Observation of the XY- abstraction products in the ion-molecule reactions X- + RY → XY- + R: An alternative to the SN2 mechanism at suprathermal collision energies

Article abstract of DOI:10.1021/ja00111a023

We report the formation of dihalide products from the endothermic gas-phase ion-molecule reaction of Cl^- with CH₃Br at suprathermal collision energies using both guided ion beam and selected ion flow drift tube (SIFDT) techniques. The cross sections for the Cl^- + CH₃Br reactions were determined using the guided ion beam apparatus over a center-of-mass collision energy range of 2-15 eV with the ClBr^- product displaying a maximum near 7 eV. This result is found to be in good agreement (when convoluted with the appropriate velocity distribution) with the rate constant measured by the SIFDT. ICl^- and I₂^- are also found for the Cl^- + CH₃I and I^- + RI reactions at elevated collision energies (≤1.5 eV) in the SIFDT. The rates for halide displacement are found to be insensitive to collision energy. These results indicate that attack on the C-X bond may not provide an efficient alternative to the S_N2 mechanism for halide exchange in the asymmetric X^- + CH₃Y systems. This conclusion is supported by ab initio calculations (MP2LANL10Z level) which indicate that ClBr^- can be formed by collinear attack at the halogen through a Cl?Br?CH₃ intermediate.