H-Tunneling Exhibiting Unexpectedly Small Primary Kinetic Isotope Effects

Probing quantum mechanical tunneling (QMT) in chemical reactions is crucial to understand and develop new possibilities of molecular design. Primary H/D kinetic isotopic effects (KIEs) beyond the semiclassical maximum values of 7‒10 are commonly used to assess QMT contributions in one-step hydrogen transfer reactions, because H-atom QMT reactions are assumed to necessarily have large primary H/D KIEs. Nevertheless, we report here the discovery of a reaction model occurring exclusively by H-atom QMT with residual primary H/D KIEs. A 2-hydroxyphenylnitrene, generated in N2 matrix, was found to isomerize to an imino ketone via domino QMT involving sequential anti to syn OH-rotamerization (rate determining step) and [1,4]-H shift reactions. This domino QMT transformation was also observed in the OD deuterated sample, and unexpected primary H/D KIEs between 3 and 4 were measured at 3 to 20 K. Analogous residual primary H/D KIEs were found in the anti to syn OH-rotamerization QMT of 2-cyanophenol in N2 matrix. Evidence suggest that the intriguing isotope insensitive QMT observations arise due to the coupling between the cryogenic N2 medium and the movement of H/D tunneling particles. Should a similar rationale be extrapolated to conventional solution conditions, then QMT may have been overlooked in many chemical reactions.