Isomeric effects in the gas-phase reactions of dichloroethene, C2H2Cl2, with a series of cations

A study of the reactions of a series of gas-phase cations (NH4+, H3O+, SF3+, CF3+, CF+, SF5+, SF2+, SF+, CF2+, SF4+, O-2(+), Xe+, N2O+, CO2+, Kr+, CO+, N+, N-2(+), Ar+, F+, and Ne+) with the three structural isomers of dichloroethene, i.e., 1,1-C2H2Cl2, cis-1,2-C2H2Cl2, and trans-1,2-C2H2Cl2 is reported. The recombination energy ( RE) of these ions spans the range of 4.7-21.6 eV. Reaction rate coefficients and product branching ratios have been measured at 298 K in a selected ion flow tube ( SIFT). Collisional rate coefficients are calculated by modified average dipole orientation (MADO) theory and compared with experimental data. Thermochemistry and mass balance have been used to predict the most feasible neutral products. Threshold photoelectron-photoion coincidence spectra have also been obtained for the three isomers of C2H2Cl2 with photon energies in the range of 10-23 eV. The fragment ion branching ratios have been compared with those of the flow tube study to determine the importance of long-range charge transfer. A strong influence of the isomeric structure of dichloroethene on the products of ion-molecule reactions has been observed for H3O+, CF3+, and CF+. For 1,1-C2H2Cl2 the reaction with H3O+ proceeds at the collisional rate with the only ionic product being 1,1-C2H2Cl2H+. However, the same reaction yields two more ionic products in the case of cis-1,2- and trans-1,2-C2H2Cl2, but only proceeds with 14% and 18% efficiency, respectively. The CF3+ reaction proceeds with 56-80% efficiency, the only ionic product for 1,1-C2H2Cl2 being C2H2Cl+ formed via Cl-abstraction, whereas the only ionic product for both 1,2- isomers is CHCl2+ corresponding to a breaking of the C=C double bond. Less profound isomeric effects, but still resulting in different products for 1,1- and 1,2-C2H2Cl2 isomers, have been found in the reactions of SF+, CO2+, CO+, N-2(+), and Ar+. Although these five ions have REs above the ionization energy (IE) of any of the C2H2Cl2 isomers, and hence the threshold for long-range charge transfer, the results suggest that the formation of a collision complex at short range between these ions and C2H2Cl2 is responsible for the observed effects.