Q1(b). Photochemical hydrogen migration in a diene

Answer in brief

Photoexcitation changes the diene occupancy from ψ₁²ψ₂² to ψ₁²ψ₂¹ψ₃¹; a photochemical [1,3]-H shift is suprafacially allowed.

Key points

• Direct π–π* absorption initially forms an excited singlet; intersystem crossing can subsequently populate a triplet.
• For the allylic fragment H–CH₂–C(R)=C(R′)–, a [1,3] shift transfers H to the other end while moving C=C to the original C–H-bearing end.
• In the excited-state correlation, same-face overlap of the migrating H 1s orbital with the two terminal lobes is constructive, so the photochemical pathway is suprafacial.
• The thermal suprafacial counterpart is symmetry-forbidden; geometry and conditions determine competition with isomerization or electrocyclization.