Anomalously slow proton transport of a water molecule

Unusually low proton-transporting ability of a water molecule has been observed in the excited-state proton transfer (ESPT) of a 7-azaindole (7AI) molecule complexed cyclically with a water molecule in diethyl ether and dipropyl ether. In contrast with ultrafast (<1 ps) proton diffusion along a systematically structured hydrogen-bond network in an aqueous solution, the proton transport of a water monomer has been observed to be extremely slow (∼1 ns) because it is hard for a monomeric water molecule alone to accept or donate a proton. Thus, polar ether molecules surrounding a cyclic hydrogen-bonded 1:1 7AI-water complex (Nc) play a crucial role in the ESPT of Nc. The proton acceptance of a water molecule from the acidic amino group of 7AI via tunneling to form a hydronium ion, which is the rate-determining step, initiates ESPT, and the subsequent rapid proton donation of the hydronium ion to the basic imino group of 7AI takes place barrierlessly to complete ESPT without accumulating any intermediate. Due to the anomalously weak proton-transporting ability of a water monomer, the elaborate reorganization of the hydrogen-bond bridge in Nc to form an optimized precursor configuration is required for proton tunneling.