Mechanisms of elimination reactions. 30. The contributions of tunneling and heavy-atom motion in the reaction coordinate to deuterium kinetic isotope effects in eliminations from 2-phenylethyl derivatives

The temperature dependence of the rates of elimination of 2-phenylethyltrimethylammonium (1) and 2-phenylethyl-2,2-d/sub 2/-trimethylammonium (1-2,2-d/sub 2/) bromides with hydroxide ion in mixtures of water and dimethyl sulfoxide and with ethoxide ion in ethanol reveals abnormalities in the Arrhenius parameters that are characteristic of tunneling. Fitting a temperature dependence calculated from the Bell theory of tunneling to the observed temperature dependence permits the evaluation of the tunnel correction to the isotope effect, Q/sub tH//Q/sub tD/, and the semiclassical isotope effect, the effect in the absence of tunneling, (k/sub H//k/sub D/)/sub s/. The tunnel corrections are modest but by no means negligible, and the semiclassical isotope effects are well below the values predicted for a transition state with a symmetrically located proton, even though k/sub H//k/sub D/ and (k/sub H//k/sub D/)/sub s/ show maximum values in 40% dimethyl sulfoxide. It is suggested that the contribution of heavy-atom motion to the motion along the reaction coordinate keeps the semiclassical isotope effect small. Examination of selected data from the literature reveals a similar pattern of moderate tunnel corrections and below-maximum semiclassical isotope effects. The implications of these results for the interpretation of deuterium kinetic isotope effects in slow proton transfers are discussed. 5 tables.