Can Cytosine, Uracil, and Thymine Be Formed from HC₃N and H₂NCO⁺ in Interstellar Space?

Syntheses of cytosine, uracil, and thymine starting from interstellar molecules were examined theoretically. Potential energy surfaces for the formation of protonated cytosine (CyH⁺), uracil (UrH⁺), and thymine (ThH⁺) from cyanoacetylene (HC₃N), protonated isocyanic acid (H₂NCO⁺), and one of NH₃, H₂O, and CH₃OH, respectively, were determined by quantum chemical calculation using the CBS-QB3 method. Barrierless pathways were found for all the three reactions. Investigation of the energetics and kinetics of further possible reactions of CyH⁺, UrH⁺, and ThH⁺ led to the conclusion that cytosine, uracil, and thymine could not be formed along the proposed pathways in the interstellar gas phase, whereas their formation might be possible on interstellar icy grain mantles.