1. Nuclear Magnetic Resonance Spectroscopy

2. 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

3. Introduction:
NMR is the most powerful tool available for organic structure determination. Studying a molecules by NMR we obtained following information.
1. The numbers of signal: How many different kinds of protons in molecules
2. Position of signal: Electronic environment of each kind of protons
3. Intensity of signal: Resolution of signal
4. Splitting patterns: Numbers of peaks neighboring protons & its environments
5. Coupling constant: molecular structure and its stereo chemical studies
E.M Purcel and F. Bloch the two physicest shared the noble prize 1952.
It is used to study a wide variety of nuclei: 1H
13C
15N
19F
31P =>
Chapter 13

4. NUCLEAR SPIN The nuclei of some atoms have a property called “SPIN”.
….. we don’t know if they actually do spin!
Each spin-active nucleus has a number of spins defined by
its spin quantum number, I.
NMR Discovered by F. Bloch and E. Purcell

5. THE ENERGY SEPARATION DEPENDS ON Bo
- 1/2
= kBo = hn DE
degenerate
at Bo = 0
+ 1/2 Bo
increasing magnetic field strength

6. g-rays IR X-rays UV Microwave Radio Visible
The entire electromagnetic spectrum is used by chemists:
Frequency, n in Hz
~1019
~1017
~1015
~1013
~1010
~105
Wavelength, l
~.0001 nm
~0.01 nm
10 nm
1000 nm
0.01 cm
100 m
Energy (kcal/mol)
> 300
300-30
300-30
~10-4
~10-6
g-rays
X-rays UV IR
Microwave
Radio
nuclear excitation (PET)
core electron excitation (X-ray cryst.)
electronic excitation (p to p*)
molecular vibration
molecular rotation
Nuclear Magnetic Resonance NMR (MRI)
Visible

7. What is NMR?
NMR – Nuclear Magnetic Resonance is a branch of spectroscopy that deals with the phenomenon found in assemblies of large number of nuclei of atoms that possess both “magnetic moments” and “angular momentum” is subjected to external magnetic field.
Resonance – Implies that we are in tune with a natural frequency of the nuclear magnetic system in the magnetic field.

8. External Magnetic Field
When placed in an external field, spinning protons act like bar magnets.
=>
Chapter 13

9. PRINCIPLE:
When energy in the form of Radiofrequency is applied the nuclei are said to be in resonance, and the energy they emit when flipping from the high to the low energy state can be measured and the obtained absorption of energy occurs and a NMR signal is recorded .
Applied frequency = Precessional frequency
frequency of
the incoming
radiation that
will cause a
transition
√ ὰ Bo
strength of the
magnetic field
The Larmor Equation!!!

10. Reference Std. Tetramethylsilane(TMS)
Chemically inert
Magnetically Isotopic
Sharp single only one peak on NMR spectrum
Molecule must be soluble in most of the organic solvent
Must be relatively volatile
High electronic density of H in TMS. (Almost all the H peaks of organic compounds appear on the left of the TMS peak.)
Since silicon is less electronegative than carbon, TMS protons are highly shielded. Signal defined as zero.

11. √sample– √ref X 106  ppm √o CHEMICAL SHIFT: ()
The separation between the frequencies for the same nucleus in different chemical environments is known as chemical shift. OR
The difference in absorption of a particular proton (nuclei) from the absorption position of reference protons called chemical shift.
√r= Frequency of the reference
√s= Frequency of the sample
√o= The frequency at which the Spectrometer operates
√sample– √ref X 106
 ppm √o

12. NMR Spectrum
A Spectrum of Absorption of Radiation Vs. Applied Magnetic Strength is called as NMR Spectrum.
Number of Signal- shows how many different kinds of protons are present.
Intensity of the Signal- shows the number of protons of each kinds.
Location of the Signals- shows how shielded or deshielded the proton is.
Signal splitting- shows the number of protons on adjacent atoms.

13. Diamagnetic shielding
In a magnetic field, the six  electrons in benzene circulate around the ring creating a ring current.
The magnetic field induced by these moving electrons reinforces the applied magnetic field in the vicinity of the protons.
The protons thus feel a stronger magnetic field and a higher frequency is needed for resonance. Thus they are deshielded and absorb downfield.
Aromatic Protons, 7-8

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

15. FACTORS INFLUENCING CHEMICAL SHIFT
Both 1H and 13C Chemical shifts are related to the following major factors:
Depends on Hydrogen bonding
Depends on adjacent group
Depends on carbon group attached
Depends on hybridization
Depends on anisotropy

16. Delta Scale

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

18. 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 =>
Chapter 13

19. Typical Values

20. Diamagnetic shielding:
In a magnetic field, the six  electrons in benzene circulate around the ring creating a ring current.
The magnetic field induced by these moving electrons reinforces the applied magnetic field in the vicinity of the protons.
The protons thus feel a stronger magnetic field and a higher frequency is needed for resonance. Thus they are deshielded and absorb downfield
Aromatic Protons, 7-8

21. Vinyl Protons, 5-6
1H NMR—Chemical Shift Values ppm
In a magnetic field, the loosely held  electrons of the double bond create a magnetic field that reinforces the applied field in the vicinity of the protons.
The protons now feel a stronger magnetic field, and require a higher frequency for resonance. Thus the protons are deshielded and the absorption is downfield.

22. Acetylenic Protons, 2.5
In a magnetic field, the  electrons of a carbon-carbon triple bond are induced to circulate, but in this case the induced magnetic field opposes the applied magnetic field (B0).
Thus, the proton feels a weaker magnetic field, so a lower frequency is needed for resonance. The nucleus is shielded and the absorption is upfield.

23. POPULATION AND SIGNAL STRENGTH
The strength of the NMR signal depends on the Population Difference of the two spin states
Radiation
induces both
upward and
downward
transitions.
induced
emission
resonance
For a net positive signal
there must be an excess
of spins in the lower state.
excess
population
Saturation = equal populations = no signal

24. A Simplified 60 MHz NMR Spectrometerhn
RF (60 MHz)
Oscillator RF
Detector
absorption
signal
Recorder
Transmitter
Receiver
MAGNET
MAGNET
~ 1.41 Tesla
(+/-) a few ppm N S
Probe

25. Instrumentation: Main features of a basic NMR include:
A radio transmitter coil that produces a short powerful pulse of radio waves
A powerful magnet that produces strong magnetic fields
The sample is placed in a glass tube that spins so the test material is subject to uniform magnetic field.
A radio receiver coil that detects radio frequencies emitted as nuclei relax to a lower energy level
A computer that analyses and record the data

26. The NMR Spectrometer:

27. 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 =>
Chapter 13

28. Carboxylic Acid Proton, 10+

29. Number of Signals
Equivalent hydrogens have the same chemical shift.

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

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

32. 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 =>
Chapter 13

33. 1,1,2-Tribromoethane Nonequivalent protons on adjacent carbons. =>
Chapter 13

34. Doublet: 1 Adjacent Proton

35. Triplet: 2 Adjacent Protons

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

37. 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 =>
Chapter 13

38. Splitting for Ethyl Groups

39. Splitting for Isopropyl Groups

40. 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 =>

41. Values for Coupling Constants

42. 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) =>

43. Splitting Tree
=>

44. Spectrum for Styrene

45. 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 =>

46. Some Nonequivalent Protons
=>

47. Resonance Frequencies of Selected Nuclei
Nuclie
Percentage Abundance
Applied field in Tesla
Precessional frequency in MHz 1H
99.98
1.0
42.6 2H
0.0156
6.5
13C
1.108
10.7
19F
100
40.0

48. 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 =>

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

50. N-H Proton
Moderate rate of exchange.
Peak may be broad.
=>

51. 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 =>

52. MODERN INSTRUMENTATION
PULSED FOURIER TRANSFORM
TECHNOLOGY
FT-NMR
requires a computer

53. FOURIER TRANSFORM n1 + n2 + n3 + ......
A mathematical technique that resolves a complex
FID signal into the individual frequencies that add
together to make it.
( Details not given here. )
converted to
DOMAINS ARE
MATHEMATICAL
TERMS
TIME DOMAIN
FREQUENCY DOMAIN
FID
NMR SPECTRUM
FT-NMR
computer
COMPLEX
SIGNAL
n1 + n2 + n
Fourier
Transform
individual
frequencies
a mixture of frequencies
decaying (with time)
converted to a spectrum

54. Fortunately, different types of protons precess at
different rates in the same magnetic field. N
Bo = 1.41 Tesla
EXAMPLE:
MHz
MHz hn
To cause absorption
of the incoming 60 MHz
the magnetic field strength,
Bo , must be increased to
a different value for each
type of proton.
MHz
60 MHz S
Differences are very small,
in the parts per million range.

55. PULSED EXCITATION (n1 ..... nn) n2 n1 n3 N S BROADBAND RF PULSE
contains a range
of frequencies n3
(n nn) S
All types of hydrogen are excited
simultaneously with the single RF pulse.

56. Spin Quantum Numbers of Some Common Nuclei
Element 1H 2H 12C 13C 14N 16O 17O 19F
Nuclear Spin
Quantum No 1/ / /2 1/2
( I )
No. of Spin
States
Elements with either odd mass or odd atomic number
have the property of nuclear “spin”.

57. n1 n2 n3 FREE INDUCTION DECAY ( relaxation )
n1, n2, n3 have different half lifes

58. COMPOSITE FID
“time domain“ spectrum
n1 + n2 + n
time

59. The Composite FID is Transformed into a classical NMR Spectrum :
“frequency domain” spectrum

60. IN THE CLASSICAL NMR EXPERIMENT THE INSTRUMENT
SCANS FROM “LOW FIELD” TO “HIGH FIELD”
LOW
FIELD
HIGH
FIELD
NMR CHART
increasing Bo
DOWNFIELD
UPFIELD
scan

61. NMR Spectrum of Phenylacetone
NOTICE THAT EACH DIFFERENT TYPE OF PROTON COMES
AT A DIFFERENT PLACE - YOU CAN TELL HOW MANY
DIFFERENT TYPES OF HYDROGEN THERE ARE

62. COMPARISON OF
CW AND FT TECHNIQUES

63. CONTINUOUS WAVE (CW) METHOD
THE OLDER, CLASSICAL METHOD
The magnetic field is “scanned” from a low field
strength to a higher field strength while a constant
beam of radiofrequency (continuous wave) is
supplied at a fixed frequency (say 100 MHz).
Using this method, it requires several minutes to plot
an NMR spectrum.
SLOW, HIGH NOISE LEVEL

64. PULSED FOURIER TRANSFORM (FT) METHOD
FAST
LOW NOISE
THE NEWER COMPUTER-BASED METHOD
Most protons relax (decay) from their excited states
very quickly (within a second).
The excitation pulse, the data collection (FID), and
the computer-driven Fourier Transform (FT) take
only a few seconds.
The pulse and data collection cycles may be repeated
every few seconds.
Many repetitions can be performed in a
very short time, leading to improved signal …..

65. IMPROVED SIGNAL-TO-NOISE RATIO
By adding the signals from many pulses together, the
signal strength may be increased above the noise level.
signal
enhanced
signal
noise
1st pulse
2nd pulse
add many
pulses
noise is random
and cancels out
nth pulse
etc.

66. 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 =>

67. 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 =>

68. Hydrogen and Carbon Chemical Shifts
=>

69. Combined 13C and 1H Spectra

70. 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 =>

71. 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 =>

72. 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 =>

73. 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 =>

74. 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 =>

75. 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. =>

76. Two 13C NMR Spectra

77. 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 =>