Curvature-induced kinetic trapping in chemical reactions

Crossing an activation energy barrier in a chemical reaction needs time because enough kinetic energy needs to redistribute into the barrier-crossing mode. Kinetic energy redistribution is also needed to get across regions with strong reaction path curvature. This latter fact is frequently overlooked, possibly because barriers and curvature peaks often coincide, and curvature is invisible in the usual, pseudo-one-dimensional reaction energy profiles. For a set of related stepwise Diels-Alder reactions, we examine reaction paths, energy profiles and molecular dynamics trajectories, demonstrating three different cases: (1) one with a traditional IUPAC-conforming potential energy intermediate, in which the surrounding energy barriers coincide with curvature peaks; (2) another case in which the second C-C bond closure is associated with small curvature and no clearly visible energy barrier, accordingly the dynamics towards full ring closure is comparatively fast; and (3) a final case also without an energy barrier for the second C-C bond closure but with a large curvature peak there, leading to exceptionally long kinetic trapping between the two C-C bond formations in the downhill-energy region. Hence, we advocate for calculating reaction path curvature as simple add-on to reaction paths, to identify cases warranting a full molecular dynamics study. Besides reaction path curvature, further features are analyzed that contribute to dynamic delays, opening up unconventional reaction control possibilities.