Heteroatom Position Controls Ultrafast Photodynamics of Oxazole and Isoxazole
Abstract
The ultrafast photochemistry of heterocyclic compounds is central to photobiology and materials chemistry, yet direct experimental observation of their structural dynamics remains rare. Here we present the first real-time structural characterization of photoinduced ring opening and subsequent fragmentation in the isomeric pair oxazole and isoxazole using MeV ultrafast electron diffraction (UED), complemented by non-adiabatic molecular dynamics simulations. Upon photoexcitation at 200 nm, both isomers undergo ring opening followed by fragmentation into various products, but with strikingly different dynamics governed by heteroatom positioning. Trajectory Surface Hopping simulations reproduce the experimental diffraction signatures, which are surprisingly similar for both isomers, and reveal distinct mechanistic pathways for the two isomers. For isoxazole, all trajectories exclusively undergo N-O bond cleavage within 40 fs, followed by sequential fragmentation into HCN + ketene and HCO + vinyl nitrene channels on the hundreds-of-femtoseconds timescale. Oxazole, by contrast, shows significantly slower ring opening (290 fs) with only 85% efficiency, proceeding primarily through O-C cleavage and accessing a richer landscape of intermediates including nitrile ylide and O-pyramidalized structures. Simulated diffraction patterns derived from trajectory ensembles, convolved with the experimental instrument response function, are in agreement with the branching ratios observed by UED. This synergy between UED and trajectory surface hopping provides an atomistic picture of how the simple interchange of heteroatom connectivity in structural isomers fundamentally reshapes excited-state potential surfaces, conical intersection accessibility, and photochemical outcome.
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BibTeX
@misc{downesward2026heteroatom,
title = {{Heteroatom Position Controls Ultrafast Photodynamics of Oxazole and Isoxazole}},
author = {Briony Downes-Ward and Paul Javed and Huynh V. S. Lam and Sajib K. Saha and Surjendu Bhattacharyya and Martin Centurion and Xinxin Cheng and R. Joel England and Casey Foley and Smita Ganguly and Patrick L. Kramer and Jinxin Lang and Randy Lemons and Ming-Fu Lin and Yusong Liu and Tu T. Nguyen and Joao Pedro Figueira Nunes and Chatura Perera and Alexander H. Reid and Ethan Ross and Artem Rudenko and Xiaozhe Shen and John Searles and Anbu Selvam Venkatachalam and Enliang Wang and Stephen P. Weathersby and Vinod Kumarappan and Arthur G. Suits and Christine Aikens and Daniel Rolles},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.31875},
eprint = {2609.31875},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.31875},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Heteroatom Position Controls Ultrafast Photodynamics of Oxazole and Isoxazole
AU - Briony Downes-Ward
AU - Paul Javed
AU - Huynh V. S. Lam
AU - Sajib K. Saha
AU - Surjendu Bhattacharyya
AU - Martin Centurion
AU - Xinxin Cheng
AU - R. Joel England
AU - Casey Foley
AU - Smita Ganguly
AU - Patrick L. Kramer
AU - Jinxin Lang
AU - Randy Lemons
AU - Ming-Fu Lin
AU - Yusong Liu
AU - Tu T. Nguyen
AU - Joao Pedro Figueira Nunes
AU - Chatura Perera
AU - Alexander H. Reid
AU - Ethan Ross
AU - Artem Rudenko
AU - Xiaozhe Shen
AU - John Searles
AU - Anbu Selvam Venkatachalam
AU - Enliang Wang
AU - Stephen P. Weathersby
AU - Vinod Kumarappan
AU - Arthur G. Suits
AU - Christine Aikens
AU - Daniel Rolles
PY - 2026
DA - 2026/09/25
PB - arXiv
DO - 10.48550/arxiv.2609.31875
UR - https://arxiv.org/abs/2609.31875
AB - The ultrafast photochemistry of heterocyclic compounds is central to photobiology and materials chemistry, yet direct experimental observation of their structural dynamics remains rare. Here we present the first real-time structural characterization of photoinduced ring opening and subsequent fragmentation in the isomeric pair oxazole and isoxazole using MeV ultrafast electron diffraction (UED), complemented by non-adiabatic molecular dynamics simulations. Upon photoexcitation at 200 nm, both isomers undergo ring opening followed by fragmentation into various products, but with strikingly different dynamics governed by heteroatom positioning. Trajectory Surface Hopping simulations reproduce the experimental diffraction signatures, which are surprisingly similar for both isomers, and reveal distinct mechanistic pathways for the two isomers. For isoxazole, all trajectories exclusively undergo N-O bond cleavage within 40 fs, followed by sequential fragmentation into HCN + ketene and HCO + vinyl nitrene channels on the hundreds-of-femtoseconds timescale. Oxazole, by contrast, shows significantly slower ring opening (290 fs) with only 85% efficiency, proceeding primarily through O-C cleavage and accessing a richer landscape of intermediates including nitrile ylide and O-pyramidalized structures. Simulated diffraction patterns derived from trajectory ensembles, convolved with the experimental instrument response function, are in agreement with the branching ratios observed by UED. This synergy between UED and trajectory surface hopping provides an atomistic picture of how the simple interchange of heteroatom connectivity in structural isomers fundamentally reshapes excited-state potential surfaces, conical intersection accessibility, and photochemical outcome.
ER -