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

We report the formation of dihalide products from the endothermic gas- phase ion-molecule reaction of Cl- with CH3Br at suprathermal collisio n 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-m ass collision energy range of 2-15 eV with the ClBr- product displayin g a maximum near 7 eV. This result is found to be in good agreement (w hen convoluted with the appropriate velocity distribution) with the ra te constant measured by the SIFDT. ICI- and I-2(-) are also found for the Cl- + CH3I and I- + RI reactions at elevated collision energies (l ess than or equal to 1.5 eV) in the SIFDT. The rates for halide displa cement are found to be insensitive to collision energy. These results indicate that attack on the C-X bond may nor provide an efficient alte rnative to the S(N)2 mechanism for halide exchange in the asymmetric X (-) + CH3Y systems. This conclusion is supported by nb initio calculat ions (MP2LANL10Z level) which indicate that ClBr- can be formed by col linear attack at the halogen through a Cl...Br...CH3 intermediate.