Translational Activation of the S_N2 Nucleophilic Displacement Reactions Cl^- + CH₃Cl (CD₃Cl) → ClCH₃ (ClCD₃) + Cl^-:  A Guided Ion Beam Study

Abstract

Guided ion beam tandem mass spectrometry techniques are used to examine promotion of the symmetric bimolecular nucleophilic substitution (S N 2) reaction 37 Cl - + CH 3 35 Cl → 35 Cl - + CH 3 37 Cl 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 CH 3 Cl is about 20% larger than for the reaction with CD 3 Cl. 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.