TRANSLATIONAL ACTIVATION OF THE S(N)2 NUCLEOPHILIC DISPLACEMENT-REACTIONS CL--]CLCH3(CLCD3)+CL- - A GUIDED ION-BEAM STUDY(CH3CL(CD3CL))

Guided ion beam tandem mass spectrometry techniques are used to examin e 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 pro cess 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 impede d by nonstatistical dynamical constraints at collision energies just a bove the barrier. A significant secondary kinetic isotope effect is ob served. 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 apparatu s and data analysis procedures are described in detail.