Jet-cooled spectroscopy of paracetamol.

We have measured the resonant 2-photon ionization spectrum of jet-cooled p-acetaminophenol (paracetamol) between 33,400 cm(-1) and 45,500 cm(-1), and analysed the results in the light of theoretical calculations of the ground-state geometry and vibrational frequencies. The experimental spectrum consists of a large number of clearly resolved vibronic transitions, and some evidence of rotational structure. It is dominated by a repeated progression with a regular spacing of 33 cm(-1) (believed to be the methyl-torsion mode) in combination with normal modes of vibration which involve the stretching of the C-C bonds in the phenyl ring and bending of the C-N-C bond in the amide group. The intensity pattern of the progressions in the methyl torsion is consistent with structural changes between the minimum-energy geometries in the ground and excited states.