The Pople notation is named after the Nobel laureate

John Pople Sir John Anthony Pople (31 October 1925 – 15 March 2004) was a British theoretical chemist who was awarded the Nobel Prize in Chemistry with Walter Kohn in 1998 for his development of computational methods in quantum chemistry. Early ...

and is a simple method of presenting second-order spin coupling systems in

NMR Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with ...

. The notation labels each (NMR active) nucleus with a letter of the alphabet. The difference in

chemical shift In nuclear magnetic resonance (NMR) spectroscopy, the chemical shift is the resonant frequency of an atomic nucleus relative to a standard in a magnetic field. Often the position and number of chemical shifts are diagnostic of the structure of ...

, δ, relative to the

J-coupling In nuclear chemistry and nuclear physics, ''J''-couplings (also called spin-spin coupling or indirect dipole–dipole coupling) are mediated through chemical bonds connecting two spins. It is an indirect interaction between two nuclear spins that a ...

between nuclei mirrors the separation of the letter labels in the

Latin alphabet The Latin alphabet or Roman alphabet is the collection of letters originally used by the ancient Romans to write the Latin language. Largely unaltered with the exception of extensions (such as diacritics), it used to write English and the o ...

. The letters used tend to be limited to A,B,M,N,X,Y. For example, AB indicates two nuclei which have similar chemical shifts (Δδ similar to or smaller than J), whereas AX indicates two which lie further apart on the spectrum (Δδ significantly larger than J). A₂B would similarly indicate a spin system containing two equivalent nuclei (A) and a third, inequivalent one (B). Nuclei which are in equivalent ''chemical'' environments (that is, symmetry-related), but inequivalent ''magnetic'' environments are distinguished with a prime; e.g. AA'. This key aspect of the notation, i.e., using a prime to differentiate between chemical equivalence only compared to full magnetic equivalence, was introduced by Richards and Schaefer in 1958. The notation can be used to represent systems of more than two nuclei, for example AMX represents three nuclei, each moderately separated from the others, and ABX represents two nuclei whose peaks are closely spaced and one other nucleus which is more distant. Examples: PHCl₂ is an AX system whereas CH₃CH₂F is an A₃M₂X system,

References

Nuclear magnetic resonance {{Chemistry-stub