William H. Casey, Professor, Departments of Chemistry and Geology, University of California Davis
Abstract: Geochemists worry a lot about two things in particular (i) thermodynamics; and (ii) reaction pathways affecting minerals. This presentation will address both and proceed in two parts---in the first half I will talk about pathways for isotope-exchange reactions between aqueous solutions and nanometer-sized clusters that look somewhat like minerals; in the second half I will introduce a new NMR probe design that extends experiments to 2.0 GPa. We've used large aqueous oxide ions as experimental models for extended mineral-solution interfaces. In large ions, both cations and anions, oxygen-isotope exchanges proceed via metastable states that form from concerted motions of large parts of the structure. We find the same general sequence of sets of steps: (i) a surface metal detaches from an underlying overbonded oxygen, forming a metastable fragment with low-coordinated oxygens; (ii) an isotopically normal oxygen adds to the metal; (iii) protons shuttle to more basic sites in the metastable structure; (iv) after an appreciable lifetime the surface-structure collapses. These 'oxygen-stuffing reactions' explain the fact that isotope-exchange rates at different oxygens exhibit the same pH dependencies that also resemble the dissociations,the broad amphoteric chemistry, and the regiospecific role of counterions.