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Substituent effects on dynamics at conical intersections: Allene and methyl allenes
Simon P Neville1, Yanmei Wang2, Andrey E Boguslavskiy1
1Department of Chemistry, University of Ottawa, 10 Marie Curie, Ottawa, Ontario K1N 6N5, Canada.
Ultrafast excited state dynamics in allenes were studied using femtosecond time-resolved photoelectron spectroscopy (TRPES) and ab initio multiple spawning (AIMS) simulations. Methylation influences relaxation pathways, with tetramethylallene showing distinct dynamics due to steric hindrance.
Area of Science:
- Physical Chemistry
- Chemical Physics
- Molecular Dynamics
Background:
- Ultrafast excited-state dynamics are crucial for understanding photochemical reactions.
- Conical intersections play a key role in efficient radiationless decay in molecules.
- Allene and its derivatives offer a model system to study structure-property relationships in excited-state dynamics.
Purpose of the Study:
- To investigate the ultrafast excited-state dynamics of allene and methylated analogues.
- To elucidate the role of conical intersections in the relaxation pathways.
- To understand how methylation affects the excited-state dynamics and relaxation mechanisms.
Main Methods:
- Femtosecond time-resolved photoelectron spectroscopy (TRPES) was used to probe excited-state dynamics.
- Ab initio multiple spawning (AIMS) simulations were employed to model reaction pathways.
- UV excitation at 200 nm (6.2 eV) initiated the dynamics.
Main Results:
- Two competing pathways, twisting and bending, were identified for ground-state relaxation.
- Methylation of allene derivatives modifies the branching ratios between these pathways.
- Tetramethylallene exhibits different dynamics due to steric hindrance from methyl groups impeding access to conical intersections.
- Observed TRPES decay times correlate with large-amplitude motions, not solely electronic evolution.
Conclusions:
- The study reveals that methylation selectively alters excited-state dynamics in allenes.
- Conical intersection-mediated relaxation is sensitive to molecular structure and steric effects.
- The interplay of inertial and potential energy surface effects governs the observed dynamical branching.
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