WHAT IS THE EFFECT OF VARIATIONAL OPTIMIZATION OF THE TRANSITION-STATE ON ALPHA-DEUTERIUM SECONDARY KINETIC ISOTOPE EFFECTS - A PROTOTYPE - CD3H + H REVERSIBLE CD3 + H2

Variational Transition state theory calculations with semiclassical transmission coefficients have been carried out for a prototype case of α-deuterium secondary kinetic isotope effects (KIEs) in a reaction involving the transformation of an sp3 carbon to sp2, in particular for the reactions of CH4 and CD3H with H and D. We also study the KIE for the reverse direction and for the reactions of CH4 and CD3H with D. We find that the variational transition states lead to significantly different nontunneling KIEs than the conventional ones, e.g., 1.22 vs. 1.07, and the inclusion of multidimensional tunneling effects increases the discrepancy even more. The origins of these variational and tunneling effects are examined in detail in terms of structures, vibrational frequencies, and the curvature of the reaction path. The conclusions have wide implications for the validity of conventional treatments of kinetic isotope effects. We predict some particularly large secondary KIEs at low temperature, and these predictions can be tested by future experiments. © 1990, American Chemical Society. All rights reserved.