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					  Nuclear Magnetic Resonance Spectrometry
                                                                           Nuclear magnetic dipole 
                                                    Precessional orbit




                                                                                        Spining proton




                                                                             Ho
   Spinning charge in proton generates magnetic
   dipole
                                                         Proton precessing in a magnetic field Ho


                                                                                       High
                                        Nuclear magnetic dipole  axis of nuclear      energy
    Precessional orbit                                             rotation            precession
    Low energy spin state (-
    1/2)

          Rotation
                                                                                     Low energy
          component of                                 Nuclear
                                                                                     precession
          magnetic field H1                            Spin

                                                                                    Reference axis
Oscillator Coil



    Precessional orbit
    High energy spin state
    (+1/2)                         Ho


       Oscillator generates rotating component of                Precession -Energy Relationship
       magnetic field H1
H1 (Magnetic component of radio frequency from oscillator coil): oscillator frequency

H1 can be resolved into 2 components rotating in opposite directions.

       (1) Rotating in the same direction in the precessional orbit of the molecular
           magnetic dipole

       (2) Rotating in the opposite direction as the precessional orbit of the nuclear
           magnetic dipole ; disregard


Magnetic Properties of Nuclei

Nuclei of certain atoms posses a mechanical spin or angular momentum. The total
angular momentum depends on the nuclear spin or spin number (spin quantum number) I.
The numerical value of the spin number ( I ) is related to the mass number and the
atomic number. Each proton and neutron has its own spin and I is a result of these spins.

                  Mass Number                Atomic Number               Spin Number
                      Odd                     Even or odd               1/2, 3/2, 5/2,----
                     Even                        Even                           0
                     Even                        Odd                       1, 2, 3, ---


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 magnetic field, where H is the
strength of the external magnetic field.
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 spin (energy) 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 different electronic chemical environment.
Fundamental NMR Equation of Radio Frequency and Magnetic Field Strengh

The energy difference between the two states is

                                            Ho
                                    V=
                                            2

               : (Magnetogyric Ratio) : Constant and a fundamental nuclear constant.
              V : Electromagnetic frequency in radio frequency
              Ho : An external magnetic field
              Wo = Ho
              Ho = 2V
              Therefore Wo = 2V



               = 2 / hI

               = Magnetic Moment (Magnetic Dipole Moment)
              h = Planck's Constant
              I = Spin Number


      Relationship between Radio Frequency and Magnetic Field Strength for Proton

           Radio Frequency (Mega Hertz)           Magnetic Field (Gauss)
                        60                               14,100
                       100                               23,500
                       300                               70,500
                       500                              117,500




                                                                      E = hv




   1.4 T              2.35 T              4.7 T               7.0 T

   60 MHz             100 MHz             200 MHz             300 MHz
                  Schematic Diagram of an NMR Spectrometer


                                        Sweep coils
                              Sample


R-F                                                                              R-F
 transmitter                                                                   receiver
                                                                                     and
                                                                                detector
                                                                                

                          Transmitter coil     Receiver coil                      Recorder
   Sweep  
   generator
                                          Magnet




   Chemical Shift

   The difference in the absorption frequency of a particular proton of the sample
   from the absorption frequency (position) of a reference proton.
          The protons at the electron rich environments (strong electronegative
          molecules such as oxygen and halogens) 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), which can be said deshielded proton.

          Therefor, the Wo of the protons in the electron rich chemical
          environments will 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.



           ppm = (reference frequency - sample frequency)  106
                           Operating instrument frequency
                        Ho
             V=         2


                    (1-)Ho
             Vi =
                        2

             Vi : Resonance Frequency of Proton i
             Ho: Applied Magnetic Field
              : Shielding Constant

     The Reference Compounds : TetraMethylSilane (TMS)
                                              H
                                         H    C    H

                                     H                  H

                             H       C        Si        C       H

                                     H                  H


                                         H    C    H
                                              H



             General Regions of Chemical Shifts


                                                                            Aliphatic alicyclic

                                                                     -Substituted
                                                                     aliphatic
                                                                     Acetyleni
                                                                     c
                                                   -Monosubstituted
                                                   aliphatic
                                             -Disubstitutid aliphatic
                                             Olefinic
     Aromatic and
     heteroaromatic
Aldehyd
ic
10       9          8            7       6         5        4       3   2       1     0
                                                                       O
                R CH C H C H2 C H CH     C H2                      C O C H3


                       5.3       2.7                         2.0           3.6 

Rest of the protons on CH3 and CH2 absorb at 0.8 - 2.0          very crowded area,

usually see a broad, big peak




Spin-Spin Coupling (Spin-Spin Splitting)

Spin-Spin Coupling is the indirect coupling of proton spins through the intervening

bonding electrons.

It occurs because there is some tendency for a bonding electron to pair its spins with the

spin of the nearest protons. The spin of a bonding electron having been thus influenced.

Coupling is ordinarily not important beyond 3 bonds unless there is ring strains as in

small rings or bridged systems, or bond delocalization as in aromatic or unsaturated

systems
Signal a is split into a doublet by coupling with one proton; signal b is split into a triplet

by two protons. Spacing in both sets is same (Jab).

                          a            b
                        C H2Br C HBr2                            a
                                   b

                                                                 Jab



                               Jab Jab




                              NMR of Fatty Acid Methyl - Ester




Information from NMR Spectrum

        The Number of signals

        The Position of signals

        The Intensity of signals

        The Splitting of signals
C 19H32O 2   Methly linolenate




    a           e     e      c        e      e                 b          O
  CH3 CH2 CH CH (CH2 CH CH)2 CH2                           (CH2)5 CH2 C
                                                                          OMe
                                                                              d
                                 Chemical shift (ppm)

                                  a       0.97   e   ca.5.38
                                  b       1.33
                                  c       2.80
                                  d       3.67

				
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posted:11/26/2011
language:English
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