Fundamental Of NMR PPT download

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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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