Deuterium kinetic isotope effects in gas phase SN2 reactions

Rate coefficients and secondary a-deuterium kinetic isotope effects (KIEs) for the nucleophilic substitution (S(N)2) reactions of Cl- + CH3Br --> CH3Cl + Br-, Cl- + CH3I --> CH3Cl + I-, Br- + CH3I --> CH3Br + I-, and their perdeuterated analogs are remeasured in the gas phase at 300 K. The measurements confirm our previous results [Gronert et al., J. Am. Chem. Soc. 113 (1991) 4009] and substantial inverse KIEs are measured with an improved accuracy: k(H)/k(D) = 0.77 (+/-0.03), 0.84 (+/-0.01), and 0.76 (+/-0.01), respectively, for the above systems. Thus, the experimentally observed effect of deuterium substitution in these reactions is considerably more dramatic than predicted previously by transition state theory where KIEs are calculated to be closer to unity. The experimental KIE values for the series of S(N)2 reactions, F-, Cl- + CH3Br and F-, Cl-, Br- + CH3I are both found to decrease (i.e. become more inverse) with an increase in the transition-state looseness parameter (RTS) for larger halide anions, in contrast to those for lighter methyl halides which show the expected positive correlations with RTs.