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.