generalchem_ls_17.pdf
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NMR spectroscopy: Shielding
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Electrons surrounding the nucleus also produce magnetic field.
Badd
= -
B
Blocal
= B + Badd
= B(1-
)
N Blocal / 2 N B / 2
Resonant frequencies being dependent on strength ofexternal magnetic field, are inconvenient to remember.
How to get rid of field-dependence?
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Chemical Shift
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Resonant frequency recorded for a reference compound (denotedby
0).
Si(CH3)
4 (Tetramethyl silane) is commonly used reference in NMR
spectra of organic compounds. (Highly shielded, single intense peak,
volatile)
Shift in the resonant frequencies of sample with respect to reference
is recorded.
Chemical shift:
= {(
)/
}
106ppm
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Chemical Shift: Ethanol
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TMS
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Factors affecting Chemical
Shift
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Reference (TMS) protons are highly shielded. Hence, larger chemical
shift indicates greater extent of deshieldinglower shielding
constant.
Shielding constant: = local
neighbour
solvent
local
: due to the shielding from the electrons immediately
surrounding the nucleus.
neighbour
: due to the shielding from the neighbouring
environment
solvent
: the contribution from solvent molecules.
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Chemical Shift: Ethanol
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TMS
Intensity of peak proportional to number of protons corresponding to
the peak: Integrate to calculate area under the peak 1:2:3
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Phenylacetone: PMR
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CH2C
O
CH3
The number of signals = number of distinct sets of equivalent protons
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Benzene: PMR spectrum
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Single peak: (all protons equivalent)
Chemical shift indicates high
deshielding
How will the specta of methane,
ethane, ethene, ethyne look like?
Position of the peak in the spectrum?
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Number of peaks, intensities
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Number of peaks equals the number of distinct sets of
equivalent protons.
Equivalent protons: Two protons are equivalent if the
compounds resulting from replacement of either of them by
identical substituent will be chemically identicalsamemolecular formula, structural formula and geometry
Intensity of a peak directly proportional to the number of
equivalent protons responsible for the peak.
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Equivalent protons
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Two protons are equivalent if the compounds resulting from
replacement of either of them by identical substituent will be
chemically identicalsame molecular formula, structural formula and
geometry.
Homotopic hydrogen atoms: If in making these replacements, we
get the same compound, the hydrogen being replaced are said to bechemically equivalent or homotopic. Homotopic atoms (or groups)
are chemical shift equivalent.
In H2C=C(CH3)2 replacement of H with Cl gives
H2C=CCH2Cl(CH3) + ClHC=C(CH3)2Six methyl hydrogens form one set; replacing any one of them with
chlorine, for example, leads to the same compound. Two vinyl
hydrogens form another set; replacing either of these leads to same
compound. So 2-metylpropene gives two 1H NMR signals
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Equivalent protons
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Enantiotopic hydrogen atoms: If replacement of each of two
hydrogen atoms by the same group yields compounds that are
enantiomers, the two hydrogen atoms are said to be enantiotopic.
Enantiotopic hydrogen atoms have the same chemical shift and give
only one 1H NMR signal.
In CH3CH2Br, two hydrogen atoms of -CH2Br group are enatiotopic.
Diastereotopic hydrogen atoms: If replacement of each of two
hydrogen atoms by the same group gives compounds that are
diastereomers, the two hydrogen atoms are said to be diastereotopic.Diastereotopic hydrogen atoms do not have the same chemical shift
and gives rise to different 1H NMR signals.
The two protons of =CH2group of ClHC=CH2 are diastereotopic.
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Examples to solve:
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How many peaks will be present in the PMR spectra of the
following molecules? What will be the ratio of intensities?
HCHO 1 -
CH3COCH
3 1 -
CH3CHO 2 3:1CH
3COCH
2CH
3 3 3:2:3
CH3CH=CHCH
3 2 3:1
(CH3)
2CH-CH
2CH
3 4 6:1:2:3
CH3COOCH3 2 1:1CH3CH
2CHClCH
3 4 3:2:1:3
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Factors affecting Chemical
Shift
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Reference (TMS) protons are highly shielded. Hence, larger chemical
shift indicates greater extent of deshieldinglower shielding
constant.
Shielding constant: = local
neighbour
solvent
Deshielding due to electronegative elements
Anisotropic effect : Shielding/deshielding due to ring currents
Other effects: Hydrogen bonding, solvent effects, etc.
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Deshielding due to
electronegative elements
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CHCl3 CH2Cl2 CH3Cl
7.27 5.30 3.05 ppm
-CH2Br -CH2CH2Br -CH2CH2CH2Br
3.30 1.69 1.25 ppm
No. of electronegative atoms Distance from the electronegative atom
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Anisotropic effect: ethene, ethyne
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deshielded
shielded
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Solvent contribution (solvent)
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A proton on the solute molecule is shielded because of the anisotropiceffect produced as a result of ring current of benzene as solvent
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Typical Chemical Shift ranges
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Fine structure (Spin-spin coupling)
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Ethanol
Each magnetic nucleus contributes to the local magnetic field of
other nuclei.
Spin- spin coupling constant J (Hz) is independent of the applied
magnetic field.
Consider AX where A and X are spin nuclei.
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Fine structure (Spin-spin coupling)
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Consider the protons A and X (or in general spin nuclei)
In absence of any interaction between A and X, the total energy of thesetwo protons in a magnetic field
B
(neglecting spin-spin coupling) is
E = -N(1-A)BmA- N(1-X)BmX
Where,
Aand
Xare the shielding constants of A and X.
If the nuclei A and X are close enough in the molecule (separated by oneto three bonds, for example), their magnetic moments will perturb eachother. The spin-spin interaction energy will be
Espin-spin= hJAXmAmX
Etotal
= E + Espin-spin
= - NB{(1-A)mA+ (1-X)mX} + hJAXmAmX
Four energy levels are obtained.
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Fine structure (Spin-spin coupling)
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In absence of spinspin coupling
In presence of spinspin coupling
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Fine structure (Spin-spin coupling)
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Fine structure (Spin-spin coupling)
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AX2
1 : 2 : 1
The two X nuclei may
have the 22= 4 spinarrangements. Themiddle twoarrangements areresponsible for the
coincident resonance ofA.
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Fine structure (Spin-spin coupling)
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AX3
1 :3 : 3 : 1
There are 23= 8arrangements of thespins of the three X
nuclei, and theireffects on the Anucleus gives rise tofour groups ofresonances.
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Fine structure (Spin-spin coupling)
N equivalent spin-1/2 nuclei split the resonance of a nearby spin
or group of equivalent spins into N+1 lines with an intensitydistribution. The relative intensities are given as coefficients of
the binomial expansion (1+x)n
This is given as Pascals triangle1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
1 5 10 10 5 1
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