The o-cyanophenol dimer as studied by laser-induced fluorescence and IR fluorescence dip spectroscopy: a study of a symmetrical double hydrogen bond

The o-cyanophenol dimer has been characterised under supersonic-jet conditions by laser-induced fluorescence and IR–UV depletion spectroscopy associated with density functional theory (DFT) calculations. The ground state structure consists of a symmetrical, planar, doubly hydrogen-bonded bridge linking the phenolic OH groups to the N atom of the CN substituents. The origin of the S0→S1 transition is red shifted by 1094 cm−1 with respect to the monomer. Active vibrations in both electronic states are measured from the excitation and dispersed fluorescence spectra and are discussed on the basis of harmonic frequency DFT calculations. The IR depletion spectrum exhibits a very strong dip at 3322 cm−1, which is assigned to the out-of-phase combination (bu) of the ν(OH) stretching modes. This main band is accompanied by three bands of lower intensity at 3391, 3406 and 3444 cm−1. Their attribution is discussed in terms of the coupling with the intermolecular modes or of possible Fermi resonances.