Nuclear Magnetic Resonance Spectroscopy

1
Nuclear Magnetic Resonance Spectroscopy
(NMR)
2
INTRODUCTION
We have seen that IR spectroscopy is a vital tool
for the identification of the functional groups
present in organic molecules. NMR provides an
image of the molecules hydrocarbon skeleton.
NMR spectroscopy dates from the 1950 - 1960s
and in the space of some 40 years has
revolutionized organic chemistry as tool for
structural elucidation, kinetic studies, and
quality control. NMR spectra result from the
absorption of radio frequency radiation when
certain atomic nuclei are placed in magnetic
fields. Why?
3
NUCLEAR SPIN
The nuclei of some atoms have a property called
SPIN.
These nuclei behave as if they were spinning.
.. we dont 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 ..
4
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 0 1/2 1 0 5/2
1/2 ( I ) No. of Spin 2 3 0 2
3 0 6 2 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.
5

• The nuclei of the elements fall into two
  categories those which have a spin and those
  which do not.
• The 1H, 13C, 19F nuclei and many others posses a
  spin.
• As they are positively charged, the spinning
  nucleus generates a tiny magnetic moment - it
  behaves like a small bar magnet.

6
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.
7
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
8
The effect of the magnetic field
In the absence of an applied magnetic field,
these little magnets are randomly
oriented. When placed between the poles of a
strong magnet, they are either aligned with
(parallel) or against (antiparallel) the field.
The parallel spin state is slightly more stable
than the antiparallel state. For every 1,000,000
protons, some 500,005 - 500,010 have the parallel
state! Not a large difference but it is the
basis of NMR!
9
When a molecule is placed in a magnetic field and
is subjected to electromagnetic radiation (radio
frequency), the nuclear spin can flip from the
parallel to the antiparallel state - the nuclei
are said to be in resonance.
h?
spin aligned with the applied field
antiparallel spin
magnetic field
10
THE RESONANCE PHENOMENON
absorption of energy by the spinning
nucleus
11
Nuclear Spin Energy Levels
N
-1/2
unaligned
In a strong magnetic field (Bo) the two spin
states differ in energy.
1/2
aligned
Bo
S
12
Absorption of Energy
quantized
Opposed
-1/2
-1/2
DE
DE hn
Radiofrequency
1/2
1/2
Applied Field
Bo
Aligned
13
THE ENERGY SEPARATION DEPENDS ON Bo
- 1/2
kBo hn
DE
degenerate at Bo 0
1/2
Bo
increasing magnetic field strength
14
The Larmor Equation!!!
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)
15
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
16
N
w
Nuclei precess at frequency w when placed in a
strong magnetic field.
RADIOFREQUENCY 40 - 600 MHz
hn
NUCLEAR MAGNETIC RESONANCE
If n w then energy will be absorbed and the
spin will invert.
NMR
S
17
Resonance Frequencies of Selected Nuclei
Isotope Abundance Bo (Tesla)
Frequency(MHz) g(radians/Tesla)
1H 99.98 1.00 42.6 267.53 1.41
60.0 2.35 100.0 7.05 300.0 2H 0.0156 1
.00 6.5 41.1 7.05 45.8 13C
1.108 1.00 10.7 67.28 2.35
25.0 7.05 75.0 19F 100.0 1.00 40.0
251.7
41
18
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
19
Schematic representation of the NMR spectrometer
sample
N
S
radio frequency detector
radio wave generator
20
1H NMR spectrometers

• The sample is placed in a glass tube which is
  then placed between the two poles of a very
  strong magnet.

• It is then irradiated using radiation of a
  constant low frequency.

• The magnetic field is varied. When it reaches
  the correct strength, the nuclei absorb energy
  and resonance occurs.

• The absorption causes a tiny electrical current
  to flow in a receiver coil surrounding the sample
  which is displayed as a peak.

21
CLASSICAL INSTRUMENTATION
typical before 1960
field is scanned
22
A Simplified 60 MHzNMR Spectrometer
hn
RF (60 MHz) Oscillator
RF Detector
absorption signal
Recorder
Transmitter
Receiver
MAGNET
MAGNET
1.41 Tesla (/-) a few ppm
S
N
Probe
23
Fortunately, different types of protons precess
at different rates in the same magnetic field.
Bo 1.41 Tesla
N
59.999995 MHz
EXAMPLE
59.999700 MHz
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.
59.999820 MHz
60 MHz
S
Differences are very small, in the parts per
million range.
24
IN THE CLASSICAL NMR EXPERIMENT THE INSTRUMENT
SCANS FROM LOW FIELD TO HIGH FIELD
HIGH FIELD
LOW FIELD
NMR CHART
increasing Bo
UPFIELD
DOWNFIELD
scan
25
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
26
MODERN INSTRUMENTATION
PULSED FOURIER TRANSFORM TECHNOLOGY
FT-NMR
requires a computer
27
PULSED EXCITATION
N
n2
n1
BROADBAND RF PULSE
contains a range of frequencies
n3
(n1 ..... nn)
S
All types of hydrogen are excited simultaneously
with the single RF pulse.
28
FREE INDUCTION DECAY
( relaxation )
n1
n2
n3
n1, n2, n3 have different half lifes
29
COMPOSITE FID
time domain spectrum
n1 n2 n3 ......
time
30
FOURIER TRANSFORM
A mathematical technique that resolves a
complex FID signal into the individual
frequencies that add together to make it.
( Details not given here. )
DOMAINS ARE MATHEMATICAL TERMS
converted to
TIME DOMAIN
FREQUENCY DOMAIN
FID
NMR SPECTRUM
FT-NMR
computer
n1 n2 n3 ......
COMPLEX SIGNAL
Fourier Transform
individual frequencies
a mixture of frequencies decaying (with time)
converted to a spectrum
31
The Composite FID is Transformed into a classical
NMR Spectrum
frequency domain spectrum
32
COMPARISON OF CW AND FT TECHNIQUES
33
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
34
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 ..
35
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
etc.
nth pulse
36
INTEGRATION
37
INTEGRATION OF A PEAK
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.
38
Benzyl Acetate
The integral line rises an amount proportional to
the number of H in each peak
METHOD 1
integral line
integral line
simplest ratio of the heights
55 22 33 5 2 3
39
Benzyl Acetate (FT-NMR)
Actually 5
2 3
33.929 / 11.3 3.00
21.215 / 11.3 1.90
58.117 / 11.3 5.14
METHOD 2
assume CH3 33.929 / 3 11.3
digital integration
Integrals are good to about 10 accuracy.
Modern instruments report the integral as a
number.
40
DIAMAGNETIC ANISOTROPY
SHIELDING BY VALENCE ELECTRONS
41
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)
fields subtract at nucleus
42
Shielding Effects
When a nucleus is placed in a magnetic field, the
electrons surrounding it are in motion about the
nucleus and create a small, localized magnetic
field which opposes the applied field.
Thus the electrons generate a small magnetic
field that shields the proton from the external
field.
43
The result is that one observes a decrease in the
intensity of the total field near to the nucleus.
The nucleus is said to be shielded. Thus the
position of an NMR absorption depends on the
electron density about the hydrogen.
44
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).
45
CHEMICAL SHIFT
46
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
0
n
47
REMEMBER
Stronger magnetic fields (Bo) cause the
instrument to operate at higher frequencies (n).
field strength
frequency
hn Bo
NMR Field Strength
1H Operating Frequency
constants
60 Mhz
1.41 T
100 MHz
2.35 T
n ( K) Bo
300 MHz
7.05 T
48
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
0
n
49
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).
50
HERZ EQUIVALENCE OF 1 PPM
What does a ppm represent?
1 part per million of n MHz is n Hz
Hz Equivalent of 1 ppm
1H Operating Frequency
1
(
)
n MHz n Hz
60 Mhz 60 Hz
106
100 MHz 100 Hz
300 MHz 300 Hz
ppm
0
1
2
3
4
5
6
7
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).
51
Chemical shifts
The absorptions are measured in relation to their
distance from the TMS peak.
These distances vary according to the strength of
the applied magnetic field. Peaks separated by
54Hz at 60MHz are separated by 72Hz at 80MHz,
270Hz at 300MHz and by 540Hz at 600MHz.
52
Proton Chemical Shift Ranges
Low FieldRegion
High FieldRegion
  For samples in CDCl3 solution. The d scale is
relative to TMS at d 0.
53
NMR Correlation Chart
-OH
-NH
DOWNFIELD
UPFIELD
DESHIELDED
SHIELDED
CHCl3 ,
TMS
d (ppm)
12
11
10
9
8
7
6
5
4
3
2
1
0
H
CH2Ar CH2NR2 CH2S C C-H CC-CH2 CH2-C-
CH2F CH2Cl CH2Br CH2I CH2O CH2NO2
C-CH-C
RCOOH
RCHO
CC
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.
54
APPROXIMATE CHEMICAL SHIFT RANGES (ppm) FOR
SELECTED TYPES OF PROTONS
R-CH3 0.7 - 1.3
R-N-C-H 2.2 - 2.9
R-CC-H
R-CH2-R 1.2 - 1.4
4.5 - 6.5
R-S-C-H 2.0 - 3.0
R3CH 1.4 - 1.7
I-C-H 2.0 - 4.0
H
R-CC-C-H 1.6 - 2.6
Br-C-H 2.7 - 4.1
6.5 - 8.0
Cl-C-H 3.1 - 4.1
R-C-C-H 2.1 - 2.4
R-C-N-H
RO-C-H 3.2 - 3.8
5.0 - 9.0
RO-C-C-H 2.1 - 2.5
HO-C-H 3.2 - 3.8
R-C-H
HO-C-C-H 2.1 - 2.5
9.0 - 10.0
R-C-O-C-H 3.5 - 4.8
N C-C-H 2.1 - 3.0
O2N-C-H 4.1 - 4.3
R-C-O-H
R-C C-C-H 2.1 - 3.0
11.0 - 12.0
F-C-H 4.2 - 4.8
C-H 2.3 - 2.7
R-N-H 0.5 - 4.0 Ar-N-H 3.0 - 5.0 R-S-H
R-O-H 0.5 - 5.0 Ar-O-H 4.0 - 7.0
1.0 - 4.0
R-C C-H 1.7 - 2.7
55
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
aliphatic C-H
alkene C-H
benzene CH
aldehyde CHO
acid COOH
XC-C-H
X-C-H
2
3
4
6
7
9
10
12
0
MOST SPECTRA CAN BE INTERPRETED WITH A KNOWLEDGE
OF WHAT IS SHOWN HERE
56
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.
57
DESHIELDING BY ELECTRONEGATIVE
ELEMENTS
58
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.
C
H
Cl
d-
d
electronegative element
NMR CHART
highly shielded protons appear at high field
deshielded protons appear at low field
deshielding moves proton resonance to lower field
59
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
4.0 3.5 3.1 2.8
2.5 2.1 1.8
Chemical shift d
4.26 3.40 3.05 2.68
2.16 0.23 0
most deshielded
TMS
deshielding increases with the electronegativity
of atom X
60
Substitution Effects on Chemical Shift
most deshielded
The effect increases with greater numbers of
electronegative atoms.
CHCl3 CH2Cl2 CH3Cl
7.27 5.30 3.05 ppm
most deshielded
-CH2-Br -CH2-CH2Br -CH2-CH2CH2Br
3.30 1.69 1.25
ppm
The effect decreases with incresing distance.
61
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.
62
fields add together
63
ANISOTROPIC FIELD IN AN ALKENE
protons are deshielded
H
H
Deshielded
shifted downfield
fields add
CC
H
H
secondary magnetic (anisotropic) field lines
Bo
64
ANISOTROPIC FIELD FOR AN ALKYNE
H
C
C
H
secondary magnetic (anisotropic) field
Shielded
hydrogens are shielded
Bo
fields subtract
65
HYDROGEN BONDING
66
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.
67
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.
68
SPIN-SPIN SPLITTING
69
SPIN-SPIN SPLITTING
Often a group of hydrogens will appear as a
multiplet rather than as a single peak.
Multiplets are named as follows
Singlet Quintet Doublet Septet Triplet Octet Quart
et Nonet
This happens because of interaction with
neighboring hydrogens and is called
SPIN-SPIN SPLITTING.
70
1,1,2-Trichloroethane
The two kinds of hydrogens do not appear as
single peaks, rather there is a triplet and a
doublet.
integral 2
integral 1
The subpeaks are due to spin-spin splitting and
are predicted by the n1 rule.
triplet
doublet
71
n 1 RULE
72
1,1,2-Trichloroethane
integral 2
integral 1
Where do these multiplets come from ?
.. interaction
with neighbors
73
MULTIPLETS
this hydrogens peak is split by its two neighbors
these hydrogens are split by their single neighbor
singlet doublet triplet quartet quintet sextet sep
tet
two neighbors n1 3 triplet
one neighbor n1 2 doublet
74
EXCEPTIONS TO THE N1 RULE
IMPORTANT !
Protons that are equivalent by symmetry usually
do not split one another
1)
no splitting if xy
no splitting if xy
Protons in the same group usually do not split
one another
2)
more detail later
or
75
EXCEPTIONS TO THE N1 RULE
The n1 rule applies principally to protons in
aliphatic (saturated) chains or on saturated
rings.
3)
or
YES
YES
but does not apply (in the simple way shown here)
to protons on double bonds or on benzene rings.
NO
NO
76
SOME COMMON PATTERNS
77
SOME COMMON SPLITTING PATTERNS
( x y )
( x y )
78
SOME EXAMPLE SPECTRA WITH SPLITTING
79
NMR Spectrum of Bromoethane
80
NMR Spectrum of 2-Nitropropane
81
NMR Spectrum of Acetaldehyde
offset 2.0 ppm
82
INTENSITIES OF MULTIPLET PEAKS
PASCALS TRIANGLE
83
PASCALS TRIANGLE
Intensities of multiplet peaks
1
singlet
1 1
doublet
1 2 1
triplet
1 3 3 1
quartet
1 4 6 4 1
quintet
1 5 10 10 5 1
sextet
1 6 15 20 15 6 1
septet
1 7 21 35 35 21 7 1
octet
84
THE ORIGIN OF SPIN-SPIN SPLITTING
HOW IT HAPPENS
85
THE CHEMICAL SHIFT OF PROTON HA IS AFFECTED
BY THE SPIN OF ITS NEIGHBORS
aligned with Bo
opposed to Bo
1/2
-1/2
50 of molecules
50 of molecules
H
H
H
H
A
A
C
C
C
C
Bo
upfield
downfield
neighbor aligned
neighbor opposed
At any given time about half of the molecules in
solution will have spin 1/2 and the other half
will have spin -1/2.
86
SPIN ARRANGEMENTS
one neighbour n1 2 doublet
one neighbour n1 2 doublet
H
H
H
H
C
C
C
C
yellow spins
blue spins
The resonance positions (splitting) of a given
hydrogen is affected by the possible spins of
its neighbour.
87
SPIN ARRANGEMENTS
two neighbours n1 3 triplet
one neighbour n1 2 doublet
methine spins
methylene spins
88
SPIN ARRANGEMENTS
three neighbours n1 4 quartet
two neighbours n1 3 triplet
methylene spins
methyl spins
89
Spin - spin coupling
Now lets look at the absorption due to the
-CHBr2 group. It is affected by the spin of the
neighbouring protons. There are 4 possible spin
combinations which are equally possible
1 2 1 a triplet
90
THE COUPLING CONSTANT
91
THE COUPLING CONSTANT
J
J
J
J
J
J
The coupling constant is the distance J (measured
in Hz) between the peaks in a multiplet.
J is a measure of the amount of interaction
between the two sets of hydrogens creating the
multiplet.
92
FIELD COMPARISON
100 MHz
200 Hz
100 Hz
Coupling constants are constant - they do not
change at different field strengths
7.5 Hz
J 7.5 Hz
1
2
3
4
5
6
200 MHz
400 Hz
200 Hz
7.5 Hz
The shift is dependant on the field
J 7.5 Hz
ppm
1
2
3
93
100 MHz
200 Hz
100 Hz
J 7.5 Hz
J 7.5 Hz
1
2
3
4
5
6
200 MHz
400 Hz
Separation is larger
Note the compression of multiplets in the 200
MHz spectrum when it is plotted on the same
scale as the 100 MHz spectrum instead of on a
chart which is twice as wide.
200 Hz
J 7.5 Hz
ppm
1
2
3
4
5
6
94
50 MHz
J 7.5 Hz
Why buy a higher field instrument?
1
2
3
Spectra are simplified!
100 MHz
J 7.5 Hz
Overlapping multiplets are separated.
1
2
3
200 MHz
J 7.5 Hz
Second-order effects are minimized.
1
2
3
95
Coupling constants
J 2-6 Hz
J 0-7 Hz
J 2-13 Hz
J 5-14 Hz
cis - J 2-15 Hz trans - J 10-21 Hz
96
NOTATION FOR COUPLING CONSTANTS
The most commonly encountered type of coupling is
between hydrogens on adjacent carbon atoms.
This is sometimes called vicinal coupling. It
is designated 3J since three bonds intervene
between the two hydrogens.
3J
Another type of coupling that can also occur in
special cases is
2J or geminal coupling
( most often 2J 0 )
Geminal coupling does not occur when the two
hydrogens are equivalent due to rotations around
the other two bonds.
2J
97
LONG RANGE COUPLINGS
Couplings larger than 2J or 3J also exist, but
operate only in special situations.
C
H
H
C
C
4J , for instance, occurs mainly when the
hydrogens are forced to adopt this W
conformation (as in bicyclic compounds).
Couplings larger than 3J (e.g., 4J, 5J, etc)
are usually called long-range coupling.
98
SOME REPRESENTATIVE COUPLING CONSTANTS
6 to 8 Hz
three bond
3J
vicinal
11 to 18 Hz
three bond
3J
trans
6 to 15 Hz
three bond
3J
cis
0 to 5 Hz
two bond
2J
geminal
Hax,Hax 8 to 14
Hax,Heq 0 to 7
three bond
3J
Heq,Heq 0 to 5
99
cis
6 to 12 Hz
three bond
3J
trans
4 to 8 Hz
4 to 10 Hz
three bond
3J
0 to 3 Hz
four bond
4J
0 to 3 Hz
four bond
4J
Couplings that occur at distances greater than
three bonds are called long-range couplings
and they are usually small (lt3 Hz) and
frequently nonexistent (0 Hz).
100
OVERVIEW
101
TYPES OF INFORMATION FROM THE NMR SPECTRUM
1. Each different type of hydrogen gives a peak
or group of peaks (multiplet).
2. The chemical shift (d, in ppm) gives a clue
as to the type of hydrogen generating the
peak (alkane, alkene, benzene, aldehyde,
etc.)
3. The integral gives the relative numbers of
each type of hydrogen.
4. Spin-spin splitting gives the number of
hydrogens on adjacent carbons.
5. The coupling constant J also gives
information about the arrangement of the
atoms involved.
102
SPECTROSCOPY IS A POWERFUL TOOL
Generally, with only three pieces of data
1) empirical formula (or composition) 2)
infrared spectrum 3) NMR spectrum
a chemist can often figure out the
complete structure of an unknown molecule.
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EACH TECHNIQUE YIELDS VALUABLE DATA
FORMULA
Gives the relative numbers of C and H and other
atoms
INFRARED SPECTRUM
Reveals the types of bonds that are present.
NMR SPECTRUM
Reveals the enviroment of each hydrogen and the
relative numbers of each type.
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Sketch a spectrum
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C6H14O
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C6H14O
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C8H8O2
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C8H8O2
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C9H12O
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C9H12O
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C8H11NO
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C8H11NO
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CH3CH2OH
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Identify the number of groups of peaks in the 1H
NMR spectra of the following
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Peak areas
The area under an NMR peak is directly
proportional to the number of protons giving rise
to the peak.
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A problem C11H16
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Spin - spin coupling CH3CH2I
There are two principal absorptions. These are
divided into three and four component peaks which
are equally spaced.
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Spin - spin coupling
Lets examine a simple molecule
Consider the absorption by the CH2Br protons in
the absence of the other proton of the -CBr2H
a single peak
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The CH2Br protons
The magnetic field experienced by these protons
at any given moment is either slightly increased
or reduced due to the spin of the CHBr2 proton.
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The CH2Br protons
The magnetic field is increased if this proton is
aligned with the applied field.
Therefore a lower external field is necessary to
maintain resonance and the peak is situated at
lower applied field
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The CH2Br protons
The magnetic field is reduced if this proton is
aligned against this applied field.
The exterior field must be increased to maintain
resonance and so we observe a peak at higher
field
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Spin - spin coupling
The signal due to the CH2Br protons is divided
into two peaks
A doublet of peaks of equal size
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Spin - spin coupling
Now lets look at the absorption due to the
-CHBr2 group. It is affected by the spin of the
neighbouring protons. There are 4 possible spin
combinations which are equally possible
1 2 1 a triplet
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Spin-spin coupling constants
J
J
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CH3CH2CH3
Predict its 1H NMR spectrum.....
TMS
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Proton exchange
Protons bonded to electronegative atoms undergo
rapid exchange and do not show spin - spin
coupling with neighbouring protons. A
time-averaged spectrum is observed, a singlet.
If such a proton is suspected, add a few drops
of D2O. The rapid exchange of H - D causes the
OH peak to disappear.
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C8H9Br
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Degree of unsaturation
Degree of unsaturation (2NC - NX NN NH
2)/2 NC number of carbons NX number of
halogens NN number of nitrogens NH number
of hydrogens
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Conclusion Each NMR Peak Contains Structural
Information in Three Forms

• Chemical Shift -- this tells us where on the
  molecule the proton is located
• Integral -- this tells us how many of each type
  of proton our molecule contains
• Splitting -- this tells us how many equivalent
  hydrogens are attached to neighboring carbons

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Taken all together, these three pieces of
information (which should be available for every
NMR peak) give us important information about the
connectivity in the molecule. They tell us
detailed information about how the atoms in the
molecule are connected to one another.
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Proton Chemical Shift Ranges
Low FieldRegion
High FieldRegion
  For samples in CDCl3 solution. The d scale is
relative to TMS at d 0.
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Remember Chemical Equivalence

• Protons in chemically identical environments have
  the same chemical shift.
• If two protons have the same chemical shift, they
  are chemically equivalent.