Revisiting the Nature of Low-Lying π Resonances in Microsolvated Uracil

Low-energy electron interactions with biomolecules play a central role in radiation-induced chemistry. Initially, electrons are attached to the building blocks of biomolecules, leading to electronic resonances. Here, we present a theoretical investigation of π resonances in microsolvated uracil anion clusters using a combination of multireference and equation-of-motion coupled-cluster electronic structure methods. In addition to the bound anionic ground state, we identify three resonances. Our results are in excellent agreement with recent experiments, predicting a progressive stabilization of all anionic states with increasing numbers of water molecules, while the excitation energies of the anion remain almost constant with solvation. Importantly, our calculations show that the second low-lying π resonance has two-particle one-hole Feshbach resonance character, rather than the one-particle shape character assigned in prior experimental interpretation. Distinguishing between shape and Feshbach resonances is crucial for understanding low-energy electron-molecule interactions, as these states exhibit very different electronic structures, lifetimes, and, therefore, decay mechanisms. Overall, these findings highlight the essential role of high-level electronic-structure theory in interpreting transient states of anions in microsolvated biomolecular systems.