Evidence for the need of a non-Born-Oppenheimer description of excited-state hydrogen transfer

The HOCHCHCHNH (enol from) to OCHCHCHNH₂ (keto form) excited-state isomerization of 1-amino-3-propenal is considered as a model system for photo-induced intramolecular hydrogen (or proton) transfer. The potential energy surfaces of the electronic ground state as well as of the lowest nπ* and ππ* excited singlet states have been characterized by ab initio compelte-active-space self-consistent-field and multireference configuration-interaction calculations. It is found that the nπ* potential energy function exhibits a significant barrier along the proton-transfer reaction coordinate, while the ππ* potential energy function is almost barrierless and intersects the nπ* surface, providing a pathway for ultrafast photo-induced proton transfer. It is argued that consideration of nπ*ππ* vibronic coupling is essential in understanding the photo-induced proton-transfer reaction.