Mechanism of Prebiotic Uracil Synthesis from Urea and HC₃O⁺in Space

The potential energy surface for the formation of protonated uracil (UH+) from urea and HC3O+ was explored by performing quantum chemical complete basis set-QB3 calculations. A barrierless pathway was found for the formation of UH+, which was estimated to occur in the interstellar medium (ISM) much faster than the timescale of chemical revolution of typical dense interstellar clouds. Investigation of further reactions of UH+ formed through the obtained pathway led to the conclusion that uracil could be produced on icy grain surfaces but not in the gas phase of the ISM.