Nonequilibrium H/D isotope effects from trajectory-based nonadiabatic dynamics.

Ground-state equilibrium kinetic isotope effects can be treated well in the framework of transition state theory, whereas excited-state nonequilibrium isotope effects are theoretically less explored. In this article we show for the first time that trajectory-based nonadiabatic dynamics simulations are able to reproduce experimental values for nonequilibrium H/D isotope effects in excited-state processes. We use high-level electronic structure calculations (MS-CASPT2, DFT/MRCI, and TDDFT) and full-dimensional OM2/MRCI-based nonadiabatic dynamics simulations to study the ultrafast intramolecular excited-state proton transfer (ESIPT) and the subsequent deactivation of 7-(2-pyridyl)indole (7PyIn) and its deuterated analogue (7PyIn-D). We evaluate a total of 1367 surface-hopping trajectories to establish the differences in the dynamical behavior of 7PyIn and 7PyIn-D. The computed H/D isotope effects for ESIPT and excited-state decay are consistent with recent experimental results from femtosecond pump-probe resonance-enhanced multiphoton ionization spectroscopy. We also analyze the influence of temperature fluctuations in the initially prepared sample on the photodynamics of 7PyIn and 7PyIn-D.