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Presented by
Suraj N. Wanjari
M.Pharm. 1st Sem. Pharmaceutical Chemistry,
University Department Of Pharmaceutical Sciences(U.D.P.S.).
Rashtrasant Tukadoji Maharaj Nagpur University,
Nagpur-440 033.
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• Introduction
• Spin quantum number
• Principle
• NMR Spectrum
• Instrumentation
• Solvent requirement in NMR
• Relaxation process
• Chemical shift
• Factors influencing chemical shift
• Coupling constant
• Electron nuclear double resonance
• C-13 NMR
• Refrences
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• Nuclear Magnetic Resonance is a form of absorption spectroscopy.
• It is concern with the absorbtion of radio frequency wave region 4 to 900 Hz by spinning
nuclei in a magnetic field when irradiated by certain energy radiation perpendicular to it .
• Absorption occurs when nuclei
undergoes transition from one
alignment in the applied field to the
opposite one .
• It thus permits identification
configuration in the molecule.
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• Spin angular momentum is a spinning charge generates a magnetic field, the
resulting spin-magnet has a magnetic moment (μ) proportional to the spin (I) .
• According to the quantum theory , a spinning nucleus can only have values for the
spin angular momentum given by the equation .
• Spin angular momentum=[I(I+1)]1/2 h/2π
where , I- spin quantum number
h- planks constant
magnetic moment (μ) = y Χ Spin angular momentum
where , y- gyromagnetic ratio
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5 • The gyromagnetic ratio or magnetogyric ratio of a particle or system is the ratio
of its magnetic moment to its angular moment .
• When nuclei place in the magnetic field the energy level become separated
corresponding to m1 = -1/2 which is antiparellel to the direction of magnetic fild
and the m2 = +1/2 which is parallel to the magnetic field for nucleus having spin
I= ½ the energies E1 and E2 .
E1 = - ½ { yh/ 2π } HO
E2 = + ½ { yh/ 2π } HO
Where
H0 = magnetic field
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6 • The frequency at which energy absorbed or emitted is given by the Bohrs
relationship .
V= E1- E2 / h
where h – planks constant
• Larmor equation follows that frequency absorb or emitted by a nucleus in moving
from one enrgy level to another is directly proportional to the applied magnetic field
.
V= y/2π H0
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• Odd mass nuclei with an odd number of nucleons have fractional spins.
• Even mass nuclei with odd numbers of protons and neutrons have integral
spins.
• Even mass nuclei composed of even numbers of protons and neutrons
have zero spin.( NMR inactive )
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• The principle is based on the spinning of nucleus and generating a magnetic
field.
• Without external magnetic (Bo) – field nuclear spin are random in direction.
• With Bo ,nuclei align themselves either with or against field of external magnetic
field.
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9 • If external magnetic field is applied , an energy transfer (∆E ) is possible between
ground state to excited state .
• When the spin returns to its ground state level , the absorbed radiofrequency
energy is emitted at the same frequency level .
• The emitted radiofrequency signal that gives the NMR spectrum okf concerned
nucleus .
• The emmited radio frequency is directly proportional to the strength of applied field
.
v= y/2π H0
y- gyromagnetic ratio
H0 = magnetic field
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NMR spectrum is a plot of intensity of NMR signals VS magnetic
field (frequency) in reference to TMS.
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1. Sample holder :- Glass tube with 8.5 cm long to 0.3 cm in diameter .
2. Permanent magnet :- It provide homogeneous magnetic field at 60 -100 MHz .
3. Magnetic coils :- These coils induce magnetic field when current flows through
them .
4. Sweep generator :- To produce equal amount of magnetic field pass through
the sample .
5. Radio frequency transmitter :- A radio transmitter coil that produces short
powerful pulse of radio waves.
6. Radiofrequency receiver :- A radio receiver coils that detect Receiver radio
frequencies emitted as nuclei relax to a lower energy level .
7. Readout Device :- A computer that analyses and record the data .
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• Proton free solvent used which does not show absorbtion of its own in NMR spectrum
• The solvent should be capable of dissolving at least 10 % of substance under
investigation .
• Characteristic of solvent used
a- Chemically inert and magnetically isotropic.
b- It should be devoid of of hydrogen atom.
c-it should dissolve the sample to a reasonable extent.
• An hydrogen bonding involve electron cloud transfer from hydrogen tom to
electronegative atom (O,N,S,) , the hydrogen atom experience deshilding .
• With increase in tempreture the extent of hydrogen bonding wan fall result in NMR apear
small δ value .
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Theire are following solvent was used in NMR spectroscopy...
a- Carbon tetrachloride-(CCL4)
b- carbon disulphide – (CS2 )
c- Deuterochloroform – CDCl3
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• Its the non-radiative transition where the nucleus in the upper transition state
returns to the lower spin state.
• Theire are three kind of relaxation processes
a- Spin-Spin relaxation – Its due to mutual exchange of spin by two
processing nuclei which are close proximity to each other.It involve transfer of
energy from one nucleus to other .
b- Spin- lattice relaxation – It was also known as longitudinal relaxation . It
involve the transfer of the energy from the nucleus in its higher energy state
to the molecular lattice . An efficient relaxation process involves a short time
and result in the broadning of absorption peak .
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c- Quadrapole relaxation-
It is prominent relaxation process for nuclei having I > ½ . The nuclei
(such as 14N , 17O , 11B etc ) due to anisotrophic interaction between non-
spherical , electrically quadrapole nuclei and the electric field gradients at
the nucleus caused by electric environments posses an assymetric
positive charge distribution on the nuclei. Hence these nuclei exhibit
electric quadrapole moment and relaxes rapidly . This process known as
electric quadrapole relaxation .
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• Rotation of electrones about the near by nuclei generates a field that can either
oppose or reinforce the applied field at the proton .
• If the induced field apposes (Diamagnetism) the applied , then the proton is said to
be shielded .
• But if the induced field reinforces applied field ( Paramagnetism) , the proton feels
a higher field strength and thus, and thus such proton is said to be deshielded .
• Shielding shifts the absorption upfield and deshielding shifts the absorption
downfiled to get an effective field strength neccesary for absorption .
• Tetramethyl silane is taken as standared because of theire less electronegativity .
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The difference in the absorption position with respect to TMS signal is called as chemical
shift .
Delta (δ) and tau (τ) scale was commonly used for the measurement of chemical shift .
δ express as parts per million and for TMS it taken as 10 and
τ = 10 – δ .
V sample – V refrence
operating frequency in megacycles
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a- Inductive effect – a proton said to be deshielded if it attached with the
electronegative atom , greater is the deshielding caused to the proton . If deshielding
more for the proton then delta value should be more.
b- Van der Waal’s deshilding - Electron cloud of a bulky group will tend to repel the
electron cloud surrounding the proton . Thus such a proton deshielded and will
resonate at slightly higher value of delta than expected in the absence of this field .
c- Anisotropic effect – The deshielding effect on protons attached to c=c is higher
than that can be accounted for by the inductive effect alone . Aldehyde and aromatic
protons are much more deshielded . Alkyne protons appear at relatively low value of
σ .
d- Hydrogen bonding – If hydrogen atom exhibits the property of hydrogen bonding
in a compound, it will get deshielded due to the strongly electronegative atom
attached to it . As such absorption shif is downfield .
e- Tempreture and concentration of solution
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• 1H nuclei are shielded by the magnetic field produced by the surrounding
electrons. The higher the electron density around the nucleus, the higher the
magnetic field required to cause resonance.
Information from 1H-nmr spectra:
1. Number of signals: How many different types of hydrogens in the molecule.
2. Position of signals (chemical shift): What types of hydrogens.
3. Relative areas under signals (integration): How many hydrogens of each type.
4. Splitting pattern: How many neighboring hydrogens
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–
• The distance between the peaks is a given multiple measure of the magnitude
of splitting effect. It is referred to as coupling constant and it is denoted by
symbol J .
• Numerical value of J expressed in Hz or cps .
• Unlike the chemical shifts , the value J was independent of the of the applied
field strength and dependent only upon the molecular structure .
• Spin-spin coupling is the interaction between the spin magnetic moments of
different electrons and/or nuclei.
• In NMR spectroscopy it gives rise to multiplet patterns, and cross-peaks in
two-dimensional NMRspectra. Between electron and nuclear spins this is
termed the nuclear hyperfine interaction. Between electron spins it gives rise
to relaxation effects and splitting of the EPR spectrum.
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Provides information about the electronic and molecular structure of paramagnetic
metal centers. Measurement of the spin state, S, the magnitude of hyperfine
interactions with metal and legend nuclei, and the zero-field splitting of half-integer
S > 1/2 electronic states, allows a researcher to identify the paramagnetic center,
and to potentially identify legating atoms.
Electron nuclear double resonance (ENDOR) is a magnetic resonance
technique for elucidating the molecular and electronic structure of paramagnetic
species. The technique was first introduced to resolve interactions in electron
paramagnetic resonance (EPR) spectra.
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• The 13C nucleus is present in only 1.08% natural abundance. Therefore,
acquisition of a spectrum usually takes much longer than in 1H NMR.
• The magnetogyric ratio of the 13C nucleus is about 1/4 that of the 1H nucleus.
Therefore, the resonance frequency in 13C NMR is much lower than in 1H NMR.
(75 MHz for 13C as opposed to 300 MHz for 1H in a 7.04 Tesla field).
• At these lower frequencies, the excess population of nuclei in the lower spin state
is reduced which in turn reduces the sensitivity of NMR detection.
24. • Unlike 1H NMR, the area of a peak is not proportional to the number of
carbons giving rise to the signal. Therefore, integrations are usually not
done.
• Each unique carbon in a molecule gives rise to a 13C NMR signal.
Therefore, if there are fewer signals in the spectrum than carbon atoms in
the compound,the molecule must possess symmetry.
• When running a spectrum, the protons are usually decoupled from their
respective carbons to give a singlet for each carbon atom. This is called a
proton-decoupled spectrum .
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• Principle of Instrumental Analysis by D.A. Skoog , F. J. Holler and S . R. Crouch seventh
edition Page number 453 – 493 .
• Instrumental method of chemical analysis by G.R.Chatwal and S.K. Anand fifth edition
page number 2.185 to 2.234
• Elementary organic spectroscopy by Y.R. Sharma page number 191 to 250
