LIF and IR dip spectra of jet-cooled p-aminophenol-M (M = CO, N2): Hydrogen-bonded or van der waals-bonded structure?

Intermolecular interaction and stable structures of the p-aminophenol-M (M=CO, N₂) 1:1 complexes have been studied by measuring the S₁ ← S₀ (ππ*) fluorescence excitation spectra and the IR dip spectra in the OH and NH stretch region combined with ab initio calculations. The S₁-S₀ electronic origin of the CO complex is 141 cm^-1 red shifted from the origin of the monomer. The red shift for the CO complex is smaller than 153 cm^-1 for the N₂ complex, although the molecular polarizability of carbon monoxide is larger than that of molecular nitrogen. The OH stretching frequency of the CO complex is 26 cm^-1 red shifted from that of the monomer in the IR dip spectrum, but the N₂ complex shows no shift. On the basis of these findings we concluded that carbon monoxide is bonded to the OH group via a hydrogen bond, whereas nitrogen is van der Waals-bonded to the π cloud of the aromatic ring.