NMR engineering chemistry pdf btech NMR

Nuclear Magnetic Resonance (NMR)
Nuclear magnetic resonance (NMR) is a physical phenomenon in
which nuclei in a magnetic field absorb and re-emit
electromagnetic radiation.
Nobel Prize for Physics in 1952
Felix Bloch, Stanford University, 1945 Edward Purcell, Harvard University, 1945
“Development of new methods for nuclear magnetic precision measurements"

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 Molecular structural determination
 Study of molecular dynamics
 Characterization of molecular materials
Structure of protein's and microorganisms etc.
NMR Spectroscopy has become an indispensable tool for
chemists, physicists and biologists…

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Theoretical Principles
1. Nuclear Spin
 No. of neutrons & No. of protons are both even - No Nuclear Spin
 No. of neutrons & No. of protons are both odd - Integer Spin
(1, 2, 3 etc.)
 No. of neutrons or No. of protons odd - Half-integer Spin
(1/2, 3/2, 5/2 etc.)x
NMR Active Nuclei
1H1 , 2H1 , 13C6 , 14N7 , 19F9 , 17O8 , 31P15 , 29Si14

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Group I - Nuclei with both At No. and No. of neutrons even
eg. 12C, 16O, 18O, 32S
Group 2 - Nuclei with Both At.No. and No. of neutrons odd
eg. 2H (I = 1), 10B (I = 3), 14N ( I=1)
Group 3 - Nuclei with Even At.No and Odd Neutrons or
vice versa
eg. 1H, 11B (I=3/2), 13C, 15N, 17O (I=5/2), 19F, 29Si, 31P
Classification of Nuclei

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Isotope Abundance Z
(At.No)
N
(Neut)
A (Mass
No)
I g
neutron - 0 1 1 1/2 -183.26
1H 99.985 1 0 1 1/2 267.512
2H 0.015 1 1 2 1 41.063
13C 1.108 6 7 13 1/2 67.264
17O 0.037 8 9 17 5/2 -36.27
31P 100 15 16 31 1/2 108.29
29Si 4.70 14 15 29 1/2 -53.142
Magnetic Properties of Selected Nuclei

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2. Nuclear Spin States
No. of spin states = 2I + 1 where I is the nuclear spin
+I, (I-1),…..(-I+1), -I
H - I = ½; Spin states: -1/2, +1/2
Cl - I = 3/2; Spin States: -3/2, -1/2, +1/2, +3/2

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Pieter Zeeman: 1902 Nobel Prize in Physics
with Hendrik Lorentz for his discovery of the
‘Zeeman effect’

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3. Nuclear Magnetic Moments
In an applied magnetic field, all protons have
their magnetic moments either aligned with
the field or opposed to it.
In the absence of an applied field, these
protons behave like bar magnets that are
randomly oriented.
The nuclear magnetic moment is the magnetic moment of an
atomic nucleus and arises from the spin of the protons and neutrons.

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• The spin states are not of equivalent energy
• The nucleus being a charged particle with a spin
generates a magnetic moment m
• For H-atom the two magnetic moments generated
by the two spin states are aligned with the applied
magnetic field or opposed to it
• The spin state +1/2 is of lower energy (aligned with
the field and -1/2 is of higher energy (aligned against
the applied field)

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Spin States of proton in the absence and presence
of an applied magnetic field

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4. Absorption of energy
Nuclear Magnetic Resonance Phenomenon occurs when nuclei aligned with
the applied magnetic field are induced to absorb energy, and change their spin
orientation with respect to the applied field
Energy absorbed is quantized = energy difference between the two states
Energy abs = E-1/2 state - E+1/2 state = hn
The spin state-energy separation as a function of the applied magnetic field; Bo

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The stronger the applied magnetic field, the greater the energy difference
DE= f (Bo)
Magnitude of energy separation depends on the particular nucleus involved
(magnetogyric ratio – ratio of the magnetic moment to angular momentum)
DE= f (gBo) = hn
Angular momentum of the nucleus is quantized in units of h/2p
DE= g (h/2p)Bo = hn
The spin state-energy separation as a function of the applied
magnetic field; Bo
n = (g/2 p) Bo

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Frequency of radiation Field strength Bo
42.6 MHz 1 T (10,000 Gauss)
60.0 MHz 1.4 T (14,000 Gauss)
100 MHz 2.35 T
300 MHz 7.05 T
500 MHz 11.1 T
n = (g/2 p) Bo
g 1H = 267.54 rad/T; Bo = 1T
n = 267.54 X 1 / 2 X 3.14 = 42.6 MHz

5. Nuclear Magnetic Resonance
When placed in a magnetic field and irradiated with radio
frequency energy, these nuclei absorb energy at frequencies
based on their chemical environments
In the applied magnetic field, nuclei begin to precess
A top precessing in earths gravitaional field
Precession of a spinning nucleus in an
applied magnetic field
Angular Frequency w or Precessional Frequency or Larmor Frequency
Precessional Frequency α Bo

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Precession of the nucleus generates an oscillating electric field of the same frequency
When a radiofrequency wave of this frequency is supplied to the precessing nucleus,
energy can be absorbed
Frequency of the oscillating electric field component of incoming radiation
= frequency of the electric field generated by the precessing nucleus
---- energy transfer to the nucleus -- spin change ---- resonance
Resonance Phenomenon

 The nucleus in the applied magnetic field precess with
angular frequency w -Larmor Frequency
 Larmor Frequency is directly proportional to field strength
Precession generates an oscillating electric field of same
frequency
If radioenergy of same frequency is supplied to the nucleus
Energy can be transferred from the incoming radiation to
the nucleus, causing a spin change -- Resonance - the
nucleus is in resonance with the incoming radiofrequency.

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 Nuclei with spin behaves like a bar magnet and has nuclear magnetic
moment
 In the absence of a magnetic field, the spin states are degenerate – of
equal energy. The spins cancel each other and will have zero magnetic
moment
 When placed in an external magnetic field, the spin states loose their
degeneracy and get separated and the nuclei begin to precess with a
frequency which is directly proportional to the external magnetic field.
When radiofrequency equal to the precessional frequency is applied,
absorption of Energy takes place
DE= g (h/2p)Bo = hn
Recap

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Shielding Effect
Protons in different environment are shielded differently by the
electrons surrounding them.
Diamagnetic anisotropy: Diamagnetic shielding of a nucleus caused
by the circulation of valence electrons.

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Chemical shift
S-electrons - spherical symmetry - secondary magnetic field
opposing the magnetic field - nuclear shielding - upfield shift
(diamagnetic shift)
P-Orbitals - no spherical symmetry - produces comparatively
Large magnetic fields at the nucleus - deshielding -
(paramagnetic shift)

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The chemical shift is the resonant frequency of a nucleus relative
to a standard in a magnetic field
Chemical shift
Exact Resonance Frequency depends on the electronic
environment of the nucleus
Beff = Bo -- sBo s= shielding constant
Larmor precession frequency (νo)
Standard: Tetramethylsilane (TMS)
Stronger the nucleus is shielded, larger s is, smaller the Beff becomes
For constant frequency the applied field Bo must be larger for resonance

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Factors Influencing Chemical Shift
Electronegativity
The greater the electronegativity of the atoms attached
to carbon atom, the more the deshielding, thus greater
is the chemical shift of the protons
CH3F>CH3Cl>CH3Br>CH3I>CH3Si –
(4.26>3.10>2.65>2.16>0)
R4N+ - Highly deshielded (high d value)
Carbanionic
centres - Highly shielded (low d value)

B. Hybridization effects
sp3 hydrogens
sp2 hydrogens
Hydrogens attached to the sp2 carbons have greater chemical shift than
hydrogens on aliphatic hydrogens on sp3 carbon

C. Acidic and exchangeable hydrogens:; hydrogen bonding
Acidic protons
Hydrogen bonding and exchangeable hydrogens

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Anisotropic Effects
 p Electrons circulate under the influence of applied magnetic field and
Create a secondary field which can either oppose or augment it.
Causes shift to higher frequency (downfield shifts or diamagnetic shift)
Or
To low frequency (upfield shift or paramagnetic shift) – Anisotropic Effect

Magnetic Anisotropy in Benzene and Acetylene
The protons on the periphery of the benzene ring happen to lie in a deshielding region of
the anisotropic field; this gives the benzene protons a d value that is greater than expected
Diamagnetic anisotropy in benzene
Anomalous shift in resonance values is due to the presence of an
unsaturated system in the vicinity of the proton in question.

Example…….
1H NMR spectrum of a-chloro-p-xylene

Diamagnetic anisotropy in acetylene
In acetylene, the magnetic field generated by induced circulation of the p
electrons has a geometry such that the acetylenic hydrogens are shielded.
Hence they have a resonance at a higher field than expected.

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Integrals and Integration
 NMR spectrum not only distinguishes the different types of protons in a
molecule, but also reveals how many of each type are contained within the
molecule.
 The area under each peak is proportional to the number of hydrogens
generating that peak.

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Spin-spin splitting (n+1) rule
Multiplicity of lines is related to the number of neighbouring protons
n+1 rule: Each type of proton “senses” the number of
equivalent protons (n) on the carbon atom(s) next to the
one to which it is bonded, and its resonance peak is split
into (n+1) components.

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The intenisty ratios of multiplets derived from the n+1 rule follow the entries in the
mathematical mnemonic device called Pascal’s triangle
Pascal’s Triangle
Spin-spin splitting (n+1) rule
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

The origin of spin-spin splitting
Spin-spin splitting arises because hydrogens on adjacent carbon
atom can “sense” one another

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COUPLING CONSTANTS (J)
 The distance between peaks in a multiplet is called coupling constant
‘J’
 ‘J’ is a measure of how strongly a nucleus is affected by the spin
states of its neighboring nuclei.
 ‘J’ is expressed in Hertz (Hz).
 ‘J’ of a group of protons that split one another must be identical.

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