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Proton-coupled electron transfer (PCET) reactions are central to a wide range of chemical and biochemical processes. The past decade has seen great strides in both experimental and theoretical understanding of PCET, but connecting these two perspectives is often challenging. This chapter shows that the Marcus cross relation is a valuable quantitative and conceptual model for solution PCET reactions that can be described as hydrogen atom transfer (HAT), X–H+Y → X+H–Y. The cross relation holds well – in many cases within an order of magnitude – for a large number of such reactions. This includes both purely organic reactions such as those involving phenoxyl radicals or ascorbate, and reactions where the electron change occurs primarily at a transition metal center, as in reactions of iron bi-imidazoline complexes. The cross relation was derived as a corollary of the Marcus Theory of electron transfer and provides a conceptual picture and an experimental entry into intrinsic barriers and other parameters. However, the cross relation is an essentially classical model that does not take into account proton tunneling or nonadiabatic effects. In addition, PCET brings a set of issues not found in electron transfer. For instance, the substantial solvent effects found in some of these reactions derive primarily from pre-equilibrium hydrogen bonding effects rather than changes in intrinsic barriers. The strengths as well as the weaknesses and limitations of using the cross relation for PCET reactions are discussed.

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