Formation of amino acid precursors in the interstellar medium. A DFT study of some gas-phase reactions starting with methylenimine

To evaluate feasibility of the formation of amino acid precursors in dense interstellar clouds from methylenimine, HCN, CN radical, water, and OH radical, stationary points for four gas-phase reactions have been computed at the B3LYP/6-311++G(d,p) level of theory. All the reactions are exothermic. For the reaction HN=CH2 + H-C equivalent toN --> H2NCH2-C equivalent toN, three high-energy transition states have been found (energies of ca. 23-33 kcal mol(-1) relative to the reactant level) which do not allow this reaction to occur as a purely gas-phase process. The same conclusion has been done for the further glycine amide formation according to the pathway H2N-CH2-C equivalent toN + H2O --> H2N-CH2 -C(=O)-NH2, where three transitions states have been obtained as well. one of them having relative energy of ca. 52. kcal mol(-1). Reaction HN= CH2 + .C equivalent toN --> H2N-C(. )H-C equivalent toN (> 70 kcal mol(-1) exothermic) exhibits no positive barriers at the theoretical level employed; it is the only one unconditionally feasible in the gas-phase. Despite the fact that for reaction H2N-CH2-C equivalent toN + . OH --> H2N-C(. )H-C(=O)-NH2 (ca. 60 kcal mol(-1) exothermic) four transition states have been found, it might also appear plausible under the interstellar conditions for the following reasons: all the barriers are associated with exclusively proton-transfer processes; only one of them is positive, and three lay below the level of reactants; for the only positive barrier, proton tunneling might facilitate the reaction. Radical products H-NC(. )HCN and H2NC(. )HC(=O)NH2 forming in the latter two cases have a free valence at the carbon atom corresponding to a-C atom in the molecules of biological amino acids. Combining them with a vast variety of open-shell interstellar species is suggested as a possible route to diverse amino acid derivatives in dense molecular clouds.