Methanol-mediated excited-state double proton transfer in 1H-pyrrolo[3,2-h]quinoline: Concerted or Sequential Mechanism?

Abstract

The formation of a 1:1 hydrogen bonded complex of 1H-Pyrrolo[3,2-h]quinoline (1HPQ) with methanol has been characterized by quantum chemical calculations. To this end, we have resorted to the application of advanced quantum chemical tools e.g., Atoms-In-Molecule (AIM) and Natural Bond Orbital (NBO) analyses to characterize and elucidate the H-bond interactions present in 1HPQ:CH3OH complex. However, the major focus of the study underlies the exploration of the methanol-mediated excited-state double proton transfer (ESDPT) process in 1HPQ:CH3OH complex. Our computational results yield compelling evidence in favor of the feasibility of an ESDPT process while in contrary a corresponding ground-state process (GSDPT) is reasonably negated. The GSDPT process is inhibited by a high energy barrier (∼17.13kcalmol−1) coupled with the lack of an energy minimum for the tautomer configuration at the ground-state. Conversely, the operation of an ESDPT process is argued on the basis of a distinct reversal of thermodynamic stabilities of the two geometries (1HPQ:CH3OH normal form and the tautomer) in the excited-state together with the remarkable lowering of the energy barrier of the process. The unique configuration of the transition state (TS) along the ESDPT potential energy surface is tested by IRC calculations. The presence of no ionic intermediate concludes in favor of a concerted mechanism.