INTRAMOLECULAR PROTON-TRANSFER IN THE GROUND AND THE 2 LOWEST-LYING SINGLET EXCITED-STATES OF 1-AMINO-3-PROPENAL AND RELATED SPECIES

The potential energy surfaces of the ground state and the two lowest-l ying singlet excited states of 1-amino-3-propenal, the cyclic complex of 1-amino-3-propenal with water, and salicylaldimine (2-iminomethylph enol) have been investigated theoretically along the proton transfer ( PT) reaction coordinate. All these three systems have in common the sa me intramolecular H-bond through an OCCCN backbone. It has been found that the PT in the pi pi excited state of 1-amino-3-propenal has a ve ry small energy barrier, which disappears after introduction of dynami c correlation, providing a pathway for an ultrafast photoinduced PT. T he energy barriers for the PT processes of the ground and n pi states increase when the transfer of the proton is carried out through a wat er 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 stu died, the ground state of salicylaldimine favors the enol structure; ( b) the near degeneracy between the n pi and pi pi* excited singlet st ates in 1-amino-9-propenal is lost in salicylaldimine, for which the p i pi excited singlet state is stabilized as compared to the n pi* sta te. These results suggest that care must be taken in generalizing to l arger molecules the conclusions obtained with models as small as 1-ami no-3-propenal. (C) 1998 Elsevier Science B.V. All rights reserved.