Many observations in hydrogen (H-) transfer reactions cannot be explained by the classical transition state theory. Rather, these reactions take place by H-tunneling effect in terms of H’s non-classical wave property. The transferring isotopes (1°) have different wave property and this difference can have an effect on the 2° kinetic isotope effect (KIE). Previous 1°/2° Hydrogen coupled motion theory cannot explain the deflated 2° KIE (as compared to the classically predicted value) in the case of D-tunneling. A new theory that can explain the observation uses the concept that the donor-acceptor distance (DAD) for D-tunneling is shorter than that for H-tunneling and thus D-tunneling has a more crowded reaction site. The latter crowding effect likely restricts the 2° H’s vibration and causes a deflation in 2° KIE. Thus our hypothesis is that H- and D-tunneling have tunneling ready structures that have different DADs, and the 1° isotope effect on 2° KIEs should be more pronounced in tunneling systems that are sterically hindered. Our group has designed several hydride transfer reactions in solution to test this hypothesis by studying the 1° isotope effect on the α- and β-2° KIEs and the results support it. This thesis work studies the 1° isotope dependence of both the near (α- and β-) and remote (ϵ-) 2° KIEs in the designed hydride transfer systems. The results are consistent with the previous observations further supporting the isotopically different DAD concept.