Vf. Deturi et al., TRANSLATIONAL ACTIVATION OF THE S(N)2 NUCLEOPHILIC DISPLACEMENT-REACTIONS CL--]CLCH3(CLCD3)+CL- - A GUIDED ION-BEAM STUDY(CH3CL(CD3CL)), The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 101(34), 1997, pp. 5969-5986
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.