Vibrational overtone spectroscopy of jet-cooled aminophenols as a probe for rotational isomers

Nonresonant ionization detected (NID) spectroscopy has been used to record the OH-, NH-, and CH-stretching vibrational overtone spectra of o-, m-, and p-aminophenol (aminohydroxybenzene; hydroxyaniline) in the collision-free environment of a supersonic jet. The OH-, NH-, and CH-stretching fundamental vibrations of jet-cooled o- and m-aminophenol have also been studied by NID and infrared-depletion techniques. We use an anharmonic oscillator local mode model, with one oscillator for each OH, NH, and CH bond, to calculate the frequencies and intensities of the stretching transitions and facilitate the assignment of the measured vibrational spectra. In the case of m-aminophenol, the presence of two rotational isomers is clearly demonstrated in the higher vibrational overtone spectra. For o-aminophenol, we find that one rotational isomer dominates, and the possible existence of a second is discussed. Electronic structure calculations are used to determine and explain the relative energies of the various rotational isomers. Our results show that vibrational overtone spectroscopy combined with jet-cooled conditions is a powerful technique for distinguishing between rotational isomers.