularprotoncomesintoresonanceis
knownasitschemicalshift.
Thetetramethylsilaneisusedasinternalstandardtorecordan
NMRspectrum.
✓Thiscausesthelesselectrondensityaroundtheprotonsandhencethe
signalproducedbytheseprotonsisshifteddownfield.
✓Thechemicalshiftofaprotonisalsosensitivetodiamagneticeffects
thatresultfromthemotionofnearbyπelectrons.Themagneticfield
causestheπelectronstocirculateandthisflowofelectronscreatesan
induced,localmagneticfield.
✓Somefactorssuchaselectronegativity,inductive,resonance,aromaticityetc.,canchangetheoriginalchemical
shiftvaluesElectronegativeatoms/groupswithdrawelectrondensityfromneighbouringatoms.
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40. Important concepts:
-protons, electrons, and neutrons all have “spin”
+a molecule can have a spin of l= +1/2 or l= -1/2
-we call l= +1/2 parallel to B0 (external magnetic field) (lower energy)
-the difference between spin state energies (l= +1/2, -1/2) is denoted ΔE
-ΔE corresponds to energy required for a spin flip (when a radio wave
photon with energy is added)
+if there is a small magnetic field there will be a small ΔE
+if there is a large magnetic field there will be a large ΔE
-we call l= -1/2 antiparallel to B0 (higher energy)
SIGNAL SPLITTING:
Why are there so many peaks all in one area? This is called signal splitting.
SPIN–SPIN SPLITTING (n +1) RULE
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41. Why does spin splitting happen?
-Each line in a signal ...
(a)has a slightly different chemical shift
(b) represents slightly different spin flip energy
(c) represents nucleus with slightly different magnetic
environment
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42. How does it work?
•The atomic nucleus is a spinning charged particle, and it generates a
magnetic field.
•Without an external applied magnetic field, the nuclear spins are random
and spin in random directions.
•But, when an external magnetic field is present, the nuclei align themselve
either with or against the field of the external magnet.
_-spin state: Protons that align with the external
magnetic field. They are in a lower energy state.
_-spin state: Protons that align against the external
magnetic field. They are in a higher energy state.
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44. Singlet : 0 equivalent hydrogen neighbours=1 peak
Doublet: 1 equivalent hydrogen neighbours=2 peaks
Triplet: 2 equivalent hydrogen neighbours=3 peaks
Quartet: 3 equivalent hydrogen neighbours=4 peaks
Quintet (Pentet): 4 equivalent hydrogen neighbours=5 peaks
Sextet: 5 equivalent hydrogen neighbours=6 peaks
Septet: 6 equivalent hydrogen neighbours=7 peaks
We classify the number of peaks a signal has by certain names:
What does spin-spin coupling mean?
-It involves the different magnetic environments that a specific
proton feels due to the presence of other protons that are (generally)
attached to adjacent carbons
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45. Tounderstanditproperly,consideramoleculeofethylbromide(CH
3CH
2Br).Thespinoftwo
protons(—CH
2—)cancouplewiththeadjacentmethylgroup(—CH
3)inthreedifferentways
relativetotheexternalfield.Thethreedifferentwaysofalignmentare:
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46. Thus,atripletofpeaksresultswiththeintensityratioof1:2:1whichcorrespondstothedistributionratioof
alignment.
Similarly,thespinofthreeprotons(CH3—)cancouplewiththeadjacentmethylenegroup(—CH2—)infour
differentwaysrelativetotheexternalfield.
Thus,quartetofpeaksresultswithanintensityratioof1:3:3:1whichcorrespondstothedistributionratioofallthe
alignments.
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47. Hence,thesplittingofasignalisduetothedifferentenvironmentoftheabsorbingproton
notwithrespecttoelectronsbutwithrespecttothenearbyprotons
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48. Let us consider the case of 1 : 1 : 2 trichloro-ethane
Thus, coupling with proton ‘b’ (one proton) gives 1 : 1 doublet.
Splitting by Proton ‘a’
Thus,asignalfor‘b’protonwillbesplitupintothreepeaks(Triplet)whichareequallyspacedwithpeakintensities1:2:1.Itis
calledoneprotontriplet.Thus,intheabovecompound,weobserve1:1doubletwhichcorrespondsto‘a’protons(—CH2—)
and1:2:1tripletwhichrepresents‘b’proton.(—CHCl2).
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49. Letusconsideranotherexample(Ethylidenedibromide),
Clearly,thesignalforproton‘b’issplitintoa
quartet(groupoffourpeaks)whichareinthe
intensityratioof1:3:3:1.
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50. Let us consider a few examples :
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51. Thepresenceofelectronegativeatomsorgroupsneartheproton,bringtheabsorptiondownfield
andtheeffectdiminisheswiththeincreasingdistanceofthesaidatomorgroupfromthe
concernedproton.
Importantnote.Spin-spincouplingtakesplacebetweennon-equivalentneighboring(onadjacent
carbonatom)protons.Non-equivalentprotonsarethosewhichhavedifferentchemical
shifts.
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53. ApplicationsofNuclearMagneticResonance(NMR)Spectroscopy
Spectroscopyisthestudyoftheinteractionofelectromagneticradiationwithmatter.
NMRspectroscopyistheuseoftheNMRphenomenontostudyphysical,chemicalandbiological
propertiesofmatter.
•Itisananalyticalchemistrytechniqueusedinqualitycontrol.
•Itisusedinresearchfordeterminingthecontentandpurityofasampleaswellasitsmolecular
structure.Forexample,NMRcanquantitativelyanalyzemixturescontainingknowncompounds.
•NMRspectroscopyisroutinelyusedbychemiststostudychemicalstructureusingsimpleone-
dimensionaltechniques.
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54. •Two-dimensionaltechniquesareusedtodeterminethestructureofmorecomplicated
molecules.
•Thesetechniquesarereplacingx-raycrystallographyforthedeterminationofproteinstructure.
•TimedomainNMRspectroscopytechniquesareusedtoprobemoleculardynamicsinsolution.
•SolidstateNMRspectroscopyisusedtodeterminethemolecularstructureofsolids.
•OtherscientistshavedevelopedNMRmethods-ofmeasuringdiffusioncoefficients.
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55. 1.WhatisNMRinorganicchemistry?
Sincethefieldsarespecialorhighlycharacteristicofindividualcompounds,thedefinitivemethod
foridentifyingmonomolecularorganiccompoundsisNMRspectroscopyinmodernorganic
chemistrypractice.Similarly,toclassifyproteinsandothercomplexmolecules,biochemistsuse
NMR.
2.WhatisprotonNMRusedfor?
ProtonnuclearmagneticresonanceistheapplicationinNMRspectroscopyofnuclearmagnetic
resonancetohydrogen-1nucleiinasubstance’smoleculestodeterminethestructureofits
molecules.
Mind Teasers…
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56. 4.HowisNMRusedinmedicine?
Itisusedbychemiststoestablishthemolecularidentityandstructure.MRI,amultidimensional
NMRimagingtechnique,isusedbymedicalpractitionersfordiagnosticpurposes.
5.HowisNMRusedinMRI?
Nuclearmagneticresonanceimaging(NMR)ismedicaltechnology.InotherNMRtechniques
suchasNMRspectroscopy,NMRcanalsobeusedforimaging.
3.WhatdoesresonancemeaninNMR?
Thoughhydrogennucleiarealwaysprecessing,nuclearmagneticresonance(NMR)isnot
continuouslyundergoing.Magneticresonanceoccurswhenexternalenergyisappliedabovethe
Larmor(resonance)frequencyintoanuclearspindevice.
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57. 6.WhyradiofrequencyisusedinNMR?
Likeallspectroscopies,NMRusesanelectromagneticradiationcomponent(radio
frequencywaves)tofacilitatetransitionsbetweenratesofnuclearenergy
(resonance).SomechemistsuseNMRtodeterminesmallmoleculesbystructure.
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