| It has been accepted that hydrogen-transfer reactions take place through a quantum mechanical tunneling mechanism, where H tunnels through its classical energy barrier in light of its wave form. There are several H-tunneling models proposed, including the contemporary Marcus-like H-tunneling model, which explains that the donor-acceptor distance (DAD) in the tunneling ready state (TRS) is shorter for a heavier isotope (e.g. deuterium (D)) then a lighter isotope (e.g., protium (H)). This model has been used to explain the kinetic isotopic effect observations in H-tunneling processes to provide mechanistic role of protein in enzyme catalysis. The purpose of the research is to test the hypothesis of "isotopically different DAD" concept by studying the hydrogen/deuterium-transfer reactions in solutions, given that hydride-transfer reactions account for over 50% of biological reactions. Our group's previous results showed that the steric hindrance and hydrogen-bonding effect played a significant role in the different hydrogen vs deuterium tunneling-ready states. In general, the shorter DAD creates more spatial crowding effect which will affect the 2° C-H vibrations and decrease the 2° KIEs. In this thesis, different reaction systems were designed to test these effects by studying the 1° isotope dependence of 2° KIEs at the near and remote positions from the reaction center. It was found that the results are consistent with the hypothesis of the "isotopically different TRS structures". |
