Theoretical investigation of Aryl/Alkyl halide reduction with hydrated electrons from energy and AIMD aspects

In this study, the reactions of hydrated electron (e^-(aq)) with alkyl and aryl halides were simulated with an ab initial molecular dynamics (AIMD) method to reveal the underlying mechanism. An original protocol was developed for preparing the proper initial wavefunction guess of AIMD, in which a single electron was curled in a tetrahedral cavity of four water molecules. Our results show that the stability of e^-(aq) increases with the hydrogen bond grid integrity. The organic halides prefer to react with e^-(aq) in neutral or alkaline environment, while they are more likely to react with hydrogen radical (the product of e^-(aq) and proton) under acidic conditions. The reaction between fluorobenzene/fluoromethane and hydrogen radical is considered as the least favorable reaction due to the highest reaction barriers. The bond dissociation energy (BDE) suggested that the cleavage of the carbon-halogen bond of their anion radical might be a thermodynamically favorable reaction. AIMD results indicated that the LUMO or higher orbitals were the e^-(aq) migration destination. The transplanted electron enhanced carbon-halogen bond vibration intensively, leading to bond cleavage. The solvation process of the departing halogen anions was observed in both fluorobenzene and fluoromethane AIMD simulation, indicating that it might have a significant effect on enthalpy. Side reactions and byproducts obtained during the AIMD simulation suggested the complexity of the e^-(aq) reactions and further investigation was needed to fully understand the reaction mechanisms. This study provided theoretical insight into the pollutant environmental fate and constructed a methodological foundation for AIMD simulation of analogous free radical reactions.

The theoretical calculation was conducted on the combination of Gaussian16 and ORCA5.0.3 software packages. The initial geometries, as well as the wavefunction initial guesses, were obtained at PBE0/ma-def2-TZVP/IEFPCM-water level in Gaussian16 unless otherwise stated. AIMD simulations were performed at the same level in ORCA. Wavefunction analysis was carried out with Multiwfn. The details methods were described in the section "Computational details" section.