Excited-state non-adiabatic dynamics simulations of pyrrole

Abstract

Non-adiabatic on-the-fly-dynamics simulations of the photodynamics of pyrrole were performed at multireference 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 S₄ 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.

Keywords:

• non-adiabatic dynamics
• conical intersection
• photochemistry
• pyrrole

Acknowledgements

This work was supported by the Austrian Science Fund within the framework of the Special Research Programme F16 (Advanced Light Sources) and Project P18411-N19. The calculations were partially performed at the Linux PC cluster Schrödinger III of the computer centre of the University of Vienna. The work in Zagreb (M.E.M and M.V.) was supported by the Ministry of Science, Education and Sport through the project 098-0982933-2920 and the COST D37 action.