Hydrogen Bonding and Intermolecular Vibrations of 7-Hydroxyquinoline-NH_3 in the S_0 and S_1 States

摘要

7-Hydroxyquinoline(NH3)~ microsolvent clusters have been shown to exhibit excited-state proton transfer (ESPT) in the S1 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’NH3 (7-HQ.NH3) complex. Mass- and rotamer-resolved S1 ~- So 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 a (stretch), fl~ and j~ (in-plane wagging), and -r (NH3 hindered internal rotation) were observed in the S0 and 51 states. Ab initio calculations using Hartree—Fock (SCF) and hybrid density functional (B3LYP) methods for the So state and the configuration interaction singles (CIS) method for the 51 state yield C,~ symmetric equilibrium structures with nearly linear 0—H’ “NH3 H-bonds. Agreement between the B3LYP/6-31 1++G(2d,2p) calculated vibrational frequencies and experimental 50-state frequencies is very good for both inter- and intramolecular modes. The ground-state effective internal rotation barrier of the NH3 group about its C3 axis was determined as V3(So) 73 cm1. S1 ~— So excitation leads to contraction of the R(O250L?‘N) distance by —0.062 A, accompanied by an increase of the H-bond dissociation energy by 2.62 kcallmol and an increase of the NH3 internal rotation barrier to V3(51) = 88 cm’. The H-bond contraction is in agreement with the SCF and CIS ab initio calculations which predict AR(0”’N) = —0.053 A. These calculations predict large intramolecular geometric changes which are not directly along the proton-transfer coordinate.