Theoretical study on excited-state multiple proton-transfer process of 7-azaindole with ammonia and mixed water-ammonia: Stepwise or concerted?

Abstract

A detailed study of the excited-state tautomerization of 7AI-(NH3)n (n = 1, 2), 7AI-H2O-NH3 and 7AI-NH3-H2O complex in the gas phase were investigated at the CASSCF(10, 9)/6-31G(d, p) level theoretically. The effect of dynamic electron correlation on the barrier height and reaction energy was considered by MRPT2. The excited-state double proton transfer in 7AI-NH3 takes place in a concerted but asynchronous protolysis pathway. However, the triple proton transfer in 7AI cluster along the H-bonded ammonia dimer prefers a two-step process with a distinct intermediate. Each proton transfer step was a concerted but asynchronous path. The formation of H-bonded chain consisting of mixed water-ammonia affected the excited-state proton transfer mechanism. With the stabilization effect of the second solvent molecule, the first solvent molecule with large or small proton-accepting ability made the ESTPT occur in a stepwise or concerted pattern.