Novel reaction pathways with the neutral radical–molecule
reaction to form cytosine, a nucleobase of DNA and RNA, which can
proceed in the interstellar environment, were discovered by automated
reaction pathway calculations based on the transition state theory.
We suggested the barrierless and exothermic formation of cytosine
in two steps. First, the reaction between the O radical and ethynamine
forms a key precursor species, NH2CCHO. Then, this radical
reacts with CN and HNCH to form cytosine. This study would contribute
to elucidating possible reactions on nucleobases under lower temperatures,
and further tests are effective by astronomical modeling/observations
and laboratory experiments.