Excited-state non-adiabatic dynamics simulations of pyrrole

Non-adiabatic on-the-fly-dynamics simulations of the photodynamics of pyrrole were performed at multi-reference configuration interaction level involving five electronic states with a simulation time of 200 fs. The analysis of the time dependence of the average state occupations shows that the deactivation of pyrrole to the electronic ground state takes place in about 140 fs. This deactivation time agrees very well with the experimentally measured time constant of 110 fs for the formation of fast hydrogen atoms. After excitation into the S-4 state, 80% of the trajectories followed the NH-stretching mechanism giving rise to a population of fast H atoms. The computed average kinetic energy is in good accord with the experimentally observed average kinetic energy of the fast hydrogen atoms. It is found that 10% of trajectories followed the ring-puckering mechanism and 3% followed the ring-opening mechanism. This latter mechanism was characterized in pyrrole for the first time and involves the conical intersection of lowest energy of this molecule.