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Introduction
Basic principle
Nuclear Spin
Chemical Shift
Factors Influencing the Chemical Shift
Coupling and Coupling Constants
Applications
Introduction
• Nuclear magnetic resonance (NMR) spectroscopy:
A spectroscopic technique that gives us
information about the number and types of atoms in a
molecule.
For example, about the number and types of
– Hydrogen atoms using 1H-NMR spectroscopy.
– Carbon atoms using 13C-NMR spectroscopy.
– Phosphorus atoms using 31P-NMR spectroscopy.
Basic Principle
• Every particle associated with spin and charge
can behaves as tiny bar magnet.
• In the presence of a strong magnetic field, the
tiny magnets due to spinning charged particles
aligns to be either with or against the external
magnetic field.
Nuclear Spin
• The nuclei of some atoms have a property called “SPIN”.
• Each spin-active nucleus has a number of spins defined by its
spin quantum number, I.
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.
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
• 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
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)
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
Nuclear Magnetic Resonance
• Resonance: In NMR spectroscopy, resonance is the
absorption of energy by a precessing nucleus and the
resulting “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.
Nuclear Magnetic Resonance
(a) Precession and (b) after absorption of
electromagnetic radiation.
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
DIAMAGNETIC ANISOTROPY
SHIELDING BY VALENCE ELECTRONS
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
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
electronega-tive
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
Interpreting NMR Spectra
– 1H-NMR spectrum of 1-propanol.
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.