Orientador: Edison Bittencourt / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Quimica / Made available in DSpace on 2018-08-15T12:07:07Z (GMT). No. of bitstreams: 1 Zangiacomi_MarcosHenrique_D.pdf: 1507466 bytes, checksum: 17472578194a52bea38f1fd549a47fb5 (MD5) Previous issue date: 2009 / Resumo: 0 mercado atual procura por materiais que apresentem boas propriedades ópticas, que satisfaçam os consumidores e os usuários em geral e que sejam de fácil manuseio e processabilidade. No geral, um polímero deve apresentar boas propriedades ópticas, resistência ao impacto e dureza, compatíveis com o seu uso como lentes oftálmicas. O processo de fabricação de lentes oftálmicas poliméricas por muito tempo foi feito por meio de processos térmicos (cura térmica, processo que ainda é usado), levando cerca de 20 horas ou mais na obtenção de lentes que não apresentassem problemas de trincas por diferença de propriedades no corpo da lente. Dessa forma a introdução da tecnologia de cura pela luz ultravioleta (UV) permitiu a obtenção de materiais com a mesma característica e até superiores aquelas obtidas através da cura térmica e com a vantagem da diminuição do tempo de processo, diminuindo de horas para minutos, resultando também em redução dos custos. Neste trabalho foram utilizados os monômeros e oligômeros listados a seguir: metil metacrilato de metila (MAM), 1,6-Hexanediol diacrilato (HDDMA), diacrilato uretano alifático tetrafuncional (Ebecryl 5129) e diacrilato epóxi bisfenol-A (Ebecryl 3700) e em cada formulação foi aplicado um sistema de fotoiniciadores (nomes comerciais) compostos por Darocur 1173, Irgacure 184, Irgacure 1300, Irgacure 2959 e Irgacure 250 e Irgacure 819. Os sistemas de fotoiniciadores foram formulados a base de 3% p/p sobre cada formulação, seguindo as orientações dos fornecedores e de resultados experimentais obtidos ao longo deste estudo, sendo o tempo de cura da formulação de 7 minutos. Foram analisadas as propriedades mecânicas relevantes que uma lente deve apresentar, entre elas, dureza e resistência ao impacto. Foram determinadas também as temperaturas de transição vítrea obtidas da formulação, e a resistência ao calor. Foram comparados com os diversos fotoiniciadores, e realizado um estudo inicial da cinética, bem como medidas as entalpias e conversões resultantes da polimerização na temperatura de trabalho de 100°C, utilizando-se um fotocalorímetro diferencial de varredura (DPC). / Abstract: The current market calls for materials presenting good optical properties which satisfy consumers and users in general and that can be easily handled and processed. Generally, a polymer must present good optical properties, impact resistance and hardness - all compatible in their use as ophthalmic lenses. For quite a long period of time, ophthalmic lenses manufacturing had been achieved by means of thermal processes (thermal curing), taking up to 20 hours to obtain lenses free of cracking and/or processing errors. In this manner, the introduction of ultraviolet light curing technology has allowed the production of materials with equal and even superior characteristics than those obtained through thermal curing, and with advantages in processing time (reduced from hours to minutes), as well as costs, which were also reduced. The following monomers and oligomers were utilized in this study: methyl methacrylate (MAM), 1,6-Hexanediol diacrylate (HDDMA), diacrylate tetra functional aliphatic urethane (Ebecryl 5129) and epoxy diacrylate bisphenol A (Ebecryl 3700-20 t). A system of photoinitiators composed by darocur 1173, Irgacure 184, Irgacure 1300, Irgacure 2959, Irgacure 250 and Irgacure 819 have been applied in each formulation. The photoinitiators systems have been formulated on a 3% p/p basis, following suppliers orientations and experimental results obtained throughout this study, with a 7-minute formulation curing time. The main mechanical properties a lens must present, such as acceptable harness and impact resistance, were analyzed in the study. Vitreous transition temperature, heat resistance and the evaluation of the photoinitiators in kinetics have been analyzed and studied, as well as the evolution of enthalpy during a 100°C polymerization process and formulation conversions by differential photocalorimetry distinguishing (DPC). / Doutorado / Ciencia e Tecnologia de Materiais / Doutor em Engenharia Química

The extent to which endogenous free radical-scavenging defense mechanisms are involved in experimental and human diabetes was investigated in various tissues of animals with chemically-induced or spontaneous diabetes (BB Wistar rats) and in erythrocytes of patients with either Type I or Type II diabetes. Diabetes was induced in female Wistar rats using alloxan (ALX) or streptozotocin (STZ), each administered in a dose of 50 mg/kg body wt., intravenously. The present study also included a group -of animals in which body wt. loss was induced by food-deprivation for 72 h. The effects of pharmacological interventions (insulin or allopurinol (ALP)), on these processes were also investigated in chemically-induced diabetes., The activities of catalase (CAT), CuZn-superoxide dismutase (CuZn-SOD), glutathione peroxidase (GSH-PX) and glutathione reductase (GSSG-RD) as well as levels of reduced glutathione (GSH) were examined in heart, pancreas, liver and kidney as well as in erythrocytes. Erythrocytes were also examined for their susceptibility to in vitro oxidative stress induced by hydrogen peroxide (H₂0₂). Criteria studied in this regard were GSH-depletion and malondi-aldehyde (MDA) production (an index of lipid peroxidation). The results obtained showed that tissue antioxidant systems are altered in experimental diabetes and that the magnitude of the alterations increased with the degree of body weight loss. Furthermore, the duration of hypoinsulinemia might contribute to the nature of alterations in antioxidant mechanisms. The complex patterns of the alterations observed varied from one tissue to another and may be the result of compensatory increases, usually involving enzymes whose activity in the particular tissue may be limiting, and direct inhibitory effects of endogenous oxidants on the enzymatic components of tissue antioxidant systems. The ability of insulin (9-12 U/kg body wt., subcutaneously) to reverse the many similar alterations of tissue antioxidant enzymes in diabetes induced by either STZ or ALX suggests that these changes are more likely attributable to hypoinsulinemia rather than to direct effects of either diabetogenic drug. The above-mentioned effects indicate that insulin can markedly influence tissue antioxidant status. However, the reason for the persistence of decreased CuZn-SOD activity in both liver and kidney of ALX-diabetic rats after 12 wk of treatment with insulin is not clear at present, and requires further investigation to determine whether this reflects the presence of a residual deficit in tissue antioxidant processes in liver and kidney despite insulin treatment, or whether it is the result of a direct effect exerted by ALX. Acute ALP administration (50 mg/kg body wt., intraperitoneally) was associated with reductions in ketonuria and early mortality among ALX-diabetic rats, and long-term ALP treatment (1.9 mg/day in drinking water) resulted in a normalization of renal CuZn-SOD activity in these animals. Comparable (although not identical) changes in tissue antioxidant status are present in insulin-dependent spontaneously diabetic BB (ISDBB) rats and in animals made diabetic by STZ or ALX administration. Our data also demonstate that the alterations in tissue GSH levels characterizing ALX-diabetes more closely paralleled changes seen in the ISDBB rat than did those in the diabetic state induced by STZ. If the alterations in antioxidant status in uncontrolled chemically-induced diabetes are attributable to a lack of insulin, the observed changes in ISDBB rats are suggestive of sub-optimal insulin therapy in these animals. The results obtained from BB rats demonstrate two types of alterations in antioxidant status: strain-related differences (increased CAT activity in pancreas and decreased GSH levels in pancreas and liver of both ISDBB and their non-diabetic littermates (NDLM)) and diabetes-related changes (mani- fested by an increase in cardiac GSH content and increases in activities of cardiac CAT and GSSG-RD, pancreatic CuZn-SOD and GSSG-RD, and renal GSH-PX). Whether or not these "strain-related" alterations in antioxidant status increase the susceptibility of these animals to developing diabetes remains unknown. Certain alterations were observed in red cells from diabetic patients and from animals with experimental diabetes suggesting that these alterations are more likely to be diabetes-related than species-dependent. Red cells in chemically-induced and clinical diabetes showed an increased resistance to peroxide-induced depletion of GSH, an effect attributed to hyperglycemia, which results in an increased supply of NADPH through the hexose monophosphate shunt for regeneration of GSH from GSSG via the GSSG-RD system. However, the susceptibility of red cells from diabetic patients and animals to lipid peroxidative damage was increased as reflected in augmented MDA production. In addition, insulin treatment did not normalize MDA production in red cells subjected to oxidative challenge and vigorous insulin treatment in both ALX- and STZ-diabetic rats resulted in a markedly decreased MDA production in response to H₂0₂. Moreover, GSSG-RD activity of red cells was increased in both uncontrolled and insulin-treated diabetic animals as well as in diabetic patients. However, some differences in erythrocyte antioxidant enzymes were also observed in erythrocytes from diabetic subjects and animals. For example, diabetic patients showed an increased activity of CuZn-SOD, while erythrocytes from diabetic animals showed no alterations in the activity of this enzyme. Erythrocyte membrane NADH-dehydrogenase activity was increased only in diabetic patients with Type I diabetes, but not in Type II diabetes or in diabetic animals. Erythrocytes from ALX- and STZ-diabetic animals showed an increase in the activity of GSH-PX and those from NDLM BB rats showed a decrease in CAT activity, alterations that were not observed in human diabetes. Finally, as far as antioxidant defense mechanisms are concerned, our results suggest that diabetes is associated with some common alterations in these mechanisms regardless of the model (chemically-induced versus the spontaneous type of diabetes) or the species used (animal versus human diabetes). Some of these alterations seem to be influenced by the degree of diabetic control, while others are apparently independent of it. Future studies will focus on the extent to which alterations in red cells of human diabetics can be used to predict the development of long-term sequelae of the disease. / Medicine, Faculty of / Anesthesiology, Pharmacology and Therapeutics, Department of / Graduate

NiSi is an attractive material in the production of CMOS devices. The problem with the utilization of NiSi, is that there is no proper method of cleaning the oxide on the surface. Sputtering is the most common method used for the cleaning, but it has its own complications. Dry cleaning methods include the reactions with radicals and these processes are not well understood and are the focus of the project. Dissociated NF3 and NH3 were used as an alternative and XPS is the technique to analyze the reactions of atomic fluorine and nitrogen with the oxide on the surface. A thermal cracker was used to dissociate the NF3 and NH3 into NFx+F and NHx+H. There was a formation of a NiF2 layer on top of the oxide and there was no evidence of nitrogen on the surface indicating that the fluorine and hydrogen are the reacting species. XPS spectra, however, indicate that the substrate SiO2 layer is not removed by the dissociated NF3 and NiF2 growth process. The NiF2 over layer can be reduced to metallic Ni by reacting with dissociated NH3 at room temperature. The atomic hydrogen from dissociated ammonia reduces the NiF2 but it was determined that the atomic hydrogen from the ammonia does not react with SiO2.

NiSi and Ni(Pt)Si, and of the effects of dissociated ammonia on oxide reduction was carried out under controlled ultrahigh vacuum (UHV) conditions. X-ray photoelectron spectroscopy (XPS) has been used to characterize the evolution of surface composition. Vicinal surfaces on NiSi and Ni(Pt)Si were formed in UHV by a combination of Ar+ sputtering and thermal annealing. Oxidation of these surfaces in the presence of either O+O2 or pure O2 at room temperature results in the initial formation of a SiO2 layer ~ 7 Å thick. Subsequent exposure to O2 yields no further oxidation. Continued exposure to O+O2, however, results in rapid silicon consumption and, at higher exposures, the kinetically-driven oxidation of the transition metal(s), with oxides >35Ǻ thick formed on all samples, without passivation. The addition of Pt retards but does not eliminate oxide growth or Ni oxidation. At higher exposures, in Ni(Pt)Si surface the kinetically-limited oxidation of Pt results in Pt silicate formation. Substrate dopant type has almost no effect on oxidation rate. Reduction of the silicon oxide/metal silicate is carried out by reacting with dissociated NH3 at room temperature. The reduction from dissociated ammonia (NHx+H) on silicon oxide/ metal silicate layer shows selective reduction of the metal oxide/silicate layer, but does not react with SiO2 at ambient temperature.

The focus of the following research was to (1) understand the chemistry involved in nitriding an organosilicate glass substrate prior to tantalum deposition, as well as the effect nitrogen incorporation plays on subsequent tantalum deposition and (2) the reduction of a native oxide, the removal of surface contaminants, and the etching of a HgCdTe surface utilizing atomic hydrogen. These studies were investigated utilizing XPS, TEM and AFM. XPS data show that bombardment of an OSG substrate with NH3 and Ar ions results in the removal of carbon species and the incorporation of nitrogen into the surface. Tantalum deposition onto a nitrided OSG surface results in the initial formation of tantalum nitride with continued deposition resulting in the formation of tantalum. This process is a direct method for forming a thin TaN/Ta bilayer for use in micro- and nanoelectronic devices. Exposure to atomic hydrogen is shown to increase the surface roughness of both air exposed and etched samples. XPS results indicate that atomic hydrogen reduces tellurium oxide observed on air exposed samples via first-order kinetics. The removal of surface contaminants is an important step prior to continued device fabrication for optimum device performance. It is shown here that atomic hydrogen effectively removes adsorbed chlorine from the HgCdTe surface.

Free radicals are produced in lignin during mechanical treatment and irradiation with light of various wavelengths. During mechanical treatment, the lignin macromolecule is degraded severely as revealed by ESR and viscosity measurements. Several types of mechano-radicals are produced in lignin during the mechanical process. Among these the phenoxy radicals are rather stable, where carbon-radicals are labile at ambient conditions. Transient mechano-radicals reacted readily with oxygen molecules to produce peroxy radicals even at 77°K, but they decayed rapidly at ambient temperature. Photodegradation of lignin was observed when macromolecule was irradiated with light of λ<3500 Å as revealed by ESR, viscosity, and weight loss. Phenoxy radicals are the predominant intermediates in the photoirradiated lignin as shown by ESR studies. Elimination of side chains of lignin phenyl propane units took effect in α-carbonyl group bearing molecules. By contrast, β aryl ether substituents adjacent to α-carbonyl groups caused ether cleavage under identical conditions of photoirradiation. This is attributed to energy transferred from excited α-carbonyl groups to the ether bonds. The α-carbonyl groups also functioned as photosensitizers accelerating photochemical reactions of lignin. Termination and decomposition reactions of mechano-radicals and photoinduced free radicals in lignin ultimately lead to the formation of para- and ortho-quinones, carbonyl groups, and double bonds which cause the color of lignin. These potential choromophoric groups can be partially removed from lignin by using ultraviolet light of λ> 4000 Å; and they can be completely removed by irradiation of lignin in the presence of dioxane-water with light of λ>3500 Å. Experimental findings suggest chat chromophoric groups in lignin were being trapped or blocked by dioxanyl radicals resulting in brightening. However, the photoreduced lignin-adduct suffered color reversion. This adverse effect can be prevented by using 2-hydroxy-4-methoxy-benzophenone as a photostabilizer. The feasibility of applying photoreduction techniques to high- yield pulps was demonstrated. However, optimal experimental conditions for photoreduction of lignin in high-yield pulps have not been established yet. / Ph. D.

Radical reactions play central roles in regulating regional air quality and global climate. Some potentially important gas phase radical reactions are being investigated in this research project, including Cl reactions with acetone, butanone, 3-pentanone, pyridine, and dimethyl selenide (DMSe), HO2 complex formation and dissociation with formic and acetic acids, and reactive and non-reactive quenching of O(1D) by the potent greenhouse gases SO2F2, NF3, and SF5CF3. The involved radicals are generated by laser flash photolysis (LFP). Temporal profiles of either the radical reactant or a product are monitored in "real time" using atomic resonance fluorescence spectroscopy (RF), time-resolved UV-visible absorption spectroscopy (TRUVVAS), or tunable diode laser absorption spectroscopy (TDLAS), allowing kinetic and mechanistic information of these reactions to be obtained. These studies provide new knowledge of the investigated radical reactions and facilitate a better understanding of their significance in atmospheric chemistry.

The present study was undertaken to determine the effects and possible mechanism of action of cimetidine in cancer and Alzheimer’s disease (AD). Throughout this study emphasis is placed on free radical levels since the magnitude of the relationship between diseases and the levels of free radicals vary from one disease to another. Studies were carried out to examine the effect of cimetidine on free radical levels using superoxide formation and lipid peroxidation as indicators of free radical levels. The experiments revealed that addition of cimetidine, especially in high concentrations (0.5 and 1.0 x10-6 M) significantly inhibited WHCO6 cancer cell growth rather than cancer cell growth, as no normal control was available. Free radical formation as well as hydroxyl radical formation were reduced in the deoxyribose assay. In addition, cimetidine exhibits properties of binding to metals such as copper and iron. To maintain consistency in the experiments, a WHCO6 (Wits Human Carcinoma of the Oesophagus) cell line was used to investigate the effect of cimetidine in cancer. Neurodegeneration was induced in the rat brain using neurotoxins such as cyanide to investigate the relationship between cimetidine in AD. A decrease in cancer cell growth was accompanied by a concomitant decrease in the levels of free radicals and lipid peroxidation, suggesting that the growth-inhibitory effects of cimetidine on WHCO6 cancer cells in vitro may be due to free radical scavenging properties. This proposal was further strengthened by determination of free radical levels in the rat brain. After treatment with neurotoxins to induce neurodegeneration, the levels of free radicals in the rat brain suggest that addition of cimetidine reduces free radical levels in the rat brain in a dosedependent manner. Further experiments were done in an attempt to uncover the underlying mechanism by which cimetidine exhibits free radical scavenging properties. Metal binding studies were done using electrochemical, HPLC and UV/Vis studies. The results show that cimetidine binds iron and copper. These metals have been implicated in free radical production via the Fenton reaction. By binding with cimetidine the metals become unavailable to produce free radicals and hence cimetidine indirectly reduces the formation of free radicals. The final experiment was the determination of cimetidine as a hydroxyl radical scavenger in the deoxyribose assay. Cimetidine was shown to act as a potent hydroxyl radical scavenger, thereby confirming its activity as a free radical scavenger. In addition, cimetidine protects against damage to the deoxyribose sugar, a component of DNA. Whilst there are many theories that explain the therapeutic role of cimetidine in degenerative disease, the actual mechanism of the role of cimetidine is emphasized as a free radical scavenger. Regardless of the mechanism of action, cimetidine does inhibit tumour growth according to this study and also reduce free radical levels in neurodegeneration, which suggests a role for cimetidine as a possible additive in treatment of patients with such disease states. These findings have important clinical implications, and needs to be investigated further.

During aerobic cell metabolism, molecular oxygen is reduced to water through electron-transfer reactions, and the oxygen not completely reduced is transformed into a set of neutral and free radical molecules with high oxidant ability, generally denominated Reactive Oxygen Species (ROS). To regulate the cellular ROS content, organisms are endowed with an efficient endogen antioxidant system. The physiological ROS levels may be excessively increased by different factors including unbalanced diets, ionizing radiations, and tobacco smoke among others, leading to the oxidative stress, term referred to the imbalance between ROS production and its neutralization by the endogenous antioxidant system. More importantly, in vitro and in vivo studies reveal the relationship between suffering oxidative stress and the development of cardiovascular and respiratory diseases, diabetes, and cancer. To prevent and treat the oxidative stress and its adverse effects, exogenous, readily oxidizable molecules may help the organism to neutralize ROS into less dangerous species. The phenolic compounds or (poly)phenols, molecules very abundant in our diet (i.e. fruits, vegetables, and beverages such as tea and wine) may be responsible for this antioxidant activity of fruits and vegetables. Currently, (poly)phenols are regarded as natural antioxidant molecules with outstanding beneficial effects, including anti-aging activity, and the prevention of cancer and diabetes. The antioxidant activity of (poly)phenols is mainly associated with their radical scavenging activity, action conferred by the transfer of a hydrogen atom or an electron to a free radical, rendering a less reactive molecule. On the other hand, chemical probes and in vitro studies have demonstrated that some highly reacting (poly)phenols are able to generate small quantities of ROS. Phenolic compounds have shown antiproliferative activity which may be caused inter alia by their ability to scavenge or generate toxic radicals. To further understand the connection between the redox reactivity of (poly)phenols and their biological actions, studies with more sensitive and selective chemical probes may help to clarify the role of redox reactions in the physiological actions of phenolics and their metabolites. In this thesis, we have focused on the utilization of two stable radicals synthesized in our laboratory, the tris(2,4,6-trichloro-3,5-dinitropehnyl)methyl (HNTTM) and the tris(2,3,5,6-tetrachloro-4-nitrophenyl)methyl)radical (TNPTM), as chemical probes to determine the electron-transfer activity of dietary (poly)phenols and some metabolites. The different reducing potential of the two stable radicals facilitates the quantitative evaluation of the radical scavenging capacity of each (poly)phenol, as well as the establishment of the most reactive moieties. TNPTM is a useful tool to determine the most reactive (poly)phenols as electron transfer donors, (poly)phenols that cannot be differentiate with any other chemosensor. The results obtained are compared with two well-established methods for the quantification of electron-transfer capacity. The action of these (poly)phenols on cell cultures of a colon cancer cell line is also presented, showing a correlation between those (poly)phenols detected with TNTPM and with those that produce the highest antiproliferative activity. This cell line is particularly relevant because dietary (poly)phenols are in contact with epithelial cells of this kind during their transit along the digestive tract and may exert some preventive action on colon cancer. In addition, a chemoenzymatic strategy to prepare glucuronated metabolites of (-)-epigallocatechin-3-O-gallate (EGCG), the most abundant and active (poly)phenol of green tea, was attempted, obtaining the acetylated and methylated precursor of the EGCG-4’’-glucuronide, the most abundant EGCG glucuronide obtained in the human metabolism. The final conjugated, the EGCG-4’’-glucruonide, was obtained albeit with not enough quantity to be purified. / "Estudi de la Transferència Electrònica de compostos fenòlics i la seva relació amb l’activitat biològica en cèl•lules canceroses" En el metabolisme i respiració dels organismes aeròbics, l’oxigen és utilitzat com a receptor electrònic reduint-se principalment a aigua. Una petita part d’aquest oxigen no és totalment reduint obtenint-se un conjunt d’espècies radicalàries i no radicalàries (ROS). Aquestes espècies en ser altament reactives poden malmetre macromolècules, activitat que està relacionada amb l’aparició de malalties com el síndrome metabòlic i el càncer. Per aquest motiu els organismes han desenvolupat un sistema de regulació de la seva concentració. En determinades situacions, aquest sistema no és suficient i per tant, antioxidants exògens poden ajudar a aquest sistema endògen. Els polifenols molècules abundants en la nostra dieta (fruita i verdures) són els principals candidats, i s’ha demostrat un efecte preventiu i terapèutic en la salut per la seva part. L’efecte beneficiós exercit pels polifenols pot tenir lloc per dos mecanismes de reacció; per transferència d’hidrogen i per transferència electrònica. Actualment, no hi ha cap mètode prou eficaç que ens permeti determinar la acció del polifenols exercida per transferència electrònica, mecanisme que també habilita als polifenols per a formar ROS. En el nostre laboratori hem sintetitzat dos radicals lliures estables, els radicals tris(2,4,6-tricloro-3,5-dinitrofenil)metil (HNTTM) i el tris(2,3,5,6-tetracloro-4-nitrofenil)metil (TNPTM) com a quimiosensors de transferència electrònica. Aquests radicals ens permeten mesurar la activitat antiradicalària dels polifenols, i el TNPTM ens permet detectar a aquells polifenols amb una reactivitat per transferència electrònica més elevada i que no poden ser diferenciats amb altres mètodes. S’han fet estudis d’antiproliferació per part dels polifenols en cèl•lules de càncer de còlon HT-29 i s’ha demostrat que els polifenols amb més activitat antiproliferativa són els que tenen activitat front al TNPTM. Per tant, el radical TNPTM permet determinar als polifenols amb una elevada reactivitat per a cedir electrons i es suggereix que els mecanismes de transferència electrònica juguen un paper important en els mecanismes d’ antiproliferació en cèl•lules de càncer de còlon HT-29. S’ha proposat metodologia sintètica per a obtenir els principals glucuronats de l’epigal•locatequin-3-O-gal•lat (EGCG) el més abundant i més actiu dels polifenols del te verd. S’ha obtingut el precursor acetil•lat i metil•lat del glucuronat majoritari de l’EGCG obtingut en el metabolisme humà, l’EGCG-4’’-glucurònid i s’ha obtingut el conjugat final però no amb una quantitat suficient com per a ser aïllat.

The Earth's atmosphere is a large photochemical reactor consisting primarily of N2 (~78%) and O2 (~21%) with Ar and water vapor being the next most abundant constituents. All of the remaining gases in the atmosphere are referred to as 'trace gases', and they play a critical role in understanding climate change, urban air quality, ozone production and depletion, and in determining the overall 'health' of the atmosphere. These trace components are present in our atmosphere with mixing ratios, i.e., mole fractions, ranging from sub parts per trillion to several hundred parts per million. One class of trace constituents that play a critical role in atmospheric chemistry are free radicals. Free radicals are highly reactive, often initiating the oxidation of natural and anthropogenic atmospheric species, thereby often controlling the fate and lifetimes of these species. The research comprising this dissertation focuses on laboratory studies of the kinetics and mechanisms of free radical (atomic halogen) reactions that can impact the levels of important trace atmospheric species. In the studies reported herein, laser flash photolysis (LFP) was coupled with time resolved atomic resonance fluorescence (RF) spectroscopic detection of Cl or Br atoms to investigate halogen atom chemistry. The research addresses three groups of reactions: Cl atom reactions with alkyl bromides, Cl and Br-initiated oxidations of small (C2-C6) alkenes, and Cl reactions with CH3SCH3 (DMS, dimethylsulfide) and CH3SeCH3 (DMSe, dimethylselenide). The alkyl bromide reactions were experimentally unique in that we were able to deduce kinetics of the Cl atom reaction with bromoethane, n-bromopropane, and 1,2-dibromoethane by monitoring the appearance of the Br product by LFP-RF. The Br is formed via elimination that occurs essentially instantaneously following β-H abstraction by the Cl atom. All three of the bromoalkanes investigated are emitted into the atmosphere primarily from anthropogenic sources and all three have been identified by the World Meteorological Organization (WMO) as very short-lived (lifetime less than 6 months) source gases with significant ozone depletion potentials (ODPs). Additionally, the bromoalkanes mentioned above have been of interest as model compounds for larger partially halogenated organics found in the atmosphere, and they have been considered as potential replacement compounds for chlorofluorocarbons (CFCs) that have been banned as a result of the Montreal Protocol. Brominated very short-lived compounds are thought to contribute 20-25% of total stratospheric bromine. Thus, there is considerable interest in understanding the atmospheric chemistry of even the most short-lived organic bromine compounds. Temporal profiles of Br atoms provided important kinetic and mechanistic insight for the reactions over a wide range of temperature and pressure. Temperature-dependent rate coefficients are determined for the alkyl bromides of interest for the first time, and the potential importance of the Cl reaction as an atmospheric degradation pathway for each alkyl bromide is qualitatively assessed. The studies of halogen atom reactions with alkenes focused on formation of weakly-bound adducts where kinetics of adduct formation and dissociation as well as non-adduct forming channels were evaluated. The elementary steps in the Br initiated oxidation of the alkenes 2-methyl-1,3-butadiene (isoprene), 2,3-dimethyl-2-butene (tetramethylethylene, TME), and 1,3-butadiene have been investigated. The experimental kinetic database for these reactions is quite sparse. The kinetic results reported herein, suggests that Br reaction with the above olefins is much faster than previously thought. Analysis of the temperature dependence of the "approach to equilibrium" kinetic data in conjunction with electronic structure calculations allows for determination of enthalpy and entropy changes associated with each addition reaction. Where possible, both forward addition and reverse dissociation channels as well as H-abstraction pathways were characterized. The enthalpy change associated with the addition reaction to give the Br−isoprene and Br−1,3-butadiene adducts has been determined for the first time and the bond dissociation enthalpy obtained for the Br−TME adduct is in reasonable agreement with the only other previously reported value. It should be noted that in the case of isoprene and 1,3-butadiene, there are multiple possible adducts that could be formed. In order to help clarify which adducts are more or less likely to be formed, we rely on electronic structure calculations (see Chapter 5) to aid in our overall understanding of the adduct forming channels. Furthermore, for the Br reactions with the three alkenes above, atomic Br kinetics have been monitored directly both in the absence and in the presence of O2 which allowed, for the first time, determination of rate coefficients for the elementary steps in the overall complex mechanism including determination of the Br−olefin + O2 rate coefficient. Also included in this group of reactions is the chlorine reaction with isoprene. In addition to the well-known fact that isoprene is emitted into the atmosphere from vegetation, a potentially significant marine source of isoprene has received considerable attention. Chlorine has long been thought to exist primarily in marine environments, however, recent findings also suggest a significant Cl production rate in the middle of the continental United States. There are numerous room temperature kinetic studies for the Cl + isoprene reaction in the literature, however, there is only one temperature dependent study reported. Current recommended 298 K rate coefficients for isoprene reactions suggest the Cl reaction is ~ 4x faster than the analogous OH reaction. If indeed this is the case, the Cl reaction could play a non-neglibible role in isoprene oxidation in atmospheric locales where Cl concentrations are relatively high. In addition, the C−Cl bond strength in Cl−C5H8 is obtained from direct measurements of the forward and reversible addition rate coefficients. Our results are compared with the literature data, and the potential importance of Cl-initiated oxidation as an atmospheric sink for isoprene is assessed. The final group of reactions investigated involves reactions of Cl with DMS and DMSe. DMS and DMSe are the most prevalent sulfur and selenium compounds emitted to the atmosphere from the oceans. The oxidation of DMS has been studied extensively due to the interest in the possible role of DMS oxidation in the formation of sulfate aerosols, however, DMSe oxidation processes have hardly been studied at all. And, DMSe oxidation products are likely to be less volatile than the analogous DMS species. Selenium is an essential nutrient for many plants and animals; however, there is a fine line between enough and excess selenium which can be toxic. Most studies suggest that atmospheric deposition is an important source of Se contamination, and it is therefore critical to evaluate the source emissions and fate of Se in the atmosphere. Since the majority of atmospheric Se exists in the form of DMSe, determination of the kinetics and oxidation mechanisms of DMSe will go a long way towards understanding the global biogeochemical cycle of Se. Both reversible addition and H-abstraction pathways have been characterized, and the first experimental determination of bond strength of the gas-phase DMS−Cl and DMSe−Cl adducts have been obtained.