C-13 AND N-15 KINETIC ISOTOPE EFFECTS ON THE DECARBOXYLATION OF 3-CARBOXYBENZISOXAZOLE - THEORY VS EXPERIMENT

Nitrogen and carbon kinetic isotope effects were measured on the decar boxylation of 3-carboxybenzisoxazole at room temperature. The nitrogen isotope effect in acetone is 1.0312 +/- 0.0006. The carbon isotope ef fect shows no dependence on the solvent polarity: 1.0448 +/- 0.0007 in 1,4-dioxane, 1.0445 +/- 0.0001 in acetonitrile, 1.0472 +/- 0.0013 in DMF, and 1.0418 +/- 0.0027 in water. These isotope effects were modele d theoretically at the semiempirical (AM1, PM3, SAM1) and ab initio (u p to B3LYP/6-31++G*) levels. The comparison of the theoretical and ex perimental results leads to the conclusion that none of the theory lev els employed is capable of quantitatively predicting these isotope eff ects.