Cooperative triple-proton/hydrogen atom relay in 7-azaindole(CH3OH)2 in the gas phase: remarkable change in the reaction mechanism from vibrational-mode specific to statistical fashion with increasing internal energy.

The 7-azaindole-methanol 1:2 cluster [7AI(CH3OH)2] undergoes excited-state triple-proton/hydrogen atom transfer (ESTPT/HT) along the hydrogen-bonded network in the gas phase. The measurements of the resonance-enhanced multiphoton ionization (REMPI) spectra of 7AI(CH3OH)2-dn (n = 0−3), where subscript n indicates the number of deuterium, and the fluorescence excitation spectrum of 7AI(CH3OH)2-d0 allowed us to investigate the ESTPT/HT dynamics. By comparing the intensity ratios of the vibronic bands between 7AI(CH3OH)2-d0 and 7AI(CH3OH)2-d3 in REMPI spectra, we obtained the lower limit of an acceleration factor (falow) of 7AI(CH3OH)2-d0, which is the ratio of the reaction rate for the excitation of a vibronic state to that of the zero-point state in S1. The falow values are 2.7 ± 0.83 and 4.0 ± 1.2 for an in-phase intermolecular stretching vibration (σ1) and its overtone (2σ1) observed at 181 cm-1 and 359 cm-1 in the excitation spectrum, respectively, while that of the vibration (ν2/σ1 or ν3/σ1) at 228 cm-1...