Conformational effects and level mixing in a doubly hydrogen-bonded 1 : 1 dimer of acetic acid and 3-fluorobenzoic acid

Vibronically resolved S1 ↔ S0 electronic spectra of a doubly hydrogen-bonded mixed dimer between acetic acid and 3-fluorobenzoic acid have been investigated by measuring laser-induced fluorescence excitation and dispersed fluorescence spectra in a cold supersonic jet expansion. Two isomeric species of the dimer have been identified, and the measurements show that their vibronic spectral patterns are significantly different. One of the isomers experiences distortion of dimer geometry upon S1 ← S0 excitation along the vibrational coordinate of a low-frequency hydrogen bond mode. Assignments of the vibronic spectral bands are suggested by correlating the observed band positions with the DFT-predicted normal mode vibrational frequencies. The methyl group of the acetic acid moiety has been found to be responsible to lower the IVR threshold of the electronically excited 3fluorobenzoic acid moiety to a significant extent. The spectral features reveal that the vibrational relaxation rate of the ring-breathing mode on S1 surface is possibly different in two conformers. The mechanism of the rotor effects, in terms of coupling of the large-amplitude rotor motion with the low-frequency hydrogen bond vibrations, has been discussed.