Molecular electronic excited states usually exhibit chemical reactivity and structures very different from those of the corresponding ground electronic state of the molecule. The goal of the following research projects is to better understand the structure and dynamics of short-lived molecular species and better understand chemical reaction mechanisms.

1. Photodissociation, Photoisomerization and Photocyclopropanation Reactions of Polyhaloalkanes

Polyhalomethanes like CFC1₃, CC1₄, CHBr₃, CHBr₂C1, CH₂I₂, CH₂Br₂ and CH₂BrI have been observed in the atmosphere and are important sources of reactive halogen species in the atmosphere. Polyhalomethanes are also of interest in synthetic chemistry for cyclopropanation reactions. We have recently shown that ultraviolet photolysis of polyhalomethanes in room temperature solutions leads to appreciable formation of novel iso-polyhalomethane photoproduct species. The iso-diiodomethane species can act as the methylene transfer agent in photocyclopropanation reactions using photolysis of diiodomethane in the presence of olefins. We are using resonance Raman and time-resolved resonance Raman spectroscopy as well as density functional theory calculations to better understand the iso-polyhalomethane species and its chemical reactivity.

 

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2. Water Catalyzed Dehalogenation Reactions of Selected Halogenated Molecules

We have recently elucidated how water assists several different types of dehalogenation reactions leading to formation of strong acid leaving groups and facile cleavage of C-H, O-H and C-X bonds. This project seeks to use a combination of experimental and theoretical studies to better understand how water is able to catalyze or assist these types of chemical reactions. This work has important implications for the phase dependent photochemistry/chemistry of a number of compounds in the natural environment and in the design of efficient photocatalysts/catalysts for degradation of pollutants in water.

3. Structure, Properties and Chemical Reactions of Arylnitrenium Ions and Selected Phototrigger Compounds

Carcinogenic aromatic amines are found in automobile fumes, tobacco smoke, broiled or fermented meat and as unwanted trace products in industrial processes. These carcinogenic compounds typically transfer an arylamine to DNA which then undergoes a chemical reaction that damages the DNA. Nitrenium ions have been found to play an important role in the metaobolism reactions of carcinogenic arylamines that damage DNA. It is important to understand the structures and reactivities of these short-lived nitrenium ions and their reaction intermediates. We are using time-resolved resonance Raman spectroscopy experiments to obtain missing structural information for selected nitrenium ions and their reaction intermediates.
Phototrigger compounds are used to release biological active species for use in physiology experiments. Several new classes of phototrigger compounds have been developed but the reaction mechanism(s) for how the photoremovable group is released remains unclear. Experimental and theoretical work is on-going to better characterize and identify the chemical reaction intermediates and to elucidate the reaction mechanism(s) involved in the release of the photoremovable group from these phototrigger compounds.

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This work has important implications for the phase dependent photochemistry/chemistry of a number of compounds in the natural environment. We hope to achieve an improved understanding of how water assists several different types of dehalogenation reactions leading to formation of strong acid leaving groups and facile cleavage of C-H, O-H and C-X bonds. This may be useful to help in the design of efficient photocatalysts/catalysts for degradation of halogenated pollutants in water.

Carcinogenic aromatic amines typically transfer an arylamine to DNA which then undergoes a chemical reaction that damages the DNA. Arylnitrenium ions have been found to play an important role in the metaobolism reactions of carcinogenic arylamines that damage DNA but their reaction mechanism and pathways are not understood. This project aims to better understand the structures and reactivities of these short-lived arylnitrenium ions and their reaction intermediates toward guanine base pairs in DNA. This will lead to an improved understanding of the molecular basis for chemical carcinogenesis of aromatic amines.

Research on these projects have resulted in more than 30 peer-reviewed articles in top international journals listed in Science Citation Index (full list will be supplied upon request). Selected articles are listed below.

Selected Publications
1. X. Zheng and D. L. Phillips, Journal of Physical Chemistry A 104, 6880-6886 (2000).
2. X. Zheng, C. W. Lee, Y.-L. Li, W.-H. Fang and D. L. Phillips, Journal of Chemical Physics 114, 8347-8356 (2001).
3. Y.-L. Li, K. H. Leung, and D. L. Phillips, Journal of Physical Chemistry A 105, 10621-10625 (2001).
4. D. L. Phillips, W.-H. Fang and X. Zheng, Journal of the American Chemical Society 123, 4197-4203 (2001).
5. P. Zhu, S. Y. Ong, P. Y. Chan, K. H. Leung, and D. L. Phillips, Journal of the American Chemical Society 123, 2645-2649 (2001).
6. P. Zhu, S. Y. Ong, P. Y. Chan, Y. F. Poon, K. H. Leung, and D. L. Phillips, Chemistry European Journal 7, 4928-4936 (2001).
7. C. Zhao, D. Wang and D. L. Phillips, Journal of the American Chemical Society 124, 12903-12914 (2002).
8. C. Zhao, D. Wang and D. L. Phillips, Journal of the American Chemical Society 125, 15200-15209 (2003).

We have also trained 5 Ph.D. and 3 M.Phil. students in the course of these research projects.

We expect to achieve an improved understanding of the key reaction intermediates and reaction mechanisms involved in the photocyclopropanation reactions of polyhalomethanes, the water-catalyzed dehalogenation reactions of selected halogenated compounds, arylnitrenium ion reactions related to chemical carcinogenesis of aromatic amines and photorelease reactions of selected phototrigger compounds.

The HPC Cluster will be useful for doing a number of ab initio and density functional theory calculations that can elucidate the structure and properties of key chemical reaction intermediates. Further calculations will also be done to investigate the chemical reaction pathways and reaction mechanism.

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