Volume-conserving mechanism of photoisomerization of a double bond in a conjugated system involving simultaneous configurational and conformational isomerization, e.g., the photoinitiated concerted rotation of two adjacent double and single bonds.
Under unconstrained conditions, the conventional @OT07432@ (OBF) process is the dominant process with the hula-twist (HT) being an undetectable higher energy process. It has been proposed that under confined conditions (e.g., a conjugated double bond @C01076@ in a protein cavity or in a solid matrix), the additional @V06627@-dependent barriers makes the OBF a less favourable process, allowing the volume-conserving HT to be the dominant process for @P04622@.
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 352 (https://doi.org/10.1351/pac200779030293)