NMR CHEMICAL SHIFT , By Dr. UMESH KUMAR SHARMA AND ARATHY S A

1. Nmrspectroscopy
By :
Dr. UMESH KUMAR SHARMA & ARATHY S A.
DEPARTMENT OF PHARMACEUTICS
MAR DIOSCORUS COLLEGE OF PHARMACY,
THIRUVANANTHAPURAM, KERALA, INDIA
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2. NMR SPECTROSCOPY
 CHEMICAL SHIFT
 FACTORS INFLUENCING CHEMICAL SHIFT
 SPIN –SPIN COUPLING
 COUPLING CONSTANT
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3. CHEMICAL SHIFT
 Chemical shift is the difference between absorption position of a sample proton &
absorption position of a reference compound.
 Theoretically for any organic compound only one NMR should be recorded for all protons
present.
 But this does not happen in practice since all hydrogen atoms are not in same environment.
 Magnetic field applied is not felt by all hydrogen atoms uniformly.
 This is due to the presence or nearness to double bond or triple bond, aromatic / hetrocyclic
/alicyclic ring systems, electronegative atoms.
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4.  Due to differences in magnetic field felt by H atoms, each proton has different
processional frequency.
 Different range of frequency are required for excitation.
 Different peaks or signals for each kind of proton are obtained.
 The signal positions in NMR spectra are chemical shifts.
 Nucleus is surrounded by constantly revolving electrons, it produce spherically
symmetrical electromagnetic charge cloud.
 When magnetic field is applied electron charge cloud circulate in a direction to produce
induced field, which is opposed to applied field.
 Induced field shied the nuclei from external field.
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5.  Induced field is directly proportional to applied field strength.
B induced=σ B applied
 Constant σ is called shielding or screening constant.
B effective = B applied-B induced
Where B effective=Bo(1-σ), Bo is applied magnetic field
 Shielding causes small difference in absorption frequency.
 Chemical shift is measured by comparing positions of peaks of reference compound.
 Frequency of proton=ϒBo/2Ϡ.
 Reference standard like Tetra methyl Silane are used.
 Resonance frequencies of sample are quoted with respect to TMS.
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6. NMR SPECTRUM
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7. Shielding of electrons:
 High electron density around nucleus shields nucleus from external magnetic field.
 Signals are up-field in NMR spectrum.
De-shielding of electrons:
 Lower electron density around nucleus de-shield nucleus from external magnetic field.
 Signals are downfield in NMR spectrum.
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8.  NMR spectra shows applied field strength increasing from left to right.
 Left part is downfield & right part is up-field.
 Nuclei which absorb on up-field side are strongly shielded & downfield side are weakly
shielded.
 Shielded nuclei do not sense magnetic field as large as de-shielded do.
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9.  As a result energy difference between α & β spin states is much lower for shielded
nuclei.
 They resonate at lower frequency.
 De-shielded nuclei have higher energy difference between α & β spin states.
 They resonate at higher frequency.
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10.  Frequency at which a nucleus come into resonance with in a magnetic field,
υ =
ϒ𝐻𝑜
2π
 Resonance frequencies of nuclei in sample is measured & compared to a reference
standard
 Position of peaks in NMR spectrum relative to reference peaks in terms of chemical
shift
𝛿 =
𝐻𝑜 𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 −𝐻𝑜(𝑠𝑎𝑚𝑝𝑙𝑒)∗106 ppm
Ho(reference)
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11. Tetra methyl Silane (TMS) as reference compound :
 TMS has 12 equivalent protons & gives a single intense signal.
 Chemically inert.
 Low boiling point.
 Volatile.
 Can be easily recovered from any sample.
 Soluble in most organic solvents.
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12. FACTORS AFFECTING CHEMICAL SHIFT
ELECTRO NEGATIVITY
 Chemical shift arises from secondary magnetic field produced by circulation of electrons in molecule.
 Electrons bonding proton passes around the nucleus in a plane perpendicular to applied field.
 It creates a secondary field which opposes applied field.
 Nucleus experience resultant field smaller than applied field.
 This shielding effect lowers processional frequency.
 In TMS since Si is electropositive, electron pushes towards methyl group by positive inductive effect.
 Powerful shielding causes protons of TMS to resonate at lower frequency.
 As electronegativity increases, electron withdrawal from methyl group increases.
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13.  Electronegative groups attached to C-H system decrease electron density
around protons.
 Its de-shielding effect causes proton to experience higher magnetic field &
higher resonance frequency.
 As electronegativity increases, de-shielding increases
 Absorption occurs downfield or at higher resonance frequency.
 Additional electronegative atoms cause increase in chemical shift.
 Effect decreases with distance.
 More electronegative atoms de-shield more & give larger shift values.
Diamagnetism.
 Produces secondary field that opposes applied field.
Paramagnetism.
 Produces secondary field in the direction of applied field.
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14. MAGNETIC ANISOTROPY:
 π electrons of aromatic compounds, alkenes, alkynes, carbonyls etc. interact with
applied field which induces magnetic field.
 They cause magnetic anisotropy.
 Causes both shielding & de-shielding.
Eg : benzene
ALKENES :
 When alkene group is so oriented that plane of double bond is at 90 to the direction of
applied field.
 Induced circulation of pie electrons generates a secondary field.
 It is diamagnetic around carbon atom but paramagnetic in region of alkene protons.
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15. ALKENES
 When alkene group is so oriented that plane of double bond is at 90 to direction of
applied field.
 Induced circulation of pie electrons generates a secondary field.
 It is diamagnetic around carbon atom but paramagnetic in region of alkene protons.
 Pie electrons of double bond generates a magnetic field that oppose applied magnetic
field in middle of molecule.
 It reinforces the applied field on outside where vinylic protons are located.
 Reinforcement de-shield vinylic protons & shift down fields.
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17. ALKYNES
 When terminal triple bond is aligned in direction of magnetic field, electrons circulate to
create induced magnetic field.
 Acetyl proton lies along the axis of field & opposes external field.
 Acetyl protons are shielded.
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18. AROMATIC COMPOUNDS:
 Pie electrons of aromatic ring are delocalized cyclically over aromatic ring.
 It induce a secondary field, that is diamagnetic at centre & paramagnetic at periphery
where protons present.
 They experience a higher magnetic field & so higher resonance frequency.
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19. HYDROGEN BONDING:
 In compounds capable of hydrogen bonding there is greater de-shielding of protons.
 Shieft down fields.
 When sample temperature increases hydrogen bond weakens & up-field shift.
 Increasing solute concentration increases de-shielding & down field shift occurs.
 Intra molecular hydrogen bonding is unchanged by dilution.
 NMR spectra of such systems is unaltered by varying concentration.
 Effect of secondary field affects chemical shift in one the ways like
a. Positive shielding b. Negative shielding
 Two types of shielding phenomena modifies resonance peak in NMR spectrum by
a. Local shielding b. Low range shielding
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20. SPIN-SPIN COUPLING:
 Interaction between spins of neighbouring nuclei in a molecule cause splitting of lines in NMR
spectrum.
 It occurs by means of a slight un-pairing of bonded electrons.
 Number of lines observed in NMR signals for a group of protons is related to number of protons
in neighbouring groups.
 Splitting is related to number of possible spin orientations that neighbouring group can adopt.
 Different spin states and resultant magnetic moment of neighbouring protons modifies actual
magnetic field experienced by a proton.
 Mutual influence between protons is not transmitted through space ,but via electrons of
intervening bonds.
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22. CAUSES OF SPLITTING:
 Splitting of signals can be determined by its environment of absorbing protons with
respect to neighbouring protons.
 Due to different spin states & resultant magnetic moments of neighbouring protons actual
magnetic field experienced by proton gets modified.
 Absorption proton experience each of the modified field.
 Its absorption signal split into different peaks.
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23. COUPLING CONSTANT(J)
 It is the distance between peaks in a given multiplet.
 It is a measure of magnitude of splitting effect.
 Value of J is independent of applied magnetic field.
 Depends on molecular structure.
 For geminal protons value of J varies from 0 - 20Hz.
 Depends on bond angle & overall structure of molecule.
 For vicinal protons J varies from 2 -18Hz depending on spatial position of protons &
molecular structure.
 In freely rotating groups, protons with anti conformation have J value 5 - 12Hz.
 Protons with gauche conformation have J value 2 - 4Hz.
 For cis protons J value is 6 - 14Hz & trans protons 11 - 18Hz.
 Coupling constants are a measure of effectiveness of spin spin coupling.
 Useful in getting information from NMR spectra of complex molecular structures.
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24. PROTON NMR SPECTRA OF
ETHANOL
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25. HNMR SPECTRA OF
BENZENE
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26. INTERPRETATION OF HNMR
SPECTRA
 The number of signals shows how many different kinds of protons are present.
 The location of the signals shows how shielded or de-shielded the proton is.
 The intensity of the signal shows the number of protons of equivalent protons.
 Signal splitting shows the number of protons on adjacent atoms.
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27. REFERENCE
 Organic spectroscopy by William Kemp
 Instrumental method of chemical analysis by Chatwal
 Principles of Instrumental analysis. Doglas A Skoog, James
Holler.5th edition
 Instrumental methods of analysis by Willard
 Wikipedia.org
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28. For Feedback / Comments
Write to umeshpanditjp@gmail.com
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