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Nuclear Magnetic Resonance Spectrometry
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Identification of Compound
H2
CH3
CH3
CH3
H3C
H2 H
HH
HH
O
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Visible
Ultra
violet
Radio
Gamma
ray
Hz
cmcm-1Kcal/mol
Electron
volts,
eV
Type
Quantum Transition
Type
spectroscopy
Type
Radiation
Frequency
Wavelength
Wave
Number VEnergy
9.4 x 107 4.9 x 106 3.3 x 1010 3 x 10-11 1021
9.4 x 103 4.9 x 102 3.3 x 106 3 x 10-7 1017
9.4 x 101 4.9 x 100 3.3 x 104 3 x 10-5 1015
9.4 x 10-1 4.9 x 10-2 3.3 x 102 3 x 10-3 1013
9.4 x 10-3 4.9 x 10-4 3.3 x 100 3 x 10-1 1011
9.4 x 10-7 4.9 x 10-8 3.3 x 10-4 3 x 103 107
X-ray
Infrared
Micro-
wave
Gamma ray
emission
X-rayabsorption,
emission
UV absorption
IR absorption
Microwave
absorption
Nuclear
magnetic
resonance
Nuclear
Electronic(inner shell)
Molecular
vibration
Electronic
(outer shell)
Molecularrotation
Magnetically
induced spin
states
Spectral Properties, Application and Interactions of Electromagnetic Radiation
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4
Number of signals
Position of signals
Intensity of signals
Splitting of signals
CH3-CH2-CH2-CH2-CH2-CH=CH-CH2-CH=CH-CH2-CH2-CH2-CH2-CH2-CH2-CH2-COOCH2
HOCH
HOCH2
4 314222 22124
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Nuclear Spins in Absence (a) and
Presence (b) of External Magnetic Field
CH3-CH=CH-CH2-CHO
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Methyl Ester of Fatty Acid
R C H C H C H2 C H C HC H
2CO
O C H 3
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Nuclear Magnetic Resonance Principle
Spinning charge in proton
generates magnetic dipole momentProton precess in a
magnetic field Ho
Ho
Precession orbit of nuclear mass
(Precession angular velocity or
precession frequency = W0)
Spinning proton
Nuclear magnetic dipole moment
= hI
W0 = 2 v
Ho
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Principles of NMR
Energy Difference
Precession -Energy RelationshipOscillator generates rotating component of magnetic field
Ho
Oscillator Coil (Radio Frequency)
Axis of nuclear rotation
High energy precession
Low energy precession
Reference axis
Nuclear Spin
(Dipole moment)
Precessional orbitLow energy spin state (+1/2)
Precessional orbit
High energy spin state (-1/2)
CH3-CH=CH-CH3
Ho
2 radian/sec = 1 Hz, 1 sec = 3 1010 cm
CH4
Nuclear Spin
(Dipole moment)
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Magnetic Properties of Nuclei
Nuclei of certain atoms posses a mechanical spin
or angular momentum. The angular momentumdepends on the nuclear spin or spin number. The
spin number (I) is related to the mass number and
atomic number.
Magnetic nucleus may assume any of (2I+1)
orientation with respect to the direction of the
applied magnetic field
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Theory of NuclearResonance
A proton in a external magnetic field assumes only twoorientations corresponding to +/- uH. It is possible toinduce transitions between two orientations.
The frequency (v) of electromagnetic radiationnecessary for such transition is given by v=2uH/h whereH is the strength of the external magnetic field.
The precession frequency of the spinning is exactlyequal to the frequency of electromagnetic radiationnecessary to induce a transition from one nuclear spin
state to another.
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Theory of NuclearResonance
There is slightly excess of nuclei in the low spinstate compared to high spin state. Boltzmansdistribution (low spin state / high spin state is
1.00001. This very small excess of lower energystate gives rise to net absorption of energy in theradio frequency. Without this small excess, therewould be no NMR.
Spin-spin relaxation and spin lattice relaxation
Lattice is the frame work of molecules containing
the precessing nuclei
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Types of Bonds
Energy
*
*
n
*
*
n
*
n
*
Antibonding
Antibonding
Nonbonding
Bonding
Bonding
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Magnetic Properties of Nuclei
Nuclei of certain atoms posses an angular momentum. The
total angular momentum depends on the spin number (I).
The spin number ( I ) is related to the mass number and the
atomic number. Each proton has its own spin and I is a
result of these spins.
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NMR Equation and Magnetic Field Strength
The energy difference between the high energy spin state and lowenergy spin state is
V = H0/ 2 , W0= 2 V is precessional frequency
As H0 increases, precessional frequency increases.
: (Magnetogyric Ratio) : a fundamental nuclear constant= 267.512 x 106 radians T-1s-1
V : Electromagnetic frequency in radio frequency
H0 : An external magnetic field
W0 = H0 , H0 = 2 V, Therefore W0 = 2 VW0 = Precessional frequency
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Nuclear magnetic dipole moment is from the rotating
nuclear charge. Angular momentum is from rotating
nuclear mass.
Nuclear Magnetic Dipole Moment &
Angular Momentum
: (Magnetogyric Ratio) : a fundamental nuclearconstant
= 267.512 x 106 radians T-1s-1
V : Electromagnetic frequency in radio frequency
H0 : An external magnetic field
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Relationship between Radio Frequency
and Magnetic Field Strength for Proton
Radio Frequency (Mega Hertz) Magnetic Field (Gauss)
60 14,100
100 23,500
300 70,500500 117,500
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Energy Difference between Spin States as a
Function of Magnetic Fields Strength
Energy
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1.4 T
60 MHz
2.35 T
100 MHz
4.7 T
200 MHz
E = hv
7.0 T
300 MHz
2
HoV=
Relationship between Applied Magnetic Field
& Radiofrequency
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R-F transmitter
Sweep generator
R-F receiver
R-F detector
Recorder
Magnet
Transmitter coil Receiver coil
Sweep coils
Sample
Schematic Diagram of NMR Spectrometer
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Chemical Shift
The difference in the absorption frequency of a particular
proton of the sample from the absorption frequency of a
reference proton. or
The separation of resonances frequencies of nuclei in different
chemical environments of molecule from some arbitrarily
chosen standard.
Operating instrument frequency
(Reference frequency - Sample frequency ) 106
ppm =
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The protons at the electron rich environments
will feel less external magnetic field strength because
the magnetic field strength generated by electrons
surrounding the proton will counteract the applied
magnetic field strength (Ho)
Chemical Shift
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The Wo (Precessional angular velocity) of the
protons in the electron rich chemical environmentswill be less and require less radio frequency to be
resonance with the applied radio frequency
compared to the protons in the electron poor
chemical environments.
Chemical Shift
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Chemical Shift
If the d(Ha) and d (Hb) differs by 1ppm, the amount in 600 MHz instrument correspond to an energy difference of600MHz or 6x10
-8cal/mole.
To measure the small difference between Ha and Hb as separate states, they would have lifetimes in each
conformation of at least t ~ 1 / 2 v = t ~ 1 / (2 E) =1 / (2 x 600 )=0.00027 sec.
The average lifetime of a given conformation is only 10-11 sec in a energy barrier of only 3kcal/mole between one
conformer to another.
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Chemical Shift
The combination of Heisenberg uncertaintyprinciple and the small energy changecharacteristic of NMR spectroscopy is that two
hydrogen states are convertible.
If separate lifetimes > 1sec, NMR can be seen as
two sharp peaks, < 1 msec as a combined singlesharp peak. The two hydrogen states aremagnetically equivalent. If the lifetimes are in anintermediate region, a broad peak results.
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Chemical Exchange (Proton Transfer)
Chemical Exchange describes the fact that in a given period of time, asingle -OH proton may attached to a number of different ethyl alcoholmolecules.
The rate of chemical exchange (proton transfer) in pure alcohol ethylalcohol is slow, this rate is very markedly increased in acidic or basicimpurities. If the rate of chemical exchange is very slow, theexpected multiplicity of hydroxyl group is observed.
If the rate of chemical exchange is rapid, a single sharp signal isobserved. An intermediate rates of proton transfer, the observationmy occur as a broad peak
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Chemical Exchange (Proton Transfer)
The rapid chemical exchange causes spin decoupling (nomultiplicity)
Heisenberg uncertainty principle quantum mechanics:
v t ~ 1 / 2 where v and t are the uncertainties inenergy and time in units of Hertz and seconds. That is, we
can not know precisely both the energy and the life time ofa given state. The longer time the state, the more preciselycan its energy content be evaluated.
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Shielding Mechanism
Ordinary proton magnetic resonance absorption frequencies are spread over7000 cps at 600MHz NMR. The magnitude of the separation of the position ofabsorption of a proton from the reference is called the chemical shift.
The shielding that a proton experiences is a combination of at least three typesof electronic circulations:
Local diamagnetic effects
Diamagnetic and paramagnetic effects from neighboring atoms
Effects from inter atomic currents.
When the nucleus experiences a smaller magnetic field than that appliedexternally, It is said to be shielded.
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Shielding Mechanism
Diamagnetic shielding always reduces the
apparent magnetic field at the proton, and
consequently is a source of positive shielding.
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Shielding Mechanism
Paramagnetic shielding arises from electronic circulationwithin the molecule when they are specifically oriented withrespect to the magnetic field.
The orientation of the protons relative to the inducedmagnetic currents are called anisotropic effects.
Aromatic nuclei contain large closed loops of electrons inwhich strong magnetic currents are induced by the magneticfield. This effects results in a paramagnetic shielding at thearomatic proton and is called ring current effects.
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Reference : TetraMethylSilane (TMS)
Si C
C
C
C H
H
H
H
HH
H
H
H
H
H
H
= 0
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Absorbance Frequency
5.3 2.7 3.6
R CH CH CH2 CH CH CH2 C
O
O CH3
60 MHz----from 59,999,280 Hz to 60,000,000 Hz 14,100 Gauss
300 MHz--- from 299,996,400 Hz to 300,000,000 Hz 70,500 Gauss600 MHz---from 599,992,800 Hz to 600,000,000 Hz 141,000 Gauss
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General Regions of Chemical Shifts
Aldehydic
Aromatic and heteroaromatic
Olefinic
-Disubstitutid aliphatic
-Monosubstituted aliphatic
Acetylenic
-Substituted aliphatic
Aliphatic alicyclic
0 1345610 2789 = TMS
CH3-CH2-CH2-CH2-CH2-CH=CH-CH2-CH=CH-CH2-CH2-CH2-CH2-CH2-CH2-CH2-COOCH2
HOCH
HOCH2
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Spin-Spin Coupling (Spin-Spin Splitting)
Spin-Spin Coupling is the indirect coupling of proton spins
through the intervening bonding electrons.
Spin-Spin Coupling occurs because there is some tendency fora bonding electron to pair its spins with the spin of the
nearest protons.
The splitting patterns is due to the magnetic fieldexperienced by the protons of one group is influenced by the
spin arrangements of the protons in the adjacent group
through the intervening bonding electrons. H-C-C-C-C-HH H H H
H H H H
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Spin-Spin Coupling (Spin-Spin Splitting)
Coupling is ordinarily not important beyond 3 bonds unless
there is ring strains as in small rings or bridged systems,
or bond delocalizaion as in aromatic or unsaturated systems
H-C-C-C-C-HH H H H
H H H H
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Spin-Spin Splitting
Signal Ha is split into a doublet by coupling with one proton. SignalHb is split into a triplet by two protons. Spacing in both sets is same
(Jab ). The number of multiplicity is n+1, n being neighboring
protons. The relative intensities of the peaks of a multiplet also
depend on n. Doublet (n=1) peaks are in the ratio of 1:1, tripletpeaks are 1:2:1 and quartets are 1:3:3:1.
Jab
Jab
Jab
b
a
H-C-C-H
Br Br
H Bra
b
010
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Number of signals
Position of signals
Intensity of signals
Splitting of signals
CH3-CH2-CH2-CH2-CH2-CH=CH-CH2-CH=CH-CH2-CH2-CH2-CH2-CH2-CH2-CH2-COOCH2
HOCH
HOCH2
4 314222 22124
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CH3
CH2
CH CH (CH2
CH CH)2
CH2
(C H2)5
CH2
C
O
OCH3
a e e c e e b
a 0.97
e 5.38
b 1.33c 2.80
d 3.67
d
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Information from NMR Spectrum
Number of signals
Position of signals
Intensity of signals
Splitting of signals
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Summary
As external applied magnetic filed increases
Spinning proton magnetic dipole moment
increases spinning proton angular momentum increases
proton precession frequency increases
the energy difference between high energy spin state and low
energy spin state increases
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The magnetic nucleus may assume any one of ( 2 I + 1)
orientations with respect to the directions of the
applied magnetic field.
Therefore, a proton (1/2) will be able to assume only
one of two possible orientations that correspond to
energy levels of + or - H in an applied magneticfield, where H is the strength of the external magneticfield.
Summary
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Summary
If proper v is introduced, the Wo will be resonance
with the properly applied radio frequency (Hi) and
the proton will absorb the applied frequency and
will be raised to the high energy spin state.
Even though the external magnetic field strength(Ho) applied to the molecule is the same, the actual
magnetic field strength exerted to the protons of
the molecule are different if the protons are in the
diff l i h i l i