IR-UV double-resonance spectroscopy of the nucleobase adenine

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.