Term: hula-twist (HT) mechanism
https://doi.org/10.1351/goldbook.HT07393

Definition:
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. HT07393.png

Note: 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 make the OBF a less favourable process, allowing the volume-conserving HT to be the dominant process for @P04622@.

Related Terms:
1) one-bond-flip (http://doi.org/10.1351/goldbook.OT07432).
2) chromophore (http://doi.org/10.1351/goldbook.C01076).
3) viscosity (http://doi.org/10.1351/goldbook.V06627).
4) photoisomerization (http://doi.org/10.1351/goldbook.P04622).
5) isomerization (http://doi.org/10.1351/goldbook.I03295).

Image: Not defined (https://goldbook.iupac.org/img/inline/HT07393.png) 

Source: PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 352 (https://doi.org/10.1351/pac200779030293)

Citation: 'hula-twist (HT) mechanism' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.HT07393

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