Gas-phase SN2 and bromine abstraction reactions of chloride ion with bromomethane: reaction cross sections and energy disposal into products.

Reaction cross sections and product velocity distributions are presented for the bimolecular gas-phase nucleophilic substitution (S(N)2) reaction Cl(-) + CH(3)Br --> CH(3)Cl + Br(-) as a function of collision energy, 0.06-24 eV. The exothermic S(N)2 reaction is inefficient compared with phase space theory (PST) and ion-dipole capture models. At the lowest energies, the S(N)2 reaction exhibits the largest cross sections and symmetrical forward/backward scattering of the CH(3)Cl + Br(-) products. The velocity distributions of the CH(3)Cl + Br(-) products are in agreement with an isotropic PST distribution, consistent with a complex-mediated reaction and a statistical internal energy distribution of the products. Above 0.2 eV, the velocity distributions become nonisotropic and nonstatistical, exhibiting CH(3)Cl forward scattering between 0.2 and 0.6 eV. A rebound mechanism with backward scattering above 0.6 eV is accompanied by a new rising feature in the CH(3)Cl + Br(-) cross sections. The competitive endothermic reaction Cl(-) + CH(3)Br --> CH(3) + ClBr(-) rises from its thermochemical threshold at 1.9 +/- 0.4 eV, showing nearly symmetrically scattered products just above threshold and strong backward scattering above 3 eV associated with a second feature in the cross section.