Decomposition Pathways of the Neutral and Protonated Formamide in Some Lower-Lying Excited States

Unimolecular decompositions of neutral (NH2CHO) and protonated (NH3CHO+) formamide, an active precursor of biomolecules in prebiotic chemistry, are investigated in the ground (S-0) and first triplet (T-1) and singlet (S-1) excited states. Different decomposition channels including the homolytic bond dissociations, dehydration, decarbonylation, dehydrogenation, etc., are explored using coupled-cluster theory (CCSD(T)/CBS method) for both S-0 and T-1 states and RASPT2(18,15)/6-31 G(d,p) computations for the S-1 state. On S-1 and T-1 energy surfaces, formamide preferentially follows C-N homolytic bond cleavages forming NH2 + HCO radical pairs. Formation of HCN and HNC from dehydration of neutral and protonated formamide via formimic acid and aminohydroxymethylene isomers has higher energy barriers. A strong stabilization upon triplet excitation of the two latter isomers significantly facilitates the interconversions between isomers, and thus considerably reduces the energy barriers for dehydration pathways. The most probable pathways for HCN and HNC generation are found to be dehydration of formamide in the T-1 state. Dehydration pathways from the neutral S-1 and protonated T-1 forms lead to stable complexes of HCN and HNC with water but are associated with large energy barriers. Overall, in the lower-lying excited states of either neutral or protonated formamide, dehydration is not competitive with homolytic C-N bond cleavages, which finally lead to formation of CO.