The Intrinsic Barrier Width and its Role in Chemical Reactivity

The Marcus dissection of the Gibbs activation energy (barrier height) into intrinsic and thermodynamic contributions, which successfully models the interplay of rate and driving force, has led to a crucial general phenomenological consequence: the well-known two reactivity paradigms of “kinetic versus thermodynamic control”. However, concepts analogous to the Marcus’ dissection for barrier widths are absent. Here we define and outline the barrier-width- counterpart of the Marcus dissection: the concept of intrinsic barrier width and driving force effect on the barrier width, and report experimental as well as theoretical studies to demonstrate their distinct roles. We present the idea of changing the barrier widths of conformational isomerizations of some simple aromatic carboxylic acids as models and use quantum mechanical tunneling (QMT) half-lives as a read-out for these changes. This sheds light on resolving conflicting trends in chemical reactivities where barrier widths are relevant, and allows us to draw some important conclusions about the general relevance of barrier widths, their qualitative definition, and the consequences for more complete descriptions of chemical reactions based on one-dimensional reaction coordinates.