A quantum chemical study of the generation of a potential prebiotic compound, cyanoacetaldehyde, and related sulfur containing species.

Cyanoacetaldehyde (NCCH 2CHO), which may have played a role in the prebiotic formation of the pyrimidine bases cytosine and uracil, is formed in water solutions by addition of water to cyanoacetylene (HCC-CN), a compound that exists in interstellar space, in comets, and planetary atmospheres. A gas-phase model of the uncatalyzed addition of water to cyanoacetylene is explored by ab initio calculations at the MP2/6-311++G** level of theory. A reaction path consisting of several steps was found in these calculations, but the activation energy of the first step is relatively high, which makes it unlikely that cyanoacetaldehyde is formed in an uncatalyzed reaction. Similar calculations were also performed for the uncatalyzed reaction of water to protonated cyanoacetylene (HCCCNH (+)), a component of the interstellar medium, forming protonated cyanoacetaldehyde (HNCCH 2CHO (+)), but a high activation energy was found for this reaction as well. Moreover, the corresponding addition reactions of hydrogen sulfide (H 2S) to HCCCN, as well as to HCCCNH (+), have been explored with similar results.