Tautomers and electronic states of jet-cooled 2-aminopurine investigated by double resonance spectroscopy and theory

We present resonant two-photon ionization (R2PI), IR-UV, and UV-UV double resonance spectra of jet-cooled 2-aminopurine (2AP) as well as Fourier transform infrared (FTIR) gas phase spectra. 2AP is a fluorescing isomer of the nucleobase adenine. The results show that there is only one tautomer of 2AP which absorbs in the wavelength range 32 300-34 500 cm(-1). The comparison with the calculated IR spectra of 9H- and 7H-2AP points to 9H- 2AP as the dominating tautomer in the gas phase but the spectra are too similar to allow an unambiguous assignment to the respective tautomer. Hence, we determined vertical and adiabatic excitation energies of both tautomers employing combined density functional theory and multi-reference configuration interaction techniques. For the 0-0 band of the first (1)pi pi* transition of 9H-2AP we obtain a theoretical value of 32 328 cm(-1), in excellent agreement with the band origin of our R2PI spectrum at 32 371 cm(-1). The first singlet pi pi* transition of the less stable 7H-2AP tautomer is predicted to be red-shifted by about 1700 cm(-1) with respect to the corresponding transition in 9H- 2AP. From the absence of experimental bands in the energy region between 30 300 and 32 350 cm(-1) we conclude that 7H-2AP is not present to an appreciable extent in the molecular beam. Our calculations yield nearly equal energies for the (1)n pi* and (1)pi pi* minima of isolated 2AP, similar to the situation in adenine. The hitherto existing argument that the energetic order of states is responsible for the different spectroscopic properties of these isomers therefore does not hold. Rather, vibronic levels close to the origin of the (1)pi pi* transition cannot access the conical intersection between the (1)pi pi* and S-0 states along a puckering coordinate of the six-membered ring, in contrast to the situation in electronically excited 9H- adenine. As a consequence, a rich vibrational structure can be observed in the R2PI spectrum of 2AP whereas the spectrum of 9H-adenine breaks off at low energies.