This research explores the role that radical-molecule complexes play in the chemistry of Earth's atmosphere. The formation of such complexes can have direct and pronounced effects on the reaction and product outcome of atmospheric chemical reactions. Some attention is also given to the formation of radial-radical pre-reactive complexes in the HO + ClO system. Peroxy radicals (RO2) can form stable complexes with polar compounds such as H2O, NH3, and CH3OH. For the simplest RO2 radical, HO2, complex formation (e.g., HO2-H2O, HO2-NH3, and HO2-722;CH3OH) gives rise to a significant increase in the HO2 self-reaction rate constant. Although this phenomenon has been observed since the mid-1970s, no satisfactory explanation has been put forward to explain this effect. Herein a rationale for the enhancement of the HO2 self-reaction is given based on extensive geometric, mechanistic and natural bond orbital (NBO) analyses. The apparent lack of a rate enhancement for the methyl peroxy (CH3O2) self-reaction is also presented. The combined insights gained from these two systems are then extended to predict if a water enhancement is expected for the 2-hydroxyethyl peroxy (HOCH2CH2O2) self-reaction kinetics. The computational results of this study are then compared to experimental work and conclusions are drawn towards a general procedure to predict the presence/absence of water initiated rate enhancements in RO2 systems as a whole. Original work regarding the formation of a series of organic RO2-H2O complexes is presented. This work established the effects of different functional groups on the stability of organic peroxy radicals and makes estimates of the associated atmospheric lifetimes and equilibrium constants. This work is further extended to the family of peroxy radicals that form from the atmospheric oxidation of isoprene (the most abundant non-methane biologically emitted hydrocarbon). For the first time, complexes of isoprene peroxy radicals with water are presented along with atmospheric lifetime estimates. Conclusions are made as to the effect of water on the product branching ratio of the isoprene peroxy radical + NO2. The oxidation of hexanal to form hexanal peroxy radicals is discussed within the context of the formation of hexanal peroxy water complexes.Aerosol formation is also perturbed as a result of complexation. Aerosol formation under atmospheric conditions is hypothesized to be initiated by radical-molecule complex formation. For example, in the absence of ammonia, the nucleation of H2SO4 in water vapor to form sulfuric acid aerosols is slow. However, as the concentration of NH3 rises, a marked increase in the rate of sulfuric acid aerosol formation is observed. This work explores the effects of the photolysis products of NH3 (NH2 and NH) on the rate of aerosol formation in systems involving H2SO4, HNO3, HC(O)OH, and CH3C(O)OH. With the exception of H2SO4-NH3 and HNO3-NH3 (geometries already published in the literature), minimum energy structures are presented here for the first time for each of the acid-NHx complexes. Thermochemical data and lifetime estimates are provided for each complex. Conclusions about the relevance of acid-NH2 and acid-NH in the formation of atmospheric aerosols are set forth. Finally, mechanistic insights into the reaction of the hydroxyl radical (OH) and Cl2O are obtained via analysis of the two potential energy surfaces that both involve the formation of HO-Cl2O pre-reactive complexes.

The importance of radical-water complexes in the atmosphere is explored in this dissertation. Radicals, although present in small concentrations in the atmosphere, play a significant role in creating and removing atmospheric pollution. As the atmosphere warms and consequently gets wetter, it is essential to understand the effects of water vapor on radical chemistry. This dissertation reports studies on the effects of water vapor on the kinetics of the self-reaction of β-hydroxyethyl peroxy radical (β-HEP), a prominent organic peroxy radical in the atmosphere. Both experimental and computational studies have been performed to examine the effects of water vapor on the kinetics of the self-reaction. The influence of water vapor and temperature on the reaction rate constant is presented. The rate of the self-reaction increases between 2 to 6 times with an increase in water vapor and decrease in temperature. The products of the self-reaction in the presence and absence of water vapor have been computed using high level ab initio calculations. Major products include alkoxy radicals, peroxides, aldehydes, alcohols and oxygen. A new reaction pathway leading to formation of hydroperoxy radical (HO2) from the self-reaction of β-HEP in the presence of water vapor was identified. In the presence of high NOx concentration HO2, forms tropospheric ozone, which is classified as a harmful pollutant by the Environmental Protection Agency (EPA). Like tropospheric ozone, aerosols are also classified as harmful pollutants by the EPA. Sulfuric acid-water complexes are estimated to be the primary reason for new aerosol formation in the atmosphere. However, the sulfuric acid concentration in the atmosphere alone is not sufficient to account for observed aerosol concentrations. Classical nucleation theory is used to explain new particle formation (NPF), which is initiated by the formation of a nucleating site (a highly polar complex). This dissertation explores the role of various radical-molecule complexes acting as the nucleating site. Experimentally, the HO2-water complex is studied as a possible nucleating site for NPF. A new instrument was developed to create and measure radical-water complex initiated particle formation. The instrument incorporates two scanning mobility particle sizers (SMPS) to measure the size distribution and number density of the aerosol particles formed. The experimental setup uses UV absorption spectroscopy and wavelength modulated spectroscopy to measurethe HO2 radical and water vapor concentrations in the reaction cell. No significant particle formation was observed at room temperature and pressure. Particle formation from the HO2-water complex, may occur at lower temperatures. Additional radical-molecule complexes have been studied computationally in an effort to identify other possible nucleating sites for particle formation. In particular, the complexes of sulfuric acid, nitric acid, acetic acid and formic acid with ammonia, amidogen radical (NH2) and imidogen radical (NH) have been studied. H2SO4-NH2 and HNO3-NH2 complexes show the potential to act as nucleating sites for formation of aerosol particles in the atmosphere. In summary, water mediated chemistry plays a significant role in the atmosphere and must be included in scientific models to better predict pollution levels in the atmosphere.

Biogenic volatile organic compounds (VOCs) react with Cl and OH radicals and the resulting radicals combine with oxygen to form peroxy radicals RO2. Organic peroxy radicals can then react with NO to form NO2, a precursor of tropospheric ozone. The work presented here explored the initial reaction between Cl and thujone, a VOC emitted by Great Basin sagebrush. The rate constant for the reaction of thujone + Cl at 296 K was measured with the method of relative rates with FTIR for detection of reactants. LEDs were used to photolyze Cl2 to generate Cl in the reaction cell. Thujone was also photolyzed by the LEDs and therefore the relative rates model was revised to account for this photolysis. With toluene as the reference compound, the rate constant for thujone + Cl at 296 K is 2.62 ± 1.90 × 10-12 molecules-1 s-1, giving an atmospheric lifetime of 0.5--2.6 minutes for thujone. Cline et al. showed that the rate of the self-reaction of HO2CH2CH2O2 (β-HEP) increases in the presence of water vapor. This enhancement has a strong temperature dependence with a greater enhancement observed at colder temperatures. The observed rate enhancement has been attributed to the formation of a β-HEP--H2O complex. In this work, the equilibrium constant for the formation of the β-HEP--H2O complex was calculated by ab initio calculations. Given the energy available at room temperature, the complex will populate three local minimum geometries and β-HEP will populate two local minimum geometries. The partition function for each of these geometries was calculated and used to calculate the equilibrium constant for complex formation as a function of temperature. Based on these computational results, the observed temperature dependence for the rate enhancement can be attributed to the strong temperature dependence for the rate constant of the reaction of β-HEP--H2O + β-HEP rather than the temperature dependence of complex formation.

This dissertation is a collection of works that investigates issues related to environmental chemistry. The first portion of this research explores the role of water vapor on the kinetics of important atmospheric reactions. Work is presented on the self-reaction of β-hydroxyethyl peroxy radical (β-HEP) and the catalytic increase in reaction rate by water vapor. β-HEP serves as a model system for investigating the possible role of water vapor in perturbing the kinetics and product branching ratio of atmospheric reactions of other alkyl peroxy radicals. The self-reaction rate coefficient of β-HEP was investigated between 276-296 K with 1.0 × 10^15 to 2.5 × 10^17 molecules cm^-3 of water vapor at 200 Torr total pressure by slow-flow laser flash photolysis coupled with UV time-resolved spectroscopy and long-path, wavelength-modulated, diode-laser spectroscopy. The overall disproportionation rate constant is expressed as the product of temperature-dependent and water vapor-dependent terms giving k(T,H2O) = 7.8 × 10^-14 (e^8.2 ^(±2.5) ^kJ/RT)(1 + 1.4 × 10^-34 × e^92 ^(±11) ^kJ/RT[H2O]). The results suggest that formation of a β--HEP-H2O complex is responsible for the observed water vapor enhancement of the self-reaction rate coefficient. Complex formation is supported with computational results identifying three local energy minima for the β--HEP-H2O complex. Both the temperature range and water vapor concentrations used were chosen because of their significance to conditions in the troposphere. As the troposphere continues to get warmer and wetter, more complexes with water will form, which in turn may perturb the kinetics and product branching ratios of atmospheric reactions. Future studies are proposed for the reaction of β-HEP + NO leading to NO2 formation. A laser-induced fluorescence cell was designed, built, and tested in preparation for studies of NO2 formation. Additionally Harriott-cell optics were manufactured and tested to detect HO2 using two-tone frequency-modulated diode-laser spectroscopy. In a related work, the breakdown of the environmental contaminants polychlorinated biphenyls (PCB's) was investigated using a new method. A new method for analyzing anaerobic digestion is also presented. The degradation rate and efficiency of digestion processes are typically measured by introducing a substrate or pollutant into a digester and then monitoring the effluents for the pollutant or substrate, a costly and slow process. A new method for rapid measurement of the rates and efficiencies of anaerobic degradation of pollutants and lignocellulose substrates from various pretreatments is described. The method uses micro-reactors (10-30 mL) containing a mixed culture of anaerobic bacteria obtained from a working anaerobic digester. The rates of degradation and metabolism of pollutants are measured in parallel sets of micro-reactors. Measurements of metabolic rate and pollutant degradation simultaneously is an effective means of rapidly examining pollutant degradation on a micro-scale. Calorimetric measurements alone allow rapid, relative evaluation of various substrate pretreatment methods. Finally calorimetric and electrophoretic methods were used to further knowledge in analytical techniques applied to important problems. In the last section of this dissertation the thermal and photolytic breakdown of promethazine hydrochloride is reported. Promethazine hydrochloride is a mediation that is commonly used as an antihistamine, a sedative, and an antiemetic, and to treat motion sickness. Perivascular extravasation, unintentional intra-arterial injection and intraneuronal or perineuronal infiltration may lead to irreversible tissue damage if the drug is not properly diluted or is administered too quickly. Data on the stability of promethazine hydrochloride diluted in sodium chloride 0.9% are lacking. This study evaluates the thermal and photolytic degradation of promethazine hydrochloride concentrations of 250 µg/mL and 125 µg/mL diluted in sodium chloride 0.9% over a period of 9 days. Degradation rates of promethazine hydrochloride were determined under UV-light, fluorescent light, and no light at various temperatures and concentrations to determine medication stability. The shelf-life (<10% degradation) at 25°C under normal fluorescent lights is 4.9 days, at 25°C protected from light, 6.6 days, and at 7°C in the dark, 8.1 days. These results may increase patient safety by improving current protocols for intravenous promethazine administration

Laboratory based temperature-dependent kinetics and product yields for alkene ozonolysis and the reaction of CH3O2 with ClO and BrO have been measured via chamber studies and a turbulent flow tube coupled to CIMS (Chemical Ionisation Mass Spectrometry). In order to gain a better understanding of the fate of the products formed during hydrocarbon oxidation and their subsequent impact on the ozone budget (and so the oxidising capacity of the atmosphere) it is imperative to know the rate at which these reactions proceed and to identify their product yields. As tropospheric temperature varies, Arrhenius parameters were determined during the ozonolysis of selected alkenes. The temperature dependent kinetic database was extended and the activation energies for the ozonolysis of selected alkenes were correlated with an existing SAR (Structure Activity Relationship). Given the myriad organic species in the atmosphere, SARs are useful tools for the prediction of rate coefficients. Inclusion of Arrhenius parameters into the SAR allows for prediction over a range of temperatures, improving the conditions reflected in models. Achieving mass balance for alkene ozonolysis has proven to be a difficult challenge considering the numerous pathways of the Criegee Intermediate (CI). The product yield of formic acid – an organic acid with significant atmospheric implications which is under predicted by models – was determined as a function of relative humidity during ethene ozonolysis. This reaction exhibited a strong water dependence which lead to the prediction of the reaction rate of the CI with water which ranges between 1 × 10-12 – 1 × 10-15 cm3 molecule-1 s-1 and will therefore dominate its loss with respect to bimolecular processes in the atmosphere. Peroxy radicals, strongly influence the total oxidising capacity of the troposphere. The reaction of peroxy radicals with halogen oxides is recognised to be responsible for considerable ozone depletion in the atmosphere, exacerbated by reactive halogens (X, XO) taking part in catalytic cycles. Arrhenius parameters were determined for ClO + CH3O2 and BrO + CH3O2. Temperature is an important parameter affecting rate, exemplified here as the reaction involving ClO exhibited a positive temperature dependence whereas for BrO a negative temperature dependence was evident. As a consequence, the impact of ClO + CH3O2 with respect to ozone loss is diminished. Global modelling predicts a reduction in ozone loss by a factor of around 1.5 and implicates regions such as clean marine environments rather than the polar stratosphere. Conversely, a more pronounced temperature dependence for the reaction of BrO with CH3O2 placed particular importance on lower stratospheric chemistry where the modelled CH3O2 oxidation is doubled. The main products for this reaction were identified to be HOBr and CH2O2. The decomposition of CH2O2 could enhance HOx in the lower and middle stratosphere and contribute to a significant source of HOx in the upper troposphere. Bimolecular reaction of CH2O2 with water could also provide a none negligible source HC(O)OH in the upper troposphere. Alkenes and peroxy radicals undergo chemical processing in the atmosphere whilst acting as a source and sink of ozone and thus can impose detrimental effects on the biosphere, climate and air quality of the Earth.