OBSERVATION THE XY(-) ABSTRACTION PRODUCTS IN THE REACTIONS X(-)+RY-]XY(-)+R - AN ALTERNATIVE TO THE S(N)2 MECHANISM AT SUPRATHERMAL COLLISION ENERGIES

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. ICI- and I-2(-) are also found for the Cl- + CH3I and I- + RI reactions at elevated collision energies (less than or equal to 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 nor provide an efficient alternative to the S(N)2 mechanism for halide exchange in the asymmetric X(-) + CH3Y systems. This conclusion is supported by nb initio calculations (MP2LANL10Z level) which indicate that ClBr- can be formed by collinear attack at the halogen through a Cl...Br...CH3 intermediate.