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 gasphase nucleophilic substitution (S(N)2) reaction Cl- + CH3Br --> CH3Cl + 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 CH3Cl + Br- products. The velocity distributions of the CH3Cl + 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 CH3Cl 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 CH3Cl + Br- cross sections. The competitive endothermic reaction Cl- + CH3Br --> CH3 + 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.