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.
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