The Effect of the Donor-Acceptor Distance on the Secondary Kinetic Isotope Effects in Hydride Transfer Reactions within the Marcus Like H Tunneling Model: A Computational Stud

We recently proposed a new physical organic chemistry direction. It examines how structure affects the 1° isotope dependence of 2° KIEs in H-transfer reactions and how this dependence provides information about the structure of the tunneling ready states (TRSs). Our hypothesis is that H- and D-tunneling have TRS structures which have different donor-acceptor distances (DADs), and pronounced 1° isotope effect on 2° KIEs should be observed in tunneling systems that are sterically hindered. Since 1° H-tunneling possesses longer DAD than 1° D-tunneling, the study of the 1° isotope effect on 2° KIEs becomes that of the DAD effect on 2° KIEs. The hypothesis becomes "pronounced DAD effect on 2° KIEs should be observed in sterically hindered systems, and the shorter the DAD, the greater the steric effects, the lower the 2° KIE." This can be studied using the computation methods. I investigated this hypothesis by calculating the DAD effect on 2° KIEs at different positions in two hydride transfer systems. The signs of the 2° KIEs and the DAD (in)dependence of the 2° KIEs were discussed. The experimental results were attempted to fit to the calculations to find the TRS structures.