Many examples are known in which hydrogen atoms migrate from one end of a system of p-bonds to the other end through pericyclic mechanisms ("sigmatropic" rearrengements during which the C-H s-bond migrates within a system of p-bonds). In these concerted processes, the hydrogen must, in the transition state, be in contact with both ends of the p-system at the same time. In accordance with the Woodward-Hoffmann rules for these reactions, the thermal 1,5-H-shift in 1,3-pentadiene is a suprafacial process, and the hydrogen atom moves either along the top or the bottom side of the p-system, but not across the corresponding plane of p-atoms (the opposite would apply for the photochemically induced reaction). The transition state is of C_S symmetry and highly aromatic based on the geometric, energetic and magnetic criteria. The animations on the left visualize the degenerate 1,5-H-shift for 1,3-pentadiene as both the start and final geometries are identical.

However, substitution including isotopes would lead to distinguishable structures. For further details see H. Jiao, P. von R. Schleyer, "Aromaticity of pericyclic reaction transition structures: magnetic evidence.", J. Phys. Org. Chem. 1998, 11, 655-662.

In analogy to the 1,5-H-shift in 1,3-pentadiene (as described above), the the thermal migration of the methylene ring protons in cyclopentadiene may occur along the ring on its top or bottom face (suprafacial "sigmatropic" rearrengement), resulting in a degenerate isomerization unless other non-hydrogen substituents are present in the molecule. Migrations of this kind which occur in rings are also called "circumambulatory rearrangements". For further details see also H. Jiao, P. von R. Schleyer, "Aromaticity of pericyclic reaction transition structures: magnetic evidence.", J. Phys. Org. Chem. 1998, 11, 655-662.

In contrast to 1,5-H-shifts (see above), the corresponding thermal 1,7-H-shifts in 1,3,5-heptatrienes are anatarfacial processes, in which the hydrogen atom must move across the plane of p-atoms either from the top to the bottom of the p-systems or vice versa; the transition state of this rearrangement is of C₂ symmetry and highly delocalized with eight electrons. Unlike the Hückel aromatic transition states with 4n + 2 electrons, thermally allowed transition structures with 4n delocalized electron are Möbius aromatics. The animations on the left display the degenerate rearrangement for 1,3,5-heptatriene; for further details see H. Jiao, P. von R. Schleyer, "Aromaticity of pericyclic reaction transition structures: magnetic evidence.", J. Phys. Org. Chem. 1998, 11, 655-662.