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

Abstract

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.

Benzene Radical Anion Microsolvated in Ammonia Clusters: Modelling the Transition from an Unbound Resonance to a Bound Species

“…One then randomly placed additional solvent molecules into the unit cell around the core so that the total number of solvent molecules in the system was 10 000 while respecting the experimental density of liquid ammonia at 223 K 33 combined with the previously estimated excluded volume of the benzene radical anion. 16 The new larger system was then equilibrated in the NVT ensemble using molecular dynamics with empirical force fields, 34,35 however, with the original core kept constrained (additional information is provided in Section S2 of the Supporting Information (SI)). Following the equilibration, clusters of various cutoff radii were then carved out of the resolvated system consistently with the approach used for the original smaller systems (Figure 1, bottom half).…”

“…However, the open-shell and dif-fuse character of the radical anion requires a high level of electronic structure theory which, in combination with the extensive modeling of the explicit bulk solution, renders the calculations computationally demanding. In this direction, we recently reported a hybrid density function theory (DFT) ab initio molecular dynamics (AIMD) simulation of the benzene radical anion in liquid ammonia in periodic boundary conditions at 223 K. 16 There, the benzene radical anion was found to retain a stable spin population over the course of the whole simulation which points to electronic stability in terms of localization of the excess electron on the aromatic ring. In a follow-up study, the one-electron binding energies were calculated for the AIMD bulk geometries employing the accurate G 0 W 0 17,18 electronic structure method 19 again in periodic boundary conditions.…”

“…The ab initio studies aimed so far at two extreme situations-the electronically unbound isolated species 6 on one hand, and the fully solvated stable system on the other hand. 16,19 Here, we bridge the gap by explicit ionization calculations employing hybrid DFT performed on clusters of increasing size. First, we address the bulk value of the VBE of clusters embedded in a polarizable continuum 20,21 employing a polarizable continuum model (PCM) augmented by a small explicit solvent region to account for local solvent-solute interactions.…”

Electronic Structure of the Solvated Benzene Radical Anion

“…Most notably, the anion solute PDOS suggests that the highest energy state, occupied by the excess electron, is fully accounted for by the solute, consistent with the previously observed spatial localization of the spin density. 19 Its mean binding energy of −2.34 eV and the absence of tails extending into the positive values prove that the excess electron is bound relative to the vacuum level, thus conclusively answering the question of stability of the molecular structure of the anion as long as it is solvated in liquid ammonia. Compared to neutral benzene (Figure 1, bottom panel), the anion solute PDOS is noticeably systematically shifted towards higher binding energies.…”

“…A first step towards the theoretical description of the condensed-phase behavior of the benzene radical anion is our recently published work on the species in the context of the Birch reduction. 19 In that study, we simulated this solute in bulk liquid ammonia under periodic boundary conditions using ab initio molecular dynamics (AIMD) based on a hybrid density functional. At this level of theory, the excess electron spontaneously localizes on the benzene ring and remains stable for the length of the simulation, indicating the presence of a bound quantum electronic state.…”