1. Nuclear Magnetic Resonance
Spectroscopy

2. 1 Spin Quantum Numbers of Some 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.

3. Nuclear Resonance
absorption of energy by the
spinning nucleus

4. 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

5. The Energy Separation Depends on Bo
- 1/2
= kBo = hn DE
degenerate
at Bo = 0
+ 1/2 Bo
increasing magnetic field strength

6. 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)

7. Resonance Frequencies of Selected Nuclei
Isotope Abundance Bo (Tesla) Frequency(MHz) g(radians/Tesla)
1H 99.98%
13C %

8. 1.3 Classical Instrumentation: The Continuous-Wave NMR
typical before 1965;
field is scanned

9. 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

10. NMR Spectrum of Phenylacetone
EACH DIFFERENT TYPE OF PROTON COMES AT A DIFFERENT
PLACE - YOU CAN TELL HOW MANY DIFFERENT TYPES OF
PROTONS THERE ARE BY INTEGRATION.

11. 1.4 Modern Instrumentation: the Fourier-Transform NMR
FT-NMR
requires a computer

12. 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.

13. 1.5 Proton NMR Spectrum

14. NMR Spectrum of Phenylacetone
EACH DIFFERENT TYPE OF PROTON COMES AT A DIFFERENT
PLACE - YOU CAN TELL HOW MANY DIFFERENT TYPES OF
PROTONS THERE ARE BY INTEGRATION.

15. Some Generalizations NMR solvents contain deuterium
Tetramethylsilane (TMS) is the reference
Spectrum of 1,2,2-trichloropropane
Chemical shift in Hz from TMS vary according to frequency of spectrometer!
Delta values (d) are independent of frequency of spectrometer (ppm)

16. 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

17. 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

18. 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).

19. 1.6 The Chemical Shift

20. Generalizations electrons shield nucleus
electronegativity: withdraws electrons to deshield nucleus
downfield (deshielding) = left side of spectrum
upfield (shielding) = right side of spectrum
delta values increase from right to left!

21. 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.

22. Overview of where protons appear in an NMR spectrum
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

23. NMR Correlation Chart d (ppm) -OH -NH DOWNFIELD UPFIELD DESHIELDED
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

24. Some approximate NMR values (part 1)

25. Some approximate NMR values (part 2)

26. 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.

27. DESHIELDING BY
ELECTRONEGATIVE ELEMENTS

28. 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

29. 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

30. 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.

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

32. fields add together

33. 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

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

35. HYDROGEN BONDING

36. 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.

37. 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.

38. 1.7 Integration

39. 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 protons that
generate the peak.

40. 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

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