The coupling constant is the distance between peaks in a multiplet in NMR spectroscopy. It is measured in Hertz and does not depend on external magnetic field strength. The value of the coupling constant provides information to distinguish multiplets and can indicate structural features like cis/trans isomers. Coupling occurs between protons close in space, known as geminal, vicinal, and sometimes long-range coupling over several bonds. The coupling constant is affected by factors like bond angle, dihedral angle, and electronegativity of substituents.

1. COUPLING
CONSTANT
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2. DEFINITION:
 The distance between the centres of the two
adjacent peaks in a multiplet is usually constant
and is called as coupling constant the
 Value of coupling constant is independent of the
external magnetic field.
 It is measured in hertz (hz) (or) in cycles per
second (cps).It is denoted by the letter (j )

4. Coupling constant
 From the value of J we can
distinguish between two singlets and
one doublet ; two doublets from one
quartert.
 This can be done by simply recording
the spectrum at two different radio
frequencies.
 The value of J generally lies between
0 and 20 hertz.

5. examples
 Methane CH4
 Ethyl bromide
CH3-CH2-Br
Here under the influence of two
equivalent protons of a the signal of
protons for b will appear as a
quartert.
 According to (n+1) rule.

6. Jab = 6.8 cps
Jbc = 7.3 cps
 The signal for b protons will be split into a
sextet.
 If the Jab and Jbc are equal, a symmetrical
sextet would have been observed.
 Since the values of J are close enough, the
signal for b protons miiltiplet will resemble as
sextet.

7.  And by the help of coupling contant we can
distinguish between cis and trans form.
 Eg : CH3CH2Cl ( ethyl chloride )
cis – J = 10 Hz
trans – J = 15Hz
 It is clearly noted that the value of coupling
constant depends partly on the number of
covalent bonds through which protons may
interact and also upon the structural
relationships between the coupled protons.

8.  They are:
1. Geminal coupling
2. Vicinal coupling
3. Substituent effects
4. Long range coupling
1.) Geminal coupling : in the case of geminal
protons, the protons are attached to the same
carbon atoms. Where the protons are in different
environment. Here the value of J depends on the
bond angle.

9.  Eg: 2,2-dimethyl 3-trimethoxy silyl butane.
 Here the two protons are attached to the same
carbon atom.
 And the double bond is present. Where the
different groups are attached to this double
bond.
 The protons are located in the different
environment. And they are not chemically
equivalent
 They couple with each other.

11.  a = 1+1 =2 ( doublet )
 b = 1+1 = 2 ( doublet )
 J = 1.4 Hz
bond angle is directly proportional to the
coupling constant. If bond angle increases value
of coupling constant increases.
bond angle J value
105˚
109˚
125˚
˃125˚
-25 cps
-12 cps
0 cps
+ cps

13. methane
• -12cps
ethylene
• +2.5 cps

14.  Substituent effect :
The increase in the electro negativity atom (or)
group, which withdraws sigma electrons, that
increases the value of coupling constant.
eg : methyl chloride -10.8cps
methyl fluoride -9.4cps

15.  The value of J decreases if an electronegative
substituent withdraws electron from the П
bonds which decreases the coupling constant.
 Eg : ethene J= +2.3 cps
vinyl fluoride J = -3.3 cps
compound J value
methane
methyl chloride
methyl fluoride
ethene
formaldehyde
-12.4 cps
-10.8 cps
-9.4 cps
+2.5 cps
+41.0 cps

16.  Vicinal coupling :
In vicinal coupling, the protons are separated by
three bonds. Here the value of J varies with dihedral
angle.
let us consider gauche and anti confermations.
gauche 2 to 4 cps
anti 5 to 12 cps
dihedral angle J value
0˚ (or) 180˚
90˚
(+)large
negative

19. Eg : cyclohexane
In cyclohexane, the absorption due to axial
and equatroial protons are different.
For the axial – axial interactions, the
dihedral angle is (180˚) nearly. So the
coupling constant is approximately 8cps.
Where as for axial- axial and equatorial –
equatorial interactions the dihedral angle is
60˚,the coupling constant is approximately
2cps.

20.  The values of vicinal coupling constants for a few
compounds are listed below:
 Long range coupling :
Generally no coupling is absorved if the distance
between the two absorbing nuclei is more than three
covalent bonds is known as long range coupling
compound J value
propane ( gauche )
propane ( anti )
dibromomethane
(gauche)
(anti)
7.2 cps
6.5 cps
15.1 cps
3.4 cps

21. unsaturated
(or) appreciable coupling
fluoro compounds observed with the
help of high resolution
spectrometers
 In pi bond system, appreciable couplings are
frequently observed between the protons
separated by even four (or) five bonds.

22.  Eg : 2,4-dichlorobenzaldehyde
coupling takes place between the
aldehydic proton and a ring proton
compounds j value
propene
butene
2,3 dimethyl butanoic acid
-1.7 cps
-2.3 cps
-1.2 cps

23. REFERENCES :
Elementary organic spectroscopy (
principles and chemical applications ) –
sharma
J.B. Leane and R.E.Rechards, Trans . Faraday
soc.55,518 (1959 )