NMR Spectroscopy (Nuclear Magnetic Resonance)

Nuclear Magnetic Resonance

1. NMR Spectroscopy

2. NMRSPECTROSCOPY
•Nuclearmagneticresonancespectroscopyhas
becomeaverypowerfultoolforstructure
elucidationtoanorganicchemist.
•Thistechniqueisbasedontransitionsbetween
nuclearspinstatesbyabsorptionofelectromagnetic
radiationsintheradiofrequencyregionofroughly4
to900MHzbycertainorganicmoleculeswhen they
areplacedinastrongmagneticfield.
•E.BlochandF.Purcell(1952)wereawardednobel
prizefordemonstratingtheNMReffectin1946.

3. In this diagram, frequency is specified in units of wavenumbers, defined as 1/λ,
whichis the number of waves per centimeter.
Wavenumbersareusedtospecifyenergyininfraredspectroscopy.

6. NuclearMagnetic
Resonance
•Resonance:InNMRspectroscopy,resonanceisthe absorption
ofenergybyaprecessingnucleusandthe resulting“flip”ofits
nuclearspinfromalowerenergy statetoahigher
energystate.
•Theprecessingspinsinduceanoscillatingmagnetic
fieldthatisrecordedasasignalbytheinstrument.
—Signal:ArecordinginanNMRspectrumofa
nuclear magneticresonance.

7. 5
NMRSchematic

8. Manynucleiundergomagneticresonance.
Ingeneral,nucleicomposed ofanodd numberof protons(
1
Handitsisotopes,
14
N
19
F, and
31
P)or anodd numberof neutrons(
13
C)show magnetic behavior.
Ifboth theproton andneutron countsareeven(
12
Cor
16
O) thenuclei arenon-‐magnetic.

9. •An NMR spectrum is a plot of the intensity of a peak against its
chemicalshift,measuredin partsper million(ppm).
1HNMRSpectra
9

10. 1
0
8
•Positionofsignals:indicateswhattypesofhydrogenthe
moleculecontains.
•Number of signals: indicates the number of different
typesofhydrogen inamolecule.
•Intensity of signals: indicates the relative amounts (how
many)ofeachkindof hydrogeninthemolecule.
•Spin-spin splitting of signals: gives further information of
theneighboringenvironmentforthevarioushydrogens
inthe molecule.
StructuralInformationfromFeaturesofa
1HNMRSpectrum

11. •Protons in a given environment absorb in a predictable
regionin an NMRspectrum.
•Two important factors: electronegativity and magnetic
anisotropy
1
1
ShieldingandChemicalShift

12. 10
ShieldingandChemicalShift
Thedegreeofshieldingof anucleusdependsupon itssurroundingelectrondensity.
Addingelectronsincreasesshielding.
Removingelectronscausesdeshielding.

13. CharacteristicChemicalShiftsRelativetoTMS(0ppm)
13

14. •The chemical shift of a C−H bond increases with increasing
alkylsubstitutionorelectronegativeatomsattached.
ElectronegativityandChemicalShift
RCH
2-‐H
14
RCH
2-‐OH

15. The absorptions ofalkanehydrogensoccur at relativelyhighfield.
Hydrogens close to an electron withdrawing group (halogen or oxygen) are shifted to
relatively lower field(deshielding).
The more electronegative the atom, the more the deshielded methyl hydrogens are
relative to methane.

16. Multiple substituentsexert a cumulativeeffect.

17. Thedeshieldinginfluenceofelectronwithdrawinggroupsdiminishesrapidlywithdistance.

18. •In a magnetic field, the six electrons in benzene circulate
aroundthe ringcreating aringcurrent.
•The magnetic field induced by these moving electrons reinforces
theappliedmagnetic fieldinthe vicinityofthe protons.
•The protons thus feel a stronger magnetic field and a higher
frequency is needed for resonance meaning they are deshielded
andabsorb downfield.
AromaticDeshieldingandAnisotropy
18

19. •In a magnetic field, the electrons of a carbon-carbon triple
bond are induced to circulate, but in this case the induced
magneticfieldopposestheappliedmagneticfield(B
0).
•Theprotonthusfeelsaweakermagneticfield,soalower
frequencyis neededfor resonance.
•Thenucleusisshieldedandtheabsorptionisupfield.
AlkyneChemicalShifts
19

20. SummaryofElectronandChemicalShift
20

21. Regionsinthe
1HNMRSpectrum
21

22. 22
20
•Positionofsignals:indicateswhattypesofhydrogenthe
moleculecontains.
•Number of signals: indicates the number of different
typesofhydrogen inamolecule.
•Intensity of signals: indicates the relative amounts (how
many)ofeachkindof hydrogeninthemolecule.
•Spin-spin splitting of signals: gives further information of
theneighboringenvironmentforthevarioushydrogens
inthe molecule.
StructuralInformationfromFeaturesofa
1HNMRSpectrum

23. Chemically equivalent hydrogens in a molecule all have identical electronic
environmentsandthereforeshow NMRpeaks atthesame position.
In the NMRspectrumof2,2-‐dimethyl-‐a-‐propanol,there arethree differentpeaks
dueto absorptions by:
Nine equivalent methyl hydrogens on the butyl group (most shielded);
Onehydrogen on the OH;
Twoequivalentmethylenehydrogens.

24. 10-5TestsforChemicalEquivalence
Molecularsymmetryhelpsestablishchemicalequivalence.
Rotational symmetry
results in equivalent
protonswhenthegroup
of protons is rapidly
rotating, as in a methyl
group.

25. Conformational interconversion may result in equivalence on the NMR
time scale.
Inthecaseoftherapidrotationofthemethylgroupinchloroethane,ortherapid
conformationflipincyclohexane,theobservedchemicalshiftsaretheaveragesof
thevaluesthatwouldbeobservedwithouttherapidrotationorflip.
For cyclohexane, the single line in the NMR spectra atδ = 1.36 ppm at room
temperature becomes two lines at a temperature of -‐90
o
C, one at δ = 1.12 ppm
forthesixaxialhydrogensandoneatδ=1.60 forthesixequatorialhydrogens.

26. 26
24
•Positionofsignals:indicateswhattypesofhydrogenthe
moleculecontains.
•Number of signals: indicates the number of different
typesofhydrogen inamolecule.
•Intensity of signals: indicates the relative amounts (how
many)ofeachkindof hydrogeninthemolecule.
•Spin-spin splitting of signals: gives further information of
theneighboringenvironmentforthevarioushydrogens
inthe molecule.
StructuralInformationfromFeaturesofa
1HNMRSpectrum

27. 1HNMRIntegration
•Modern NMR spectrometers automatically calculate and plot
thevalue ofeach integralin arbitraryunits.
•The ratio of integrals to one another gives the ratio of
absorbingprotons inaspectrum.
•Note that this gives a ratio, and not the absolute number, of
absorbingprotons.
27

28. 2.35
28
1.0 1.43

29. 29
27
•Positionofsignals:indicateswhattypesofhydrogenthe
moleculecontains.
•Number of signals: indicates the number of different
typesofhydrogen inamolecule.
•Intensity of signals: indicates the relative amounts (how
many)ofeachkindof hydrogeninthemolecule.
•Spin-spin splitting of signals: gives further information of
theneighboringenvironmentforthevarioushydrogens
inthe molecule.
StructuralInformationfromFeaturesofa
1HNMRSpectrum

30. 1HNMR—Spin-SpinSplitting
•Thespectrauptothispointhavebeenlimitedtosingle
absorptionscalled singlets.
•Often signals for different protons are split into more
thanone peak.
30

31. 10-7Spin-‐SpinCoupling:TheEffectofNon-‐Equivalent
NeighboringHydrogens
Whennon-‐equivalenthydrogenatomsarenotseparatedbyatleastonecarbon
oroxygenatom,anadditionalphenomenoncalled“spin-‐spinsplitting”or“spin-‐
spincoupling”occurs.
Insteadofsinglepeaks(singlets),morecomplexpatternsoccurcalledmultiplets
(doublets,tripletsorquartets).
The number and kind of hydrogen atoms directly adjacent to the absorbing nuclei
canbe deduced from the multiplicityof the peak.

32. Spin-‐spinsplittingisusuallyobservedonlybetweenhydrogenatomsboundto the
samecarbon (geminalcoupling)or to adjacentcarbons (vicinalcoupling).
Hydrogen nuclei separatedby more thantwo carbon atoms(1,3coupling)is
usuallynegligible.
Finally, equivalent nuclei do not exhibit
mutual spin-‐spinsplitting.Ethaneexhibits
onlyasingleline atδ=0.85ppm.
Splitting is observed only between nuclei with
differentchemical shifts.

34. Local-‐fieldcontributionsfrom more than onehydrogen areadditive.
Consider the triplet above. It corresponds to the methyl protons being split by
themethylene protons.
The methylene proton spins will statisticallyorient in the external magneticfield
as,,and.Eachmethylprotonwillseeanincreasedfield25%of
the time (), no change 50% of the time (and ), and a decreased field
25%of the time().

35. The integrated intensity of the triplet will be 6 since there are a total of 6
equivalentmethyl protons.

36. In the case of the methylene protons, the methyl proton spins will statistically
distributeas,,,,,,,and.
Thiswillresultina1:3:3:1quartetofpeaks.
The integrated intensity of the quartet will be 4, corresponding to the 4
equivalentmethylene protons.

37. In many cases,spin-‐spinsplittingisgiven bythe N+1rule.
Asimplesetofrules:
Equivalent nuclei located adjacent to one neighboring hydrogen resonate as a
doublet.
Equivalent nuclei located adjacent to two hydrogens of a second set of
equivalentnuclei resonate asatriplet.
Equivalent nuclei located adjacent to a set of three equivalent hydrogens
resonateasaquartet.

38. This table illustrates the N+1 rule: Nuclei having N adjacent equivalent neighbors
splitinto N+1 peaks. The heights of the N+1peaks follow Pascal’s triangle.
In many cases,spin-‐spinsplittingisgiven bythe N+1rule.

39. Itis important tonote that non-‐equivalentnucleisplit each other.
A split in one requires a split in the other. In addition, the coupling constants will
bethe same for each typeof nuclei.
Twoadditionalexamples:

41. 10-8Spin-‐SpinSplitting: Some Complications
The N+1 rule may not apply in a direct way if several neighboring hydrogens
havingfairlydifferentcouplingconstantsarecoupledtotheresonating nucleus.
Inthiscase,theruleisrevisedto(n1+1)*(n2+1).

42. 40
1HNMRofVinylAcetate

43. Inthe caseof 1-‐bromopropane,thehydrogens on C2arealsocoupledto two non-‐
equivalentsetsofneighbors.Atheoreticalanalysisofthisresonancewould
predictasmany as12lines (a quartet of triplets).
Because the coupling constants are very similar, however, many of the lines
overlap,thus simplifying the pattern.

44. 10-9Carbon-‐13NuclearMagneticResonance
The NMR spectroscopy of
13
Cis very useful for structural determination.
1
H-‐Decoupling

45. •
13
C Spectra are easier to analyze than
1
H spectra because the
signalsplitting can beavoided.
•Each type of carbon atom appears as a single peak with 1H-
decoupling.
13CNMRSpectrumExample
43

46. The numbers of non-‐equivalent carbons in the isomers of C
7H
14are clearly
demonstratedby thenumbers of
13
Cpeaks intheir NMRspectra.

47. ChemicalShiftsin
13CNMR
•In contrast to the small range of chemical shifts in
1
H NMR (1-10
ppm usually),
13
C NMR absorptions occur over a much broader
range(0-220 ppm).
•The chemical shifts of carbon atoms in
13
C NMR depend on the
sameeffectsasthechemical shiftsofprotonsin
1
HNMR.
Common
13C-‐NMRchemical shiftvalues
47

48. 13CNMRof1-Propanol
48

49. 13CNMRofMethylAcetate
49

50. MagneticResonanceImaging(MRI)
50

51. Howtodeterminestructuresofcompoundsfromtheirspectra?
1.Determinethemolecularformulaanddegreeofunsaturation
2.From
13
C-‐NMR,determine#ofsignalsandchemical shifts
3.From
1
H-‐NMR,determine#ofsignals,#ofprotonsineach signal,
peaksplittingpatternsandchemical shifts

52. Molecularformula:C4H7O2Cl
O
O
Cl

53. Molecularformula:C5H10O
O

54. Molecularformula:C7H7Br
Br

55. Formula
C5H10O2