Download 0_2_NMR Spectroscopy_1

Nuclear Magnetic Resonance
ANIMATED ILLUSTRATIONS
MS Powerpoint Presentation Files
Uses Animation Schemes
as available in MS XP or MS 2003 versions
A class room educational material
http://ugc-inno-nehu.com/links_from_web.html
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
Single NMR line
Spectral features
Besides these
individual line
features, it is the
characteristic
groups and patterns
of lines in a
spectrum which is
useful for structure
elucidation and the
INTEGRATED
INTENSTIEIS
Amplitude
Δ=FWHH
Full Width at
Half Height
δ0
Line position
Reference line
δ ppm
0 ppm
This NMR spectral line can be attributed with a characteristic
shape and is describable by a mathematical equation.
There are in general two distinct kind of shapes known for the
spectral lines 1. Gaussian line shape 2. The Lorentzian shape.
Both are typically symmetric line shapes:: symmetric about the line
centre
NOTE that the previous slide contained data for ethyl
alcohol spectrum in acidic medium
medium.
Deuterium exchange can cause the Proton
signal to disappear
The problem seems to be that the position of the -OH peak varies dramatically
depending on the conditions - for example, what solvent is used, the concentration,
and the purity of the alcohol - especially on whether or not it is totally dry.
The above is in CDCl3
NMR spectrum for an alcohol like ethanol: When a few drops of
deuterium oxide, D2O, is added to the solution, and allowed to
settle, the -OH peak disappears!
2nI+1 and
with I=1/2
2-spins
cause a line
to split into 3
lines
3-Spins cause
a line to split
into 4 lines
0 singlet
1
1 doublet
1 1
2 triplet
1 2 1
3 quartet
1 3 3 1
4 pentet
1 4 6 4 1
5 sextet
1 5 10 10 5 1
6 septet
1 6 15 20 15 6 1
7 octet
1 7 21 35 35 21 7 1
8 nonet
1 8 28 56 70 56 28 8 1
This phenomenon has been found to be capable of revealing the nature of
nuclear environments in molecules (chemical compounds) because of the
changes in the electronic structures due to the bonding criteria for the atoms
forming the molecules.
These are essentially the variations in the resonance frequencies due to
electron circulations within molecules. And these variations called ‘Chemical
Shifts’ are in the order of parts per million of the applied field/frequency.
1H;
1 ppm (δ) = 1 part per million
( 1 part in 106 ) of the
Spectrometer frequency ν0 .
The spectrometer
frequencies can be
usually in MHz mostly
above 200 MHz ( 1 MHz =
106 ) for protons
I=1/2
13C
I=1/2
15N
I=1/2
At 300 MHz 1ppm =300 Hz
At 400MHz
1ppm =400 Hz
At 75 MHz
1ppm = 75Hz
At 100 MHz
1ppm= 100Hz
At 30 MHz 1ppm = 30 Hz
At 40 MHz 1ppm= 40 Hz
Other nuclei commonly chosen for NMR study are 31P (I=1/2), 19F = (I=1/2)
NMR of several of the other nuclei occurring in the chemical periodic table can
be studied with a multi nuclear facility
Thus if proton nuclei has a characteristic resonance frequency of
300MHz corresponding to a applied magnetic field of 7.05 Tesla, then
the total range for the variation of the proton resonance frequency due
to differences in molecular electron circulations (the Chemical shift
range) is 10ppm. This corresponds to a total variation of 3 KHz in 300
MHz (since 1ppm=300Hz).
This implies a stringent stability criterion for the Magnetic field
and RF frequency sources and the required ratio must be also
maintained to the same accuracy to obtain reliable readout
parameters from the spectrum obtained from spectrometers. This
is the requirement of field-frequency lock in NMR spectrometers.
With that good stability ensured, the magnetic field must be
shimmed to get high homogeneity of the field in the sample
region. By such techniques a reproducibility of the chemical shift
to the accuracy of 0.0001ppm are possible with the current
generation of spectrometers.
1cc of water contains proton spins of the order of 1022 spins and the
actual sample of water in the detectable region of nmr-probe would
contain about 1021 spins corresponding to 100 micro liter of water
sample.
A typical spectrometer of the 300MHz frequency can detect
conveniently a spin count of 1018 which amounts to volumes in a
few ‘micro liter’ range.
But the present generation of Spectrometers at as much high
field as corresponding to 900MHz can be sensitive enough
detect 1011 spins which in terms of sample volume in the ‘pico
liter’ range.
All this is due to the advances in instrumentation on the rf detection side during
the continuous wave mode of NMR detection and subsequently and the
improvements in tuning of sample coils simultaneously used for the transmitter
and receiver purposes with High Power [up to 3KW peak power for solid
samples] pulsing and detecting the response, possibly in the range of 10μv,
induced RF in the coil due to NMR induction in pulsed NMR detection. Up to
100MHz proton resonance frequency, Electromagnets (23 KG) can be used but
for fields higher than this value Supercon Magnet Systems (with
superconducting current carrying elements) are necessary. The possibility of
realizing superconducting magnet systems has brought about a total revolution
in what was possible by NMR Spectroscopic Technique.
Ethyl benzene
Ethyl benzene