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Insights into Chemical Reactivity


Primary versus secondary ¹⁸O equilibrium isotope effects on the acidity of a variety of carbon, boron, nitrogen, and phosphorus acids were computated by DFT using the B3LYP/aug-cc-pVTZ basis set. The results show that while both isotope effects are normal for the deprotonation of a Brønsted acid, the magnitude of the secondary equilibrium isotope effect is larger than the magnitude of the corresponding primary equilibrium isotope effect. This does not hold for hydroxide addition to Lewis acids, where the primary equilibrium isotope effects are inverse and larger in magnitude than the secondary equilibrium isotope effects, which are normal. The symmetry of the hydrogen bond of hydrogen cyclohexene-1,2-dicarboxylate monoanion was determined in chloroform using the NMR method of isotopic perturbation. The observed increase in the chemical shift separation as the temperature decreases suggests that this monoanion is a mixture of tautomers in rapid equilibrium, and has an asymmetric hydrogen bond. The mechanism for the base-catalyzed decomposition of malonic anhydride to ketene and carbon dioxide was studied computationally. The DFT methods included the B3LYP/6-31G+ (d) basis set with and without the polarized continuum model of chloroform and the MP2/cc-pVDZ basis set. The decomposition proceeds via deprotonation of malonic anhydride by the base catalyst. The malonic anhydride anion undergoes ring-opening to form a ketene carboxylate intermediate. Decarboxylation of this intermediate results in a ketene anion which rapidly deprotonates a molecule of malonic anhydride to form the ketene product and continue the catalytic cycle

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