Abstract
Theoretical approaches on the ground- and excited-state proton (or hydrogen atom) transfer in the 3-formyl-7-azaindole (3FAI)/formic acid dual hydrogen-bonded complex were performed. In the ground state, the analysis of the transition-state geometry led us to conclude a concerted, asynchronous proton-transfer pattern that correlates with the hydrogen-bonding strength. The lowest singlet excited state in the 3FAI/formic acid complex was calculated to be in an nπ* configuration. On the basis of frontier molecular orbital analyses, the n → π* transition was concluded to originate from the carbonyl lone-pair electrons of the formyl substitute. A highly endergonic proton-transfer reaction barrier of ∼16.7 kcal/mol was calculated in the 1nπ* state at the CIS/6-31G(d‘,p‘) level of theory. The second excited singlet state possesses a ππ* configuration in which the excited-state double proton transfer (ESDPT) takes place with a negligible energy barrier. The results provide a theoretical rationalization of the competitive internal conversion/ESDPT mechanism previously proposed for the 3FAI hydrogen-bonded complexes (J. Phys. Chem. A 2000, 104, 8863).
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