Study of the Primary Isotope Dependence of the Secondary Kinetic Isotope Effects and Hammett Correlations in Hydride Transfer Reactions in Solutio

This research aims to understand the quantum tunneling mechanism in hydride (H)-transfer reactions in solution. Previous studies have shown that the transferring primary (1°) H isotope substitution with deuterium (D) affects the kinetic isotope effect (KIE) at the in-place secondary (2°) H/D positions, i.e.; the 2° KIE is inflated and more in H-transfer than in D-transfer. This has been explained in terms of an H-tunneling mechanism and the 1°/2° H coupled motions model. This model is, however, not able to justify the deflated 2° KIEs in D-tunneling observed in other systems. Recent explanation uses the concept that the donor-acceptor distance (DAD) at tunneling ready state (TRS) is longer in H-tunneling than in D-tunneling. The shorter Donor-Acceptor distance creates spatial crowding effect that solidifies the 2°-H vibrations and decreases the 2° KIE. This brings on a novel research direction that analyzes how structure affects the 1° isotope dependence of 2° KIEs and how this dependence provides information about the structure of the TRS. The hypothesis is that H- and D-tunneling have TRS structures which have different DADs, and prominent 1° isotope effect on 2° KIEs should be observed in sterically hindered tunneling systems. Our group has designed several hydride-transfer reactions to investigate the hypothesis and results substantiate the hypothesis. This research focuses on the 1° isotope effect on the 2° KIEs at near (alpha- and beta-) and remote (epsilon-) positions from the reaction center. The results are consistent with the previous observations and computational studies further supporting the isotopically different conformations. Second approach of studying the DAD concept is to employ Hammett correlations to study the substituent effect on the Primary KIEs. This will allow distinguishing the isotopically different electronic TRS structures.