Atmospheric Reaction Complex Anions
Oxidation of volatile organic compounds: We are building a photoelectron imaging experiment coupled with an ion gun/pulsed molecular beam valve for the production of adducts formed by the addition of hydroxyl radical to isoprene (and other VOCs). These anion adducts are stable precursors to reactive radical neutral complexes. The anion is often a closed shell weakly bound complex, but its analgous neutral is a reactive radical complex. Adducts formed by the addition of hydroxyl radical to isoprene are believed to be a critical step in the production of tropospheric ozone in rural areas. With the low temperatures accessible in molecular beam expansions, we will explore the role of water clustering on the complex, which is presently not a well-established component of atmospheric chemical models.
Aromatic compounds are particularly troublesome pollutants found in diesel-fueled motor vehicle exhaust. We have studied the interactions between aromatic compounds and molecular oxygen anion/neutrals as well as the electronically complex O4 anion/neutrals. The complexes observed apear to have a co-planar structure which facilitates a dramatic charge delocalization resulting in a higher sensitivity to photochemistry.
All of these projects start with negative ions. Initiating studies with anions has numerous advantages. First, these systems are inherently mass selectable, thereby eliminating any ambiguity regarding species identification. Second, depending on the system, the anions’ “excess” electron is generally located in places that are sensitive to how the constituent atoms are bound, or even the dipole moment of the resulting neutral. So by detaching the electron from the anion, we learn about the properties of the associated neutral. And the density functional theory calculations provide an invaluable tool for the analysis of the spectra of complicated systems. Using this combination of tools, we extract detailed information regarding the structure, chemical and electronic properties of this broad range of systems.