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