Intermolecular 13C kinetic isotope effects in the decarboxylation of oxalic acid in dioxane solution

Abstract

Intermolecular 13C kinetic isotope effects in the decomposition in dioxane solution of oxalic acid to carbon dioxide and formic acid were determined at several temperatures between 80 and 132 °C. The isotope effect is normal in sign (k12C ≳ k13C), and approximately 3% in magnitude; the temperature dependence (small over the range of experimental temperatures) may be complex, if real. The method of three-element reaction coordinates (with ν‡1=0) was employed to expose possible properties of the transition state. The reaction coordinate eigenvectors yielding best correspondence to the experimental data consist of C–C bond extension, C–O bond contraction, and either (a) O–H bond extension or (b) contraction of the angle O–C–C to linearize O–C–O, these displacements being sited about the C atom which will appear in carbon dioxide product. The differences in the same region between reactant and transition state force fields which yield calculated results in best agreement with the experimental measurements are F‡C–C = 0.125 F0C–C, F‡C–H = 0.5 F0O–H, F‡C–O = 0.875 F0C–O, and F‡bend = (C‡/C0)F0bend, where C is the product of the bond orders of the stretching coordinates defining the bond angle.