Fig. 2.

Marcus-like model of a reaction with H tunneling. At the TRS (‡), the reactant (Black) and product (Gray) surfaces are degenerate, which allows the probability density of the hydrogen (Shaded) to spread from the donor well to the acceptor well (i.e., quantum mechanical tunneling) at a rate dependent on the DAD and the particle’s mass. Once degeneracy is broken, the hydride wave function can collapse into the acceptor well, giving a net transfer (i.e., dissipative tunneling). The Marcus coordinate is a complex amalgamation of many modes, some of the most important of which are discussed in the text.