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 A,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 symmetrgamma-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.
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