Large Temperature Dependence of Large Kinetic Isotope Effects of Multistep Hydride Reduction of <i>p</i> -Chloranil by NADH Models in Acetonitrile: Proton Tunneling within Loose Radical Ion-Pairs

The temperature dependence of H/D kinetic isotope effects (KIEs) was determined for electron-transfer initiated multistep hydride-transfer reactions of NADH models in acetonitrile to investigate our hypothesis that more rigid systems exhibit smaller isotopic activation energy differences (Δ<i>E</i>_a = <i>E</i>_aD - <i>E</i>_aH). Proton-tunneling occurs in close contact or partially solvent-separated radical ion-pair intermediates. Large KIEs and large Δ<i>E</i>_a's were observed, in contrast to the relatively small KIEs and Δ<i>E</i>_a's typically observed for one-step mechanisms where hydride-tunneling occurs within much tighter charge-transfer complexes. Results support the hypothesis.