Theoretical study of kinetic isotope effects on hydrogen abstraction reactions

Isotope effects (IEs) on chemical reaction rates due to tunneling can be classified as three kinds: primary, secondary, and reaction-path curvature effects. The primary IE originates from the effective mass for the reaction coordinate and is seen when an atom in a bond that is formed or broken is replaced with an isotope. The secondary IE originates from variation in the zero-point energy (ZPE) along the reaction coordinate and is seen when a “spectator” atom is replaced with an isotope. This effect is usually small as compared to the primary IE and often shows an inverse effect, i.e., an isotopic substitution reduces the rate constant. The reaction-path curvature effect originates from the corner cutting of the tunneling path. The curvature of the reaction path depends only on the mass combination of the system; for example, it is large for a heavy-light-heavy system and small for a light-heavy-light system.