Coupling Constants - Spectroscopy Chemistry Notes | EduRev

Organic Chemistry

Chemistry : Coupling Constants - Spectroscopy Chemistry Notes | EduRev

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Coupling constant is a quantitative measure of spin-spin splitting. It denotes how strongly a nucleus is affected by the spin-state of its neighbour. It is

  • Distance between the peaks of multiplet
  • Measured in Hz
  • Not dependent on strength of the external field
  • Multiplets with the same coupling constants may come from adjacent groups of protons that split each other.
  • Coupling constant is a constant. It does not change in magnitude with change in instrument frequency.
  • Increase in instrument frequency increases only the chemical shift of the nuclei, which increases the resolution of the spectrum.
  • J can be either positive or negative, but only the absolute value is considered.

Types of Coupling Constant

1. Based on the coupling partner:

  • Homonuclear Coupling: Coupling between two nuclei of the same atom.
  • Heteronuclear Coupling: Coupling between two nuclei of different

atoms.

2. Based on the number of intervening bonds:

  • One Bond Coupling (1J): Strong coupling, mainly heteronuclear.
  • Two Bond Coupling (2J): Geminal coupling, high in magnitude.
  • Three Bond Coupling (3J): Vicinal coupling, lower in magnitude, but

highly significant for analysis.

3. Others: Specific couplings of higher order can be extremely useful

in NMR analysis. Ex: meta coupling, allylic coupling, W-coupling.

One Bond (1J) Coupling

Adjacent nuclei prefer to be in opposite spin states in the ground state. For a typical C—H bond, two peaks are observed in the 13C spectrum, due to the two shown transitions.

Coupling Constants - Spectroscopy Chemistry Notes | EduRev

Some common one-bond coupling constants have been displayed below:

Coupling Constants - Spectroscopy Chemistry Notes | EduRev

Two Bond (2J) Coupling

Commonly called as geminal coupling, these are usually smaller in magnitude than 1 bond coupling.

Coupling Constants - Spectroscopy Chemistry Notes | EduRev

Some common 2 bond coupling constants:

Coupling Constants - Spectroscopy Chemistry Notes | EduRev

Coupling Constants - Spectroscopy Chemistry Notes | EduRev


Variation in 2J Coupling

Geminal coupling increases in magnitude as the decreases, due to higher electron spin correlation.


Coupling Constants - Spectroscopy Chemistry Notes | EduRev

Coupling Constants - Spectroscopy Chemistry Notes | EduRev


Three Bond (3J) Coupling

Commonly called as vicinal coupling, these usually follow the (n+1) rule in simple aliphatic hydrocarbon chains. Nuclear and electronic spin interactions carry the spin information from one hydrogen to its neighbour.

Coupling Constants - Spectroscopy Chemistry Notes | EduRev

As a result, the best overlap occurs when the C—H bonds are at a dihedral angle of 0°. 


Variation in 3J Coupling

The magnitude of vicinal coupling is directly related to the dihedral angle between the C—H bonds in question. The Karplus equation relates dihedral angle to coupling constant between two vicinal hydrogen nuclei.

Coupling Constants - Spectroscopy Chemistry Notes | EduRev 

Coupling Constants - Spectroscopy Chemistry Notes | EduRev

cis/trans alkenes:

Coupling Constants - Spectroscopy Chemistry Notes | EduRev

Cyclohexane systems:

Coupling Constants - Spectroscopy Chemistry Notes | EduRev

Cyclopropane/Epoxide Systems: Jcis > Jtrans (unlike in alkenes)


Coupling Constants - Spectroscopy Chemistry Notes | EduRev


Long Range Coupling

Very rarely, but often significantly, coupling takes place between two atoms separated by four bonds or more (nJ; n≥4). The common systems where this is exhibited are: allylic systems, rigid bicyclic systems and aromatic rings. Since these couplings are observed through a large number of bonds, a highly specific stereochemical arrangement is essential.

Coupling Constants - Spectroscopy Chemistry Notes | EduRev


Stereochemical Nonequivalence:

  • Usually, two protons on the same C are equivalent and do not split each other.
  • If the replacement of each of the protons of a —CH2 group with an imaginary “Z” gives stereoisomers, then the protons are non-equivalent and will split each other.

Coupling Constants - Spectroscopy Chemistry Notes | EduRev


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