MECHANISMS OF ELIMINATION-REACTIONS .30. 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-d2-trimethylammonium (1-2, 2-d2) bromides with hydroxide ion in mixtures of water and dimethyl sulfoxide and with ethoxide ion in ethano! 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, QtH/QtD, and the semiclassical isotope effect, the effect in the absence of tunneling, (kH/kD)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 kH/kD, and (kH/kD)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. © 1979, American Chemical Society. All rights reserved.