Title: hula-twist mechanism
Long Title: IUPAC Gold Book - hula-twist mechanism
DOI: 10.1351/goldbook.HT07393
Status: current

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. 

Note
Under unconstrained conditions, the conventional one-bond-flip (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 chromophore in a protein cavity or in a solid matrix), the additional viscosity-dependent barriers make the OBF a less favourable process, allowing the volume-conserving HT to be the dominant process for photoisomerization.

Related Terms
- chromophore: https://goldbook.iupac.org/terms/view/C01076
- isomerization: https://goldbook.iupac.org/terms/view/I03295
- one-bond-flip: https://goldbook.iupac.org/terms/view/OT07432
- photoisomerization: https://goldbook.iupac.org/terms/view/P04622
- viscosity: https://goldbook.iupac.org/terms/view/V06627

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)

Other Outputs
- html: https://goldbook.iupac.org/terms/view/HT07393/html
- json: https://goldbook.iupac.org/terms/view/HT07393/json
- xml: https://goldbook.iupac.org/terms/view/HT07393/xml

Citation: Citation: 'hula-twist mechanism' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.HT07393
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Accessed: 2026-05-11T04:24:38+00:00