1. NUCLEAR MAGNETIC RESONANCE

2. WHAT IS SPECTROSCOPY ?
The study of molecular structure and dynamics through the absorption, emission, and scattering of light.

3. THE ELECTROMAGNETIC SPECTRUM
high
Frequency (n)
low
high
Energy
low
MICRO-
WAVE
X-RAY
ULTRAVIOLET
INFRARED
RADIO
FREQUENCY
Nuclear
magnetic
resonance
Vibrational
infrared
Ultraviolet
Visible
2.5 mm
15 mm
1 m
5 m
200 nm
400 nm
800 nm
BLUE
RED
short
Wavelength (l)
long

4. SPECTROSCOPIC TECHNIQUES AND THEIR USES IN ORGANIC CHEMISTRY
Radiation
Absorbed
Effect on the Molecule
Information Deduced
Ultraviolet-visible
, nm and
Changes in electronic energy levels within the molecule
Extent of -electron systems. Presence of conjugated unsaturation, and conjugation with nonbonding electrons
Infra-red (mid infra-red)
, mm , cm-1
Changes in the vibrational and rotational movements of the molecule
Detection of functional groups, which have specific vibration frequenciesfor example, C=O, NH2, OH, etc.
Microwave
Electron spin resonance or electron paramagnetic resonance; induces changes in the magnetic properties of unpaired electrons
Detection of free radicals and the interaction of the electron with, for example, nearby protons

5. SPECTROSCOPIC TECHNIQUES IN ORGANIC CHEMISTRY AND THEIR USES
Radiation
Absorbed
Effect on the Molecule
Information Deduced
NMR Radio frequency
, 1-5m
Nuclei placed under the static magnetic field change their spins after absorption of radiofrequency radiations
The electronic environment of nuclei, their numbers and number of neighboring atoms.

6. Radio waves Longest wavelength EM waves Uses: TV broadcasting
AM and FM broadcast radio
Heart rate monitors
Cell phone communication
MRI (MAGNETIC RESONACE IMAGING)
Uses Short wave radio waves with a magnet to create an image
Click the little rainbow box in the top right corner to return to the overview EM spectrum diagram

7. ALL SPECTROMETERS HAVE SOME COMMON ESSENTIAL FEATURES
A source of electromagnetic radiations of the appropriate frequency range.
A sample holder to permit efficient irradiation of the sample.
A frequency analyzer which separates out all of the individual frequencies generated by the source.
A detector for measuring the intensity of radiations at each frequency, allowing the measurement of how much energy has been absorbed at each of these frequencies by the sample; and
A recorder – either a pen recorder or computerized data station, with a VDU for initial viewing of the spectrum, with the possibility of manipulation.

8. NMR NUCLEAR- MAGNETIC- RESONANCE- Study of nuclear spins
Under the influence of applied
magnetic field
Record the resulting resonance in
nuclear spin through the absorption
of RF

9. COMPARISON BETWEEN THE PRINCIPAL SPECTROSCOPIC METHODS: GOOD FEATURES
13C-NMR
1H-NMR IR MS
UV/VIS
Identification of functional groups **
*** *
Measurement of molecular complexity
Sensitivity
(sample size needed)
Quantitative information
Interpretability of the data
Theory needed to interpret spectra
Ease of instrument operation
Instrument cost, running cost
Method
Feature
*** = Good ** = Average * = Poor

10. CHARACTERISTICS OF PRINCIPAL SPECTROMETRIC METHODS
1H-NMR
13C-NMR MS IR
Scale
0-15 ppm
1-220 ppm
amu
cm-1
Sample
1 mg
5-10 mg
< 1 mg
Molecular formula
Partial
Yes No
Functional group
~ yes
Limited
Substructure
yes
Very limited
C-Connection
Stereochem. & regiostereo-chemistry

11. NMR NUCLEAR- MAGNETIC- RESONANCE- Study of nuclear spins
Under the influence of applied
magnetic field
Record the resulting resonance in
nuclear spin through the absorption
of RF

12. NUCLEAR SPIN The nuclei of some atoms have a property called “SPIN”.
These nuclei behave as if
they were spinning.
….. we don’t know if they actually do spin!
This is like the spin property
of an electron, which can have
two spins: +1/2 and -1/2 .
Each spin-active nucleus has a number of spins defined by
its spin quantum number, I.
The spin quantum numbers of some common nuclei follow …..

13. Spin Quantum Numbers of Some Common Nuclei
The most abundant isotopes of C and O do not have spin.
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”.
The number of spin states is 2I + 1,
where I is the spin quantum number.

14. THE PROTON Although interest is increasing in other nuclei,
particulary C-13, the hydrogen nucleus (proton)
is studied most frequently, and we will devote
our attention to it first.

15. NUCLEAR SPIN STATES - HYDROGEN NUCLEUS
The spin of the positively
charged nucleus generates
a magnetic moment vector, m. m + +
The two states
are equivalent
in energy in the
absence of a
magnetic or an
electric field. m
+ 1/2
- 1/2
TWO SPIN STATES

17. Precession of Spinning Top
A rapidly spinning top will precess in a direction determined by the torque exerted by its weight. The precession angular velocity is inversely proportional to the spin angular velocity, so that the precession is faster and more pronounced as the top slows down.
The direction of the precession torque can be visualized with the help of the right-hand rule.
Spin a top on a flat surface, and you will see it's top end slowly revolve about the vertical direction, a process called precession. As the spin of the top slows, you will see this precession get faster and faster. It then begins to bob up and down as it precesses, and finally falls over. Showing that the precession speed gets faster as the spin speed gets slower is a classic problem in mechanics.

19. Gyromagnetic ratio
In physics, the gyromagnetic ratio or  magnetogyric ratio of a particle or system is the ratio of its magnetic dipole moment to its angular momentum, and it is often denoted by the symbol γ, gamma. Its SI units are radian per second per tesla (rad s−1·T -1) or, equivalently, coulomb per kilogram (C·kg−1).

20. Gyromagnetic ratio for a classical rotating body
Consider a charged body rotating about an axis of symmetry. According to the laws of classical physics, it has both a magnetic dipole moment and an angular momentum due to its rotation. It can be shown that as long as its charge and mass are distributed identically (e.g., both distributed uniformly), its gyromagnetic ratio is
γ q  2m
\where q is its charge and m is its mass.

21. Gyromagnetic ratio and Larmor precession
Any free system with a constant gyromagnetic ratio, such as a rigid system of charges, a nucleus, or an electron, when placed in an external magnetic field B (measured in teslas) that is not aligned with its magnetic moment, will precess at a frequency  (measured in hertz), that is proportional to the external field:
 =γ

22. THE “RESONANCE” PHENOMENON
absorption of energy by the
spinning nucleus

23. Nuclear Spin Energy Levels
-1/2
unaligned
In a strong magnetic
field (Bo) the two
spin states differ in
energy.
+1/2
aligned Bo S

24. Absorption of Energy Bo quantized -1/2 -1/2 DE DE = hn +1/2 +1/2
Opposed
-1/2
-1/2 DE
DE = hn
Radiofrequency
+1/2
+1/2
Applied
Field Bo
Aligned

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

26. The Larmor Equation!!! n = n = Bo 2p
DE = kBo = hn can be transformed into
gyromagnetic
ratio g g
n = 2p
gB0
n = 2p
frequency of
the incoming
radiation that
will cause a
transition Bo
strength of the
magnetic field
g is a constant which is different for
each atomic nucleus (H, C, N, etc)

27. A SECOND EFFECT OF A STRONG MAGNETIC FIELD
WHEN A SPIN-ACTIVE HYDROGEN ATOM IS
PLACED IN A STRONG MAGNETIC FIELD
….. IT BEGINS TO PRECESS
OPERATION OF AN NMR SPECTROMETER DEPENDS
ON THIS RESULT

28. w N Nuclei precess at frequency w when placed in a strong
magnetic field.
RADIOFREQUENCY
MHz hn
NUCLEAR
MAGNETIC
RESONANCE
If n = w then
energy will be
absorbed and
the spin will
invert.
NMR S

29. Effect of Static Magnetic Bo Field on Magnetic Moment

30. Resonance Frequencies of Selected Nuclei
Isotope Abundance Bo (Tesla) Frequency g10-6(radians/Tesla/sec)
(MHz)
1H 99.98%
2H %
13C %
19F 100.0%
4:1

31. 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

32. 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.

33. 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

34. DIAMAGNETIC ANISOTROPY
SHIELDING BY VALENCE ELECTRONS

35. Diamagnetic Anisotropy
The applied field
induces circulation
of the valence
electrons - this
generates a
magnetic field
that opposes the
applied field.
valence electrons
shield the nucleus
from the full effect
of the applied field
magnetic field
lines
B induced
(opposes Bo)
Bo applied
fields subtract at nucleus

36. PROTONS DIFFER IN THEIR SHIELDING
All different types of protons in a molecule
have a different amounts of shielding.
They all respond differently to the applied magnetic
field and appear at different places in the spectrum.
This is why an NMR spectrum contains useful information
(different types of protons appear in predictable places).
UPFIELD
DOWNFIELD
Highly shielded
protons appear here.
Less shielded protons
appear here.
SPECTRUM
It takes a higher field
to cause resonance.

37. CHEMICAL SHIFT

38. PEAKS ARE MEASURED RELATIVE TO TMS
Rather than measure the exact resonance position of a
peak, we measure how far downfield it is shifted from TMS.
reference compound
tetramethylsilane
“TMS”
Highly shielded
protons appear
way upfield.
TMS
Chemists originally
thought no other
compound would
come at a higher
field than TMS.
shift in Hz
downfield n

39. REMEMBER FROM OUR EARLIER DISCUSSION
Stronger magnetic fields (Bo) cause
the instrument to operate at higher
frequencies (ν).
field
strength
frequency γ 2π
hν = Bo
NMR Field
Strength
1H Operating
Frequency
constants
1.41 T
60 Mhz
2.35 T
100 MHz
ν = ( K) Bo
7.05 T
300 MHz

40. HIGHER FREQUENCIES GIVE LARGER SHIFTS
The shift observed for a given proton
in Hz also depends on the frequency
of the instrument used.
Higher frequencies
= larger shifts in Hz.
TMS
shift in Hz
downfield n

41. THE CHEMICAL SHIFT
The shifts from TMS in Hz are bigger in higher field
instruments (300 MHz, 500 MHz) than they are in the
lower field instruments (100 MHz, 60 MHz).
We can adjust the shift to a field-independent value,
the “chemical shift” in the following way:
parts per
million
shift in Hz
chemical
shift
= δ =
= ppm
spectrometer frequency in MHz
This division gives a number independent
of the instrument used.
A particular proton in a given molecule will always come
at the same chemical shift (constant value).

42. HERZ EQUIVALENCE OF 1 PPM
What does a ppm represent?
1 part per million
of n MHz is n Hz
1H Operating
Frequency
Hz Equivalent
of 1 ppm 1
n MHz = n Hz ( )
60 Mhz Hz
106
100 MHz Hz
300 MHz Hz 7 6 5 4 3 2 1
ppm
Each ppm unit represents either a 1 ppm change in
Bo (magnetic field strength, Tesla) or a 1 ppm change
in the precessional frequency (MHz).

43. NMR Correlation Chart d (ppm)
-OH
-NH
DOWNFIELD
UPFIELD
DESHIELDED
SHIELDED
CHCl3 ,
TMS
d (ppm) 12 11 10 9 8 7 6 5 4 3 2 1 H
CH2F
CH2Cl
CH2Br
CH2I
CH2O
CH2NO2
CH2Ar
CH2NR2
CH2S
C C-H
C=C-CH2
CH2-C-
C-CH-C
RCOOH
RCHO
C=C C
C-CH2-C
C-CH3 O
Ranges can be defined for different general types of protons.
This chart is general, the next slide is more definite.

44. R-CH3 0.7 - 1.3 R-N-C-H 2.2 - 2.9 R-C=C-H R-CH2-R 1.2 - 1.4 4.5 - 6.5
APPROXIMATE CHEMICAL SHIFT RANGES (ppm) FOR SELECTED TYPES OF PROTONS
R-CH
R-N-C-H
R-C=C-H
R-CH2-R
R-S-C-H
R3CH
I-C-H H
R-C=C-C-H
Br-C-H O
Cl-C-H
R-C-C-H O O
RO-C-H
R-C-N-H
RO-C-C-H
HO-C-H O O O
R-C-H
HO-C-C-H
R-C-O-C-H O
N C-C-H
O2N-C-H
R-C-O-H
R-C C-C-H
F-C-H
C-H
R-N-H Ar-N-H R-S-H
R-O-H Ar-O-H
R-C C-H

45. YOU DO NOT NEED TO MEMORIZE THE PREVIOUS CHART
IT IS USUALLY SUFFICIENT TO KNOW WHAT TYPES
OF HYDROGENS COME IN SELECTED AREAS OF
THE NMR CHART
C-H where C is attached to an
electronega-tive atom
CH on C
next to
pi bonds
acid
COOH
aldehyde
CHO
benzene CH
alkene
=C-H
aliphatic
C-H
X=C-C-H
X-C-H 12 10 9 7 6 4 3 2
MOST SPECTRA CAN BE INTERPRETED WITH
A KNOWLEDGE OF WHAT IS SHOWN HERE

46. DESHIELDING AND ANISOTROPY
Three major factors account for the resonance
positions (on the ppm scale) of most protons.
1. Deshielding by electronegative elements.
2. Anisotropic fields usually due to pi-bonded
electrons in the molecule.
3. Deshielding due to hydrogen bonding.
We will discuss these factors in the sections that
follow.

47. DESHIELDING BY
ELECTRONEGATIVE ELEMENTS

48. Cl C H d- d+ DESHIELDING BY AN ELECTRONEGATIVE ELEMENT d- d+
Chlorine “deshields” the proton,
that is, it takes valence electron
density away from carbon, which
in turn takes more density from
hydrogen deshielding the proton. Cl C H d- d+
electronegative
element
NMR CHART
“deshielded“
protons appear
at low field
highly shielded
protons appear
at high field
deshielding moves proton
resonance to lower field

49. Electronegativity Dependence of Chemical Shift
Dependence of the Chemical Shift of CH3X on the Element X
Compound CH3X
CH3F CH3OH CH3Cl CH3Br CH3I CH4 (CH3)4Si
Element X
F O Cl Br I H Si
Electronegativity of X
Chemical shift d
most
deshielded
TMS
deshielding increases with the
electronegativity of atom X

50. Substitution Effects on Chemical Shift
most
deshielded
The effect
increases with
greater numbers
of electronegative
atoms.
CHCl3 CH2Cl2 CH3Cl
ppm
most
deshielded
-CH2-Br CH2-CH2Br -CH2-CH2CH2Br
ppm
The effect decreases
with incresing distance.

51. ANISOTROPIC FIELDS DUE TO THE PRESENCE OF PI BONDS
The presence of a nearby pi bond or pi system
greatly affects the chemical shift.
Benzene rings have the greatest effect.

52. fields add together

53. H H C=C H H ANISOTROPIC FIELD IN AN ALKENE Bo protons are deshielded
shifted
downfield
fields add
C=C H H
secondary
magnetic
(anisotropic)
field lines Bo

54. H C C H ANISOTROPIC FIELD FOR AN ALKYNE Bo secondary magnetic
Shielded Bo
hydrogens
are shielded
fields subtract

55. HYDROGEN BONDING

56. HYDROGEN BONDING DESHIELDS PROTONS
The chemical shift depends
on how much hydrogen bonding
is taking place.
Alcohols vary in chemical shift
from 0.5 ppm (free OH) to about
5.0 ppm (lots of H bonding).
Hydrogen bonding lengthens the
O-H bond and reduces the valence
electron density around the proton
- it is deshielded and shifted
downfield in the NMR spectrum.

57. SOME MORE EXTREME EXAMPLES
Carboxylic acids have strong
hydrogen bonding - they
form dimers.
With carboxylic acids the O-H
absorptions are found between
10 and 12 ppm very far downfield.
In methyl salicylate, which has strong
internal hydrogen bonding, the NMR
absortion for O-H is at about 14 ppm,
way, way downfield.
Notice that a 6-membered ring is formed.

58. INTEGRATION

59. NMR Spectrum of Phenylacetone
RECALL
Each different type of proton comes at a different place .
You can tell how many different types of hydrogen
there are in the molecule.
from last
time

60. INTEGRATION OF A PEAK Integration = determination of the area
Not only does each different type of hydrogen give a
distinct peak in the NMR spectrum, but we can also tell
the relative numbers of each type of hydrogen by a
process called integration.
Integration = determination of the area
under a peak
The area under a peak is proportional
to the number of hydrogens that
generate the peak.

61. Benzyl Acetate 55 : 22 : 33 = 5 : 2 : 3 METHOD 1 integral line
The integral line rises an amount proportional to the number of H in each peak
METHOD 1
integral line
integral
line
55 : 22 : = : 2 : 3
simplest ratio
of the heights

62. Benzyl Acetate (FT-NMR)
Actually :
/ 11.3
= 5.14
/ 11.3
= 1.90
/ 11.3
= 3.00
METHOD 2
assume CH3
/ 3 = 11.3
digital
integration
Integrals are
good to about
10% accuracy.
Modern instruments report the integral as a number.

63. CLASSICAL INSTRUMENTATION
typical before 1960
field is scanned

64. 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

65. 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

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

68. 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.

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

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

71. 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

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

73. COMPARISON OF
CW AND FT TECHNIQUES

74. 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

75. 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 …..

76. 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.