Non-zero rates at cryogenic temperatures and a curved Arrhenius plots are two indications that a reaction occurs by quantum mechanical tunneling. However, many reactions occur by tunneling that is thermally activated. In such reactions a highly temperature-dependent primary H/D kinetic isotope effect (KIE) and primary and secondary KIEs that are non-multiplicative, provide reliable and, in general, more easily obtained indications of tunneling than curved Arrhenius plots. Examples will be drawn from calculations on a variety of organic and organometallic reactions.
Calculations predict that cyclopropylcarbinyl radical should undergo rapid ring opening, even at cryogenic temperatures, and that substitution of deuterium for hydrogen at different carbons should lead to the observation of both normal and inverse kinetic isotope effects. Both of these experimentally testable predictions will be discussed.