On the N1-H and N3-H Bond Dissociation in Uracil by Low Energy Electrons: A CASSCF/CASPT2 Study.

The dissociative electron-attachment (DEA) phenomena at the N-1-H and N-3-H bonds observed experimentally at low energies (<3 eV) in uracil are studied with the CASSCF/CASPT2 methodology. Two valence bound pi(-) and two dissociative sigma(-) states of the uracil anionic species, together with the ground state of the neutral molecule, are proven to contribute to the shapes appearing in the experimental DEA cross sections. Conical intersections (CI) between the pi(-) and sigma(-) are established as the structures which activate the DEA processes. The N-1-H and N-3-H DEA mechanisms in uracil are described, and experimental observations are interpreted on the basis of two factors: (1) the relative energy of the (U-H)(-) + H fragments obtained after DEA with respect to the ground-state equilibrium structure (S-0) of the neutral molecule (threshold for DEA) and (2) the relative energy of the CIs also with respect to S-0 (band maxima). The pi(-)(1) state is found to be mainly responsible for the N-1-H bond breaking, whereas the pi(-)(2) state is proved to be involved in the cleavage of the N-3-H bond.