Goals
The research project is focussed on the experimental quantitative characterization of the UV absorption spectra, kinetics and thermochemistry of hydroxycyclohexadienyl radicals using laser pulsed photolysis &endash; transient UV-VIS absorption spectroscopy. Fundamental study on the properties, thermochemistry and reactivity of these transient intermediates is the main objective of the research. These free radical species ((alkyl)-hydroxycyclohexadienyl radicals) are formed in the primary step in the oxidation of aromatic compounds in the atmosphere via addition of hydroxyl to the aromatic rings. Understanding of the fundamental chemistry, kinetics and thermodynamics of the intermediate steps of complex reactions of oxidation of airborne organic compounds is crucial for the development of predictive models of ozone and smog formation. Modeling of the further transformations of these intermediates currently heavily relies upon the calculated thermochemical properties and estimated rate constants.
The anticipated results of the research include quantitative UV spectra (absorption cross-sections) of alkyl hydroxycyclohexadienyl radicals, kinetic parameters (rate constants, temperature and pressure dependencies) of reactions of hydroxyl radicals with benzene and toluene, thermochemistry of the adducts of hydroxyl radicals with benzene and toluene (alkyl hydroxycyclohexadienyl radicals) and the rate data on the reactions of these radicals with O2, NO and NO2. The data obtained in this study will improve the accuracy and reliability of the current models of aromatic compound oxidation in the atmosphere.
Background
Airborne organic compounds contribute to air pollution in both urban and rural areas. Many of them are directly harmful to human health and the environment and play critical role in the ozone and smog formation. Understanding of the fundamental chemistry, kinetics and thermodynamics of the intermediate steps of complex reactions of oxidation of airborne organic compounds is crucial for the development of predictive models of ozone and smog formation.
Aromatic hydrocarbons are relatively stable at atmospheric conditions and do not react directly with an appreciable rate with molecular oxygen or even with much stronger oxidizer &endash; ozone. The oxidation of these compounds requires initial "activation" &endash; conversion to free radicals either via addition or abstraction reactions. "Activation" of aromatic compounds in the atmosphere occurs in reactions of free radical addition to the aromatic ring(s) or in the reactions of H-atom abstraction by free radicals
In the atmosphere, hydroxyl radical, OH, plays the major role in the processes of activation of hydrocarbon molecules. Reactions of hydroxyl radicals with hydrocarbons represent the main contribution to the primary oxidation step. In the case of aromatic compounds (such as common solvents benzene and toluene) hydroxyl radical can attach to the aromatic ring forming an intermediate complex (substituted hydroxycyclohexadienyl radical) or to abstract hydrogen atom:
At atmospheric conditions the first step (addition) is the major one. At 298 K the rate of addition is higher than the rate of abstraction 16 times in the case of benzene and 12 times in the case of toluene. Therefore, the intermediates formed in these addition reactions (hydroxycyclohexadienyl and methyl hydroxycyclohexadienyl radicals, respectively) are of the central importance for the whole oxidation process.
These intermediate radicals &endash; products of addition of hydroxyl to the aromatic ring &endash; are relatively weakly bonded. They can undergo further bimolecular (with O2, NO and NO2) as well as unimolecular reactions. Formation of intermediate adducts in reaction (2a) as well as in subsequent reactions of these radicals with oxygen, NO and NO2 leads to chemical activated systems. The modeling of such systems based on statistical theories heavily relies on the thermochemistry of these and subsequent intermediates. There have been substantial theoretical efforts on the characterization of the intermediates in the oxidation of aromatic compounds. Limited experimental data are currently available to support the theoretical findings. While the overall rates and the branching ratios of reactions (2a,b) are reasonably well established, the intermediates formed as well as the processes of their further transformations are still poorly characterized. This is mainly due to the lack of simple and clean way of generation of these intermediates and convenient and sensitive experimental methods of detection.
Experimental studies of oxidation of aromatic compounds until recently were mainly of the smog-chamber type, with UV irradiation of the reaction mixture and analysis of the stable or semi-stable products of the reaction. No direct observation of intermediates and determination of rate constants of elementary reactions under the conditions of isolation are performed in such studies.
Zetzsch and co-workers and recently Lee with co-workers employed sensitive technique of laser induced fluorescence (LIF) to monitor OH radical after the laser photolysis of O2/aromatics/NOx mixtures. The rate constants of the adducts formation and the further adducts reactions with oxygen and nitrogen oxides are derived based on the fitting of the experimentally observed double-exponential decays of OH radicals to a hypothetical reaction mechanism. No direct detection of the adduct intermediates were performed and the data obtained depend upon the proposed (hypothetical) mechanism.
The further advancement in the understanding of the thermochemistry and the reactivity of the intermediates in the oxidation of aromatic compounds requires development and characterization of direct methods of their detection. One of the possibilities is transient UV absorption. This is one of the few methods, which can be used at atmospheric and higher pressures without the loss of sensitivity. There have been some recent advances both in theoretical and experimental characterization of UV absorption of the adduct of OH to benzene. Thermochemistry and other properties of a simpler cyclohexadienyl radical (the product of addition of H-atom to benzene) is characterized somewhat better. However, the UV absorption spectrum was reliably determined only recently. The radical has strong absorption with maximum at 300 nm and the absorption cross-section 2.55x10-17 cm2 molecule-1. The fact that the absorption is strong and is located in the near ultraviolet allows relatively sensitive detection of this radical even with traditional light sources. Absorption spectra of the radicals of interest in this study are expected to have similar parameters.
Another approach is recently being developed by Prof. M. Molina. In this approach, chemical ionization mass spectrometry technique is used to detect substituted hydroxycyclohexadienyl radicals. The technique is very sensitive and is used to study the reactivity of these radicals in combination with sub-atmospheric and atmospheric pressure turbulent flow systems.
Method of Approach and Facilities
Experimental
The experimental approach is based on the combination of Excimer Laser Pulsed Photolysis with UV-VIS Transient Absorption Spectroscopy. Significant modifications were incorporated into the existing experimental facility during the initial phase of the project. First, a powerful industrial scale excimer laser (Lumonics, Index-200) was acquired and installed. Second, a fast Gated Intensified CCD Camera (Princeton Instruments, ICCD-Max) together with a imaging spectrograph (Acton, 320i) was purchased and incorporated into the experimental facility (Fig. 1). In addition to the Lumonics Index-200 excimer laser, a low energy (10 mJ) laser from Lambda Physik (Optex) is available for the experiments. A pulse from the excimer laser (Lumonics, Index-200, 100 - 150 mJ at 193 nm, ArF) is directed along the axis of a flow reactor. Selective dielectric mirrors (Newport, OptoSigma) are used to merge the photolytic pulse with the CW or quasi-CW monitoring light.
Xenon, mercury arc lamps (Oriel) as well as current boosted hollow cathode lamps (home-made, based on the Photron Superlamp) are used as sources of monitoring light. Current boosted hollow cathode lamps (current 4 A, duration 10 msec) provide higher intensity light compared to other traditional sources at wavelengths shorter than 230 nm. An imaging spectrograph equipped with a fast gated CCD camera (Princeton Instruments, ICCD-Max, 256x1024 CCD) and a photomultiplier tube is used in conjunction with the camera controller, digital storage oscilloscopes (LeCroy) and a PC for digital data acquisition and processing. The spectrograph is equipped with three computer controlled gratings (150, 300 and 1200 groove/mm) which allow low resolution wide range and high resolution narrow range spectra and a computer controlled diverter.
The experimental facility allows both kinetic and time resolved spectral modes of operation. In the kinetic mode, the deflector mirror is set to divert light onto the slit equipped with a photomultiplier tube. In this arrangement, the whole kinetic curve at a single (tunable) wavelength is acquired. In the gated spectral mode, the dispersed light is enters the gated ICCD camera. In a single pulse, this allows acquisition of a whole spectrum at a specific delay time. In repetitive pulse experiments, the temporal behavior of the whole spectra is acquired.
Flow cells and high-pressure flow system (based on Brooks high-pressure mass-flow controllers and back-pressure controllers) allow kinetic measurements from sub-atmospheric (ca. 0.001 atm) to high (up to 100 atm) pressures. Experiments at elevated pressures are considered in view of possible pressure dependence of association/dissociation reaction forming relatively weakly bonded adducts, especially at elevated temperatures required for the equilibrium studies
Generation of Hydroxycyclohexadienyl Radical
The main difficulty in the quantitative kinetic studies of cyclohexadienyl radicals is in the "clean" production of these species. Up to date, these radicals are produced indirectly via attachment of hydroxyl radicals to a proper aromatic molecule (reaction 2b in the case of benzene). The existence of a competing route of H-atom abstraction (reaction 2b) already represents a kinetic complication. Quantitative kinetic measurements of absorption cross-sections and radical-radical reaction rates require reliable determination of the absolute concentration of the free radical species. Interfering reactions such as reaction 2b require additional measurements of the branching ratio and introduce additional uncertainties.
Production of relatively high concentrations of hydroxyl radicals (required for the absorption spectra characterization) under conditions when aromatic compounds are present is challenging due to strong absorption of aromatic compounds in the far UV.
Several indirect photochemical generators (which differ by the mechanism of OH radical production and by the relative importance of the secondary chemistry) were considered and evaluated. All these indirect methods suffer from the competition between the desired reaction which forms the target radical species and reactions which consume hydroxyl and the target radical. To overcome this competition, high concentrations of the aromatic compounds are required. This leads to attenuation of photolytic light as well as to the production of undesirable products of their photolysis.
The main additional problems of the peroxide based mechanism are in the fast reaction of hydroxyl radical with the precursor molecule and in the difficulties of operating with highly concentrated peroxide.
Photolysis of nitric oxide is a good source of OH at longer wavelengths. However, at 193 nm the quantum yield of hydroxyl radicals is only 0.33. The product of photolysis (NO2) as well as the products of undesirable routes (NO, NO3, etc.) complicate the secondary chemistry. A complete kinetic model for this system was build and used to process the kinetic profiles. In addition, a Cl-atom based precursors ((COCl)2, COCl2 and Cl2 photolysis were analyzed as well as a possibility of using a softer UV light (248 nm, 308 nm, 351 nm). None of these additional analyses lead to a satisfactory generator of cyclohexadienyl radicals.
Modeling of a kinetic system showed that the approach based on the photo generation of the excited metastable oxygen O(1D) atoms is the best indirect source of cyclohexadienyl radicals. Photodissociation of nitrous oxide at 193 forms O(1D) atoms with the 100% quantum yield. These atoms are quantitatively converted to two hydroxyl radicals in reaction with water molecule. This approach is chosen as a main indirect source of cyclohexadienyl radicals in the planned research.
Development of a Direct Photolytic Source of Hydroxycyclohexadienyl Radicals
The modeling performed in this study as well as the literature survey suggest that the experimental studies on the spectroscopy, kinetics and thermochemistry of these important intermediate species &endash; cyclohexadienyl-type radicals &endash; are hindered by the absence of a clean and convenient primary photolytic precursor of these species. Because of this reason an attempt is undertaken to design and synthesize such a precursor. The suggested precursor molecule is a halogenated substituted cyclohexadien. For example, for generation of hydroxycyclohexadienyl radical 4-Bromo-1-Hydroxy-2,5-cyclohexadiene is considered:
These compounds are not commercially available. Thermal stability is unknown. Currently, the synthesis of the simplest precursor (bromocyclohexadiene) of this class was performed. The separation and characterization of the compound is underway. The preliminary data on the thermal stability is encouraging.
Using a direct photolytic precursor will allow to avoid of the majority (if not all) of the problems encountered in the indirect methods. The necessity of high concentrations of aromatic compounds, of the generation of hydroxyl radical and accounting for the interfering secondary chemistry could be avoided. Therefore, the development of direct photolytic precursors of cyclohexadienyl-type radicals will allow to significantly simplify the experimental approach and analysis of the experimental data.
Summary of Progress and Accomplishments
The experimental facility (Laser Photolysis-Transient UV-Vis Spectroscopy) was modified and upgraded to incorporate a powerful excimer laser and a fast Gated ICCD camera.
Several methods of indirect photolytic production of (substituted)cyclohexadienyl radicals for quantitative kinetics studies were modeled and evaluated. These methods include hydroxyl radical production in laser photolysis of H2O2, HNO3, and N2O (with subsequent reaction of O(1D) atom with H2O). The target radical is formed in the attachment reaction of hydroxyl radical to a proper aromatic molecule. The decision is made to focus on the system based on the nitrous oxide/water/aromatics mixtures.
A direct photolytic precursor was suggested. The synthesis of the required compound (commercially unavailable) was initiated and currently is half-way to completion.
Temporal profiles of hydroxyl and hydroxycyclohexadienyl radicals in photolysis of (H2O2 or HNO3)/benzene mixtures are recorded. A reaction kinetic model was developed and compared with the experimental data.
Interactions with other CAO Projects
The research proposed is related most closely and is complementary to the projects of Prof. M. Molina "Laboratory Studies of Intermediate Steps in the Atmospheric Oxidation of Organic Compounds", where the main accent is made on the characterization of the kinetics of the oxidation intermediates using chemical ionization mass-spectrometry, to the project of Prof. Bozzelli "Pathway Analysis and Elementary Reaction Mechanism for Atmospheric Reactions of Aromatics &endash; Benzene and Toluene" (modeling study), and to the experimental studies of Prof. J. Seinfeld and R. Flagan "Atmospheric Transformation of Volatile Organic Compounds: Photooxidation and Gas-to Particle Conversion". This latter research is mainly focussed on the final photooxidation products and particulate matter formation.
At the current stage, the main results are in the experimental developments. The interactions are anticipated at the later stages of the research.
Status of Original Plan
Essentially, the research follows the original plan. Some modifications and improvements were introduced in the experimental approach due to the emerged opportunities. New excimer laser is incorporated. The new gated ICCD camera allows much more efficient acquisition of the spectral data. Substantial reconsideration was given to the photochemical precursors of the target radical species. The development and characterization of a direct photochemical precursor is new.
Future Plans
In the nearest future it is planned to:
Handling of QA/QC
The data currently generated in this research are of spectroscopic (UV absorption spectra and cross-sections) and kinetic (rate constant) nature obtained in the laboratory. No field samples are to be collected under the research project. The quality of the laboratory data is warranted by using the modern approaches in the elementary chemical kinetics (monochromatic light photolysis coupled to a sensitive time-resolved transient monitoring), by modern techniques of the data acquisition and processing (digital data acquisition, signal accumulation, numerical curve fittings, etc.), by the experimental data interpretation based on the modern procedures and theories of chemical reactions, and by the experience and qualification of the investigators.
The quality of the data is evaluated via the error assessment, comparison of the results with the results of other researchers and with the literature data.
Specifically, in the research performed a computer controlled excimer laser in combination with a computer controlled imaging spectrograph and gated ICCD camera is used. This allows stabilization and/or proper correction for the both laser and monitoring light variations in the time and wavelength domains. Both the digital oscilloscope used (LeCroy) and the gated ICCD camera provide highly accurate time and wavelengths bases as well as the signal/noise improvement through the signal accumulation. Numerical data processing using Scientist software provides possibility to fit experimental curves with a numerical solution of a system of differential equations which gives more flexibility and reliability in the data processing (in contrast to typical software such as Origin, SigmaPlot and some other which allow only analytical functions and therefore only these models which can be solved analytically). Complete statistical analysis of the errors and the dependencies of the extracted parameters is provided and performed on regular bases. High accuracy mass-flow controllers (Brooks), high accuracy pressure gauges (Matheson, Baratron) together with accurate Omega temperature controllers are used to control and monitor the experimental conditions in the reactor.
For all experimentally measured quantities a complete error assessment (including the statistical and systematic errors due to the accuracy of the measurements for all relevant parameters (temperature, total pressure, reactants flow rates, contribution of undesirable reactions) is performed. The data will be reported with the errors assessed in peer reviewed journals.
A post-doctoral associate who performs the experimental studies records all experiments in a laboratory notebook. The notebook is stored in the research laboratory (305 Tiernan Hall) of the P.I. at NJIT. All original kinetic data (oscilloscope traces) are stored on a hard drive of the laboratory PC as well as on floppy diskettes.
[Source and Control Projects] [Transport and Transformation Projects] [Monitoring and Source Attribution Project]