Photofragmentation of cyclobutanone at 200 nm: TD-DFT vs CASSCF electron diffraction

TL;DR

This study compares TD-DFT and CASSCF computational methods in simulating the ultrafast photofragmentation dynamics of cyclobutanone at 200 nm, revealing significant differences in predicted electron diffraction spectra.

Contribution

It demonstrates the impact of different electronic structure methods on simulating ultrafast molecular dynamics and electron diffraction spectra in photochemical processes.

Findings

TD-DFT predicts bond cleavage and ultrafast deactivation accurately.

CASSCF results differ significantly in electron diffraction spectra.

Triplet and higher singlet states are negligible in early dynamics.

Abstract

To simulate a 200 nm photoexcitation in cyclobutanone to the n-3s Rydberg state, classical trajectories were excited from a Wigner distribution to the singlet state manifold based on excitation energies and oscillator strenghts. Twelve singlet and twelve triplet states are treated using TD-B3LYP-D3/6-31+G$^{**}$ for the electronic structure and the nuclei are propagated with the Tully Surface Hopping method. Using TD-DFT, we are able to predict the bond cleavage that takes place on the S$_1$ surface as well as the ultrafast deactivation from the Rydberg n-3s state to the n$\pi^*$. After showing that triplet states and higher-lying singlet states do not play any crucial role during the early dynamics (i.e., the first 300 fs), the SA(6)-CASSCF(8,11)/aug-cc-pvDZ method is used as an electronic structure and the outcome of the non-adiabatic dynamic simulations is recomputed. Gas-phase ultrafast electron diffraction (GUED) spectra are computed for both electronic structure methods, showing significantly different results.