Can cytosine, thymine and uracil be formed in interstellar regions? A theoretical study

Abstract

This theoretical study investigates possible synthetic routes to cytosine, uracil and thymine in the gas phase from precursor molecules that have been detected in interstellar media. Studies at the CCSD(T)/6-311++G(d,p)//B3LYP/6-311++G(d,p) level of theory suggest that: The reactions between :CCCNH and :CCCO with monosolvated urea may constitute viable interstellar syntheses of cytosine and uracil. No low energy equilibration between cytosine and uracil has been demonstrated. The interaction of :CH₂ with the 5 C–H bond of uracil may form thymine in an energetically favourable reaction, but competing reactions where :CH₂ reacts with double bonds and other CH and NH bonds of uracil, reduce the effectiveness of this synthesis. The reaction between the hydrated propional enolate anion and isocyanic acid may produce thymine, in a reaction sequence where ΔG_reaction(298 K) is −22 kJ mol⁻¹ and the maximum energy requirement (barrier to the first transition state) is only 47 kJ mol⁻¹.