Are conical intersections responsible for the ultrafast processes of adenine, protonated adenine, and the corresponding nucleosides?

Excited-state potential energy surfaces of adenine, protonated adenine, and their N9-methylated analogs are explored by means of a complete active space (CAS) and time-dependent density functional theory (TD-DFT) study to understand the dynamics associated with internal conversion. After photoexcitation of the ground-state molecules to the S₁ state, the nuclear motions that are responsible for taking the wavepacket out of the Franck-Condon region are either an H-N9/C-N9 stretch or a ring-puckering motion that leads to pyramidalization. These motions lead to accessible conical intersections with the groundstate surface. The results are used to successfully interpret previous measurements on the photodissociation of adenosine 5′-monophosphate nucleotide onions and cations, where the latter react in a highly nonstatistical manner.