3. Nuclerar Magnetic Resonance Spectroscopy

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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