Carbon-thirteen kinetic isotope effect modeling studies on the decompositions of formic acid, dimethyl ether, and oxalic acid

The formic acid molecule was used as a model for the survey of the influence on calculated heavy-atom kinetic isotope effects (KIE's) of barrier curvature. Restrictions employed were: invariant geometry and basis diagonal force field; light species reaction coordinates eigenvector (L1) consisting of one or two nonzero elements only. The formic acid molecule was used as a model for a computational comparison of the Willi--Wolfsberg curvature parameter (K) method (Type K) and the Johnston--Bonner--Wilson method (Type II), both including bond order (N) and barrier curvature effects, for calculation of 13C KIE's involving L1 consisting nominally of two nonzero elements. Results obtained by the two methods covered similar ranges of value of calculated k/k'. Intermolecular and intramolecular 13C KIE's in the pyrolysis of dimethyl ether were analyzed. The isotope effects arise in the destruction of the symmetry of the reactant in hydrogen transfer reactions of the form R + C2H3OC1H3 → RH + .C2H2--OC1H3, where R = CH3, H, or NO. The best match of model predictions and experimental data was obtained when: (a) transition state force constants were very product-like; (b) the relative internal coordinate displacements comprising L1 are: C--C bond extension, C--O bond contraction, and O--H bond extension