Transcript 1_NMR_ElementaryDefinition

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FT NMR WORKSHOP/===/ S.A.I.F./===/ NEHU/==/ Shillong
INTRODUCTORY LECTURE
S.ARAVAMUDHAN
Distinct Spectroscopic context for the NMR Spectroscopy
A quick exposition of the Magnetic Resonance Phenomena
An animated elaboration of the Single spin Interactions
Concluding Slide with a Definition of N.M.R.
Slides # 2- 4
Slide # 5
Slides #6-7
Slide # 8
In a Scheme of an Atom
Electrons circulate in Orbits
Chemical Molecular structure
depends on the electronic structural
changes due to bonding between
atoms. Nucleus plays no role in
determining the optimum geometry
except that they get an assigned
place as they occupy in the
molecule.
Molecular spectroscopic studies
involve studies assuming an
equilibrium structure, but the
molecular phenomenon responsible
for spectroscopic absorptions
require changes in the electron
dispositions around the equilibrium
geometries
NUCLEUS is stationery at the center
One of the aspect to be reckoned with is the fact that all the
chemical consequences are because of the electrons present
in the elemental atom or ion because of which these elements
exhibit chemically binding characteristics.
It is known that the atoms and ions of such elements
have their characteristic nucleus around which the
electrons of the system revolve in orbits. For the
chemical consequences there is not any significant
role assigned to the nuclear characteristics unless it is
a radio active element and the nuclear radiations can
make it possible to be tracked by radio active tracer
techniques.
The radio activity itself can be hazardous
besides the toxic effects of such elements
by chemical reactivities
3
When all the extra nuclear electrons
are in such continuous motion and
participate in the bonding, if there
can be a stimulation of nucleus,
which does not in any way affect the
electronic dispositions, then, would
that be useful studies of molecular
electronic structure?
In magnetic resonance, the nuclei are
stimulated in such a way that the
electronic dispositions are influenced by
the perturbation so that the resulting
stimulated response is indicative of the
specifics about equilibrium free electron
dispositions in the molecular structures
This unique value of ‘γ’ for each
element’s nucleus different from
every other element is what makes
multi nuclear NMR possible
Magnetic
moment
Field
Spin=1/2
+1/2
-1/2
Energy
Magnetic
moment
+1/2
-1/2
SPIN
SPIN
These tiny
nuclear magnetic
moments are
similar to the bar
magnets which
are influenced by
Externally applied
magnetic fields
Spin Quantum
number value= 3/2
The value of
γ
differs from one
nucleus to the other.
The central nucleus of
some of the elements posses intrinsic spin
and hence also can be
associated with a
magnetic moment
2π ν = γ H
Electromagnetic radiation with
frequency ν can cause
transition between these levels
and this is the resonance
phenomenon
Similar effects are possible with electrons
also but only in PARAMGNETIC IONS or
Applied
Molecules when there are unpaired electrons
magnetic
present. Here the reference is only to
Diamagnetic Samples (compounds) to
field
introduce NMR exclusive of any other
Discrete orientations
effects.
RESULTS in discrete
energy levels
Illustration is
a case of
or SPIN=3/2
results in 4
equally
or spaced
energy
levels
or
Only Discrete
orientations of
the spin are
possible due
to the
quantization
criteria at
atomic
regimes
Spin and Magnetic moment:
Illustration:
Nucleus rotates about an axis
within itself, which is referred to
as the spinning of nuclei.
Spin axis
The spinning object has an angular momentum
Nucleus has electrical Charge. Thus a rotating
charge has associated magnetic moment.
Spin Angular
momentum
The NUCLEI thus possess angular momentum; and the
angular momentum in atomic system are quantized. Due to
this quantization, the angular momentum component in any
chosen direction can take only specified discrete values.
NOTE: that the
single spin is
invariably
associated with
an XY component
perpendicular to
the magnetic field
direction
Ih
+1/2ħ
For protons: Spin
quantum number=1/2
The angular momentum
and magnetic moment
are in the same direction
because of the positive
charge of the nucleus
In the quantized
orientations the spins
experience a torque due
to which they must be
precessing (Larmor
precession)
±1/2
-1/2ħ
Ih
Magnetic Field
Electromagnetic
radiation causes
the spin flipping
A single spin description cannot be complete
without the associated XY component
The Larmor precession
frequency depends on the
strength of external field
+1/2 ħ
hν=gβH
If a rotating magnetic field of
relatively small magnitude is
present in the perpendicular
plane at frequency ν , then the
resonance occurs and the spin
undergoes a flipping transition to
another orientation.
-1/2 ħ
Photon energy absorbed;
transition occurs
Radiation
Quantized
Energy
level
descriptio
n
Field
Spin=1/2
+1/2
-1/2
hν=gβH
+1/2
-1/2
For proton spin of
½, there are two
allowed
orientations so that
the component
along z-axis is
either +1/2 or -1/2
+1/2 ħ
Vector
diagramatic
description
hν=gβH
+1/2 ħ
Induced Transition or stimulaed transition
Energy
-1/2 ħ
Lower
energy
Upper
energy
A definition:
Magnetic Resonance Phenomenon is
a manifestation due to the presence of
INTRINSIC SPIN angular momentum
and the associated Magnetic Moment
characteristically in electrons and
Nuclei
Single line
NMR spectrum
When the experimental conditions are
set for the NUCLEI to resonate, then it
is the Nuclear Magnetic Resonance.
NMR spectrum of a sample of
spin ensemble……..
10ppm
PMR
spectrum
0 ppm
TMS