American Chemical Society
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Excited-State Triple-Proton Transfer in 7-Azaindole(H2O)2 and Reaction Path Studied by Electronic Spectroscopy in the Gas Phase and Quantum Chemical Calculations

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journal contribution
posted on 2010-10-28, 00:00 authored by Kenji Sakota, Christophe Jouvet, Claude Dedonder, Masaaki Fujii, Hiroshi Sekiya
We have investigated the excited-state multiple-proton/hydrogen atom transfer reactions in the 7-azaindole water clusters, [7AI](H2O)n (n = 2,3), in the gas phase by combining electronic spectroscopy and quantum chemical calculations. The fluorescence excitation (FE) spectrum of 7AI(H2O)2 has been observed by monitoring visible emission. In contrast, no vibronic bands are detected in the FE spectrum of 7AI(H2O)3 when the visible emission is monitored. The dispersed fluorescence spectra of 7AI(H2O)n (n = 2,3) have been measured. The excitation of +180 cm−1 band from the electronic origin of 7AI(H2O)2 enhances the visible emission as compared with the 0−0 excitation. The +180 cm−1 band is assgined to an intermolecular mode (σ(1)) of the cyclic hydrogen-bonded ring structure. The calculated S1−S0 absorption spectrum for the cyclic hydrogen-bonded structure is in agreement with the FE spectrum around the 0−0 region. The excitation of σ(1) significantly promotes the reaction and generates the tautomeric form of 7AI(H2O)2. These experimental results on 7AI(H2O)n (n = 2,3) are very similar to those on 7AI(CH3OH)n (n = 2,3) and 7AI(C2H5OH)n (n = 2,3). We conclude that the excited-state triple proton/hydrogen atom transfer (ESTPT/HT) occurs in 7AI(H2O)2. Cuts of the potential energy surfaces along the proton/hydrogen atom transfer coordinates of 7AI(H2O)n (n = 2,3) and 7AI(CH3OH)n (n = 2,3) are comparatively calculated by quantum chemistry calculations (RI-CC2/cc-pVDZ and TD-DFT(B3LYP)/cc-pVDZ) to explore the mechanism of the ESTPT/HT reaction. The calculated results suggest that concerted proton transfers occur in 7AI(H2O)2 as well as in 7AI(CH3OH)2, whereas the potential barrier for the excited-state quadruple proton transfer in 7AI(H2O)3 and 7AI(CH3OH)3 is higher than those for ESTPT. The theoretical results are consistent with the observation of ESTPT/HT in 7AI(H2O)2.