Mechanisms of elimination reactions. XXXIII. Carbon isotope effects in E2 reactions of (2-phenylethyl-2-14C)trimethylammonium ion: the role of tunneling

Carbon isotope effects have been determined for the reactions of (2-phenylethyl-2-14C)trimethylammonium ion with hydroxide ion in 10, 25, 40, and 60% Me2SO-water and with ethoxide ion in dry ethanol. Comparison of these values with earlier results on 13C isotope effects shows that the relation between the 14C and 13C effects is close to that predicted from theory. The magnitudes of the isotope effects suggest significant contributions from tunneling, and this suggestion is supported by temperature-dependence studies that give ΔE/sub a/ values too large and A/sub a(12)//A/sub a(14)/ values too small to account for without tunneling. Temperature dependences calculated from the Bell theory of tunneling can be fitted to the experimental data. The results indicate that the semiclassical (without tunneling) carbon isotope effects are small or even inverse and that tunnel corrections make major contributions to the observed isotope effects