The valence electron affinity of uracil determined by anion cluster photoelectron spectroscopy†

The unoccupied π* orbitals of the nucleobases are considered to play important roles in low-energy electron attachment to DNA, inducing damage. While the lowest anionic valence state is vertically unbound in all neutral nucleobases, it remains unclear even for the simplest nucleobase, uracil (U), whether its valence anion (U⁻) is adiabatically bound, which has important implications on the efficacy of damage processes. Using anion photoelectron spectroscopy, we demonstrate that the valence electron affinity (EA_V) of U can be accurately measured within weakly solvating clusters, U⁻(Ar)_n and U⁻(N₂)_n. Through extrapolation to the isolated U limit, we show that EA_V = −2 ± 18 meV. We discuss these findings in the context of electron attachment to U and its reorganization energy, and more generally establish guidance for the determination of molecular electron affinities from the photoelectron spectroscopy of anion clusters.