Calculations of Substituent and Solvent Effects on the Kinetic Isotope Effects of Menshutkin Reactions

Nitrogen, deuterium, halogen, and carbon kinetic isotope effects have been modeled for the
Menshutkin reaction between methyl halides and substituted N,N-dimethylaniline at the HF/6-31G(d) level of theory augmented by the C-PCM continuum solvent model for several solvents.
Systematic changes in geometries of the transition states and Gibbs free energies of activation
have been found with phenyl ring substituents, solvent, and the leaving group. Kinetic isotope
effects also change systematically; however, these changes are predicted to be small, inside the
usual precision of the experimental measurements. On the contrary, no correlation has been found
between the kinetic isotope effects and the Hammett constants for para substituents. Thus opposite
to previous assumptions, our results indicate that kinetic isotope effects on the Menshutkin reaction
cannot be used to predict the position of the transition state on the reaction coordinate.