Translational activation of the S(N)2 nucleophilic displacement reactions Cl-+CH3Cl(CD3Cl)->ClCH3(ClCD3)+Cl-: A guided ion beam study

Guided ion beam tandem mass spectrometry techniques are used to examine promotion of the symmetric bimolecular nucleophilic substitution (S(N)2) reaction Cl-37(-) + (CH3Cl)-Cl-35 --> Cl-35(-) + (CH3Cl)-Cl-37 by translational energy. The translational energy threshold for this process is 45 +/- 15 kJ/mol, well above the previously reported potential energy barrier height of 10-13 kJ/mol for the S(N)2 transition state. The collisionally activated process involves conventional S(N)2 back-side attack at the carbon atom, but passage over the barrier is impeded by nonstatistical dynamical constraints at collision energies just above the barrier. A significant secondary kinetic isotope effect is observed. The cross section for reaction with CH3Cl is about 20% larger than for the reaction with CD3Cl. At high energies, >410 +/- 40 kJ/mol, diatomic Cl-2(-)-products are observed. The guided ion beam apparatus and data analysis procedures are described in detail.