Water-chain clusters: Vibronic spectra of 7-hydroxyquinoline centre dot (H_2O)_2

摘要

Mass- and ismoer-selected S_1<-S_0 resonant two-photon ionization and S_1->S_0 fluorescence spectra were obtained for the supersonically cooled 7-hydroxyquinoline centre dot (H_2O)_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 centre dot (H_2O)_2 “up/down” water-chain form is the most stable cluster. The pxperimentally 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.