chapter 13- structure determination: nuclear magnetic ... · 7/27/16 1 chapter 13- structure...
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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