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To understand how constitutional isomerism dictates non-adiabatic photochemistry, the authors mapped the real-time photoinduced ring opening and fragmentation of oxazole and isoxazole using MeV ultrafast electron diffraction (UED) coupled with trajectory surface hopping (TSH) molecular dynamics. Although both isomers produce deceptively similar UED signatures upon 200 nm excitation, TSH reveals fundamentally divergent mechanisms: isoxazole undergoes exclusive, ultrafast N–O bond cleavage within 40 fs, whereas oxazole proceeds via slower O–C cleavage over 290 fs with only 85% efficiency through a wider landscape of intermediates. This work provides an atomistic benchmark elucidating how shifting heteroatom connectivity reshapes conical intersection topography, excited-state decay channels, and fragmentation branching ratios.
Swapping adjacent heteroatoms in identical five-membered rings collapses ring-opening times from 290 fs to 40 fs, revealing that deceptively similar diffraction signatures can conceal entirely divergent non-adiabatic reaction pathways.
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.