The normal range for secondary Swain-Schaad exponents without tunneling or kinetic complexity

An analysis is presented of the range of secondary Swain-Schaad exponents to be expected at 25 °C in the absence of tunneling or kinetic complexity. From 15996 sets of exact harmonic semiclassical equilibrium isotope effects for simple C-H/D/T exchange reactions and 954 sets of exact harmonic semiclassical secondary H/D/T kinetic isotope effects for C-H positions in simple organic reactions, the distribution of Swain-Schaad exponents versus magnitude of the isotope effect is determined. This distribution defines when a secondary Swain-Schaad exponent may be considered to implicate nonsemiclassical behavior, revises the expected Swain-Schaad exponent for extrapolation of secondary isotope effects, and serves as a guide to the uncertainty in such extrapolations.