Molecular orbital and transition state theory modeling of isotope exchange in aqueous systems

Molecular orbital (MO) modeling combined with transition state theory (TST) provides a means to describe the progress of reactions at the atomic scale and to estimate the order of magnitude of reaction rate constants. In this thesis, the combined method, referred to as MO-TST, is used to elucidate the geochemically relevant kinetics of three isotope exchange reactions in two aqueous systems: the oxygen and hydrogen exchange between orthosilicic acid and water, and the carboxylic carbon exchange between acetic acid and bicarbonate.; The isotopes of hydrogen and oxygen are extensively used in studies where water interacts with rocks or evolves from a source body. These isotopes are generally assumed to exchange between water and dissolved silica species at higher rates than fluid transport or mineral silica-water exchange; however, there is no established basis for this assumption and the magnitudes of the exchange rates have not been measured. MO-TST modeling of water and orthosilicic acid reveal that these reactions are indeed relatively fast, with computed reactant half-lives of <1 s at room temperature. The results furthermore show how such fast exchanges may proceed: through concerted proton transfers facilitated by water molecules, and by way of symmetric transition states (hydrogen) or intermediates (oxygen) through symmetric reactions.; Carbon isotopes are key tools for understanding an important topic in oil exploration and production—the sources and history of short chain organic acids in oil field waters. Lately, carbon isotope studies on individual short chain organic acids have found evidence suggesting the pH dependent mixing of dissolved inorganic carbon signatures with the carboxyl group of the acids. However, there is no generally recognized reaction by which this exchange takes place. MO-TST is used to determine the feasibility of two proposed symmetric pathways for the exchange: one by way of malonic acid, and another by a hydroxylated derivative of malonic acid. The results show rates consistent with experimental data and the correct pH dependence suggesting that these are good candidates for the exchange reactions. The rate determining steps found are also concerted proton transfers facilitated by water molecules.