NUGIE ARIMI AGNEMG RESONANCE
SPIE@IROSCOPY
RESENTED BY
Ss CMUNUIN
SAL CHEMISTRY
WBa Pat e
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CONTENTS
e Introduction
e Fundamental principles of NMR
e Interpretation
¥ Chemical shift
Y Number of signals
¥ Spin-Spin coupling: Splitting of signals
Y Coupling constant
V Integrals
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NMR Spectroscopy
e Nuclear Magnetic Resonance is_ a branch of
spectroscopy in which radio frequency waves induce
transitions between magnetic energy levels of nuclei
of a molecule.
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_ Highest energy Lowest energy
Wavelength (nm) a Longer Wavelength
10°? 10° 10° 104 10° 108 19! 10!
| | | | | | | | | | | | | | |
T } T I
I I | |
Gamma | | Ulira- |= Io | ;
ray X-ray violet |’z Infrared Microwave ! Radio frequency
| } > | |
1 i l l
1 | | | | | | | | | | | |
10° 19!8 1o!° 10!4 10! 10! 108 10° 107
High Frequency —€— Frequency (s“’)
400 500 600 700 750 nm
Visible region
The Electromagnetic Spectrum
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The frequency of radio waves lies between 107 and 10®cps
The energy of radio frequency ( rf ) radiation can be
calculated by using the equation :
E=hy
h = Planck’s constant = 6.6 x 1027 erg sec
v = frequency = 107- 10° cps(cycles per sec).
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E = 6.6 x 10-77 x 107 ( or 10° ergs)
= 6.6 x 10-° ( or 6.6 x 10-9 ergs )
Energy of rf radiation is very small to vibrate, rotate , or
excite an atom or molecule. But this energy is
sufficient to affect the nuclear spin of the atoms of a
molecule.
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~ NUCLEAR SPIN
The nuclei of some atoms have a property called “SPIN”.
(a These nuclei behave as if
they were spinning.
See
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 number of Spin states = 21 +1
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e If the number of neutrons and the number of protons are
both even, then the nucleus has NO spin. »C, «O ,32S etc.
e If the number of neutrons plus the number of protons is
odd, then the nucleus has a half-integer spin (i.e. 1/2, 3/2,
5/2) 1H,19F, 3:P
e If the number of neutrons and the number of protons are
both odd, then the nucleus has an integer spin (i.e. 1, 2, 3)
2H, yN
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Element H 2H 13 14 15 16 197 31p 32g
Nuclear spin
quantum Le ee ee eS eA ee
number ( J )
Number of Z 3 1 2 3 Z 1 2 DN
spin states
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Principle
e NMR spectroscopy is the interaction of magnetic field with spin
of nuclei and then absorption of radio frequency. For example,
the nucleus of proton ,H* has two spin rotations : clockwise
rotation with a spin quantum number I = +% and |
counterclockwise rotation with a spin quantum number I = – 4
e The number of spin sates is 2]+1 which is 2x (1/2) +1 = 2 state
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The two states
iv are equivalent
in energy in the
+1/2 -1/2 absence of a
magnetic or an
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e Without the magnetic field the spin states of nuclei
possess the same energy, and energy level transition is
not possible.
e When a magnetic field is applied, the separate levels
and radio frequency radiation can cause transitions
between these energy levels.
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Energy Differences Between Nuclear Spin S
oO
28 |
OY
= increasing field strength ASSEN
no difference in absence of magnetic field
proportional to strength of external magnetic field
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= Some important relationships in NMR
Units
The frequency of absorbed
Hz
electromagnetic radiation
is proportional to
the energy difference between kJ/mol
two nuclear spin states (kcal/mol)
which is proportional to
the applied magnetic field tesla (T)
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Magnetic properties of nuclel
° When a charged particle such as a proton spins on
its axis, it creates a magnetic field. Thus, the
nucleus can be considered to be a tiny bar magnet.
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® But when magnetic field is applied, the proton (H)
posses spin & their own magnetic field align
themselves either or opposite to magnetic field.
° For e.g. 1H has +1/2 & -1/2 spin state, the proton (H)
have +1/2 spin state align themselves with field
(Lower energy) and with -1/2 spin state align
opposite to field (Higher energy).
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A spinning proton re
crea tmagenestic field.
o
The nuclear | are
oriwieth nor tagaeinsdt B ..
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Nuclear Spin
Copyright © The McGraw-Hill Companies, inc. Permission required for reproduction or display
O s
Ho
(a) No external magnetic field (b) Apply external magnetic field #¢
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Higher
energy state Spin —4
(aligned against
the applied field)
Lower
energy state Spin +4
(aligned with
the applied field)
© Grooks/Cole, Cengage Leaning
Change in spin state energy separation with increase by applied
magnetic field ,B,
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External Magnetic Field
When placed in an external field, spinning
protons act like bar magnets.
GG
lower energy higher energy
more stable less stable
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THE “RESONANCE PHENOMENON
absorption of energy by the
spinning nucleus
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N
> -1/2
unal igned In a strong magnetic
field (B,) the two
spin states differ in
energy.
+1/2
aligned
B
O
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Two Energy States
The magnetic fields of wo
(| B state —
the spinning nuclei ‘ bit p Es
Y
will align either with
the external field, or Bo hv = AE
against the field.
A
A photon with the right
( | ) & state—
amount of energy can : Le
be absorbed and
cause the spinning
proton to flip.
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Excited state = High energy
N S
7) s 7
Add Energy
Energy Released
Aligned = Low Energy Back to low energy ground state
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e According to the quantum theory, a spinning nucleus
can only have values for the spin angular momentum
given by the equation :
Spin angular momentum = [I(I+1)]2/2 h / 2u
| = Spin quantum number
h= Plancks constant
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® But U=y~x spin angular momentum
LL = magnetic moment of the nucleus
y = gyro magnetic ratio
e If a nucleus having a magnetic moment is introduced into a
magnetic field , Hy the two energy levels become separate
corresponding to m , = -1/2 (anti-parallel to the direction of
magnetic field) and m, = +1/2 (parallel to the direction of
magnetic field).
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e Fora nucleus with I = 1/2, the energies E, and E, for the
two states with m , = +1/2 and m , = -1/2, respectively,
are
B= r/o |v hy |
E,=+1/2|yh2/p] H,
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——
T=1/2,m=+1/2,-1/2
Energy
No field
m=- 1/2
a
a a
m=+1/2 ‘
Ҥ
‘
BS
m=+1/2
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° When the nucleus absorbs energy, the nucleus will be
promoted from the lower energy state E,, to the
higher energy state E, by absorption of energy , AE,
equal to the energy difference, E, —E, .
e It means that the absorption of energy AE changes
the magnetic moment from the parallel state m , =
+1/2 to the anti-parallel state (m , = -1/2).
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