Water-wire clusters:: Vibronic spectra of 7-hydroxyquinoline•(H2O)3

The supersonically cooled 7-hydroxyquinoline . (H2O)(3) cluster was investigated by mass- and isomer-selected S-1<--S-0 resonant two-photon ionization and S-1-->S-0 fluorescence spectroscopy. UV(ultraviolet)/UV-holeburning measurements prove that a single cluster isomer is formed, although different tautomers (7-keto- and 7-hydroxyquinoline), rotamers (cis and trans) and isomers (cyclic and water-wire) are possible. Ab initio calculations of structures and vibrations of different tautomers and isomers of this cluster predict that the cis-enol-7-hydroxyquinoline . (H2O)(3) "water-wire" cluster is the most stable species. The experimental S-0 and S-1 inter- and intramolecular vibrational frequencies are in good agreement with the calculated harmonic frequencies. S-1<--S-0 excitation leads to contraction of all four hydrogen bonds along the hydrogen bonded water wire, inducing intense intermolecular stretching vibrations of the O-H . . .O and HO-H . . .N(quinoline) hydrogen bonds which terminate the water-wire. There are no spectroscopic signs of proton transfer. (C) 2000 American Institute of Physics. [S0021-9606(00)00944-2].