Pople nomenclature
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.
literature
- M. Hesse, H. Meier, B. Zeeh: Spectroscopic methods in organic chemistry . 6th edition, Thieme, Stuttgart 2002, ISBN 3-13-576106-1 .
- Jürgen T. Renze: Investigations and mechanistic interpretations of the hydrolysis behavior of benzyl-functionalized cycloSal nucleotides . Dissertation, Hamburg 2002, urn : nbn: de: gbv: 18-6769 .
Web links
- Fundamentals of quantum chemistry / Spartan , slide 28 (PDF file; 539 kB)