Dissociative electron capture of halocarbon caused by the internal electron transfer from water trimer anion

Abstract

Dissociative electron capture dynamics of halocarbon absorbed on water cluster anion, caused by internal electron transfer from the water trimer anion to the halocarbon, have been investigated by means of the direct density functional theory (DFT)–molecular dynamics (MD) method. The CF₂Cl₂ molecule and a water trimer anion e⁻(H₂O)₃ were used as a halocarbon and a trapped electron, respectively. First, the structure of trapped electron state, expressed by e⁻(H₂O)₃–CF₂Cl₂, was fully optimized. The excess electron was trapped by a dipole moment of water trimer. Next, initial geometries were randomly generated around the equilibrium point of the trapped electron state, and then trajectories were run. The direct DFT–MD calculations showed that the spin density distribution of excess electron is gradually changed from the water cluster (trapped electron state) to CF₂Cl₂ as a function of time. Immediately, the Cl⁻ ion was dissociated from CF₂Cl₂⁻ adsorbed on the water cluster. The reaction was schematically expressed bye⁻(H₂O)₃–CF₂Cl₂ → [(H₂O)₃–CF₂Cl₂]⁻ → (H₂O)₃ + CF₂Cl + Cl⁻where [(H₂O)₃–CF₂Cl₂]⁻ indicates a transient intermediate state in which the excess electron is widely distributed on both the water cluster and CF₂Cl₂. The mechanism of the electron capture of halocarbon from the trapped electron in water ice was discussed on the basis of the theoretical results. Also, the dynamics feature was compared with those of the direct electron capture reactions of CF₂Cl₂ and CF₂Cl₂–(H₂O)₃, i.e. e⁻ + CF₂Cl₂, and e⁻ + CF₂Cl₂–(H₂O)₃, investigated in our previous paper [Tachikawa and Abe, J. Chem. Phys., 2007, 126, 194310].