Electronic manipulating in hydrated electron simulation of the carbon-halogen bond reductive cleavage

Abstract

Hydrated electron e−(aq) reaction with the alkyl halide and aryl halide was simulated synergistically with ab initial molecular dynamics (AIMD) in this study to reveal the morphological and dynamics mechanism. An original method was developed for preparing the proper initial wavefunction guess of AIMD, in which the extra electron was curled properly in a tetrahedral cavity of four water molecules. Our results indicated that the tetrahedral structure of e−(aq) (THE*) is more stable than the prism structure e−(aq) (PHE*) from the energy aspect. The interior weak interaction in THE* is mainly between the hydroxyl group with the extra electron, while the PHE* structure stability is attributed to the weak H-H interaction. The extra electron, with a significant sigma characteristic, collapses in a cavity composed of water molecules in these two structures and has a probability of collapsing to a certain sole water, this probability is inversely proportional to the number of waters; Organic halides prefer the direct reaction with e−(aq) in a neutral or alkaline environment while the hydrogen radical would be the dominant reaction species in an acidic solution. Fluorobenzene and fluoromethane are the hardest molecules to accept the extra electron and also have the highest reaction barriers during the hydrogen radical reactions; AIMD suggested that the LUMO or higher orbitals were the e−(aq) migration destination. The transplanted electron enhanced C-halogen bond vibration before the cleavage actually occurred. The solvation of the departing halogen anions was observed in both fluorobenzene and fluoromethane AIMD, indicating it might have a significant effect on enthalpy. A deformation of fluoromethane product, the methane radical, was detected from the sp3 structure to the sp2 plane structure, resulting in larger energy differences during the reaction than aryl halides. The study provided theoretical insight into the pollutant environmental fate and placed a methodological foundation for AIMD simulation of analogous free radical reactions.