On the Absence of Isotope Effects in the Absence of Force Constant Changes

Calculations have been carried out to demonstrate that there is usually very little quantum-mechanical contribution to an isotope effect on a rate (equilibrium) in the absence of force constant changes, between reactant and transition state (product), at the position(s) of isotopic substitution. The usual upper limits to the deviations of such secondary isotope effects [(k1/k2)(ν1L‡/ν2L‡)−1 or K1/K2] from unity, at room temperature, have been estimated to be 1.5% per α deuterium, 0.2% per β deuterium, and even less for heavy-atom isotopes or for deuteriums further removed from the reaction site. Some exceptional circumstances under which there may be large ``no-force-constant-change'' isotope effects have been found. However, usually it is quite proper to interpret the existence of sizable quantum-mechanical isotope effects as indicative of force constant changes at the isotopic position(s).