1H NUCLEAR MAGNETIC RESONANCE

CHEMICAL SHIFT AND ITS FACTOR EFFECTS, COUPLING CONSTANT, FIRST ORDER TO NON FIRST ORDER, SPIN SYSTEMS, CHEMICAL EQUIVALENCE AND NON EQUIVALENCE, TIRUMALA SANTHOSHKUMAR S

1. CENTRAL TRIBAL
UNIVERSITY
VIZIANAGARAM
Mr. S. TIRUMALA SANTHOSH KUMAR
ASSISTANT PROFESSOR IN CHEMISTRY
E-mail ID : [email protected]
PAPER-IV
ORGANIC SPECTROSCOPY
ON
1
H-NMR SPECTROSCOPY

2. Introduction of NMR
•Aspectroscopictechniquethatgivesusinformation
aboutthenumberandtypesofatomsinamolecule.
•Nuclearmagneticresonancespectroscopyisapowerful
analyticaltechniqueusedtocharacterizeorganic
moleculesbyidentifyingcarbon-hydrogenframeworks
withinmolecules.
MagneticNuclear Resonance
In the
Nucleus
Involves
Magnets
In the
Nucleus
2

3. Chapter 13 3
•Phenomenon of NMR was first proposed in
1946 by two groups of physicists:
i) Bloch, Hansen and packard-detected a signal from the
protons of water
ii) Purcell, torrey & Pound observed a signal from the proton
in PARAFIN WAX
•The NMR is the most powerful tool available for
organic structure determination.
•It is used to study a wide variety of nuclei:

1
H

13
C

15
N

19
F

31
P
=>

4. •TwocommontypesofNMRspectroscopyareusedto
characterizeorganicstructure:1HNMRisusedto
determinethetypeandnumberofHatomsinamolecule;
13CNMRisusedtodeterminethetypeandnumberofC
atomsinthemolecule.
•ThesourceofenergyinNMRisradiowaveswhichhave
longwavelengths,andthuslowenergyandfrequency.
•Whenlow-energyradiowavesinteractwithamolecule,
theycanchangethenuclearspinsofcertainatomsin
presenceofstrongmagneticfields,including1Hand
13C.
•Alltheatomscontainsnucleiandallnucleicontains
protons(+ve)chargeinwhichsomechargenuclei
posses“Spin”ontheirownaxis.

5. Chapter 13 5
Nuclear Spin
•A nucleus with an odd atomic number or
an odd mass number has a nuclear spin.
•The spinning charged nucleus generates
a magnetic field.
=>

6. •SpinnucleiarethosewhichcontainseitherOddatomic
numberoroddatomicmassnumberorbothe.g.1H,2H,
13C,14N,17O,35CletcareusefulforNMR.
•ThosenucleicontainsEvennumberofatomicandmass
numberarenotusefulforNMRe.g.12C,16Oetc.
•Thenucleipossesspin,theyspinontheirnuclearaxis
leadstogeneratemagneticdipole‘µ’sotheangular
momentumofthisspinningchargeisquantifiedand
describedbyQuantumSpinNumber“I”.1
H
2
H
12
C
13
C
14
N
16
O
31
P
32
S
15
N
19
F
Element
Nuclear spin
quantum
number (I )
Number of
spin states
1/210 0 01/21
23123 1
1/2
21
1/2
2
1/2
2
6

7. •Theindividualprotonshavespinquantumnumber+1/2or-1/2.
•i.e.Hydrogenhavespinquantumnumber(I)=+1/2or-1/2.
•These spin states have equal amount of energy (degenerated)
in the absence of magnetic field.
•Whenachargedparticlesuchasaprotonspinsonitsaxis,it
createsamagneticfield.Thus,thenucleuscanbeconsidered
tobeatinybarmagnet.
+1/2 -1/2
Fig. 1. Two spin states allowed for
proton (H)
7

8. •Butwhenmagneticfieldisapplied,theproton(H)posses
spin&theirownmagneticfieldalignthemselveseitheror
oppositetomagneticfield.
•Fore.g.1Hhas+1/2&-1/2spinstate,theproton(H)
have+1/2spinstatealignthemselveswithfield(Lower
energy)andwith-1/2spinstatealignoppositetofield
(Higherenergy).
8

9. Fig.2. Change in spin state energy separation with
increase by applied magnetic
field, B
0
9

10. •Strongertheappliedmagneticfield,greaterwillbethe
differencebetweenthespinstate,∆E=f(B
0)eq.------1
•AsE
absorbed(∆E)=differenceinenergylevelsoftwo
energystates=hv.
•Eachnucleusdependingonitschargeandmasshasits
ownmagnetogyricratio‘Ý’whichisratioofmagnetic
dipolemomenttoangularmomentum,anditconstantfor
eachnucleus,therefore,
• ∆E=f(ÝB
0)=hv------------2
• ∆E=Ý(h/2πB
0)=hv--------3
• v=(Ý/2π)B
0----------------4
•Here,visfrequencyofelectromagneticradiation.
10

11. •Precessionalfrequency:-Maybedefinedasrevolutions
persecondmadebythemagneticmomentvectorofthe
nucleusaroundtheexternalmagneticfieldBo&
Alternativelyprecessionalfrequencyofspinningbar
magnetmaybedefinedasequaltothefrequencyofEMR
inmegacyclespersecondnecessarytoinduceatransition
fromonespinstate(α-spinstate)toanotherspinstate(β-
spinstate).TheenergyrequiredtobringtransitioninNMR
(∆E=hv)ortoflipprotondependsuponthestrengthof
externalmagneticfield.Thisphenomenoniscalled
flippingofproton
•So,strongerthefieldgreaterwillbethetendencyof
nucleusmagnettoremainlinedupwithitandhigherwill
bethefrequencyofradiationneededtoflipprotonto
higherenergystate. 11

12. •Theproton(Nucleus)+vechargeparticlegeneratesan
magneticfieldonitsaxiswithsamefrequencyasthatof
appliedmagneticfield,iscalledprecessionalfrequency.
•Thefrequencyofprecession(proton)isdirectly
proportionaltothestrengthofappliedmagneticfield.
•Iftheprecessingnucleusisirradiatedwith
electromagneticradiationofthesamefrequencyasthat
frequencyofprecessionnucleus,then
Thetwofrequenciescouple
Energyisabsorbed
Thenuclearspinisflippedfromspinstate+1/2(with
theappliedfield)to-1/2(againsttheappliedfield).
12

13. •The flipping of protons takes place until population
b/w two spin states becomes equal. This state is called
Stationary state
•When the population b/w two spin state become equal, No
peak will be observed, but in normal NMR spectrum the
population b/w these state can not be equal. Because
the nuclei in higher energy state will tend to reach the lower
energy state with lose of energy by two radiation less process.
1. Spin-LaticeRelaxation process
2. Spin –Spin Relaxation process
1.Spin-LaticeRelaxation process(Longirudinalrelaxation):
The higher energy nuclei can undergo energy lose by
transferingobserving energy to some electromagnatic
vector present in surrounding environment. This relaxation
is called ‘ Spin-LaticeRelaxation’

14. Chapter 13 14
Spin-Spin Relaxation process(Transverse relaxation):
This relaxation process involves the transfer of energy
to the neighbouring nuclei provided that the energy is common
To both the nuclei. This mutual exchange of spin energy is
Called ‘Spin-Spin Relaxation process”.

15. Nuclear Magnetic
Resonance
•NMRisphenomenonwhichoccurswhenthosenuclei
whicharealignedwithappliedmagneticfieldareinduce
toabsorbenergyandchangetheirspinstatetoopposite
state,itiscalledasResonance.
•Fig.3.shows
15

16. 16
Fig.4. A nucleus is in resonance when it absorbs RF
radiation and spin flips to higher energy states.

17. To determine the NMR spectrum the following
characteristic must be known.
1.Number of Signals
2.Position of peaks (chemical shift)
3.Intensity of peak
4.Splitting of peaks (spin-spin coupling)
5.Coupling constant
CHARACTERSITC OF
NMR SPECTROSCOPY

18. 1. Number of peaks:
The Number of peaks in the NMR spectrum indicates
the presence of different types of protons which
chemically non equivalent.
Chemically Equivalent Protons:
18
•ChemicallyEquivalentprotons&Magneticequivalent;
•If the compound containing two or more nuclei are
equivalent by symmetry, is chemically equivalent
protons
•Theseequivalentprotonsdonotspliteachother.
•Equivalentprotonshavethesamechemicalshift.

19. 19
Equivalentprotonshavesamechemicalshiftwithout
splittingoccurs
•Compound must be contains Plane of Symmetry
EXAMPLE:

20. Chapter 13 20O
2.09
2.09
ChemNMR H-1 Estimation
Estimation Quality: blue = good, magenta = medium, red = rough
Protocol of the H-1 NMR Prediction:
Node Shift Base + Inc. Comment (ppm rel. to TMS)
CH3 2.09 0.86 methyl
1.23 1 alpha -C(=O)C
CH3 2.09 0.86 methyl
1.23 1 alpha -C(=O)C
012
PPM
EXAMPLE 2: ACETONE

21. Chapter 13 21
•In some molecules chemically equivalent protons
may not be magnetically equivalent.
•Magnetically equivalent protons must follow two rules
Magnetically equivalent nuclei must be isochronous.
that means it must be have identical chemical shift vlaues
Magnetically equivalent nuclei must have identical coupling
constant values to all other nuclei present in the molecule.
Examples:
•This molecule has a plane of symmetry, so the hydrogen on
the opposite side adjacent to groups X & Y are chemically
equivalent and 1HNMR spectrum gives doublets for each
protons.X
Ha'
Hb'
Y
Hb
Ha

22. 22
•Let us lable the chemically equivalent hydrogens
has Ha, Ha’ & Hb, Hb’. We expect Ha & Ha’, Hb & Hb’
has same chemical shift values and same coupling constant.
•but in this nuclei that coupling constants are not equivalent
and hence thus two proton Ha & Ha’, Hb & Hb’ are
magnatically non equivalent. Because Ha with Hb’the couping
constant distance is 3J, but Ha’ with Hb’, the coupling constant
distance is 5J. Thus they cannot be magnaticaly equivalent
even through they are chemically equivalent.X
X X
X X
X X
XX X
Y X Y
Y
X
ZZ
Y
Z
X

23. •Nonequivalent protons are like this;
•IfHaandHbarenotequivalent,thesplittingisobserved.
•Nonequivalentprotonsonadjacentcarbonshavemagnetic
fieldsthatmayalignwithoropposetheexternalfield.
•Thismagneticcouplingcausestheprotontoabsorbslightly
downfieldwhentheexternalfieldisreinforcedandslightly
upfieldwhentheexternalfieldisopposed.
•Allpossibilitiesexist,sosignalissplit.
23

24. CHEMICAL NON EQUIVALENT
24
Nonequivalentprotonshavedifferentchemicalshiftwith
splittingoccurs

25. CHEMICAL NON EQUIVALENT
Splittingisnotgenerallyobservedbetweenprotons
separatedbymorethanthreebonds.
25

26. 2. Position of signels
( chemical shift)
•Positionofsignalsinspectrumhelpustoknownatureof
protonsi.e.aromatic,aliphatic,acetylinic,vinylic,
adjacenttoelectronreleasingorwithdrawinggroup.Thus
theyabsorbatdifferentfieldstrength.
•Thepositionofpeakscanbeobtainedbydeshielding
andshieldingeffect
•Whenmoleculeplacedinmagneticfield,soits
surroundingelectroncirculate&generatessecondary
fieldi.einducedmagneticfieldwhichopposestheapplied
magneticfieldonprotonsothat,fieldfeelsbyprotonis
reducedandthatprotoncalledastheShieldedproton.
•Theprotonwhichareshieldedresonatesatlowerδvalue
andhighfieldi.eupfield.
26

27. CHEMICAL SHIFT
•Rotationofelectrons(π)tonearbynucleigeneratefield
thatcaneitheropposeorstrongthefieldonproton.
•Ifsecondarymagneticfieldisreinforcestoapplied
magneticfieldonprotonandiffieldisstrongtheapplied
fieldthen,protonfeelshighmagneticfieldstrengthand
suchprotoncalledasDeshieldedproton.
•Theprotonwhicharedeshieldedresonatsathighδvalue
andlowerfieldi.eDownfield.
•So,shieldedprotonshiftsabsorptionsignaltorightside
(upfield)anddeshieldedprotonshiftsabsorptionsignalto
leftside(downfield)ofspectrum.
•So,electricenvironmentsurroundingprotontellsus
whereprotonshowsabsorptioninspectrum.
27

28. CHEMICAL SHIFT
•Theseparationb/wtwospectralpeaksassociatedwith
thehydrogennucleipresentindifferentchemical
environment.
•SuchshiftinginpositionofNMRabsorptionsignalswhich
ariseduetotheshieldingordeshieldingofprotonby
surroundingelectronsarecalledasChemicalshift.
28

29. Shielding or Deshielding Protons In
Molecule
•Aswehaveseen,dependingonelectronicenvironment
protonsinmoleculesareshieldedordeshieldedby
differentamounts.
29

30. •So,shieldedprotonshowsabsorptionofsignalstoright
sideanddeshieldedprotonsatleftsideofspectrum.
30

31. Tetra Methyl Silane
•Formeasuringchemicalshiftsofvariousprotonsina
molecule,thesignalofTMSistakenasreference,dueto
lowelectro-negativityofSiatomandshieldingof
equivalentprotonsgives1NMRsignalisgreaterthan
mostoforganiccompounds.
•TheNMRsignalforparticularprotonswillappearat
differentfieldstrengthcomparedtosignalfromTMS.
•Thisdifferenceintheabsorptionpositionofthe
protonwithrespecttoTMSsignaliscalledas
chemicalshift(δ–value).
•Inmajorityoforganiccompound,protonsresonateat
lowerfieldthanprotonsofTMSbcz,itssignalsappears
attheextremerightsideofspectrum,thusassigning
delta(δ)valueforTMSequaltozero(0).
31

32. •Anyprotonorsetofprotonswhichabsorbatlowerfield
thanTMSisgivenapositivevaluefor(δ).
•Thevalueofδforasubstancew.r.ptoTMScanbe
obtainedby
Chemicalshift,ppmδ=ShiftdownfieldfromTMS(inHz)
Spectrometerfrequency(inMHz)
•Thevalueofchemicalshift,δexpressedinppmand
theirvalueisbetween0to15intheδscale.
•Intheτscale,signalforstandardreference,TMStaken
as10ppm.
•Thetwoscalesarerelatedbytheexpression
τ=10-δ
32

33. Chapter 13 33

34. REFERENCE TMS
Its is chemically inert due to its inert nature it will not
form inter molecular association with the sample.
It give signal sharp peak because it has 12 protons with
same chemical environment.
It is volatile and soluble in most of the organic
compound.
It is boiling point is 21
0
C, due to its low boiling point it
can be separated immediately after recording the
spectrum.
If the CH
3protons in TMS are well shielded by their
electrons form external magnetic field.
TMS is insoluble in water and D
2O in this case Na salt of
Tri Methyl Silane propane sulphonid and is used. 34Si
CH3
CH
3
CH
3
H
3C

35. Chapter 13 35
CHEMICAL SHIFT VALUES:

36. Factors affecting chemical
shift
Followingarethefactorswhichinfluencechemical
shift;
•Inductive effect
•Mesomeric effect
•Van der Waal’s deshielding
•Anisotropic effect
•Hydrogen bonding
36

37. INDUCTIVE EFFECT
Chapter 13 37
•Displacement of sigma electrons towards more electonegative
atom is called Inductive effect.
•The shielding or deshielding experienced by nuclei is directly
related to the electron density.
•If the electron density on the carbon decreases, then the proton
present on the carbon experiences deshielding effect.
•If the electron density on the carbon increases, then the proton
experiences shielding effect.
Ex: CH
3-CH
2-F > CH
3-CH
2-Cl
The electronegative atom withdraws the electron density from
The neighbouring proton, that means low induced magnetic field
Is observed. So less field is required to resonated that proton.
There fore δvalue will be increased.

38. Chapter 13 38
INDUCTIVE EFFECT
•F > Cl > Br > I
Greater electro negativity of halogen which is directly related
to the methyl proton bonded to F will resonate at higher δvalue.
Ex: CH
3-F > CH
3-Cl > CH
3-Br > CH
3-I
4.6 3.2 2.8 2.16
•The distance of the proton from the electronegative increases,
deshielding effect decreases.
CH
3-Cl CH
3-CH
2-Cl
3.2 1.3
•If O,N, C are taken, the electronegative order is O > N > C.
•If the No.of electronegative atom on the carbon increases, then
the deshielding effect on the proton increases.
CH
3-Cl < CH
2-Cl
2< CHCl
3

39. MESOMERIC EFFECT
•Mesomeric effect may be an electron
withdrawing or electro releasing effect.
•When –M effect groups are present on
aromatic, then the proton are more deshielded.
•When +M effect groups are present on the
aromatic, then the proton will be shielded.
39N
+
OO
-
NH
2

40. ANISOTROPIC EFFECT
•A Physical property having different values, in
different directions is called Anisotropic effect.
Chapter 13 40

41. •Alkenes are oriented in a such a way that the plane of double
bond is right angle to the applied magnetic field.
•The central pi bonds will experiences to diamagnetic field.
•The alkene protons will experience to paramagnetic field.
Resonates at more δvalue 4-5ppm

42. •The alkynes are oriented in such way that the axis of triple bond
is parllel to the applied magnetic field, so alkyne proton
experiences Diamagnetic shielding. Resonates at lower δvalue
2-3ppm

43. •pi e-Cyllendrically
delocalised arround the
aromatic ring which produce
the ring current.
•The ring current is induced in
such way that the periferall-
-Protons will
Experiances para
Magnetic
Deshielding.

44. Aldehyde Proton, 9-10
=>
In Aldehydic proton is present away from the conical shap of
the πelectron density and hence experiences deshielding
effect. Hence the chemical shift for aldehydic protons apears
b/w 9-10ppm.

45. Chapter 13 45
=>
[18] Annulene,

46. =>
1,4-DecaCyclophane
•In this molecule δ0.3ppm protons are
embedded in circulating πe-cloud,
hence they become shielded.
•δ2.6ppm protons are un-effected by
circulating πe-cloud, hence they so
a chemical shift value equal to that
aliphatic protons of a methyle group
and showing chemical shift value at
δ2.6ppm.

47. CYCLO ALKANES
Chapter 13 47
•The equitorial protons will shows lower δvalue
Than axial proton, due to anisotropic deshielding which
produced by the ciruculating the sigma electronic bonds.

48. HYDROGEN BONDING
O-H and N-H Signals
•Chemical shift depends on concentration.
•When concentration is increased then
deshielding is increased. Due to increase
in intermolecular hydrogen bonding.
•Hydrogen bonding in concentrated
solutions deshield the protons, so signal
is around 3.5 for N-H and 4.5 for O-H.
•Proton exchanges between the molecules
broaden the peak.
=>

49. 49
=>
Van der Waal’s deshielding
Steric Effect
•In trans stilbenthe two protons
are deshieldedby two phenyl
groups.Theco-planarity
of the proton associated with
aryl and alkenewill influences
the deshieldedeffect on proton
•In cisstilbenethe two protons
are shieldedbecause of two
phenyl rings are adjacent to
each other due to stericfactor.

50. Intensity of Signals
•The intensity of the peaks indicates no.of
protons present in a identical group.
•The area under each peak is proportional to
the number of protons.
•Shown by integral trace.
=>

51. Chapter 13 51
How Many Hydrogens?
When the molecular formula is known,
each integral rise can be assigned to a
particular number of hydrogens.
=
>

52. Spin-Spin Splitting
•The interaction of spin of the neighbouring
nuclei in a molecule will split into no.oflines in
the NMR spectrum is called spin-spin splitting.
•Nonequivalent protons on adjacent carbons
have magnetic fields that may align with or
oppose the external field.
•This magnetic coupling causes the proton to
absorb slightly downfield when the external field
is reinforced and slightly upfieldwhen the
external field is opposed.
•All possibilities exist, so signal is split. =>

53. N + 1 Rule
•If a signal is split by Nequivalent protons,
it is split into N+ 1 peaks.
•If a signal is split by no.of N equivalent protons, it
is split into (Na+1)(Nb+1)(Nc+1) lines
•If the no.of splitting lines more than 4, then the
split is called “Multiplet”
•the intensity of
multiplet can be obtain
from coefficient of
binomial expansion:
(X+1)
n =>

54. Chapter 13 54
1,1,2-Tribromoethane
Nonequivalent protons on adjacent carbons.
=>

55. Chapter 13 55
Doublet: 1 Adjacent Proton
=>

56. Chapter 13 56
Triplet: 2 Adjacent Protons
=>

57. Chapter 13 57
Range of Magnetic
Coupling
•Equivalent protons do not split each other.
•Protons bonded to the same carbon will
split each other onlyif they are not
equivalent.
•protons adjecent proton=3, it appears as
quartet with intensity 1:3:3:1 as follows
•(X+1)
3
means 3C1, 3C2, 3C3
=>

58. Chapter 13 58
Splitting for Ethyl Groups
=>

59. Chapter 13 59
Splitting for
Isopropyl Groups
=>

60. Coupling Constants
•Distance between the two splitted peaks.
•The symbol is J, Measured in Hz or Cycle/sec.
•Not dependent on strength of the external field
•Multiplets with the same coupling constants
may come from adjacent groups of protons that
split each other.
•Coupling interactions are two types:
i) The coupling between two nuclei of the same
type is called Homo nuclear coupling.(
1
H-
1
H)

61. Coupling Constant
ii) The coupling between two different nuclei is
called Hetero nuclear coupling (
13
C-
1
H)
•Depending upon the distance between two
nuclei , the coupling interactions are divided
into two types:
i)The distance b/w two interact hydrogen nuclei
is more than 3 bonds,isa Long range
Coupling. Ex: allylic, homoallylic, meta, para.
ii)The distance b/w two interact nuclei is less
than 3 bonds, is a Short range coupling.
ex: Geminal, vicinal, ortho, cis-trans

62. Homo Nuclear Coupling Constant
Geminal coupling
Vicinal coupling
Homoallylic coupling
Allylic coupling
Aromatic coupling
Cis-Trans coupling
Virtual coupling
Meta
Ortho
Para

63. Chapter 13 63
Values for
Coupling Constants
=>

64. Geminal coupling
•The coupling b/w two protons separated by two
bonds or two protons present on same carbon
atom having different chemical environment.
•Strong coupling and high J-value 10-18Hz
Effect of electronegative group:
•When an electronegative group is present on
the carbon having geminal hydrogen, the J
value decreases at 12-9 Hz.
ex: ClC electron density on carbon
•Difficult to determine the geminal peaks, can
over come by using Deuterium process.(D
2O).
H
H

65. Geminalcoupling
J
H,
H= J
H,
DX 6.53
One of the hydrogen is replaced by D2O, then
we can calculate J
H,
Dcoupling constant, by
knowing the J
H,
Dvalue we can also calculate J
H,
H
value by using following formulae.
EFFECT OF RING SIZE:
•In cycloalkanes angular stain increases with decrease in ring size
of the geminal compound, then Jgem value is also decrease, due
to steric hinderence.10-14Hz 8-9Hz 4-6Hz

66. Vincinalcoupling
.
•Vicinal coupling constant range depends on bond angle,
bond length, electronegative group attached to the carbon.
Jvic α1/bond angle α1/bond length α1/electronegative
Effect of dihydral:
Protons attached to the adjacent carbon atoms are called as
vicinal protons and thus are separated by three bonds. Vicinal
coupling constant always has the positive value and depends on
the dihedral angle (angle of rotation, HCCH), the carbon-carbon
bond length, and the effects of electronegative atoms.

67. Vincinalcoupling
Karpusequation
.
The highest coupling constants will occur between protons that
have a dihedral angle of either 0°or 180°,
and the lowest coupling constants will occur at 90°. This is due to
orbital overlap –when the orbitals are
at 90°, there is very little overlap between them, so the hydrogens
cannot affect each other’s spins very much.
θ = 0-90 = 8.5cos2 θ –0.28
θ = 90-180 = 9.5cos2 θ –0.28

68. Chapter 13 68
CIS –TRANS COUPLING
•The coupling constant value of trans hydrogens is more
than cis hydrogen.
•The largest vicinal coupling arises with proton in the
trans co-plannar positions.(θ = 180).
J = 12-18Hz J = 5-10Hz

69. AROMATIC COUPLING
69
In aromatic coupling the ortho protons having more coupling
constant value bcz in this case the protons are separates by 3
bonds. Hence short range and J-value is more.
3J = 7-10Hz
Meta and Para protons coupling is less by increase in number
of protons, then J-value decreases.
2 -3Hz 0-1HzH
H H
H
H
H

70. ALLYLIC COUPLING
•In this case allylic compounds for coupling constant is weaker,
due to 4 bonds separation between two nulei, then J value
decreases with increasing in no. of bonds.
H-C=C-C-H
J=0-2Hz
HOMOALLYLIC COUPLING
This type of coupling when two interacting nuclei separated by
5 bonds, due to the more separation J-value decreases.
H-C=C-C-C-H
J=0-1.6Hz

71. Chapter 13 71
VIRTUAL COUPLING
Coupling interaction b/w two protons is far off, then the
Coupling is called virtual coupling. Due to the confirmation
Flexibility in the molecule the two protons are coupled.
H H

72. Chapter 13 72

73. Chapter 13 73

74. Chapter 13 74

75. Chapter 13 75

76. Chapter 13 76

77. Chapter 13 77

78. Chapter 13 78

79. Chapter 13 79

80. Chapter 13 80

81. INSTUMENTATION METHOD

82. Chapter 13 82

83. Chapter 13 83

84. DOUBLE IRRADIATION
(OR)
SPIN DECOUPLING
•If we take AX type of proton contains two different protons,
due to these protons a pair of doublet is observed, the
reason for the formation of pair of doublets for each is due to
different spin orientation.C C
H
COOH
C
6H
5
H
(X)
(A)
•If we take a proton ‘A’ it experiences two orientations on ‘X’, they
are Parallel ad Anti-parallel orientation of X”

85. DOUBLE IRRADIATION
Chapter 13 85
•If the spin of the proton ‘X’ parallel to proton ‘A’, then less field is
required for resonance and the proton is deshielded.
•If is Anti-parallel to the proton ‘A’ then high field is required for
resonance and proton is said to be shielded, due to two different
orientation absorption takesplace at two different position as a
result doublet will be observed.
•If we make a proton ‘A’ to feel only one orientation of ‘X’, then a
singlet can be observed. This can down by using double
irradiation technique.
•Double irradiation technique means in addition to ordinary
frequency, we supply a second frequency which is
appropriate to proton ‘X’

86. DOUBLE IRRADIATION
•Proton ‘X’ absorbs this radiation and get excited to the higher
energy state and return back to the lower energy state. i.e
induces rapid transitions between the two spin states, therefore
there will be no sufficient time for proton ‘X’ to coupled with
proton ’A’. As a result proton ‘A’ experiences an average time
view of proton ‘X’., hence a singlet can observed for proton ‘A’
instead of doublet. In this way we determine the position of ‘A’.
•Similarly we can determine the position of ‘X’ by double
irradiating the proton ‘A’.
•In this technique the coupling is less, so this called spin
decoupling technique.

87. SPIN SYSTEMS
The analysis of complex NMR patterns is assisted by a general
labellingmethod for spin systems introduced by Pople.
NOMENCLATURE:
A Simple nomenclature to designate a spin system is based on
the relative chemical shifts and the size of the chemical shift
difference with respect to the coupling constant(J).
The symbols used to designate individual nuclei and spin system
are given below:
A group of protons for which chemical shift difference
comparable to the spin spincoupling are denoted by letters
A, B,C, etc..,
Deshieldedproton is present given labeled as ‘A’
Ex: SYSTEM IS A
2B
3CH
3CH
2(CH
2)
4X
BA

88. SPIN SYSTEMS
The group of the protons having resonance position far apart
from A,B,C.., they are denoted by letter X,Y,Z...etc.,
the chemical shift difference between A & X is more
ex: SYSTEM IS A
2X3
Prime symbols are used to designated protons which are
chemically equivalent ,but not magnetically equivalent.
Eg: CH
3COOCH
2CH
3
XA C
C
C
C
H
B
H
C
H
A
H
A'
H
C'
H
B'

89. TYPES OF SPIN SYSTEMS
TWO SPIN SYSTEMS
AX : First order. Significant parameters: JAX. A and X are each
doublets.
AX
X A
AB:J is directly measurable, nA and nB must be calculated.
Intensities are distorted: the doublets are not 1:1; the inner lines
are larger, the outer lines smaller
AB

90. TYPES OF SPIN SYSTEMS
THREE SPIN SYSTEMS
AX
2:First order. Significant parameters: JAX. A is a triplet, X is a
doublet
AMX: First order. Significant parameters: JAM, JAX, JMX. A, M
and X are each doublet of doublets (assuming all three couplings
are large enough to detect). Typical systems are trisubstituted
benzenes, vinyl groups, and monosubstituted furans and
thiophenes.

91. Chapter 13 91
TYPES OF SPIN SYSTEMS
THREE SPIN SYSTEMS
ABX:Second order. This is a very common pattern. JAB is
directly measurable. The parameters JAX, JBX, nA and
nB can be calculated from the line positions of the spectrum
once it has been properly analyzed.
ABC: Second order. This pattern can only be accurately solved
using computer simulation methods. Manual analysis as a
distorted ABX or even AMX pattern will lead to approximate
values of coupling constants, which in severe cases can be
drastically wrong.
ABXABX
ABC

92. 92
TYPES OF SPIN SYSTEMS
FOUR SPIN SYSTEMS
AX3First order. Significant parameters: JAX. Commonly seen
in CH3CHXY group.
AA'XX'Second order. Common pattern. Can be solved by hand,
but there are several ambiguities. For example, one cannot
distinguish JAA' from JXX'. Significant parameters: JAA', JXX',
JAX, JAX'. The AA' and XX' patterns are each centrosymmetric,
and they are identical to each other, hence the inability to
distinguish A from X parameters. A common type is X-CH2-CH2-
Y, which is often just two triplets. Also common are p-
disubstituted benzenes: and dioxolanes:
AX
3

93. Chapter 13 93
Carbon-13
•
12
C has no magnetic spin.
•
13
C has a magnetic spin, but is only 1% of
the carbon in a sample.
•The gyromagnetic ratio of
13
C is one-
fourth of that of
1
H.
•Signals are weak, getting lost in noise.
•Hundreds of spectra are taken, averaged.
=>

94. Chapter 13 94
Fourier Transform NMR
•Nuclei in a magnetic field are given a
radio-frequency pulse close to their
resonance frequency.
•The nuclei absorb energy and precess
(spin) like little tops.
•A complex signal is produced, then
decays as the nuclei lose energy.
•Free induction decay is converted to
spectrum. =>

95. Chapter 13 95
Hydrogen and Carbon
Chemical Shifts
=>

96. Chapter 13 96
Combined
13
C
and
1
H Spectra
=>

97. Chapter 13 97
Differences in
13
C Technique
•Resonance frequency is ~ one-fourth,
15.1 MHz instead of 60 MHz.
•Peak areas are not proportional to
number of carbons.
•Carbon atoms with more hydrogens
absorb more strongly.
=>

98. Chapter 13 98
Spin-Spin Splitting
•It is unlikely that a
13
C would be
adjacent to another
13
C, so splitting by
carbon is negligible.
•
13
C willmagnetically couple with
attached protons and adjacent protons.
•These complex splitting patterns are
difficult to interpret.
=>

99. Chapter 13 99
Proton Spin Decoupling
•To simplify the spectrum, protons are
continuously irradiated with “noise,” so
they are rapidly flipping.
•The carbon nuclei see an average of all
the possible proton spin states.
•Thus, each different kind of carbon
gives a single, unsplit peak.
=>

100. Chapter 13 100
Off-Resonance Decoupling
•
13
C nuclei are split only by the protons
attached directly to them.
•The N+ 1 rule applies: a carbon with N
number of protons gives a signal with
N+ 1 peaks.
=>

101. Chapter 13 101
Interpreting
13
C NMR
•The number of different signals indicates
the number of different kinds of carbon.
•The location (chemical shift) indicates the
type of functional group.
•The peak area indicates the numbers of
carbons (if integrated).
•The splitting pattern of off-resonance
decoupled spectrum indicates the number
of protons attached to the carbon. =>

102. Chapter 13 102
Two
13
C NMR Spectra
=>

103. Chapter 13 103
MRI
•Magnetic resonance imaging, noninvasive
•“Nuclear” is omitted because of public’s
fear that it would be radioactive.
•Only protons in one plane can be in
resonance at one time.
•Computer puts together “slices” to get 3D.
•Tumors readily detected.
=>

104. Chapter 13 104
THANK YOU