Spectroscopic study on the structural isomers of 7-azaindole(ethanol)(n) (n=1-3) and multiple-proton transfer reactions in the gas phase.

The resonance-enhanced two-photon ionization (RE2PI) and laser-induced fluorescence excitation spectra were recorded for the S(1)-S(0)(pipi( *)) region of the 7-azaindole(ethanol)(n) (n=1-3) [7AI(EtOH)(n) (n=1-3)] clusters in the gas phase to investigate the geometrical structures and the multiple-proton/hydrogen atom transfer reaction dynamics. Four and two structural isomers were identified for 7AI(EtOH)(2) and 7AI(EtOH)(3), respectively. Density functional theory calculations at the B3LYP/6-31++G( * *)/6-31G( *) level predicted four different conformations of the ethyl group for 7AI(EtOH)(2), in good agreement with the observation of the four structural isomers in the RE2PI spectra. Visible fluorescence from the tautomeric forms was observed in the S(1) states for all isomers of 7AI(EtOH)(2), but no sign of double-proton/hydrogen atom transfer and quadruple-proton/hydrogen atom transfer has been obtained in the electronic spectra of 7AI(EtOH)(1) and 7AI(EtOH)(3), respectively. These results suggest that the multiple-proton transfer reaction is cluster-size selective, and the triple-proton/hydrogen atom transfer potential is dominated by the cyclic hydrogen-bonded network in 7AI(EtOH)(2). The excitation of the in-phase intermolecular stretching vibration prominently enhances the excited-state triple-proton/hydrogen atom transfer reaction.