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Introduction to NMR Spectroscopy
• The study of absorption of radiofrequency radiation by nuclei in a magnetic field is called Nuclear Magnetic Resonance.
• When a charged particle such as a proton spins on its axis, it creates a magnetic field.
• Thus, the nucleus can be considered to be a tiny bar magnet.
• Normally, these tiny bar magnets are randomly oriented in space. However, in the presence of a external magnetic field, they can absorb electromagnetic radiation in radio-frequency region.
• They are oriented with or against this applied field.
• More nuclei are oriented with the applied field.
• The angular momentum associated with the spin quantum number, I (in unit of h/2п)
• The I for isotopes would be an integral values or a half-integral values.
• I value equal to zero indicates no spin.• Spin Number of isotopes determine by
these rules:1. Nuclei with even number of proton and
neutron=zero spin2. Nuclei with odd number of proton and
neutron=integral spin3. Nuclei with odd mass number= half-
integral spin
Spin quantum number for various nucleiNumber of protons
Number of Neutrons
Spin Quantum Number (I)
Examples
Even Even 0 12C, 16O, 32S
Odd Even 1/2 1H, 19F, 31P
" " 3/211B,35Cl, 79Br,
127I
Even Odd 1/2 13C
" " 5/2 17O
Odd Odd 1 2H, 14N
PRINCIPLE• Keeping the magnetic field
constant;• Varying the radiation frequency
through the substance.• Observing the frequency at
which radiation is absorbed is the principle way of NMR Spectroscopy.
PRINCIPLE• Keeping the radiation frequency constant;• Varying the magnetic field through the
substance.• At some value of the field strength where
the energy required to flip the proton matches the radiation energy,
• Absorption occurs and signal is observed.• Due to convenience it is the practical way
of NMR Spectroscopy.
Abso
rpti
on o
f rad
iati
on
Low field high field
Downfield direction
The NMR spectrum
Magnetic field sweep
INSTRUMENTATION• There are two general types of
NMR instrument; continuous wave and Fourier transform.
• Early experiments were conducted with continuous wave (C.W.) instruments, and in 1970 the first Fourier transform (F.T.) instruments became available.
• This type now dominates the market.
Schematic NMR Spectrometer
MagnetNormally superconducting. Some electromagnets and permanent magnets (EM-360, EM-390) still around.
RADIOFREQUENCY OSCILLATOR• The rf field is provided by a transmitter
coil whose magnetic vector component moves in a plane perpendicular to the direction of HO the rf field induces nuclear transition when its frequency equal to angular precissional velocity
RADIO FREQUENCY RECEIVER (DETECTOR)
• The flipping of nuclei as a result of irradiation induces a voltage in receiving coil
RECORDERThe voltage from the receiving coil is amplified and observed in a recorder. The peaks of an NMR spectrum are result of plotting intensity of absorption vs frequency of strength (field strength).
ADVANTAGES OF FTNMR• Rapid functioning with repetition of every 2
secs.• FTNMR can easily take: The spectra of 16 samples at very low conc. NMR studies on nuclei with very low natural
abundance (13C). NMR studies on nuclei with low abundance
and small magnetic moments (13C,15N).
NMR SPECTRUM• All protons absorbed at the
same effective field strengths but they absorb at different applied field strengths.
• The applied field strength is measured and the graph is plotted between the applied field strength and the absorption.
• This plot is called NMR spectrum.
THE NUMBER OF THE SIGNALS• The number of signals depends on
the number of equivalent protons (hydrogens)
• In a molecule, protons in the same magnetic environment absorb at the same applied magnetic field strength
• For example: CH₃-CH₂-Cl CH₃-CH₂-CH₂-Cl
2 NMR signals 3 NMR signals Ethyl Chloride n-Propyl chloride
THE POSITION OF THE SIGNALS
• The position of a signal in the spectrum helps to reveal
• What "type" of proton(s) gives rise to the signal. The
• position of a signal – its chemical shift – is measured in ppm (parts per million) relative to the proton signal
• Equivalent protons have the same chemical shift.
• Also, protons in similar environments, but in different molecules, will absorb at about the same place in the spectrum.
REFERENCE POINT• The reference point from which chemical shifts
are measured is, for practical reason, not the signal from the naked proton but the signal from the actual compound, usually Tetra Methyl Saline (TMS) is used because;
TMS is chemically inert, Has low boiling point, Easily removed from a recoverable sample of
valuable organic compound, Soluble in most organic solvents, Can be added to the sample solution as an
internal standard, TMS is not soluble in H20 or D2O, for solution in
these solvent the sodium salt of 3propane sulfonic acid is used.
Scale• The most commonly used scale is the δ
(delta) scale on which the TMS signal is taken as 0.0ppm.Small δ value = Small down field shiftLarge δ value = Large down field shift
• There is another scale known as τ (tau) on which the TMS is taken as 10.0ppm.
• The two scales are related by the expression
τ = 10-δ
INTENSITIES OF THE SIGNALS (PEAK AREA AND PROTON
COUNTING):Consider NMR spectra of toluene and p-xylene
(p-xylene) Each compound possess two type of proton:(1)Methyl (2)aromatic protonsThese protons shows two signal in NMR spectra nearly δ 2.3 and δ 7.2 valuesIntensities of methyl proton and aromatic proton signal in NMR spectra on comparison based on the areas under the peak show that they have peak in 3:5 of toluene while 6:4 (3:2) of p-xylene Area under NMR signal are measured by electronic integrator and are usually given on the spectrum chart in the form of stipped curve.
THE SPLITTINGS OF THE SIGNALS• (SPIN-SPIN COUPLING)• Splitting is a phenomenon which result by the
interaction of proton(H)with the adjacent proton in a compound or molecule.
E.g.: 1,1,2tri bromomethaneBr-CH-CH2-Br
• When secondary proton feel magnetic field by the spin of neighbouring tertiary proton inc if the tertiary proton is aligned with the applied field r dec if tertiary proton against the field
• For half the molecule absorption by a secondary proton shift downfield and other half of the molecule the absorption shift up field the signal is split into two peaks a doublet with equal peak intensities
• Similarly absorption of tertiary proton is affected by the spin of the neighbouring secondary proton…
Sample Handling• The sample which are use in NMR are must
be clear liquid or non-viscous• Material which are use they should be near
to liquid• The solvent which are use they must be not
contain any of that molecule which may contain proton(H) because in process this proton give there peak which cause error so avoid the use of such molecule like alcohol, water so we prefers the solvent like deuterated chloroform(CDCL3), deuterated benzene(C6D6), D2O, D6-DMSO (Dimethyl sulphoxide)in order to diminish the error.
APPLICATIONS OF NMR• The most important application of proton
NMR is identification, structural elucidation of organic, metal-organic and biochemical molecules.
• It is use for the identification of compounds.• It is useful in quantitative analysis of
absorbing species.• The number of nuclei in the spectrum α the
peak area.• NMR is useful in determination of functional
groups such as Aldehydes, Ketones, carboxylic acids, alcohols and phenols.
Elemental Analysis• Elemental analysis is done for
the elements like carbon, hydrogen, nitrogen.
• Sometimes it is done for the analysis of elements like sulpher and oxygen.
• NMR spectroscopy can be employed to determine the total concentration of given kind of magnetic nucleus in a sample.
Carbon13 NMRThere are two basic advantages of C13 NMR:• Provide information about the backbone of molecule (proton NMR gives periphery information.)
• The scale limit in C13 NMR is about 200ppm (proton NMR limits about 10ppm – 15ppm)
CONCLUSION
ANY QUESTION??
?
THANK YOU
