An investigation of the reaction of O^- with CH₂F₂ with ab initio molecular orbital calculations

Abstract

A selected ion flow tube (SIFT) study of the O^- + CH₂F₂ reaction is reported which gives the following negatively charged products at 298 K: CF₂ (37%), OH^- (25%) and electrons (38%). Ab initio molecular orbital calculations are carried out at various levels of sophistication to investigate the reaction paths and the relative energies of the possible reaction channels.Both the reactant-like (O^-.CH₂F₂) and product-like (H₂O.CF^- ₂) ion–molecule intermediates are located. However, it is found that the OH^-.CHF₂ product-like intermediate is not a stationary point on the (P)UMP2/6-31 + + G** hypersurface. A proton abstraction from CHF₂ by OH^- gives the H₂O.CF^- ₂ intermediate without a barrier, which in turn gives rise to the product channel H₂O + CF^- ₂. Extensive transition state searches, followed by intrinsic reaction coordinate (irc) path calculations, locate a number of transition state structures and minima, but none of them is connected with any of the observed products. Nevertheless, one transition state search leads to the surface for abstraction of a proton from CH₂F₂ by O^-. A careful search on the intersection between the OH.CHF^- ₂ and the OH^-.CHF₂ surfaces indicates that these two surfaces cross in a region with the OC distance between 2⋅8Å and 3⋅2Å at energies close to the total reactant energy (at the UMP2/6-31 G** level). Optimization and transition state search on the OH^-.CHF₂ surface in the region near the crossing with the OH.CHF₂ surface lead to the H₂O.CF₂ intermediate and the products, OH^- and CHF₂, respectively. These observations suggest that both the negative ion products observed, OH^- and CF^- ₂, are produced from the same reaction surface. Initially OH.CHF^- ₂ is produced via proton abstraction by O^-. This is followed by an electron transfer between OH and CHF^- ₂ at the crossing between the OH.CHF^- ₂ and OH^-CHF₂ surfaces. Later in the reaction, the OH^-.CHF₂ surface is crossed by the H₂O.CF^- ₂ surface. The only exothermic product channels which could produce electrons from the O^- + CH₂F₂ reaction are found to be H₂O.CF₂ + e ^- and H₂O + CF₂ + e ^-. Negative ion and electron relative concentrations determined as functions of temperature in SIFT experiments could be understood in terms of these reactions.