Predicting the photodynamics of cyclobutanone triggered by a laser pulse at 200 nm and its MeV-UED signals—A trajectory surface hopping and XMS-CASPT2 perspective

Abstract:

This work is part of a prediction challenge that invited theoretical/computational chemists to predict the photochemistry of cyclobutanone in the gas phase, excited at 200 nm by a laser pulse, and the expected signal that will be recorded during a time-resolved megaelectronvolt ultrafast electron diffraction (MeV-UED). We present here our theoretical predictions based on a combination of trajectory surface hopping with XMS-CASPT2 (for the nonadiabatic molecular dynamics) and Born–Oppenheimer molecular dynamics with MP2 (for the athermal ground-state dynamics following internal conversion), coined (NA+BO)MD. The initial conditions were sampled from Born–Oppenheimer molecular dynamics coupled to a quantum thermostat. Our simulations indicate that the main photoproducts after 2 ps of dynamics are CO + cyclopropane (50%), CO + propene (10%), and ethene and ketene (34%). The photoexcited cyclobutanone in its second excited electronic state S2 can follow two pathways for its nonradiative decay: (i) a ring-opening in S2 and a subsequent rapid decay to the ground electronic state, where the photoproducts are formed, or (ii) a transfer through a closed-ring conical intersection to S1, where cyclobutanone ring opens and then funnels to the ground state. Lifetimes for the photoproduct and electronic populations were determined. We calculated a stationary MeV-UED signal [difference pair distribution function—ΔPDF(𝑟)] for each (interpolated) pathway as well as a time-resolved signal [ΔPDF(𝑟,𝑡) and Δ𝐼/𝐼(𝑠,𝑡)] for the full swarm of (NA+BO)MD trajectories. Furthermore, our analysis provides time-independent basis functions that can be used to fit the time-dependent experimental UED signals [both ΔPDF(𝑟,𝑡) and Δ𝐼/𝐼(𝑠,𝑡)] and potentially recover the population of photoproducts. We also offer a detailed analysis of the limitations of our model and their potential impact on the predicted experimental signals.

Additional Resources

DOI: 10.1063/5.0203105

Bibtex:

@article{jan24predicting,
    author = {Janoš, Jiří and Figueira Nunes, Joao Pedro and Hollas, Daniel and Slavíček, Petr and Curchod, Basile F. E.},
    title = {Predicting the photodynamics of cyclobutanone triggered by a laser pulse at 200 nm and its MeV-UED signals—A trajectory surface hopping and XMS-CASPT2 perspective},
    journal = {The Journal of Chemical Physics},
    volume = {160},
    number = {14},
    pages = {144305},
    year = {2024},
    month = {04},
    issn = {0021-9606},
    doi = {10.1063/5.0203105},
    url = {https://doi.org/10.1063/5.0203105},
    eprint = {https://pubs.aip.org/aip/jcp/article-pdf/doi/10.1063/5.0203105/19873879/144305\_1\_5.0203105.pdf},
}