L5 Coupling Constants and Spin Systems in NMR

8/10/2019 L5 Coupling Constants and Spin Systems in NMR

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L5CH521

Suvarn Kulkarni


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Examples

S stem N+1Quin, 2H,1:4:6:4:1

, ,

2


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Example AMX system

dd

Doublet of a doublet S, d, t, dd, dt, q, quin

How will you

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v sua y st ngu sBetween dd and q


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,

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ac ors n uenc ng coup ng cons an

What determines the size of the couplingconstant?

There are three factors.

protons

ng e e ween e wo on s Electronegative substituents

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0o

Shorter bonds lead to larger J

6Bond distance


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Electronegative atoms pull the electrons from the C-H and

Electronegative atom

ecreasescoupling constant

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There are 2 in ic rotons in 4 4-di eth c c ohe -2-ene-1-one and they are cis. The coupling constant for ciscoupling is approximately 10 Hz so the peaks should be

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.


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HH

a c

C C Hb

Si nals ma be s lit b ad acent rotons,different from each other, with

different cou lin constants. Example: Ha of styrene which is split by

=an adjacent H cisto it (J = 11 Hz).

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a c C CHb

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H-NMR S ectrum of St rene

This is another exam le of AMX s stem.6.73

5.25

J = 17 Hz J = 11 Hz J = 1 Hz

125.75

All are dd; two with very small coupling constant


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ParaDi-substitutedbenzene

It looks like a sextuplet because the intensity ratios of the lines in an octupletwould be 1:7:21:35:35:21:7:1 (from Pascals triangle) and it is hardly surprising thatthe outside lines disa ear.

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AB

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dd18,8

dq,15,

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Chapter13 17


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H bonding Intermolecular hydrogen bonding decreases

frequency of proton .

it appears downfield

shift to lower frequency region.

Intramolecular H bonding does not change byu on u c anges y ea ng. So NMR is a good technique to study H

.

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DMSO d6 or acetone d6 is used to observe splitting.Exchan e rate can also be decreased b usin low tem dilution and havin drneutral conditions, treating with anhyd. Al2O3 or Na2CO3 and filtering thro glass

wool


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OHpeak

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Overlappingquartetofdoublets

J=5Hz(CH2andOH)and7Hz(CH2andCH3)

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Stereochemical Nonequivalence

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Homotopic, enantiotopic and

diastereoto ic rotons

Chiral solvents/reagents cannot distinguis

Chiral solvents/reagents can distinguish

Chemicall non e uivalent rotons

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Doublet of doublet


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The two protons on the CH2Cl group arediastereotopic; their imaginary replacements give

. Diastereotopic protons are usually vicinal to

stereocenters (chiral carbons). But it is not a

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.


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1,2-dichloropropane

Proton NMR spectrum o 1,2- ic oropropane s owsdistinct absorptions for the methylene protons on C1.

These h dro en atoms are diastereoto ic and are

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chemically non-equivalent.


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Cyclic systemsCage compoundsRestricted rotation

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Chemical shift equivalence

y

structures

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Keto-enol interconversionIt is a s ow enoug process at RT

(101-103 Hz) at NMR time scaleSo, both forms can be detected.

At higher temp averaged spectrumis obtained.

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Interconversion around a partial double bond

DMF has two methyl signals, at 2.89 and 2.98 p.p.m., each 3H,and these are the two methyl groups on nitrogen. Restricted

groups different.

At RT, the rate of rotation around the hindered partial double bond is slow.At ~123 oC the rate of exchange becomes so rapid that the two peaks merge andwe get only one averaged peak.

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Interconversion around a single bond of ring

Molecules are tumbling relative to the magnetic field,so NMR is an averaged spectrum of all the orientations.

.

Axial and equatorial protons on cyclohexaneinterconvert so rapidly that they give a single signal.

Freezing out of conformation at -89oC 2 conformersare seen. Frozen chair conformation.

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, ,


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AAXXsystem

Neither two A rotons or two X rotons are ma neticall e uivalent

This is not a first order spin system; written as AAXX (not A2X2)Will not conform to intensity pattern in Pascal triangleThe distance between eaks is not the cou lin constantBut, spectra do not become first order by changing magnetic field

It becomes deceptively simple at low field.


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paraNitrotoluenehastwopairsofequivalentaromaticprotonsa.

approximately8Hz,

the

peaks

of

the

signal

will

be

separated

by

around8Hz.

Meta and ortho show more com lex NMR.

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ExampleofAAXXsystem

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ExampleofAAXXsystem

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The Kar lus correlation

DihedralVicinal coupling constants

Newman projection

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Calculations are Approximations. It does not take into account the electronegativityvalues, bond length and bond angles.


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Geminal cou lin constantsDepends on H-C-H bond angle

Sp2 or sp carbon increase it

Jis usually negativeu on y or eory.

Coupling is seen only if theCH2 protons are diastereotopic

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Hintsforproblemsolving

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L5 Coupling Constants and Spin Systems in NMR

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