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·H 2 O 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 S 1 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·H 2 O 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·H 2 O were measured. The results are consistent with H 2 O 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·H 2 O complexes and give C s symmetric structures with trans-linear hydrogen bonds. The calculated torsional transition states for H atom exchange on the H 2 O subunit are nonplanar with C 1 symmetry. The calculated H-bond dissociation energies are ≈ 5.9 kcal/mol for the cis and ≈ 6.0 kcal/mol trans rotamer, slightly stronger than those of 2-naphthol·H 2 O (5.8 kcal/mol) and phenol·H 2 O (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 β 2 wag mode.