Nuclear Magnetic Resonance
Spectroscopy
An
Introduction
Over the past fifty years nuclear magnetic resonance
spectroscopy, commonly referred to as NMR, has become
the preeminent technique for determining the structure of
organic compounds.
Nuclear magnetic resonance (NMR) spectroscopy:
A spectroscopic technique that gives us information
about the number and types of nuclei in a molecule.
For example, about the number and types of
– Hydrogen nuclei using 1H-NMR spectroscopy.
– Carbon nuclei using 13C-NMR spectroscopy.
– Phosphorus nuclei using 31P-NMR spectroscopy.
Nuclear Magnetic Resonance spectroscopy involves the
transition of a nucleus from one spin state to another with
the resultant absorption of electromagnetic radiation by
spin active nuclei when they are placed in a magnetic field.
The following features lead to the
NMR phenomenon:
1. A spinning charge generates a
magnetic field, as shown by the
animation on the right.
The resulting spin-magnet has a
magnetic moment (μ) proportional
to the spin.
The nuclei of many elemental isotopes have a characteristic
spin (I).
Some nuclei have integral spins (e.g. I = 1, 2, 3 ....),
some have fractional spins (e.g. I = 1/2, 3/2, 5/2 ....), and
a few have no spin, I = 0 (e.g. 12C, 16O, 32S, ....).
Isotopes of particular interest and use to organic chemists
are 1H, 13C, 19F and 31P, all of which have I = 1/2. Since the
analysis of this spin state is fairly straightforward, our
discussion of NMR will be limited to these and other I = 1/2
nuclei.
NUCLEAR SPIN STATES - HYDROGEN NUCLEUS
The spin of the positively
charged nucleus generates
m
a magnetic moment vector, m.
+
+
m
+ 1/2
- 1/2
TWO SPIN STATES
The two states
are equivalent
in energy in the
absence of a
magnetic or an
electric field.
In the presence of an external magnetic field (B0), two spin states
exist, +1/2 and -1/2.
The magnetic moment of the lower energy +1/2 state is alligned
with the external field, but that of the higher energy -1/2 spin
state is opposed to the external field. Note that the arrow
representing the external field points North.
Nuclear Spins in Strong External
Magnetic Fields
N
-1/2
In a strong magnetic
field (Bo) the two
spin states differ in
energy.
+1/2
Bo
S
THE ENERGY SEPARATION DEPENDS ON Bo
- 1/2
DE
= kBo = hn
degenerate
at Bo = 0
+ 1/2
Bo
increasing magnetic field strength
The Larmor Equation!!!
DE = kBo = hn
can be transformed into
gyromagnetic
frequency of
the incoming
radiation that
will cause a
transition
n =
ratio g
g
2p
Bo
strength of the
magnetic field
g is a constant which is different for
each atomic nucleus (H, C, N, etc)
• More nucleons will be in the lower energy
state aligned with the magnetic field.
• A nucleon can absorb a quantum of energy in
the radio frequency range and align against
the magnetic field.
• It emits a radio frequency when it drops back
to its original position.
Absorption of Energy
quantized
Opposed
-1/2
-1/2
DE
DE = hn
Radiofrequency
+1/2
Applied
Field
Bo
Aligned
+1/2
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
If rf energy having a frequency matching the Larmor frequency is
introduced at a right angle to the external field (e.g. along the xaxis), the precessing nucleus will absorb energy and the magnetic
moment will flip to its I = -1/2 state. This excitation is shown in
the following diagram.
Nuclear Magnetic Resonance
• Resonance: In NMR spectroscopy, resonance is due to
the absorption of energy by a precessing nucleus and
the results in “flip” of its nuclear spin from a lower
energy state to a higher energy state.
• The precessing spins induce an oscillating magnetic
field that is recorded as a signal by the instrument.
– Signal: A recording in an NMR spectrum of a nuclear
magnetic resonance.
Strong magnetic fields are necessary for NMR spectroscopy.
The earth's magnetic field is not constant, but is approximately 10-4 T at ground
level. Modern NMR spectrometers use powerful magnets having fields of 1 to 20
T.
Even with these high fields, the energy difference between the two spin states is
less than 0.1 cal/mole.
Resonance Frequencies of Selected Nuclei
Abundance
1H
99.98%
1.00
1.41
2.35
7.05
42.6
60.0
100.0
300.0
2H
0.0156%
1.00
7.05
6.5
45.8
41.1
13C
1.108%
1.00
2.35
7.05
10.7
25.0
75.0
67.28
100.0%
1.00
40.0
19F
Bo (Tesla)
Frequency(MHz)
g(radians/Tesla)
Isotope
267.53
251.7
4:1
For NMR purposes, this small energy difference (ΔE) is
usually given as a frequency in units of MHz (106 Hz),
ranging from 20 to 900 Mz, depending on the magnetic
field strength and the specific nucleus being studied.
Irradiation of a sample with radio frequency (rf) energy
corresponding exactly to the spin state separation of a
specific set of nuclei will cause excitation of those nuclei in
the +1/2 state to the higher -1/2 spin state.
The nucleus of a hydrogen atom (the proton) has a
magnetic moment μ = 2.7927, and has been studied more
than any other nucleus
NMR SPECTROMETER - INSTRUMENTATION
DIAMAGNETIC ANISOTROPY
SHIELDING BY VALENCE ELECTRONS
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
Bo applied
B induced
(opposes Bo)
fields subtract at nucleus
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).
DOWNFIELD
Less shielded protons
appear here.
SPECTRUM
UPFIELD
Highly shielded
protons appear here.
It takes a higher field
to cause resonance.
CHEMICAL SHIFT
• To standardise measurements on different
NMR instruments, a standard reference
sample is used in each experiment. This is
tetramethylsilane (TMS).
This is a symmetrical and inert molecule. All H
atoms have the same chemical environment and a
single peak is produced from this molecule.
• The difference in energy needed to change the spin state in
the sample is compared to TMS and is called the CHEMICAL
SHIFT.
• The chemical shift of TMS is defined as zero
• The symbol d represents chemical shift and is measured in
ppm. The chemical shift scale is measured from right to left
on the spectrum.
parts per
million
chemical =
shift
d
=
shift in Hz
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).
= ppm
NMR Correlation Chart
-OH
-NH
DOWNFIELD
UPFIELD
DESHIELDED
SHIELDED
H
CHCl3 ,
TMS
12
11
10
9
8
7
6
H
RCOOH
RCHO
C=C
5
4
CH2F
CH2Cl
CH2Br
CH2I
CH2O
CH2NO2
3
2
CH2Ar
CH2NR2
CH2S
C C-H
C=C-CH2
CH2-C-
1
0
d (ppm)
C-CH-C
C
C-CH2-C
C-CH3
O
Ranges can be defined for different general types of protons.
IT IS USUALLY SUFFICIENT TO KNOW WHAT TYPES
OF HYDROGENS COME IN SELECTED AREAS OF
THE NMR CHART
acid
COOH
12
aldehyde
CHO
10
benzene
CH
9
7
alkene
=C-H
6
C-H where C is
attached to an
electronegative
atom
CH on C
next to
pi bonds
3
4
X-C-H
MOST SPECTRA CAN BE INTERPRETED WITH
A KNOWLEDGE OF WHAT IS SHOWN HERE
aliphatic
C-H
2
X=C-C-H
0
Factors influencing the Chemical Shift
• Inductive effect by Electronegative groups
• Magnetic Anisotropy
• Hydrogen Bonding
DESHIELDING BY AN ELECTRONEGATIVE ELEMENT
d-
Cl
d+
C
d-
electronegative
element
H
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.
NMR CHART
“deshielded“
protons appear
at low field
highly shielded
protons appear
at high field
deshielding moves proton
resonance to lower field
Electronegativity Dependence
of Chemical Shift
Dependence of the Chemical Shift of CH3X on the Element X
Compound CH3X
Element X
Electronegativity of X
Chemical shift
d
most
deshielded
CH3F CH3OH CH3Cl CH3Br CH3I
CH4 (CH3)4Si
F
O
Cl
Br
I
H
Si
4.0
3.5
3.1
2.8
2.5
2.1
1.8
4.26
3.40
3.05
2.68
2.16
0.23
0
TMS
deshielding increases with the
electronegativity of atom X
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.
Ring Current in Benzene
Circulating p electrons
H
Bo
H
Deshielded
fields add together
Secondary magnetic field
generated by circulating p
electrons deshields aromatic
protons
ANISOTROPIC FIELD IN AN ALKENE
protons are
deshielded
Deshielded
fields add
H
H
shifted
downfield
C=C
H
Bo
H
secondary
magnetic
(anisotropic)
field lines
ANISOTROPIC FIELD FOR AN ALKYNE
H
C
C
H
Bo
Shielded
fields subtract
hydrogens
are shielded
secondary
magnetic
(anisotropic)
field
HYDROGEN BONDING DESHIELDS PROTONS
The chemical shift depends
on how much hydrogen bonding
is taking place.
R
O
H
H
O
H
O R
Alcohols vary in chemical shift
from 0.5 ppm (free OH) to about
5.0 ppm (lots of H bonding).
R
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.
NMR Spectrum of Acetaldehyde
O
CH3 C
offset = 2.0 ppm
H
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
Doublet
Triplet
Quartet
Quintet
Septet
Octet
Nonet
This happens because of interaction with neighboring
hydrogens and is called SPIN-SPIN SPLITTING.
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
Cl H
H C C Cl
Cl H
integral = 1
triplet
doublet
The subpeaks are due to
spin-spin splitting and are
predicted by the n+1 rule.
this hydrogen’s peak
is split by its two neighbors
these hydrogens are
split by their single
neighbor
H
H
H
H
C
C
C
C
H
two neighbors
n+1 = 3
triplet
H
one neighbor
n+1 = 2
doublet
MULTIPLETS
singlet
doublet
triplet
quartet
quintet
sextet
septet
SOME COMMON SPLITTING PATTERNS
X CH CH Y
CH3 CH
(x=y)
CH2 CH
X CH2 CH2 Y
(x=y)
CH3 CH2
CH3
CH
CH3
INTENSITIES OF
MULTIPLET PEAKS
PASCAL’S TRIANGLE
PASCAL’S TRIANGLE
Intensities of
multiplet peaks
1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
1 5 10 10 5 1
1 6 15 20 15 6 1
1 7 21 35 35 21 7 1
The interior
entries are
the sums of
the two
numbers
immediately
above.
singlet
doublet
triplet
quartet
quintet
sextet
septet
octet
THE CHEMICAL SHIFT OF PROTON HA IS
AFFECTED BY THE SPIN OF ITS NEIGHBORS
aligned with Bo
50 % of
molecules
opposed to Bo
+1/2
-1/2
H
HA
H
HA
C
C
C
C
Bo
downfield
neighbor aligned
upfield
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.
50 % of
molecules
SPIN ARRANGEMENTS
one neighbor
n+1 = 2
doublet
one neighbor
n+1 = 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 neighbor.
SPIN ARRANGEMENTS
two neighbors
n+1 = 3
triplet
one neighbor
n+1 = 2
doublet
H
H
H
H
C
C
C
C
H
methylene spins
H
methine spins
SPIN ARRANGEMENTS
three neighbors
n+1 = 4
quartet
H
H
C
C
H
H
methyl spins
H
two neighbors
n+1 = 3
triplet
H
H
C
C
H
H
H
methylene spins
THE COUPLING CONSTANT
H H
J
J
C C H
J
H H
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.
J
APPLICATIONS
• GEOPHYSICAL: Used to determine the water
content in the geophysical samples.
• Engineering Applications: It is used to study the
Process engineering aspects like the kinetic and
equilibrium studies of Formaldehyde – water –
methanol systems.
• Non- destructive testing of DNA, proteins etc.
• This is very much useful in Data acquisition in
Petroleum Industry. Here NMR probes are
developed and Used.
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.