2020ApJ...898...13C

The potential energy surfaces for the formation of cytosine (Cyt) and a protonated cytosine (CytH⁺) from reactions of urea with cyanoacetylene (CA), cyanoacetaldehyde (CAA), or their protonated ions, CAAH⁺ and CAH⁺ with or without H₂O, have been determined from quantum chemical calculation using the CBS-QB3 method. The overall activation energies of the formation of Cyt or CytH⁺ from urea + CA, urea + CAA and, urea + CAH⁺ are 127, 211, and 31 kJ mol^–1, respectively, which are too high for the thermal reactions to occur in the interstellar medium (ISM). The barrierless reaction pathways have been proposed for the formation of CytH⁺ from urea + CAH⁺+ H₂O and urea + CAAH⁺. A kinetic analysis shows that the synthesis of Cyt through the formation of CytH⁺ from urea, CAH⁺, and H₂O would be possible in the ISM.