Phenomenological manifestations of large-curvature tunneling in hydride-transfer reactions

An important consequence of recent dynamical theories of tunneling is that, because of large curvature of the reaction path in a typical H, H⁺, or H^- transfer, light-isotope transfer occurs in more extended nuclear frameworks than heavy-isotope transfer. This is incorporated here into a modified version of the Marcus phenomenological theory relating reaction rate constants to equilibrium constants. It leads to Brønsted slope parameters that depend on the isotope transferred. The new theoretical formulation is tested on experimental data for hydride and deuteride transfer between nicotinamide adenine dinucleotide analogues and on computational data for hydrogen-atom and deuterium-atom transfer between pseudoatoms. The experimental kinetic isotope effects (KIE's) are shown to vary with reaction equilibrium constant (K_ij) in a way that is quantitatively consistent with the theory. The critical configurations generated by the calculations vary from the saddle point and from each other in the way anticipated by theory. However, the calculated KIE values are a rather scattered function of K_ij because the tunneling corrections are large and somewhat system specific. Overall, we believe that this combination of experimental and calculated results provides considerable support for the idea that large-curvature tunneling needs to be considered in hydrogen-transfer reactions.