Dissociative Electron Attachment to Formamide: Direct and Indirect Pathways from Resonant Intermediates

Dissociative electron attachment (DEA) to formamide (HCONH2), the smallest molecule with a peptide bond, is investigated with electron-molecule scattering calculations. At the equilibrium geometry we identify two resonances of A′′ and A′ symmetry at 3.77 and 14.90 eV, respectively. To further assess potential bond-breaking pathways for the transient negative ions (TNIs), the behavior of the resonances upon bond stretching of the C−H and C−N bond is investigated. While along the C−H dissociation coordinate neither resonance changes significantly, we find instead that both resonances are stabilized upon stretching the peptide C−N bond, with their resonance energy and width coming down rapidly, most strongly so for the A′ resonance. The A′ resonance is thus seen to disappear when the C−N bond is stretched for more than 1 Å, where it presumably smoothly connects to a bound anion state, a direct DEA pathway for the A′ TNI to yield NH2 − and HCO. The A′′ resonance is found instead not to be purely dissociative along the C−N coordinate but to evolve into forming a low-lying resonance on the NH2 fragment. Furthermore, symmetry considerations dictate here that the incoming electron attaches itself to an orbital of A′ symmetry of the NH2 − and HCO asymptotic fragments. Therefore, DEA from the A′′ TNI has to occur via a symmetry-breaking, nonadiabatic curve crossing which connects to the purely dissociative A′ metastable anionic state that is coming down in energy as the bond stretching occurs.