Jet-cooled spectroscopy of paracetamol

Abstract

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.