title: Theoretical study on water-mediated excited-state multiple proton transfer in 7-azaindole: significance of hydrogen bond rearrangement.
abstract: Excited-state multiple proton transfer (ESMPT) in the cluster of 7-azaindole with three water molecules [7-azaindole(H(2)O)(3)] is theoretically investigated by the TDDFT, CASPT2, and CC2 methods. Examination of the potential energy surface in the first excited state indicates that ESMPT in 7-azaindole(H(2)O)(3) proceeds initially with the rearrangement of hydrogen bond structure of water molecules from a bridged-planar isomer to a cyclic-nonplanar isomer, followed by triple proton transfer in the latter. This reaction is found to be energetically more favorable than quadruple proton transfer in the bridged-planar isomer without hydrogen bond reorganization. It is also shown that all proton-transfer processes follow a concerted mechanism rather than a stepwise mechanism. The computational results show good consistency with the unexpected experimental observations as to the electronic spectra and excited-state lifetime. In particular, the barrier of the hydrogen bond rearrangement is found to be less than 1 kcal/mol, consistent with the missing vibronic bands for 7-azaindole(H(2)O)(3) with an excess energy of more than 200 cm(-1) in the S(1) state.
authors.affiliation: Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan.
authors.forename: Xue-fang
authors.initials: XF
authors.lastname: Yu
authors.forename: Shohei
authors.initials: S
authors.lastname: Yamazaki
authors.forename: Tetsuya
authors.initials: T
authors.lastname: Taketsugu
mesh_terms: D006860:Hydrogen Bonding; D007211:Indoles / Q000737:chemistry*; D007536:Isomerism; D008956:Models, Chemical; D008958:Models, Molecular; D011522:Protons; D011789:Quantum Theory; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
chemical_list: C023422:7-azaindole dimer; D007211:Indoles; D011522:Protons; D014867:Water