open access

https://hdl.handle.net/2115/98109

Substituent Effects on Electrocyclic Reactions : Ultrafast Ring-Opening of α-Phellandrene Stimulated by Impulsively Excited Molecular Vibrations

英語

2025-09-25

American Chemical Society

Journal of physical chemistry A

129(38)

8733-8743

The ring-opening reaction of 1,3-cyclohexadiene (CHD), involving a concerted electrocyclic process with simultaneous electronic and structural rearrangement, has been widely studied for its ability to modulate molecular functionality and confer specific biological activity. In this study, we investigate how substituents influence the ring-opening dynamics of (-)-alpha-phellandrene (alpha PH), a naturally occurring CHD derivative, following two-photon photoexcitation to Rydberg states using 3.1 eV pump pulses. The dynamics are probed by time-resolved photoelectron spectroscopy (TRPES) with extreme ultraviolet light (29.5 eV) in combination with quantum chemical calculations. Unlike CHD, which undergoes ballistic ring opening, alpha PH exhibits a stepwise reaction pathway in the ground state. This difference arises from the steric and mass effects of the isopropyl substituent, which elevate the energy of the first transition state and create a potential barrier. Ultrafast excitation to Rydberg states impulsively triggers coherent intramolecular vibrations, resulting in periodic isomerization between equatorial (eq) and axial (ax) conformers in the ground state of alpha PH at intervals of 436 fs. These oscillations facilitate the gradual accumulation of the ring-opened product. Intrinsic reaction coordinate (IRC) analyses further reveal that substitution on the CHD moiety substantially alters the potential energy surface, thereby modulating the reaction mechanism. These findings highlight the crucial role of structural substituents in governing ultrafast ring-opening dynamics in conjugated cyclic molecules.

学術雑誌論文

Accepted Manuscript

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of Physical Chemistry A, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://pubs.acs.org/articlesonrequest/AOR-YM6NZUCFW9X9CUMKZSEN.

1089-5639