above deuterium racemizes faster than the hydrogen isotopologue resulting in a steric isotope effect. A model for the steric isotope effect was developed by Bartell. A steric isotope effect is usually small, unless the transformations passes through a transition state with severe steric encumbrance, as in the racemization process shown above. Another example of the steric isotope effect is in the deslipping reaction of rotaxanes. The deuterium isotope, due to its smaller effective size, allows easier passage of the stoppers through the macrocycle, resulting in faster rates of deslipping for the deuterated rotaxanes. Inverse kinetic isotope effects Reactions are known where the deuterated species reacts faster than the undeuterated analogue, and these cases are said to exhibit inverse kinetic isotope effects (IKIE). IKIE's are often observed in the reductive elimination of alkyl metal hydrides, e.g. (Me2NCH2CH2NMe2)PtMe(H). In such cases the C-D bond in the transition state, an agostic species, is highly stabilized relative to the C–H bond. An inverse effect can also occur in a multistep reaction if the overall rate constant depends on a pre-equilibria prior to the rate-determining step which has an inverse equilibrium isotope effect. For example, the rates of acid-catalyzed reactions are usually 2-3 times greater for reactions in D2O catalyzed by D3O+ than for the analogous reactions in H2O catalyzed by H3O+ This can be explained for a mechanism of specific hydrogen-ion catalysis of a reactant R by H3O+ (or D3O+). H3O+ + R RH+ + H2O RH+ + H2O → H3O+ + P The rate of formation of products is then d[P]/dt = k2[RH+] = k2K1[H3O+][R] = kobs[H3O+][R]. In the first step, H3O+ is usually a stronger acid than RH+. Deuteration shifts the equilibrium toward the more strongly bound acid species RD+ in which the effect of deuteration on zero-point vibrational energy is greater, so that the deuterated equilibrium constant K1D is greater than K1H. This equilibrium isotope effect in the first step usually outweighs the kinetic isotope effect in the second step, so that there is an apparent inverse isotope effect and the observed overall rate constant kobs = k2K1 decreases. Solvent hydrogen kinetic isotope effects For the solvent isotope effects to be measurable, a finite fraction of the solvent must have a different isotopic composition than the rest. Therefore, large amounts of the less common isotopic species must be available, limiting observable solvent isotope effects to isotopic substitutions involving hydrogen. Detectable kinetic