Kinetic Isotope Effects are only distantly related to the increased mass
(reduced mass in the Schrödinger Wave Equation). KIE is a quantum mechanical
Research zero-point energy and think about the strength of the bonds involved.
OK, well I know that the wavenumber (proportional to both frequency and energy of the vibration) varies inversely with the reduced mass. => which would lead to a higher ZPE for H-H than for D-D for example.
Now in the case of CH, NH and OH, I totally missed the point that the reduced mass takes into account both the isotope and the atom to which it is bonded. I have calculated the reduced masses for each of CH, CD, NH, ND, OH and OD. I found that going from C to N to O, the reduced masses all increase, as does the difference between the reduced masses. So for OH/OD there is a bigger difference in the reduced mass than for CH/CD. As difference in reduced mass increases, so does difference in ZPEs (from Hooke's Law, by way of wavenumber, frequency).
Given that in each case the maximum KIE value occurs when the transition state contains no vibrational structure involving the isotope, we can assume that magnitude of maximum KIE values are dictated solely by the difference in ZPEs. => This would lead to OH having a larger maximum KIE than CH, I think
This is what I've come up with but I don't know if it is correct.
About bond strengths, in what way are they important for the Kinetic Isotope Effect? I have found bond energies that suggest the order of strength in these 3 bonds as OH (strongest) > CH > NH (weakest). This does not correlate with the order of OH > NH > CH for maximum kinetic isotope effects.http://www.kentchemistry.com/links/Kinetics/BondEnergy.htm
Do you think I have made a mistake or on the right track? I really appreciate that people come on this to share their knowledge it's great!!