Tautomers and electronic states of jet-cooled adenine investigated by double resonance spectroscopy

We present resonant two-photon ionization (R2PI) and IR-UV double resonance spectra of the nucleobase adenine seeded in a supersonic jet. We show that there are two tautomers of adenine which absorb in the wavelength range 35400 to 36700 cm(-1). The IR spectra, measured in the range 3200 to 3700 cm(-1) via hole burning at 36105 (35824) cm(-1), show bands at 3452 (3437), 3508 (3507/3514) and 3569 (3550) cm(-1), which we assign to the symmetric NH2, N-H and antisymmetric NH2 stretching vibrations of the 9H-(7H-) tautomers of adenine by comparison with ab initio based normal mode calculations. The IR spectrum taken via a much weaker band at 35 497 cm(-1) agrees completely with the 9H spectrum analysed at 36 105 cm(-1) so that an earlier hypothesis (N.J. Kim, G. Jeong, Y.S. Kim, J. Sung and S. K. Kim, J. Chem. Phys., 2000, 113, 10 051, ref. 7) of an npi* transition contributing to the red part of the spectrum is very probable. It is shown that the conventional IR absorption spectrum of adenine in the gas phase is a superposition of the IR spectra of the 9H- and 7H- tautomers and possibly other tautomers. Despite the short lifetime of electronically excited adenine we were able to perform two-color R2PI to obtain the ionization threshold of the 9H tautomer (69400+/-50 cm(-1)=8.606+/-0.006 eV).