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Marion E. CassProfessor of Chemistry
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Education:B.S. Chemistry, 1979 Ph.D. Chemistry, 1984 American Cancer Society |
Contact Information:Department of Chemistry Carleton College 1 North College Street Northfield, MN 55057E-Mail: mcass |
Advanced Inorganic Chemistry, Molecular Orbital Theory, Advanced Inorganic Chemistry Laboratory (which includes inert atmosphere synthesis), Introductory Chemistry, Chemistry for Non-Science Majors, and Scientific Glassblowing
Since 2004, I have been doing computational research in collaboration with my colleague Professor Henry Rzepa from Imperial College, London. In several projects we have examined the mechanisms by which molecules scramble atom positions without breaking chemical bonds. A major emphasis of our work has been to develop interactive visualizations to facilitate the understanding of these molecular processes.
In our first published project we used computational methods to find the transition states and to visually examine the molecular motion for the Berry Pseudorotation (in PF5, SF4, and Sn(amidinate)2), the Lever Mechanism (in ClF3 and in one high energy pathway for SF4), the Turnstile Mechanism (which results in cis/trans isomerization for the computational model SF4Cl2), and the Bartell Mechanism (in IF7). In addition, although square pyramidal molecules were also known to undergo intramolecular rearrangements, no mechanistic analysis had been performed. We located the transition states and computed the activation barriers for several AX5 molecules and discovered that they passed through one of two types of transition states; one of Cs symmetry (for BrF5, IF5) or one of C2v symmetry (for ClF5, ICL5 and IBr5). Although these two mechanisms share some similar types of motions, the two rearrangement processes are distinctly different.
In our second published project, we examined a series metal complexes of acetylacetonate ligand analogs that racemize the &Lambda and &Delta chiral forms via a Bailar Twist and/or a Rây-Dutt mechanism (variants of the twist mechanism). We then extended the project to look for computational verification of the experimental observation that the bite angle of the ligand in the tris-chelate will influence whether a Bailar Twist mechanism has a lower energy barrier relative to a Rây-Dutt.In a third project, I examining the relative stabilities of the four conformers (that result from &lambda and &delta ligand twists) for each of the enantiomers (&Delta and &Lambda) of [Co(en)3]3+. The relative stabilities are known to be solvent and counter ion dependent. Although these systems have been extensively studied in the past, I have been working with my Advanced Inorganic Chemistry Laboratory class (Chem 352) to develop projects that support our synthetic work. In this class, offered each spring at Carleton, we have carried out the synthesis, separation and purification of the two enantiomers &Delta and Λ-[Co(en)3]I3. We then convert each enatiomer to its corresponding [Co(1,8-dinitro-3,6,10,13,16,19-hexaazabicyclo-(6.6.6-icosane)]Br3 complex via a synthetic procedure reported by A. Sargeson and his co-workers. We have explored different spectroscopic techniques to characterize the products and different methods to determine the enantiomeric excesses.
Previously my research interests were focused in two general areas. When I first came to Carleton, I examined the relationship of molecular structure to specificity in binding of metal ions by naturally occurring metal ion sequestering agents. Specifically, I aspired to learn more about the specificity acheived in binding the vanadium(IV) ion by the N-hydroxy-2,2'-iminodipropionic acid ligand found in the mushroom Amanita muscaria. We designed and synthesized new chelating agents and then examined the coordination chemistry of our ligands with vanadium(IV) and other metal ions. I also have been interested in the design and synthesis of inorganic dye molecules to be used as sensitizers in titanium dioxide dye-sensitized solar cells. During the 1994-95 and 1995-96 academic years, I was on leave from Carleton at the California Institute of Technology working with Professor Nate Lewis and his research colleagues. Our research involved synthesizing and characterizing new and previously known dye sensitizers. We fabricated solar cells with these dyes to be used in a variety of comparative tests to help us gain an understanding of the fundamental limitations of these types of solar cells in energy conversion processes.
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