Decomposition pathways of formamide in the presence of vanadium and titanium monoxides.

Abstract

Thermally feasible decomposition pathways of formamide (FM) in the presence of vanadium VO(X(4)Σ(-)) and titanium TiO(X(3)Δ) monoxides are determined using density functional theory (BP86 functional) and coupled-cluster theory (CCSD(T)) computations with large basis sets. These diatomic metal oxides have been shown to be present in the prebiotic conditions. The dehydration, decarbonylation and dehydrogenation reactions of the molecular and dissociative complexes of FM and MO (M = V, Ti) turn out to be more favourable than those of the ground state isolated FM. The effect of addition of one or two water molecules on energy barriers is also probed for these reaction pathways. In some cases, a combined catalytic effect when adding water is observed. This enhanced catalytic effect was not observed in previously reported cases of FM transformation, for example, when adding water molecules into the mineral-catalyzed isomerizations of FM. The dehydration process of MO-FM complexes without the presence of water is found to be more feasible than the decarbonylation and dehydrogenation. The overall energy barrier for the non-water VO-FM dehydration is ∼3 kcal mol(-1) lower than the reference energy of the separated systems, whereas those of the two latter reactions are higher than the reference. Although the TiO-FM dehydration has a larger overall barrier of 14 kcal mol(-1) as compared to the VO-FM counterpart, the two other decomposition pathways still have much higher energy barriers. Direct formation of urea and H2CO from a FM dimer and indirect formation of urea from FM via the intermediate HNCO are also established. Urea formation in an indirect pathway is preferred. These low-energy-barrier pathways leading to the formation of important prebiotic molecules suggest that metal monoxides MO could play an important catalytic role in the prebiotic reactions of FM.