Nmr spectroscopy full details.

1. Nuclear Magnetic
Resonance Spectroscopy

2. 2
1nm 10 102 103 104 105 106 107
(the wave) X-ray UV/VIS Infrared Microwave Radio
Frequency
(the transition) electronic Vibration Rotation Nuclear
(spectrometer) X-ray UV/VIS Infrared/Raman NMR
Fluorescence
NMR Spectroscopy
Where is it?

3. 3
Felix Bloch
1905-1983
Edward M. Purcell
1912-1997
Kurt Wuthrich
1938-
Richard R. Ernst
1933-
CW NMR 40MHz
1960

4. 4
800 MHz

5. 5
The problem the we want to solve by NMR
What we “really”
see
What we want to “see”
NMR

6. Chapter 13 6
Introduction
• NMR is the most powerful tool available for
organic structure determination.
• It is used to study a wide variety of nuclei:
1H
13C
15N
19F
31P
=>

7. 7
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.
=>

8. 8
Nuclear spin
 Nuclear spin is the total nuclear angular momentum quantum number.
This is characterized by a quantum number I, which may be integral,
half-integral or 0.
 Only nuclei with spin number I  0 can absorb/emit electromagnetic
radiation. The magnetic quantum number mI has values of –I, -I+1, …..+I
.
( e.g. for I=3/2, mI=-3/2, -1/2, 1/2, 3/2 )
1. A atom with an even mass no. A and even atomic no. Z  nuclear spin I
is zero
Example: 12C, 16O, 32S  No NMR signal
2. A atom with an even mass no. A and odd atomic no. Z  integer value
I
Example: 2H, 10B, 14N  NMR detectable
3. A atom with odd mass no. and odd or even atomic no. A  I=n/2,
where n is an odd integer
Example: 1H, 13C, 15N, 31P  NMR detectable
 Properties of the Nucleus

9. 9
External Magnetic Field
When placed in an external field, spinning
protons act like bar magnets.
=>

10. 10
Two Energy States
The magnetic fields of
the spinning nuclei
will align either with
the external field, or
against the field.
A photon with the right
amount of energy
can be absorbed
and cause the
spinning proton to
flip. =>

11. 11
Nuclear Zeeman effect
• Zeeman effect: when an atom is placed in an external magnetic field,
the energy levels of the atom are split into several states.
• The energy of a given spin sate (Ei) is directly proportional to the value
magnetic field strength B0
ΔE= E-1/2-E+1/2
m=–1/2
m=+1/2
The Zeeman splitting is proportional to the strength of the magnetic
field

12. Chapter 13 12
E and Magnet Strength
• Energy difference is proportional to the
magnetic field strength.
• E = h =  h B0
2
• Gyromagnetic ratio, , is a constant for
each nucleus (26,753 s-1gauss-1 for H).
• In a 14,092 gauss field, a 60 MHz
photon is required to flip a proton.
• Low energy, radio frequency. =>

13. 13
 The Nucleus in a Magnetic Field
Precession and the Larmor frequency
• The magnetic moment of a spinning nucleus precesses with a
characteristic angular frequency called the Larmor frequency w, which is a
function of r and B0
Larmor frequency or precessional frequency
w=rB0
When precession frequency = radio frequency supplied from outside, NMR
signal is obtained.

14. 14
Number of signals
=>

15. Chapter 13 15
Number of signals
Nonequivalent protons on adjacent carbons.
=>

16. Chapter 13 16
Number of Signals
Equivalent hydrogens have the same
chemical shift.
=>

17. Chapter 13 17
Chemical Shift
(Position of signal)
• It is the difference between the
absorption position of a sample proton
and the absorption position of the
reference compound, TMS.
• Measured in parts per million.
• Called the delta scale.
=>

18. Chapter 13 18
Tetramethylsilane
• TMS is added to the sample.
• Since silicon is less electronegative
than carbon, TMS protons are highly
shielded. Signal defined as zero.
• Organic protons absorb downfield (to
the left) of the TMS signal.
=>
Si
CH3
CH3
CH3
H3C

19. Chapter 13 19
Delta Scale
=>

20. Chapter 13 20
Location of Signals
• More electronegative
atoms deshield more and
give larger shift values.
• Effect decreases with
distance.
• Additional electronegative
atoms cause increase in
chemical shift.
=>

21. Chapter 13 21
Magnetic Shielding
• If all protons absorbed the same
amount of energy in a given magnetic
field, not much information could be
obtained.
• But protons are surrounded by electrons
that shield them from the external field.
• Circulating electrons create an induced
magnetic field that opposes the external
magnetic field. =>

22. Chapter 13 22
Shielded Protons
Magnetic field strength must be increased
for a shielded proton to flip at the same
frequency.
=>

23. Chapter 13 23
Protons in a Molecule
Depending on their chemical
environment, protons in a molecule are
shielded by different amounts.
=>

24. Chapter 13 24
NMR Signals
• The number of signals shows how many
different kinds of protons are present.
• The location of the signals shows how
shielded or deshielded the proton is.
• The intensity of the signal shows the
number of protons of that type.
• Signal splitting shows the number of
protons on adjacent atoms. =>

25. Chapter 13 25
The NMR Spectrometer
=>

26. 26
NMR Solvents
• Solvents for NMR should not contain H-
atom – it will interfere
• Deuterated solvents should be used
• Deuterated methanol (CD3OD),
• Deuterated water (D2O),
• Deuterated chloroform (CDCl3),
• Carbon tetrachloride (CCl4),
• Deuterated acetic acid (CD3COOD)

27. Chapter 13 27
Typical Values
=>

28. Chapter 13 28
Aromatic Protons, 7-8
=>

29. Chapter 13 29
Vinyl Protons, 5-6
=>

30. Chapter 13 30
Acetylenic Protons, 2.5
=>

31. Chapter 13 31
Aldehyde Proton, 9-10
=>
Electronegative
oxygen atom

32. Chapter 13 32
O-H and N-H Signals
• Chemical shift depends on concentration.
• 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.
=>

33. Chapter 13 33
Carboxylic Acid
Proton, 10+
=>

34. Chapter 13 34
Number of Signals
Equivalent hydrogens have the same
chemical shift.
=>

35. Chapter 13 35
Intensity of Signals
• The area under each peak is
proportional to the number of protons.
• Shown by integral trace.
=>

36. Chapter 13 36
How Many Hydrogens?
When the molecular formula is known,
each integral rise can be assigned to a
particular number of hydrogens.
=>

37. 37
Spin-Spin Splitting(Coupling)
• Splitting of a NMR signal for one type of proton
due to presence of another type of proton nearby,
is known as spin-spin coupling.
• 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 upfield when the external
field is opposed.
• All possibilities exist, so signal is split.

38. Chapter 13 38
1,1,2-Tribromoethane
Nonequivalent protons on adjacent carbons.
=>

39. Chapter 13 39
Doublet: 1 Adjacent Proton
=>

40. Chapter 13 40
Triplet: 2 Adjacent Protons
=>

41. 41
The N + 1 Rule
If a signal is split by N equivalent protons,
it is split into N + 1 peaks.
=>

42. 42
Have we understood Spin-spin
coupling?
1. CH3-CH(Cl)(Cl)
1. CH3-CO-CH2-CH3
1. CH3-CO-CH(CH3)2

43. 43
Range of Magnetic
Coupling
• Equivalent protons do not split each other.
• Protons bonded to the same carbon will
split each other only if they are not
equivalent.
• Protons on adjacent carbons normally will
couple.
• Protons separated by four or more bonds
will not couple.

44. 44
Applications of NMR
• Structure elucidation – types of protons,
environment of protons, number of protons,
• Identification of positional isomers
• Quantitative analysis of single component
and multicomponent formulations
• Identification of isolated plant molecules
• In hyphentation with LC as LC-NMR
• Determination of keto-enol tautomerism
• Determination of optical purity

45. 45
Applications of NMR
• Purity and composition of sample
• Impurity profiling
• Raw materials finger printing
• Reaction kinetics monitoring
• Reaction mechanism investigation
• Molecular characterisation
• Formulations investigation

46. 46
Carbon-13 NMR
• 12C has no magnetic spin.
• 13C has a magnetic spin, but is only 1.1%
of the carbon in a sample.
• The gyromagnetic ratio of 13C is one-
fourth of that of 1H.
• Signals are weak, getting lost in noise.
• Hundreds of spectra are taken, averaged.

47. 47
Differences in
13C Technique
• Resonance frequency is ~ one-fourth,
15.1 MHz instead of 60 MHz. WHY?
• Peak areas are not proportional to
number of carbons.
• Carbon atoms with more hydrogens
absorb more strongly.

48. 48
Spin-Spin Splitting
• It is unlikely that a 13C would be
adjacent to another 13C, so splitting by
carbon is negligible.
• 13C will magnetically couple with
attached protons and adjacent protons.
• These complex splitting patterns are
difficult to interpret.

49. Chapter 13 49
Splitting for Ethyl Groups
=>

50. Chapter 13 50
Splitting for
Isopropyl Groups
=>

51. Chapter 13 51
Coupling Constants
• Distance between the peaks of multiplet
• Measured in Hz
• 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.
=>

52. Chapter 13 52
Values for
Coupling Constants
=>

53. Chapter 13 53
Complex Splitting
• Signals may be split by adjacent
protons, different from each other, with
different coupling constants.
• Example: Ha of styrene which is split by
an adjacent H trans to it (J = 17 Hz) and
an adjacent H cis to it (J = 11 Hz).
=>
C C
H
H
H
a
b
c

54. Chapter 13 54
Splitting Tree
C C
H
H
H
a
b
c
=>

55. Chapter 13 55
Spectrum for Styrene
=>

56. Chapter 13 56
Stereochemical
Nonequivalence
• Usually, two protons on the same C are
equivalent and do not split each other.
• If the replacement of each of the protons of
a -CH2 group with an imaginary “Z” gives
stereoisomers, then the protons are non-
equivalent and will split each other.
=>

57. Chapter 13 57
Some Nonequivalent
Protons
C C
H
H
H
a
b
c
OH
H
H
H
a
b
c
d
CH3
H Cl
H H
Cl
a b =>

58. Chapter 13 58
Time Dependence
• Molecules are tumbling relative to the
magnetic field, so NMR is an averaged
spectrum of all the orientations.
• Axial and equatorial protons on
cyclohexane interconvert so rapidly that
they give a single signal.
• Proton transfers for OH and NH may occur
so quickly that the proton is not split by
adjacent protons in the molecule.
=>

59. Chapter 13 59
Hydroxyl
Proton
• Ultrapure samples
of ethanol show
splitting.
• Ethanol with a small
amount of acidic or
basic impurities will
not show splitting.
=>

60. Chapter 13 60
N-H Proton
• Moderate rate of exchange.
• Peak may be broad.
=>

61. Chapter 13 61
Identifying the O-H
or N-H Peak
• Chemical shift will depend on
concentration and solvent.
• To verify that a particular peak is due to
O-H or N-H, shake the sample with D2O
• Deuterium will exchange with the O-H
or N-H protons.
• On a second NMR spectrum the peak
will be absent, or much less intense.
=>

62. Chapter 13 62
Carbon-13
• 12C has no magnetic spin.
• 13C has a magnetic spin, but is only 1% of
the carbon in a sample.
• The gyromagnetic ratio of 13C is one-
fourth of that of 1H.
• Signals are weak, getting lost in noise.
• Hundreds of spectra are taken, averaged.
=>

63. Chapter 13 63
Differences in
13C 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.
=>

64. Chapter 13 64
Spin-Spin Splitting
• It is unlikely that a 13C would be
adjacent to another 13C, so splitting by
carbon is negligible.
• 13C will magnetically couple with
attached protons and adjacent protons.
• These complex splitting patterns are
difficult to interpret.
=>

65. Chapter 13 65
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. =>

66. Chapter 13 66
Hydrogen and Carbon
Chemical Shifts
=>

67. Chapter 13 67
Combined 13C
and 1H Spectra
=>

68. Chapter 13 68
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.
=>

69. Chapter 13 69
Off-Resonance Decoupling
• 13C 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.
=>

70. Chapter 13 70
Interpreting 13C 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. =>

71. Chapter 13 71
Two 13C NMR Spectra
=>

72. Chapter 13 72
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.
=>