Hydrogen Bonding and Intermolecular Vibrations of 7-Hydroxyquinoline·NH₃ in the S₀ and S₁ States

7-Hydroxyquinoline·(NH 3 ) n microsolvent clusters have been shown to exhibit excited-state proton transfer (ESPT) in the S 1 state for n ≥ 4 [Bach, A.; Leutwyler, S. J. Chem. Phys . 2000, 112, 560.]. We present a combined spectroscopic and ab initio theoretical investigation of the first member of this cluster series, the hydrogen-bonded 7-hydroxyquinoline·NH 3 (7-HQ·NH 3 ) complex. Mass- and rotamer-resolved S 1 ↔ S 0 vibronic spectra of supersonic jet-cooled 7-HQ·NH 3 were obtained by two-color resonant two-photon ionization and dispersed fluorescence spectroscopy. Both the trans and cis rotamers are present in the jet, at a trans/cis ratio of 1:40. The H-bond vibrations σ (stretch), β 1 and β 2 (in-plane wagging), and τ (NH 3 hindered internal rotation) were observed in the S 0 and S 1 states. Ab initio calculations using Hartree−Fock (SCF) and hybrid density functional (B3LYP) methods for the S 0 state and the configuration interaction singles (CIS) method for the S 1 state yield C s symmetric equilibrium structures with nearly linear O−H···NH 3 H-bonds. Agreement between the B3LYP/6-311++G(2d,2p) calculated vibrational frequencies and experimental S 0 -state frequencies is very good for both inter- and intramolecular modes. The ground-state effective internal rotation barrier of the NH 3 group about its C 3 axis was determined as V 3 (S 0 ) = 73 cm -1 . S 1 ← S 0 excitation leads to contraction of the R (O···N) distance by −0.062 Å, accompanied by an increase of the H-bond dissociation energy by 2.62 kcal/mol and an increase of the NH 3 internal rotation barrier to V 3 (S 1 ) = 88 cm -1 . The H-bond contraction is in agreement with the SCF and CIS ab initio calculations which predict Δ R (O···N) = −0.053 Å. These calculations predict large intramolecular geometric changes which are not directly along the proton-transfer coordinate.