Triple proton transfer after water rearrangement in (2,6-aza)Ind•(H2O)2

We have studied the electronic equilibrium structures and the tautomerization process in (2,6-aza)Ind center dot(H2O)(2-3) cluster in the gas phase and in solvent water by DFT and TDDFT methods. In (2,6-aza)Ind center dot (H2O)(2) cluster, we found three novel stable clusters (NCHB-I1, NCHB-I2 and NCHB-O) with water line hydrogen boned in the molecular plane and under the molecular plane, respectively. Especially, the water line in NCHB-O is connected with the proton donor and proton acceptor, which provides a route for proton transfer. Atoms in molecules (AIM) and non-covalent interactions (NCI) analysis were used to analyze the interaction strength and interaction type in them. We also found that NCHB-I1 and NCHB-I2 could isomerize to NCHB-O via water rearrangement, from where triple proton transfer takes place. Further analysis shows that excited-state triple proton transfer follows an asynchronous concerted mechanism but with considerably high energy barrier in both gas phase and in water which makes it difficult to occur. In (2,6aza)Ind center dot(H2O)(3), the ADMP simulations show that water rearrangement could occur from the bridged normal cluster to the 2 + 1 type cluster (2,6-aza)Ind center dot(H2O)(2+1) with two water molecules in the first shell and a single water in the second shell. The excited-state IRC calculation shows that energy barrier for the excited-state triple proton transfer in (2,6-aza)Ind center dot(H2O)(2+1) is slightly lowered after a single water in the second shell compared with the one in (2,6-aza)Ind center dot H2O)(2). On the other hand, the energy barrier for the excited-state quadruple proton transfer in (2,6-aza)Ind center dot(H2O)(3) is considerably lower than the ones for triple proton transfer in (2,6-aza)Ind center dot(H2O)(2) and (2,6-aza)Ind center dot(H2O)(2+1), suggesting that the quadruple proton transfer mechanism is more preferred instead of triple proton transfer. This study provides significant insight into the proton transfer behavior in similar cluster systems, also shows the significance of water rearrangement in the proton transfer processes. (C)2022 Elsevier B.V. All rights reserved.