Dr.Sarish Chemistry factors affecting Chemical shift in NMR.pdf

NMR

1.

NMR SPECTROSCOPY

Dr. Sarish S
Assistant Professor
PG Dept. Chemistry
N.S.S. College
Pandalam
N.S.S. COLLEGE–PANDALAM
2017-2018
Part I

2. 2
INTRODUCTION
Spectroscopy is concerned with interactions
between electromagnetic radiations and matter.

NMR spectroscopy use NMR phenomenon to
study physical, chemical and biological properties
of matter

NMR phenomenon was discovered in 1945
Research groups at Stanford (Bloch, Hansen and
Packard )and Harvard University(Purcell, Torrey
and Pound) , in the USA.

3. 3
INTRODUCTION
NMR spectrum of ethanol was first recorded
in 1946.(First Observation of the Chemical
Shift)





F. Bloch, W. W. Hansen, M. E. Packard, Phys. Rev. 1946, 69, 127

Purcell and Bloch received Nobel prize in Physics
in 1952 for the discovery of NMR.

4. 4
INTRODUCTION
The utility of NMR spectroscopy for structural
elucidation arises because different atoms in
a molecule experience slightly different
magnetic fields and therefore transitions at
different resonance frequencies.

In addition , splitting of spectral lines arises
due to interactions between different nuclei
provides valuable information about the
proximity of different atoms in a molecule.

5. 5
INTRODUCTION
NMR spectroscopy is considered as
one of most powerful for the structural
elucidation of organic compounds



NMR spectroscopy uses
radiofrequency radiations to induce
transitions between different nuclear
spin states of a molecule, which are
created by keeping the sample in a
magnetic filed

6. 6
THEORY OF NMR
.

Nuclear spin is the total nuclear angular
momentum quantum number. This is
characterized by a quantum number I, which may
be integral, half-integral or 0.
Only nuclei with spin number I  0 can
absorb/emit electromagnetic radiation. The
magnetic quantum number m
I
has values of –I, -
I+1, …..+I .
(fI3/2 3/21/21/23/2)
The NMR phenomenon is based on the fact that
nuclei of atoms have magnetic properties that can
be utilized to yield chemical information.
Atomic nucleus has mass and it spins on its own axis

Due to the spin, it possesses angular momentum (P)

7. 7
THEORY OF NMR

Mass
number
Atomic
number
Spin Examples
Even Even zero I(12C) = 0
Even Odd multiple
of 1
I(2H) = 1
Odd Even or
Odd
multiple
of 1⁄2
I(1H) =
1/2

8. 8
THEORY OF NMR

Nuclear magnetic moments
Magnetic moment  is another important
parameter for a nuclei
 =  I (h/2)
I: spin number; h: Plank constant; :
gyromagnetic ratio
The ratio of magnetic momentum to angular
momentum is called “Gyromagnetic ratio”.
It is very characteristic of a given nuclei. It is a
constant for a given nucleus.

9. 9
THEORY OF NMR
The Nucleus in a Magnetic Field
Precession and the Larmor frequency
The magnetic moment of a spinning nucleus
processes with a characteristic angular frequency
called the Larmor frequency w, which is a function
of r and B
0
Larmor frequency  = rB
0
 
Linear precession frequency  = /2 =  B
o
/
2

10. 10
THEORY OF NMR

Nuclear Zeeman effect
Zeeman effect: when an atom is placed in an
external magnetic field, the energy levels of the
atom are split into several states.
The energy of a give spin sate (E
i
) is directly
proportional to the value of m
I
and the magnetic
field strength B
0
Spin State Energy E
I
=- . B
0
=-m
I
B
0
r(h/2)
Notice that, the difference in energy will always be
an integer multiple of B
0
r(h/2). For a nucleus with
I=1/2, the energy difference between two states is
 ΔE=E
-1/2
-E
+1/2
= B
0
r(h/2)

11. 11
THEORY OF NMR

Application of radio frequency causes the
absorption of the same due to excitation of nuclear
spins from lower energy level to upper energy level
when the two energies match (resonance condition)
The applied magnetic field causes an energy
difference between the aligned (α) and
unaligned (β) nuclei

12. 12
THEORY OF NMR

Distribution of nuclear spins:
Maxwell Boltzmann distribution
N
α
/ N
β
= exp(-ΔE/kT)
N
α
/ N
β
is the population ratio of the excited state
to the ground state
NMR Signal
NMR signal results from the transition of spins
from the α to β state
Strength of the signal depends on the population
difference between the α and β spin states

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RELAXATION PROCESS

(a)
Nuclear spins relax to maintain the population
difference. The spins in the excited state return
back to ground state by

spin lattice relaxation
Transfer of energy from the nuclear spin in high
energy state to molecular lattice.
It is also called longitudinal relaxation
Helps to maintain excess population in the
ground state
And is characterized in terms of time constant T
1

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RELAXATION PROCESS

(b) spin-spin relaxation
Mutual exchange of spins by two precessing
nucleus I the close proximity of each
It is also called transverse relaxation
Helps to decrease the half life of the excited
state
And is characterized in terms of time constant T
2

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CHARACTERISICS OF NMR SPECTRUM
Position of the Signal :
Chemical shift
Type or Chemical
nature of the
proton
Intensity of Signal:
Area under the
peak
Number of NMR Signal
Number of different
types of protons
Spin-Spin Coupling :
Nature of the signal:
Splitting of Signal
Interaction
between Nuclei
Number of protons
of a particular kind

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CHEMICAL SHIFT
ν = (B
o
γ)/ 2π
All the nuclei of a given type in a molecule don't
resonate at the same frequency
Because the nuclei in molecule are not bare -
Surrounded by electrons – modify the applied filed
The resonance condition for nuclei

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CHEMICAL SHIFT
Nuclear magnetic field interact with the local
magnetic filed


Thus the local magnetic field experienced by nucleus
depends on the electronic structure the nucleus of
interest.

The nuclei are either shielded or deshielded from the
applied field.

The shift in resonance frequency due to chemical
enviroment of the nuclei is chemical shift.  
                                                                                                      

18. 18
CHEMICAL SHIFT

Depending on the electron density around the
protons , the resonance condition gets modified as

ν = (B
eff
γ) / 2π = [B
o
(1-σ) γ / 2π]

Nuclear shielding
Local field is different from applied filed because
the induced magnetic field (B
ind
) produced by the
spinning electron opposes the external magnetic
field(B
0
). This is diamagnetic shielding.
the effective magnetic field felt by nucleus,
B
eff
=B
o
-B
ind
= B
o
(1-σ)
B
ind=
σB
o
σ is the shielding constant.

19. 19
CHEMICAL SHIFT
Chemical shift expressed in δ instead
Instead of actual frequencies of resonances
δ = (ν sample – ν reference) x 10
6
(in ppm)
spectrometer frequency
Variation in absorption frequencies for different
nuclei in a molecule due to electronic shielding and
deshielding (Chemical Shift)
Molecules containing certain nuclei or pi
electrons, the induced field amplify the applied
field
Nuclear deshielding

20. 20
REFERENCE COMPOUND FOR
1
H-NMR SPECTROSCOPY
Tetramethylsilane (TMS) is used as a reference

Due to large diamagnetic shielding, the chemical
shift of TMS is lower than most protons in organic
molecules.
All the 12 protons in TMS are equivalent and
hence very sharp only one signal
TMS is a liquid and is soluble in most solvents
It is also volatile and hence easy to remove from the
sample
It is chemically inert

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FACTORS AFFECTING CHEMICAL SHIFT
1. Electronegativity, inductive and resonance effects
CH
4
= 0.23
CH
3
I - 2.10 , CH
3
Br - 2.65, CH
3
Cl -3.10, CH
3
F 4.26





Any factor affecting the electronic environment
around the active nuclei would alter its chemical
shift
Factors
Electronegativity, inductive and resonance effects
Hybridization
Vander Waals deshielding
Bond anisotropy
Hydrogen Bonding
With increasing electronegativity, the protons get deshie

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FACTORS AFFECTING CHEMICAL SHIFT
As the number of EN atoms increases, the protons
get deshielded.
CH
4
- 0.23, CH
3
Cl - 3.10 , CH
2
Cl
2
-5.33, CHCl
3
-7.24
2. Hybridization
Chemical shifts of protons attached to a carbon
atom depends on the hybridization
The electronegativity of different hydridised
carbon atoms is different
SP
3
< SP
2
< SP

23. 23
FACTORS AFFECTING CHEMICAL SHIFT
3. Vander Waals deshielding
Sterically hindered molecules, The electron cloud
of the hindering group will tend to repel the electron
cloud of the neighboring protons
The proton will be deshielded
4. Bond anisotropy
Non-spherical electron density – induced
magnetic field will be non-uniform in space –
anisotropic effects
These effects are paramagnetic in certain
directions and diamagnetic in other around the π
electron cloud
π electron cloud of aromatic ring and unsaturated
compounds of the type, C=C and C=O type

24. FACTORS AFFECTING CHEMICAL SHIFT
24
Diamagnetic anisotropy in benzene
Reference; Introduction to spectroscopy D.L .Pavia

25. FACTORS AFFECTING CHEMICAL SHIFT
25Reference; Introduction to spectroscopy D.L .Pavia
Diamagnetic anisotropy in acetylene
5. Hydrogen Bonding
A hydrogen bonded proton is highly deshielde
d .Such a proton will resonate at lower field strength
Eg. Phenolic protons usually at δ=5-6 ppm

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