Speaker
Description
The symmetry of the molecular structure of the benzene radical anion ($C_6H_6^{\bullet -}$) leads to a rich and intriguing palette of quantum behavior manifestations. Moreover, this simplest aromatic anion possesses a great importance in organic chemistry. In particular, one of the most well-known appearances in this field is as the first reactive intermediate in the Birch reduction used to reduce benzene to 1,4-cyclohexadiene using the blue solution of alkali metals in liquid ammonia.
In this work, we perform and analyze ab initio molecular dynamics (AIMD) simulations of the benzene radical anion in liquid ammonia to shed light on its condensed phase behavior never explored before.
As the title of this contribution suggests, this solvation is crucial for the existence and stability of the anion. In fact, it was demonstrated in previous works that in the gas phase (i.e., without solvent) the excess electron of the radical anion is unbound. However, it was suggested and, in this work, we show that as soon as it is submerged in a solvent such as liquid ammonia, it becomes a true bound state. In the context of this phenomenon, we discuss the methodological aspects necessary to capture a bound species in the simulation: we conclude that an expensive hybrid DFT electronic structure calculation is needed to obtain a "well-behaved" radical anion.
With these computational settings, we obtained 100 ps of production dynamics of the radical anion and a neutral benzene reference in liquid ammonia. Based on these trajectories, we are able to discuss structural features of the radical anion itself such as the dynamic Jahn-Teller distortions and the structure of the surrounding solvent including the actively discussed phenomenon of $\pi$-hydrogen bonding. We show that benzene and its anion both form a characteristic hydrogen bond with the solvent which possibly correlates with macroscopic effects such as the elevated benzene solubility in protic solvents such as ammonia and further extending into liquid water.
