Pople nomenclature

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The Pople nomenclature is named after the Nobel laureate in chemistry, John Pople , and enables the simple representation of repeatedly occurring spin coupling patterns in NMR . It can be derived from symmetry considerations and provides information about how many spectral lines and with what multiplicity can be expected in the NMR spectrum.

Definitions

A letter is defined for each nuclear spin:

  • A, B, M, N, X, Y

If the chemical shift of two nuclei is similar, we write:

  • AB, MN, XY
  • A 2 B 2 , AB 3 , ...

The chemical shift is significantly different, as shown by the use of letters that are far apart in the alphabet:

  • AM, ON
  • A 2 M, AM 4 , ...

If the chemical shift differs by orders of magnitude, this is indicated as follows:

  • AX, AY
  • A 2 X 2

With the same chemical shift but coupling with each other one writes:

  • AA ', XX'
  • AA'XX '

If two chemically equivalent nuclei can be converted into one another by a symmetry operation and all other spin-bearing atoms are left alone, they do not couple.

Examples

a) In the case of 1,1-difluoroethene , the proton shifts are identical (chemical equivalence), but there are coupling between them. Reason: If two proton nuclei are chemically equivalent, but couple with each other, it is called an AA 'spin system. Are there two other spin-bearing atoms present, e.g. B. Fluorine , then one also has XX '. The overall system is therefore AA'XX '. Consequence: You have one octet in 1 H-NMR.

b) methane : The four methane protons do not couple with each other, one has a A 4 system. Result: a singlet is obtained in H-NMR.

c) 1,1-dichloroethene has an A 2 system. Reason: There is a symmetry operation (here: mirror plane) that converts the two protons into one another. The chlorine atoms are also moved, but are not spin-bearing. Consequence: 1 H-NMR shows a singlet.

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