Hydrogen bonding and intermolecular vibrations of 6-hydroxyquinoline H2O in the S0 and S1 states

A combined spectroscopic and ab initio theoretical study of the hydrogen-bonded 6-hydroxyquinoline . H2O complex was performed. 6-Hydroxyquinoline (6-HQ) is bifunctional, acting as an H-bond donor at the O-H group and as an acceptor at the N atom in bulk aqueous solution. Excited-state proton transfer (ESPT) from 6-HQ to the solvent occurs adiabatically in the SI state, involving proton transfer at both functional groups. We have measured the mass- and rotamer-resolved S-1 <-- S-0 vibronic spectra of cis- and trans rotamers of 6-HQ and of both the cis- and trans-6-HQ . H2O complexes in supersonic jets, using two-color resonant two-photon ionization spectroscopy and UV/UV spectral hole-burning techniques. Following the identification of the cis and trans rotamer electronic origins, dispersed fluorescence emission spectra of both rotamers of 6-HQ and 6-HQ . H2O were measured. The results are consistent with H2O bonded exclusively to the hydroxyl group and not to the N atom. Equilibrium structures were calculated by ab initio SCF methods for both rotamers of 6-HQ and both the cis- and trans-6-HQ . H2O complexes and give C-s symmetric structures with trans-linear hydrogen bonds. The calculated torsional transition states for H atom exchange on the H2O subunit are nonplanar with C-1 symmetry. The calculated H-bond dissociation energies are approximate to 5.9 kcal/mol for the cis and approximate to 6.0 kcal/mol trans rotamer, slightly stronger than those of 2-naphthol . H2O (5.8 kcal/mol) and phenol . H2O (5.6 kcal/mol). Harmonic normal-mode frequencies allow detailed assignments of the observed inter- and intramolecular vibronic transitions. Agreement between theory and experiment is generally good, with the exception of the beta(2) wag mode.