Four-dimensional quantum study on exothermic complex-forming reactions: Cl-+CH3Br -> ClCH3+Br-

Abstract

The exothermic gas-phase bimolecular nucleophilic substitution (S(N)2) reaction Cl-+CH3Br (upsilon(1)('),upsilon(2)('),upsilon(3)('))-> ClCH3 (upsilon(1),upsilon(2),upsilon(3))+Br- and the corresponding endothermic reverse reaction have been studied by time-independent quantum scattering calculations in hyperspherical coordinates on a coupled-cluster potential-energy surface. The dimensionality-reduced model takes four degrees of freedom into account [Cl-C and C-Br stretching modes (quantum numbers upsilon(3)(') and upsilon(3)); totally symmetric modes of the methyl group, i.e., C-H stretching (upsilon(1)(') and upsilon(1)) and umbrella bending vibrations (upsilon(2)(') and upsilon(2))]. Diagonalization of the Hamiltonian was performed employing the Lanczos algorithm with a variation of partial reorthogonalization. A narrow grid in the total energy was employed so that long-living resonance states could be resolved and extracted. While excitation of the reactant umbrella bending mode already leads to a considerable enhancement of the reaction probability, its combination with vibrational excitation of the broken C-Br bond, (0, 1, 1), results in a strong synergic effect that can be rationalized by the similarity with the classical transitional normal mode. Exciting the C-H stretch has a non-negligible effect on the reaction probability, while for larger translational energies this mode follows the expected spectatorlike behavior. Combination of C-Br stretch and symmetric C-H, (1,0,1), stretch does not show a cooperative effect. Contrary to the spectator mode concept, energy originally stored in the C-H stretching mode is by no means conserved, but almost completely released in other modes of the reaction products. Products are most likely formed in states with a high degree of excitation in the new C-Cl bond, while the internal modes of the methyl group are less important. Reactants with combined umbrella/C-Br stretch excitation, (0, 1, 1), may yield products with two quanta in the umbrella mode. (C) 2005 American Institute of Physics.