Benzene Radical Anion in the Context of the Birch Reduction: When Solvation Is the Key.

The benzene radical anion is an important intermediate in the Birch reduction of benzene by solvated electrons in liquid ammonia. Beyond organic chemistry, it is an intriguing subject of spectroscopic and theoretical studies due to its rich structural and dynamical behavior. In the gas phase, the species appears as a metastable shape resonance, while in the condensed phase, it remains stable. Here, we approach the system by ab initio molecular dynamics in liquid ammonia and demonstrate that the inclusion of solvent is crucial and indeed leads to stability. Beyond the mere existence of the radical anion species, our simulations explore its condensed-phase behavior at the molecular level and offer new insights into its properties. These include the dynamic Jahn-Teller distortions, vibrational spectra in liquid ammonia, and the structure of the solvent shell, including the motif of a π-hydrogen bond between ammonia molecules and the aromatic ring.