Prebiotic Cytosine Synthesis from Urea in Interstellar Space: A Computational Mechanistic Study

Abstract

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 H 2 O, 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 + + H 2 O and urea + CAAH + . A kinetic analysis shows that the synthesis of Cyt through the formation of CytH + from urea, CAH + , and H 2 O would be possible in the ISM.