Radical–Radical Interactions among Reduced Uracil Bases, Including Uracil Anion Radical and Hydrogenated Uracil Radical

Employing density functional theory (DFT), we systematically investigate the structural, electronic, and magnetic spin-coupling properties of a series of homogeneous and heterogeneous dimers of reduced uracil radicals (U^•-, U6H^•, U5H^•, U4H^•). Different hydrogen-bonding (Watson-Crick, Hoogsteen, and minor-groove) and π-π stacking configurations are examined. DFT and complete active space self-consistent field calculations reveal that the double-electron reduced, hydrogen-bonded base pairs (U6H^•U6H^•, U5H^•U5H^•, U6H^•U5H^•, U4H^•U4H^•) exhibit diradical character with tunable ferromagnetic (FM) or antiferromagnetic (AFM) coupling. Hydrogen-bonded dimers linked through Watson-Crick sites typically form weakly coupled open-shell singlets, while Hoogsteen or minor-groove connections significantly enhance AFM coupling strength. These magnetic interactions are governed by a balance of hydrogen bonding, electrostatic repulsion, and radical coupling. Notably, the U6H^•U^•- and U5H^•U^•- pairs form stable, nonmagnetic closed-shell complexes under strong hydrogen bonding. For the U^•-U^•- dimer, double-electron reduction induces metastability, leading to a negative yet barrier-hindered dissociation energy, an unusual phenomenon arising from competing hydrogen-bond attraction and electrostatic repulsion. In contrast, π-π stacked systems exhibit significantly stronger magnetic coupling and richer magnetic behavior. This work provides the first theoretical prediction of the electronic properties of potentially doubly reduced uracil-uracil base pairs, offering new insights into their magnetic tunability.