A Theoretical Investigation of Nonadiabatic Dynamics and Time-Resolved NEXAFS Spectra of Uracil.

Uracil, a fundamental RNA nucleobase, exhibits complex excited-state dynamics governed by nonadiabatic processes that prevent photodamage. Upon photoexcitation, population initially reaches the bright ππ* (S2) state and subsequently relaxes to the dark nπ* (S1) state via conical intersections, before returning to the ground state on picosecond-nanosecond time scales. In this work, we investigate the ultrafast S2 → S1 relaxation pathway using time-dependent density functional theory (TD-DFT)-based nonadiabatic surface hopping dynamics in combination with simulated time-resolved near-edge X-ray absorption fine structure (TR-NEXAFS) spectroscopy of oxygen, nitrogen and carbon K-edges. This combined approach allows us to capture both electronic and nuclear motion during the ultrafast relaxation and to identify atom-specific contributions to the decay process. In particular, ππ* to nπ* transitions occurring within the first 140 fs leave traces in O 1s and C 1s NEXAFS signals, indicating CO bond elongations and torsional motion along the CC bond, respectively. Overall, our results provide a detailed picture of population transfer, electronic-state trapping, and structural evolution in photoexcited uracil.