IR–UV double-resonance spectroscopy of the nucleobase adenine

Abstract

We present R2PI and IR–UV double resonance spectra of the nucleobase adenine seeded in a supersonic jet. We show that there is only one tautomer of adenine which absorbs in the wavelength range 36 050 to 36 700 cm⁻¹. The IR spectra, measured in the range 3200 to 3700 cm⁻¹, show bands at 3452, 3508 and 3569 cm⁻¹, which we assign to the symmetric NH₂ , N–H and antisymmetric NH₂ stretching vibrations of a single tautomer of adenine. We compare the experimental IR–UV double resonance spectra with ab initio based normal mode calculations. The observed tautomer is most probably the 9H amino-form of adenine.