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

Abstract

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 1pipi* 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 pipi* 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 1npi* and 1pipi* 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 1pipi* transition cannot access the conical intersection between the 1pipi* 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.

Adenine and 2-aminopurine: Paradigms of modern theoretical photochemistry

“…The intensity of the bands in the low-energy range of the absorption spectra of 2-aminopurine seems to be distributed between the two 1 (*) transitions. Regarding band origins, the resonant two-photon ionization spectrum of 2-aminopurine in the gas phase displayed an extended vibronic structure, with a band origin (T 0 ) for the recorded lowest-lying state at 4.01 eV (36,52), red-shifted 0.5 eV below the corresponding feature in adenine. The band origin in water is red-shifted to Ϸ3.70 eV (41).…”

“…Recently, the main role in the ultrafast relaxation of the nucleobases of low-energy CIs between the ground and a low-lying 1 (*) state has been established in cytosine, uracil, and adenine, whereas other states (n*, *) have been proposed to contribute to the different decay pathways (28)(29)(30)(31)(32)(33)(34)(35). Particularly relevant and valuable for the present work are recent multiconfigurational second-order perturbation theory (CASPT2) studies on adenine (34,35) and the analyses of several structures of adenine and 2-aminopurine at the density functional theory ͞multireference configuration interaction level (33,36). The most recent evidence highlights the relevance of out-of-plane distortions in all nucleobases (31,(33)(34)(35).…”

UV-Excitation from an Experimental Perspective: Frequency Resolved

“…Adenine, which is 6-aminopurine (6AP), also absorbs about 3500 cm -1 to the blue of its isomer 2-aminopurine (2AP). Comparing those two isomers, one notices not only the large difference in origin, but also the fact that the adenine spectrum features only a few lines while the 2-aminopurine spectrum is very extensive and covers a large frequency range 43 . These observations are consistent with a model that is 8 summarized schematically in Figure 5 42 .…”

The low-lying excited states of neutral polyacenes and their radical cations: a quantum chemical study employing the algebraic diagrammatic construction scheme of second order

“…Nevertheless, all existing methods possess individual weaknesses and drawbacks [1,2]. The success of the available methods is manifested by ample excellent applications in the fields of photochemistry and photobiology [13][14][15][16][17][18][19][20][21][22][23][24]. Even more than for closed-shell molecules, theoretical methods are urgently needed to reliably provide information about excited states of open-shell molecules, since neutral and charged radicals are of crucial importance in a broad range of chemical and biological processes.…”