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Summary

Description
English: Fermi interaction of NMR.

Green arrow: nuclear spin ±½.

Blue arrow: electron spin ±½.

1: Direct J-coupling in hydrogen gas. +½ spin 1H polarizes electron close to it to -½ spin state. Antiparallel spins can exist within the same orbital as described by Pauli exclusion principle. Following the same principle, this -½ electron polarizes the other σ-bond electron to +½ state. +½ electron polarizes 1H nucleus to -½ state. 1H nuclei are now coupled and antiparallel. Coupling constant is positive. As gyromagnetic ratio γ>0 for 1H nuclei, magnetic moment vectors μ are also antiparallel. μ of one nucleus is aligned withe magnetic field of the other nucleus. This is also energeticilly favorable in addition to spin antiparallel alignment.

2: 1H nuclei are parallel. This form is more unstable (has higher energy E) than the form 1. So, E1 < E2.

3: Positive vicinal J-coupling of 1H nuclei via 12C or 13C nuclei. Electron spins on p-orbitals of the carbons are parallel as described by Hund's rules (orbitals are filled with parallel spins).

Sources for the description and the adapted pictures:

  • RS Macomber: A complete introduction to modern NMR spectroscopy. p. 135 John Wiley & sons, Inc., 1998. ISBN 9780471157366
  • M Balcı: Basic ¹H- and ¹³C-NMR spectroscopy. p. 103-105 Elsevier, 2005. OCLC: 162130159. ISBN 9780444518118
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Author Keministi

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