NMR - Nuclear magnetic resonance (NMR).pptx

NMR
Nuclear magnetic
resonance (NMR)
spectroscopy
Name – Muskaan
Subject – MPAT

• Nuclear magnetic resonance (NMR) spectroscopy
is the study of molecules by recording the
interaction of radiofrequency (Rf) electromagnetic
radiations with the nuclei of molecules placed in a
strong magnetic field.
• It is concerned with absorption of certain amount of
energy
by spinning nuclei in a magnetic field when irradiated
with
radiofrequency radiation (RFR) of equivalent energy.

• NMR gives the information about the number and
configuration of
magnetically active atoms, like positions of different types
of Hydrogen over the C- skeleton of an organic molecule.
• Absorption of RFR occurs when the nucleus undergoes
transition from one alignment in the applied magnetic field
to the opposite alignment, i.e. from parallel (ground state)
orientation to anti-parallel (excited state) orientation.

RESONANCE
When the frequency of the oscillating electric field of the
incoming RFR just matches the frequency of the electric field
generated by the precising nucleus, then the 2 fields can
couple, and the energy can be transferred from the
incoming radiation to the nucleus, thus causing a spin change
(clock-wise to anti-clock-wise).
This condition is called "resonance", and the nucleus is said to
have resonance with the incoming electromagnetic wave
(RFR).

NMR SPECTRUM
• In NMR technique, the frequency of the RFR is kept constant
(60MHz) and the strength of magnetic field is varied.
• At certain value of the applied field strength, depending
upon the nature of proton or nucleus, the energy required to
flip the proton matches the energy of radiation.
• As a result, absorption takes place and a signal is observed
in the spectrum. Such a signal or peak is called an NMR
Spectrum.
• NMR spectrum is graphical plot of relative intensity
(Y axis) and the δ value (x axis).

FUNDAMENTAL PRINCIPLE &
THEORY
• The nucleus of a hydrogen atom (proton) behaves as a spinning bar magnet
because it possesses both electric and magnetic spin.
• Like any other spinning charged body, the nucleus of hydrogen atom also generates
a magnetic field.
• Nuclear magnetic resonance Involves the interaction between an oscillating
magnetic field of electromagnetic radiation and the magnetic energy of the
hydrogen nucleus or some other type of nuclei when these are placed in an external
static magnetic field.
• The sample absorbs electromagnetic radiations in radio wave region at different
frequencies since absorption depends upon the type of protons or certain nuclei
contained in the sample)

Processional Motion
• Consider a spinning top. It also performs a slower waltz like motion,
in which the spinning axis of the top moves slowly around
the vertical.
• This is processional motion & the top is said to be processing around the
vertical axis of earth's gravitational field.
• The precession arises from the interaction of spin with earth's gravity acting
vertically downwards.
• It is called Gyroscopic motion.
• Proton will be showing processional motion due to interaction of Spin &
Gravitational force of Earth

Processional frequency - The processional frequency may be
defined as the number of the nucleus around the external field
(Ho).
Spinning frequency of Proton will be same.
OR
Alternatively the processional frequency of spinning bar
magnet (nucleus) may be defined as equal to the frequency of
electromagnetic radiations in megacycles per second
necessary to induce a transition from one spin state to the
another

Processional frequency (ν) α External Magnetic Field (H0)
ΔE=hν
(h= Planck’s Constant, ν= Frequency of EMR)
Angular Processional velocity (ω) α External Magnetic Field (H0)
ω α H0
ω = γ H0 …………..(1)
γ = Gyromagnetic ratio = 2πμ / HI
Where , μ= magnetic moment of the spinning bar Magnet
I is spin quantum number of the spinning magnet
h is Planck's constant.
Larmour Equation for NMR , ν = γH0/2π
γH0 = 2πν ………………………(2)
Put the value of γH0 in eq. 1
ω = 2πν…………………..(3)
Angular Processional velocity (ω) = Angle Covered/Time
ω = 2π/T We know, 1/T = v
ω = 2πν
Angular processional velocity ω = 2πv
The value of this frequency (v) inserted is called as Processional
Frequency.

Spin Quantum Number
• All nuclei carry a charge.
• So, they will possess Spin Angular Momentum.
• The moment of the spin angular momentum is quantized, i.e., only those nuclei which
have a finite value of spin quantum number (I > 0) will precis along the axis of rotation.
• It is known that the spin quantum number (I) is associated
with Mass Number and Atomic Number of the nuclei.
• The Spin Quantum Number is the fourth quantum number which is
introduced to describe the orientation of the electron spin(rotation) in
space.
• It can be clockwise or anticlockwise.

To understand spin quantum no. Nuclear Shell Model is
important.
❑ Protons / Neutrons are filled as per the spin multiplicity.
❑ Nucleus shell is different from electron shell.
❑ Spin multiplicity = 2s+1
❑ Here s = spin of Proton or Neutron and it may be:
1/2
3/2
5/2
7/2
and so on..............

» Spin is a property of an atomic nucleus which has either odd
mass no., or odd atomic no., or both.
» Common nuclei which have spin are 1H1, 13C6, 19F3, 31P15.
» Ordinary isotopes of Carbon and Oxygen (12C6 and 1608)
do not have spin property.
» For each nucleus with spin, the number of spin states can be
quantized and is determined by its 'nuclear spin quantum
number, (I). the number of spin states allowed could be

Instrumentation
The nuclear magnetic resonance (NMR)
spectrometer consists of
❑ Sample Holder
❑ Magnet
❑ Sweep Generator
❑ RF-Transmitter
❑ Receiver Coil & Amplifier
❑ Detector

Sample Holder
❑ 5mm glass tube is used; which can hold 0.4mL liquid sample.
❑ Microprobes are used for low volume.
❑ The Sample Cell is a small cylindrical glass tube that is
suspended in the gap between the faces of the pole pieces of
the magnet.
❑ The sample is spun around its axis to ensure that all parts of the solution
experience a relatively uniform magnetic field.
Magnet
❑ Accuracy and quality of the instrument is dependent on
its strength.
❑ Resolution increases with increase in the field strength.
❑ Conventional Magnets: 30 to 60 MHz
❑ Permanent or Electromagnet: 60 MHz, 90 MHz, 100
MHz (Coiling System & Temp. sensitive).
❑ Super Conducting Solenoids: 470 MHz

Sweep Generator
❑ A Set of Helmholtz coils located parallel to the magnet; which alters the applied
magnetic field over a small range.
❑ By varying a direct current through these coils, the effective field can be changed by
milligauss.
❑ Such a field is automatically changed linearly with time.
❑This change is synchronized with the linear drive of chart recorder.
❑For a 60MHz 1H instrument; the sweep
range is 235 milligauss.
RF-Transmitter
❑ It is fed into a pair of coils mounted perpendicular (at 900) to the path of the field. A fixed
oscillator with a frequency of 60, 90 or 100 MHz is used in high resolution
NMR; the power output is 100 which is plane polarized.

Receiver Coils & Amplifier
❑ Amplifies the received EMR by 105 times.
Detector
❑The RF signal produced by resonating nuclei is detected by
means of a coil that surrounds the sample.

 CCl4(Carbon Tetrachloride)
❑ CS2(Carbon di-sulphide)
❑ CDCl3(Deuterochloroform)
❑ [(CCl3)2CO] (Hexachloroacetone)
❑ [(CD3)2SO] (Deuterodimethyl sulfoxide/ Deuterium containing DMSO)
❑ C6D6 (Deuterobenzene)
Commonly used Solvents in
NMR spectroscopy are:

Characteristics of Solvents
❑ Chemically inert solvent is used in NMR Spectroscopy.
❑ Solvent should be magnetically isotropic (no preferred direction) in nature.
❑ It should be free from any 1H atom, pure and easily available.
❑ Examples of I=1/2 are 1H, 13C, 19F, 31P etc.
❑ Hydrogen bonding:
• It will increase the δ value.
• Due to hydrogen bonding de-shielding occurs.
• Higher concentration of OH or NH group leads to hydrogen bonding and electron
transfer from hydrogen; ultimately de-shielding takes place.
• δ value of same sample will be different.

Number of Signals in 1H NMR
 The number of signals produced tells us about the number of different
sets of equivalent protons in a molecule.
 Number of Signals in the NMR spectrum gives the information about
different sets
of equivalent proton in a molecule.
❑ Each signal in the NMR spectrum represents a set of equivalent proton.
❑ Magnetically equivalent protons are known as chemically equivalent and
gives a
single signal in NMR Spectroscopy.
 'Equivalent protons' are those set of protons which have same
environment around themselves.
 Now the protons of same environment absorb at the same applied
magnetic field and protons of different environment absorb at different
field strengths.

Methane has four protons, they are all connected to the same atoms and have the
same neighbors on all sides – in other words, they are equivalent because they are in
the same environment.
So Methane will give Single signal
41
Equivalent protons give one NMR signal

Benzene:
»All six protons are chemical equivalent (have the same bonding and in the
same chemical environment) to each other and have the same resonance
frequency in an 1H NMR experiment, therefore show only one signal

Chemical Shift in 1H NMR
• Different type of protons are present in the compounds & they have different
electronic environment; due to variation in electronic environment protons gets
absorb at different applied magnetic field.
• When a molecule is placed in a magnetic field, its electrons are caused to circulate and
thus, they produce secondary magnetic fields i.e., induced magnetic field.
• Rotation of electrons about the proton itself generates a field in such a way that at the
proton, it opposes the applied field.
• Thus, the field felt by the proton, is diminished and the proton is said to be shielded.

❑ Each proton in a molecule is shielded from the applied magnetic field to an
extent that depends on the electron density surrounding it.
❑ Greater the electron density around a nucleus, the greater the shielding.
❑ So, less net external applied magnetic field will be experienced by nucleus.
❑ As a result, the nucleus precises at lower frequency.
❑ Each proton in a molecule is in a slightly different chemical environment. So,
different amount of shielding & resonance frequency

If the induced field opposes the applied field, then proton is said to be shielded
But if the induced field reinforces the applied field, the proton feels a higher
field strength and thus, such a proton is said to be deshielded.
Shielding shifts the absorption upfield
Deshielding shifts the absorption downfield
69

Thus, it is quite obvious that the shielded protons
require a higher applied magnetic field for the
absorption of RFR to take place.
Likewise, the deshielded protons require a lesser
applied magnetic field.
70
The shift in position of absorption of the RFR by
the nucleus due to shielding or
deshielding effect is called "Chemical shift“.
The difference in the absorption position of a
particular proton from the absorption of a
reference proton (proton of TMS) is said to be the
chemical shift of that proton.
71

• Chemical shift is measured as field or frequency (because field and
frequency have linear relationship), it is actually the ratio of change in
the magnetic field to the applied field or the ratio of the change in
frequency to the standard (reference) frequency.
• Thus it is a dimensionless constant, usually denoted as δ and
expressed in ppm.
72

For measurement of chemical shift of proton in a
molecule the signals of Tetra Methyl Silane (TMS) is
taken as reference.
74

Why TMS is used as reference
standard?
❑ δ-value of TMS is considered as 0.
❑ It has 12 equivalent protons and gives intense single signal.
❑ Electronegativity of silicon is very low (1.8 as compared to
carbon 2.5). So, shielding is more than max. organic compounds.
❑ All signals appear downfield from TMS.
❑ Chemically inert, low Boiling Point (300k)
❑ TMS is not soluble in aqueous solution.
❑ For water soluble substance: DSS (2, 2 dimethyl-2-silapentane-5-sulphonate) is
used as reference compound.
 Since all the Hs are equivalent, a strong signal or peak is produced.
It is easy to recover as TMS is highly volatile.
 Silicon is poorly electro negative.
 Thus, the electron density around the protons is weakly attracted by silicon.

Hence, the electrons remain around the individual protons itself causing a powerful
shielding effect.
Hence, the protons resonate at high field (δ = 0) and all other protons resonate at
lower field than the protons of TMS.

FACTORS AFFECTING ON
CHEMICAL SHIFT
Electronegativity effects
»A proton is said to be deshielded if it is attached to the electronegative
atom or group.
»Greater is the electronegativity of the atom, greater is the deshielding
caused to the proton.
»Larger is the deshielding of a proton, larger is the chemical shift value
(δ).
Electronegativity increases – Deshielding increases.
Hydrogen Bonding
»In general hydrogen bonded protons show higher value of chemical
shift than the non-hydrogen bonded protons.
»Due to the high electro-negativity of the atom to which proton is
hydrogen bonded, the electron cloud around the proton is decreased
and thus causes deshielding of the proton.
Greater is the degree of hydrogen bonding of proton, greater the

Solvent Effects
»Chloroform (CDCl3) is the most common solvent for NMR measurements.
»Other deuterium labelled compounds, such as deuterium oxide (D2O),
benzene-d6 (C6D6), acetone-d6 (CD3COCD3) and DMSO-d6 (CD3SOCD3) are
also used as NMR solvents.
All these solvents vary considerably in their polarity and magnetic
susceptibility.
»Hence the NMR spectrum recorded in one solvent may be slightly different
from that recorded in other solvent.
» Hence it is important to mention the solvent used.
»The NMR signals for protons attached to carbon are generally shifted slightly
by changing solvent except where significant bonding or dipole-dipole
interaction might arise.

NMR - Nuclear magnetic resonance (NMR).pptx

NMR - Nuclear magnetic resonance (NMR).pptx

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