Combined density functional and multireference configuration interaction methods have been used to calculate the electronic spectrum of 9H-adenine, the most stable tautomer of 6-aminopurine. In addition, constrained minimum energy paths on excited potential energy hypersurfaces have been determined along several relaxation coordinates. The minimum of the first [nπ*]1 state has been located at an energy of 4.54eV for a nuclear arrangement in which the amino group is pyramidal whereas the ring system remains planar. Close by, another minimum on the S1 potential energy hypersurface has been detected in which the C2 center is deflected out of the molecular plane and the electronic character of S1 corresponds to a nearly equal mixture of [ππ*]1 and [nπ*]1 configurations. The adiabatic excitation energy of this minimum amounts to 4.47eV. Vertical and adiabatic excitation energies of the lowest nπ* and ππ* transitions as well as transition moments and their directions are in very good agreement with experimental data and lend confidence to the present quantum chemical treatment. On the S1 potential energy hypersurface, an energetically favorable path from the singlet nπ* minimum toward a conical intersection with the electronic ground state has been identified. Close to the conical intersection, the six-membered ring of adenine is strongly puckered and the electronic structure of the S1 state corresponds to a ππ* excitation. The energetic accessibility of this relaxation path at about 0.1eV above the singlet nπ* minimum is presumably responsible for the ultrafast decay of 9H-adenine after photoexcitation and explains why sharp vibronic peaks can only be observed in a rather narrow wavelength range above the origin. The detected mechanism should be equally applicable to adenosine and 9-methyladenine because it involves primarily geometry changes in the six-membered ring whereas the nuclear arrangement of the five-membered ring (including the N9 center) is largely preserved.

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