Water-chain clusters:: Vibronic spectra of 7-hydroxyquinoline•(H2O)2

Mass- and isomer-selected S-1<-- S-0 resonant two-photon ionization and S-1--> S-0 fluorescence spectra were obtained for the supersonically cooled 7-hydroxyquinoline .(H2O)(2) cluster. UV/UV-holeburning measurements show that > 98% of the spectrum is due to a single "water-chain" cluster isomer, although two different tautomers (7-keto- and 7-hydroxyquinoline), two different rotamers (cis- and trans-hydroxy), and two torsional conformers of the chain are possible. Ab initio calculations of structures and vibrations of five different tautomers/ rotamers/ conformers of this cluster are reported. These predict that the cis-7-hydroxyquinoline .(H2O)(2) "up/down" water-chain form is the most stable cluster. The experimentally observed S-0 and S-1 state vibrational frequencies agree well with those calculated for this isomer. We find no evidence for either the trans-rotamer or the keto tautomer clusters. S-1<-- S-0 excitation leads to contraction of all three hydrogen-bonds along the hydrogen-bonded water chain, inducing intermolecular stretching vibrations, but no proton transfer. (C) 2000 American Institute of Physics. [S0021-9606(00)01803-1].