An investigation of the reaction of O-with CH2F2withab initiomolecular 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.