Intramolecular proton transfer in the ground and the two lowest-lying singlet excited states of 1-amino-3-propenal and related species Original Research Article

The potential energy surfaces of the ground state and the two lowest-lying singlet excited states of 1-amino-3-propenal, the cyclic complex of 1-amino-3-propenal with water, and salicylaldimine (2-iminomethylphenol) have been investigated theoretically along the proton transfer (PT) reaction coordinate. All these three systems have in common the same intramolecular H-bond through an OCCCN backbone. It has been found that the PT in the ππ* excited state of 1-amino-3-propenal has a very small energy barrier, which disappears after introduction of dynamic correlation, providing a pathway for an ultrafast photoinduced PT. The energy barriers for the PT processes of the ground and nπ* states increase when the transfer of the proton is carried out through a water molecule bridge. Calculations reveal two main differences between 1-amino-3-propenal and salicylaldimine: (a) while in 1-amino-3-propenal the keto tautomer is the most stable in all the electronic states studied, the ground state of salicylaldimine favors the enol structure; (b) the near degeneracy between the nπ* and ππ* excited singlet states in 1-amino-3-propenal is lost in salicylaldimine, for which the ππ* excited singlet state is stabilized as compared to the nπ* state. These results suggest that care must be taken in generalizing to larger molecules the conclusions obtained with models as small as 1-amino-3-propenal.