The Reid group is investigating the environment-dependent photochemistry of halooxides (OClO, ClOCl), nitrosyl halides (ClNO), and other halogen-containing compounds (HCCs). HCC reactivity is a central issue in environmental chemistry due to the role these compounds play in many processes including stratospheric ozone layer depletion and aquifer contamination. The environmental impact of these compounds originates from their ability to photochemically produce atomic halogens with the quantum yield for this process dependent on the environment in which the chemistry occurs.
Figure 1: Photoexcitation of chlorine dioxide can lead to either photoisomerization or dissociation to form ClO and atomic oxygen.
The photochemistry of OClO provides an excellent example of environment-dependent reactivity, where the quantum yield for production of atomic chlorine is about 0.04 in the gas phase, but increases in condensed environments reaching unity in low-temperature matrices. The mechanistic details of this phase dependence are unknown; however, this information is requisite in understanding the reactivity of HCCs in condensed environments (e.g., polar stratospheric clouds and aquifers). The dramatic difference between the gas and condensed-phase reactivity of OClO demonstrates the need for investigating the condensed-phase reaction dynamics directly. In addition, the photochemical dynamics occur on a short timescale; therefore, knowledge of the ultrafast reaction dynamics of HCCs is required to understand the mechanism of halogen production. Although some recent work in this area has been performed, critical mechanistic issues such as the nuclear dimensionality of the initial excited state evolution, the excited-state decay rate, the kinetics of photoproduct formation, and the nature of solvent effects on photochemical quantum yields remain unexplored. In our research, sophisticated spectroscopic techniques are utilized to address these issues. In addition to understanding HCC reactivity, the information gained from these studies can be used in developing predictive models of the condensed-phase environmental reactivity of HCCs, a major goal in environmental chemistry.
A brief description of current projects is provided below:
Halooxides. Two resonance Raman spectroscopic techniques, resonance Raman intensity analysis and time-resolved resonance Raman spectroscopy, are combined with femtosecond time-resolved transient absorption spectroscopy to elucidate the solution-phase reaction dynamics of halooxides from the initial excited state nuclear evolution to the appearance and relaxation of the ground-state photoproducts. We are currently focusing on the use of time-resolved vibrational spectroscopic techniques (Raman and infrared absorption) to study the geminate recombination and vibrational relaxation dynamics of OClO. In addition, we are exploring the dependence of the OClO reaction dynamics on photolysis wavelength. We are also exploring the photochemistry of ClOCl using time-resolved resonance Raman. Previous femtosecond pump-probe studies performed in our lab demonstrated that following ClOCl photolysis, a protoproduct with an absorption maximum at 320 nm is produced; therefore, we are using time-resolved vibrational spectroscopy to identify the structure of this product.
Nitrosyl Halides. With our work on the photochemistry of OClO and ClOCl, we were interested in determining if the reaction dynamics of these halooxides was a more general pattern of reactivity. Therefore, we have begun to explore the chemistry of a related set of compounds, nitrosyl halides. In particular, we have recently studied the photochemical reaction dynamics of ClNO using femtosecond pump-probe spectroscopy. Very little is known about the condensed-phase reactivity of this compound, and our studies represent the first look at this reactivity in solution. Similar to the behavior observed for the halooxides, a photoproduct is produced on the ~10-ps timescale (see figure below). It has been proposed that this species is the photoisomer, ClON. We are currently using time-resolved resonance Raman and IR absorption spectroscopy to test this hypothesis by looking for a vibrational signature of this species.
Figure 2. Contour plot of the transient absorption spectrum of ClNO dissolved in acetonitrile. The absolute change in optical density is indicated by the contour lines, with the color scale shown above the plot. The plot shows the blue-shifting and band narrowing typical of vibrational relaxation. This behavior is not observed for the 330 nm band produced from the photolysis of ClNO dissolved in chloroform.
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