1. NUCLEAR MAGNETIC RESONANCE

2. 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

3. 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.

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

5. The Larmor Equation!!! g n = Bo n = ( K) Bo
g yromagnetic ratio g is a constant
which is different for each atomic nucleus (H, C, N, etc)
frequency g 2p
n = Bo
NMR Field
Strength
1H Operating
Frequency
constants
field
strength
1.41 T
60 Mhz
2.35 T
100 MHz
n = ( K) Bo
7.05 T
300 MHz

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

7. 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

8. 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

9. 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

10. 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

11. 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

18. COMPARISON OF
CW AND FT TECHNIQUES

19. 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

20. 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 …..

21. 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.

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

23. 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.

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

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

26. 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

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

28. DIAMAGNETIC ANISOTROPY
SHIELDING BY VALENCE ELECTRONS

29. 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

30. 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.

31. CHEMICAL SHIFT

32. 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

33. 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

34. 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
= d =
= 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).

35. 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).

36. 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.

37. 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

38. 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

39. 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.

40. DESHIELDING BY
ELECTRONEGATIVE ELEMENTS

41. 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

42. 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

43. 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.

44. 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.

45. fields add together

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

47. HYDROGEN BONDING

48. 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.

49. 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.

50. INTEGRATION

51. 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

52. 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.

53. 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

54. 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.