Hydrogen bonding and intermolecular vibrations of 7-hydroxyquinoline•NH3 in the S0 and S1 states

7-Hydroxyquinoline(.)(NH3)(n) microsolvent clusters have been shown to exhibit excited-state proton transfer (ESPT) in the S-i state for n greater than or equal to 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.NH3) complex. Mass- and rotamer-resolved S-i <-- S-o vibronic spectra of supersonic jet-cooled 7-HQ.NH3 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 sigma (stretch), beta (1) and beta (2), (in-plane wagging), and tau (NH3 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 fur the SII state and the configuration interaction singles (CIS) method for the S-1 state yield C-s symmetric equilibrium structures with nearly linear O-H...NH3 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 NH3 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 Angstrom, accompanied by an increase of the H-bond dissociation energy by 2.62 kcal/mol and an increase of the NH3 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 DeltaR(O...N) = -0.053 Angstrom. These calculations predict large intramolecular geometric changes which are not directly along the proton-transfer coordinate.