Hydrogenation of Uracil in Solid Para -Hydrogen: Infrared Spectra of Monohydrogenated Radicals (5 H -C4H5N2O2, 6 H -C4H5N2O2, and 4 OH -C4H5N2O2) and Their Implications for RNA-Related Chemistry in the Interstellar Medium

Uracil (C4H4N2O2), an essential RNA nucleobase central to the RNA World hypothesis and the prebiotic emergence of life, has been detected in pristine asteroid samples, yet its survival mechanism in nonenergetic interstellar environments remains an elusive paradox. Backed by high-level CCSD(T)//B3LYP quantum-chemical calculations, we successfully captured and characterized the IR spectra of three transient, monohydrogenated radicals for the first time: the dominant 5H-C4H5N2O2(1) and minor 6H-C4H5N2O2(2) radicals, resulting from H-addition to the C═C moiety, alongside minor features of the 4-hydroxy-uracil radical 4OH-C4H5N2O2(3), resulting from H-addition to the carbonyl oxygen at the C4 position, under hydrogen-rich conditions. Under H-rich conditions, the formation of minor features of the dihydrogenated product 5,6-dihydrouracil (DHU), resulting via secondary H-addition to (1) or (2), was also observed. The assignments for (3) and DHU are tentative because only a limited number of bands were observed due to low intensities and interference. Crucially, real-time temporal profiling uncovered a striking, counterintuitive recycling mechanism: while both secondary H-addition (forming DHU) and secondary H-abstraction (regenerating uracil) are barrierless, the radical-to-parent regeneration pathway is significantly favored. This unexpected preference suggests that repeated collisions with cosmic hydrogen atoms might not efficiently destroy uracil, thereby drastically extending its chemical lifetime, which might have implications for how uracil could survive and be exogenously seeded onto young planets.