Tritium and deuterium Arrhenius parameter effects in a base-promoted elimination reaction: evidence for tunnelling
The theory of isotope effects, based on absolute reaction rate theory, provides what is probably the single most powerful tool for the study of transitio:n. state structure. Thus, isotope rate effects are most frequently ol:served. for the purpose of elucidating mechanism. However, sufficiently detailed observations ofkinetic isotope effects on well~characterized reactions provide a valuable test not only of kinetic isotope effect theory but also of absolute reaction rate theory upon which it is based. A relatively small nurober of isotope effects have been studiedas a function of temperature; an even smaller nurober are known for two isotopes in the sa:me reaction; in only the reaction of chlorirre atoms with hydrogen molecules is there available information on temperature-dependence of two isotope f!.ffects in the same reaction. This system has recently been treated theoretically in so:ne detaill. The results "constitute a positive test of the transition state fonnulation of the effect of isotopic substitution on the rates of chemical re.lctiom;". It was not possible to conclude whether or not tunnelling is a si§:nificant feature of the process. Tunnel corrections, however, have been calculated tobe generally large in a theoretical treatment ofhydrogen atom transfer reactions and the results confirmed by reference to measured hydrogen-deuterium isotope effects as a function of temperature in the re;lctions of methyl radicals with organic compounds2. In a recent survey ofisotope effects in proton transfer reactions in solution, Be ll3 found only one study, ofthe bromination of carbethoxycyclopentanone, in which hydrogen-deuterium isotope effects had been measured as a fu:1ction of temperature in a reaction simple enough to be amenable to quantitative treatment. A detailed theoretical analysis, based on a onedi:mensional transition state model, indicated substantial tunnelling by hydrogen with a smaller but significant amount by deuterium. It was pointed out that while tunnelling might lead to recognizable abnormalities in isotope rate ratios or in activation energy differences, usually knowledge of transition state structure is insufficient to allow definitive conclusions in in·:erpreting these parameters. On the other hand, the suggested most likely abnormality diagnostic of tunnelling was that the deuterium Arrhenius "A factor" was larger than that of hydrogen. In the theoretical