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Chapter 13- Structure Determination: Nuclear Magnetic Resonance Spectroscopy

AshleyPiekarski,Ph.D.

The Use of NMR Spectroscopy

•  Usedtodeterminerela;veloca;onofatomswithinamolecule

•  Mosthelpfulspectroscopictechniqueinorganicchemistry

•  Mapscarbon-hydrogenframeworkofmolecules

•  Dependsonverystrongmagne;cfields•  hHp://www.youtube.com/watch?v=uNM801B9Y84

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Why do I care?

•  NMRisthemostvaluablespectroscopictechniqueusedforstructuredetermina;on

•  MoreadvancedNMRtechniquesareusedinbiologicalchemistrytostudyproteinstructureandfolding

13.1 Nuclear Magnetic Resonance Spectroscopy

•  1Hor13Cnucleusspinsandtheinternalmagne;cfieldalignsparalleltooragainstanalignedexternalmagne;cfield

magne;cfield

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13.1 Nuclear Magnetic Resonance Spectroscopy

•  Radioenergyofexactlycorrectfrequency(resonance)causesnucleitoflipintoan;-parallelstate

•  Energyneededisrelatedtomolecularenvironment(propor;onaltofieldstrength,B)

13.2 The Nature of NMR Absorptions •  Electronsinbondsshieldnucleifrommagne;cfield•  Differentsignalsappearfornucleiindifferent

environments

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The NMR Measurement

•  Thesampleisdissolvedinasolventthatdoesnothaveasignalitselfandplacedinalongthintube

•  Thetubeisplacedwithinthegapofamagnetandspun•  RadiofrequencyenergyistransmiHedandabsorp;onis

detected•  Speciesthatinterconvertgiveanaveragedsignalthatcanbe

analyzedtofindtherateofconversion•  Canbeusedtomeasureratesandac;va;onenergiesofvery

fastprocesses

NMR Measurement

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13.3 Chemical Shifts

•  Therela;veenergyofresonanceofapar;cularnucleusresul;ngfromitslocalenvironmentiscalledchemicalshi+

•  NMRspectrashowappliedfieldstrengthincreasingfromleatoright

•  LeapartoftheNMRspectrumisdownfield

•  RightpartoftheNMRspectrumisupfield

•  Nucleithatabsorbonupfieldsidearestronglyshielded.•  Chartcalibratedversusareferencepoint,setas0,

tetramethylsilane[TMS]

Measuring Chemical Shift

•  Numericvalueofchemicalshia:differencebetweenstrengthofmagne;cfieldatwhichtheobservednucleusresonatesandfieldstrengthforresonanceofareference•  Difference is very small but can be

accurately measured •  Taken as a ratio to the total field and

multiplied by 106 so the shift is in parts per million (ppm)

•  Absorp;onsnormallyoccurdownfieldofTMS,totheleaonthechart

•  Calibratedonrela;vescaleindelta(δ)scale•  Independent of instrument’s field strength

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13.4 13C NMR Spectroscopy: Signal Averaging and FT-NMR

•  Carbon-13:onlycarbonisotopewithanuclearspin•  Natural abundance 1.1% of C’s in molecules •  Sample is thus very dilute in this isotope

•  Sampleismeasuredusingrepeatedaccumula;onofdataandaveragingofsignals,incorpora;ngpulseandtheopera;onofFouriertransform(FT-NMR)

•  Allsignalsareobtainedsimultaneouslyusingabroadpulseofenergyandresonancerecorded

•  Frequentrepeatedpulsesgivemanysetsofdatathatareaveragedtoeliminatenoise

•  Fourier-transformofaveragedpulseddatagivesspectrum

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Increasing Signal/Noise Ratio

13C Chemical Shifts

WhydoyouthinkC=Oissodownfield?

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•  Spectrumof2-butanoneisillustra;ve-signalforC=Ocarbonsonleaedge

13C spectrum of 2-butanone

13.6 DEPT 13C NMR Spectroscopy

•  Improvedpulsingandcomputa;onalmethodsgiveaddi;onalinforma;on

•  DEPT-NMR(distor'onlessenhancementbypolariza'ontransfer)

•  NormalspectrumshowsallC’sthen:•  Obtain spectrum of all C’s except quaternary

(broad band decoupled) •  Change pulses to obtain separate information for

CH2, CH •  Subtraction reveals each type

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13C DEPT rules

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13.7 Uses of 13C NMR Spectroscopy

•  Providesdetailsofstructure

•  Example:productorienta;oninelimina;onfrom1-chloro-methylcyclohexane

•  Differenceinsymmetryofproductsisdirectlyobservedinthespectrum

•  1-chloro-methylcyclohexanehasfivesp3resonances(δ20-50)andtwosp2resonancesδ100-150

13.8 1H NMR Spectroscopy and Proton Equivalence

•  ProtonNMRismuchmoresensi;vethan13Candtheac;venucleus(1H)isnearly100%ofthenaturalabundance

•  Showshowmanykindsofnonequivalenthydrogensareinacompound

•  Theore;calequivalencecanbepredictedbyseeingifreplacingeachHwith“X”givesthesameordifferentoutcome

•  EquivalentH’shavethesamesignalwhilenonequivalentaredifferent•  There are degrees of nonequivalence

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Nonequivalent H’s

•  ReplacementofeachHwith“X”givesadifferentcons;tu;onalisomer

•  ThentheH’sareincons>tu>onallyheterotopicenvironmentsandwillhavedifferentchemicalshias–theyarenonequivalentunderallcircumstances

Equivalent H’s

•  TwoH’sthatareiniden;calenvironments(homotopic)havethesameNMRsignal

•  TestbyreplacingeachwithX•  if they give the identical result, they are equivalent •  Protons are considered homotopic

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Enantiotopic Distinctions

•  IfH’sareinenvironmentsthataremirrorimagesofeachother,theyareenan>otopic

•  ReplacementofeachHwithXproducesasetofenan;omers

•  TheH’shavethesameNMRsignal(intheabsenceofchiralmaterials)

Diastereotopic Distinctions

•  Inachiralmolecule,pairedhydrogenscanhavedifferentenvironmentsanddifferentshias

•  Replacementofapro-RhydrogenwithXgivesadifferentdiastereomerthanreplacementofthepro-Shydrogen

•  Diastereotopichydrogensaredis;nctchemicallyandspectroscopically

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13.9 Chemical Shifts in 1H NMR Spectroscopy

•  Protonsignalsrangefromδ0toδ10

•  LowerfieldsignalsareH’saHachedtosp2C

•  HigherfieldsignalsareH’saHachedtosp3C

•  Electronega;veatomsaHachedtoadjacentCcausedownfieldshia

13.10 Integration of 1H NMR Absorptions: Proton Counting

•  Therela;veintensityofasignal(integratedarea)ispropor;onaltothenumberofprotonscausingthesignal

•  Thisinforma;onisusedtodeducethestructure•  Forexampleinethanol(CH3CH2OH),thesignalshavetheintegrated

ra;o3:2:1•  Fornarrowpeaks,theheightsarethesameastheareasandcanbe

measuredwitharuler

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13.11 Spin-Spin Splitting in 1H NMR Spectra

•  Peaksareoaensplitintomul;plepeaksduetointerac;onsbetweennonequivalentprotonsonadjacentcarbons,calledspin-spinspliBng

•  ThespliqngisintoonemorepeakthanthenumberofH’sontheadjacentcarbon(“n+1rule”)

•  Therela;veintensi;esareinpropor;onofabinomialdistribu;onandareduetointerac;onsbetweennuclearspinsthatcanhavetwopossiblealignmentswithrespecttothemagne;cfield

•  Thesetofpeaksisamul;plet(2=doublet,3=triplet,4=quartet)

Simple Spin-Spin Splitting

•  AnadjacentCH3groupcanhavefourdifferentspinalignmentsas1:3:3:1

•  Thisgivespeaksinra;ooftheadjacentHsignal

•  AnadjacentCH2givesara;oof1:2:1

•  Thesepara;onofpeaksinamul;pletismeasuredisaconstant,inHz•  J (coupling constant)

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Rules for Spin-Spin Splitting

•  Equivalentprotonsdonotspliteachother•  ThesignalofaprotonwithnequivalentneighboringH’sissplitinton+1peaks

•  Protonsthatarefartherthantwocarbonatomsapartdonotspliteachother

13.12 More Complex Spin-Spin Splitting Patterns

•  Spectracanbemorecomplexduetooverlappingsignals,mul;plenonequivalence

•  Example:trans-cinnamaldehyde

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13.13 Uses of 1H NMR Spectroscopy

•  Thetechniqueisusedtoiden;fylikelyproductsinthelaboratoryquicklyandeasily

•  Example:regiochemistryofhydrobora;on/oxida;onofmethylenecyclohexane