Gas-Phase S_N2 and Bromine Abstraction Reactions of Chloride Ion with Bromomethane:  Reaction Cross Sections and Energy Disposal into Products

Abstract

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.

Quantum Dynamics of Gas‐Phase SN2 Reactions

“…They found, as in ref. [39], other reaction channels which, however, are not of concern for the present Review.…”

“…Graul and Bowers [33] found that the translational energy distribution of the products (Br À and CH 3 Cl) is smaller than the phase space theory (PST) prediction, which indicates that the product molecule is vibrationally hot. Recently, Angel and Ervin [39] have measured reaction crosssections and product distributions for the reaction Cl À + CH 3 Br for translational energies between 0.06 and 24 eV. At the lowest energies, the S N 2 reaction exhibits the largest cross-sections, and a symmetrical forward/backward scattering of the CH 3 Cl + Br À products is observed.…”

Thermochemistry of Hydrocarbon Radicals

“…S N 2 reaction. 25 Although not of thermochemical or combustion chemistry interest, this exothermic reaction has a classic double-well potential and the kinetic energy release had previously been studied by metastable ion dissociation, 26,27 providing an interesting test system for comparison with literature results. [26][27][28][29][30][31][32][33][34] At the lowest energies (near room temperature), the product velocity distributions are statistical as modeled by classical phase space theory (which disagrees with the metastable ion dissociation work but might be explained by the different experimental conditions).…”

“…At still higher kinetic energies, a new rebound mechanism is indicated by backward scattering. 25 We further applied product velocity distribution measurements to reaction (10) with RH = CH 4 and C 2 H 6 . 23 These experiments showed definitively that the reactions are non-statistical.…”

“…Both bimolecular reactions and collision-induced dissociation processes are modeled with statistical rate theory. 22,23,25,57,58 The transition state parameters including geometries and frequencies are obtained using electronic structure calculations, typically density function theory (e.g., using the hybrid functional B3LYP). Calculated potential energy surfaces for our reaction help inform our statistical rate theory analyses and understand reaction mechanisms and dynamics.…”