The kinetics of the gas-phase decomposition of the 2-methyl-2-butoxyl and 2-methyl-2-pentoxyl radicals

The kinetics of the title reactions have been studied by relative-rate methods as a function of temperature. Relative-rate coefficients for the two decomposition channels of 2-methyl-2-butoxyl have been measured at five different temperatures between 283 and 345 K and the observed temperature dependence is consistent with the results of some previous experimental studies. The kinetics of the two decomposition channels of 2-methyl-2-pentoxyl have also been investigated, as a function of temperature, relative to the estimated rate of isomerisation of this radical. Room-temperature rate coefficient data for the two decomposition channels of both 2-methyl-2-pentoxyl and 2-methyl-2-butxoyl (after combining the relative rate coefficient for this latter with a value for the rate coefficient of the major channel, extrapolated from the data presented by Batt et al., Int. J. Chem. Kinet., 1978, 10, 931) are shown to be consistent with a non-linear kinetic correlation, for alkoxyl radical decomposition rate data, previously presented by this laboratory (Johnson et al., Atmos. Environ., 2004, 38, 1755-1765).

Thermochemical kinetics: does it still give insights?

This tutorial review revisits the subject of the seminal book written by Sidney Benson in 1968. A short summary of the nature of the subject is presented, including its place in the wider world of quantitative chemistry. A number of themes are selected to illustrate its previous and continuing usefulness in evaluating numerical values of important quantities, and probing ideas of reaction mechanism. These include strain enthalpies for biradical combination, chain reactions, why some reactions don't occur and the involvement of carbenes in hydrocarbon rearrangements.

Gas-phase kinetics of chlorosilylene reactions. I. ClSiH + Me3SiH: absolute rate measurements and theoretical calculations for prototype Si−H insertion reactions

Time-resolved studies of chlorosilylene, CISiH, generated by the 193 nm laser flash photolysis of 1-chloro-1-silacyclopent-3-ene, have been carried out to obtain rate constants for its bimolecular reaction with trimethylsilane, Me3SiH, in the gas phase. The reaction was studied at total pressures up to 100 torr (with and without added SF6) over the temperature range 297-407 K. The rate constants were found to be pressure independent and gave the following Arrhenius equation: log(k/cm(3) molecule(-1) s(-1)) = (-13.97 +/- 0.25) + (12.57 +/- 1.64) kJ mol(-1)/RT In 10. The Arrhenius parameters are consistent with a mechanism involving an intermediate complex, whose rearrangement is the rate-determining step. Quantum chemical calculations of the potential energy surface for this reaction and also the reactions of CISiH with SiH4 and the other methylsilanes support this conclusion. Comparisons of both experiment and theory with the analogous Si-H insertion processes of SiH2 and SiMe2 show that the main factor causing the lower reactivity of ClSiH is the secondary energy barrier. The calculations also show the existence of a novel intramolecular H-atom exchange process in the complex of ClSiH with MeSiH3.

Direct time-resolved study of the kinetics of the reaction of silylene with phenylsilane in the gas phase. Does SiH2 react with the aromatic ring?

Laser flash photolysis studies of silylene, SiH2, generated by the 193 nm laser flash photolysis phenylsilane, PhSiH3, have been carried out to obtain rate constants for its bimolecular reaction with PhSiH3 itself, in the gas phase. The reaction was studied in SF6 (mostly at 10 Torr total pressure) over the temperature range 298-595 K. The rate constants (also found to be pressure independent) gave the following Arrhenius equation: log(k/cm(3) molecule(-1) s(-1)) = (-9.92 +/- 0.04) + (3.31 +/- 0.27) kJ mol(-1)/RT ln 10 Similar investigations of the reaction of silylene with benzene, C6H6, (295-410 K) gave data suggestive of the fact that SiH2 might be reacting with photochemical products of C6H6 as well as with C6H6 itself. However, in the latter system, apparent rate constants were sufficiently low to indicate that in the reaction of SiH2 with PhSiH3 addition to the aromatic ring was unlikely to be in excess of 3% of the total. Quantum chemical calculations of the energy surface for SiH2 + C6H6 indicate that 7-silanorcaradiene and 7-silacycloheptatriene are possible products but that PhSiH3 formation is unlikely. RRKM calculations suggest that 7-silanorcaradiene should be the initial product but that it cannot be collisionally stabilized under experimental conditions

A kinetics and mechanistic study of the atmospherically relevant reaction between molecular chlorine and dimethyl sulfide (DMS)

A gas-phase kinetics study of the atmospherically important reaction between Cl2 and dimethyl sulfide (DMS)Cl2 + CH3SCH3 → productshas been made using a flow-tube interfaced to a photoelectron spectrometer. The rate constant for this reaction has been measured at 1.6 and 3.0 torr at T = (294 ± 2) K as (3.4 ± 0.7) × 10−14 cm3 molecule−1 s−1. Reaction (1) has been found to proceed via an intermediate, (CH3)2SCl2, to give CH3SCH2Cl and HCl as the products. The mechanism of this reaction and the structure of the intermediate were investigated using electronic structure calculations. A comparison of the mechanisms of the reactions between Cl atoms and DMS, and Cl2 and DMS has been made and the relevance of the results to atmospheric chemistry is discussed.

Kinetic multi-layer model of gas-particle interactions in aerosols and clouds (KM-GAP): linking condensation, evaporation and chemical reactions of organics, oxidants and water

We present a novel kinetic multi-layer model for gas-particle interactions in aerosols and clouds (KM-GAP) that treats explicitly all steps of mass transport and chemical reaction of semi-volatile species partitioning between gas phase, particle surface and particle bulk. KM-GAP is based on the PRA model framework (Pöschl-Rudich-Ammann, 2007), and it includes gas phase diffusion, reversible adsorption, surface reactions, bulk diffusion and reaction, as well as condensation, evaporation and heat transfer. The size change of atmospheric particles and the temporal evolution and spatial profile of the concentration of individual chemical species can be modelled along with gas uptake and accommodation coefficients. Depending on the complexity of the investigated system, unlimited numbers of semi-volatile species, chemical reactions, and physical processes can be treated, and the model shall help to bridge gaps in the understanding and quantification of multiphase chemistry and microphysics in atmo- spheric aerosols and clouds. In this study we demonstrate how KM-GAP can be used to analyze, interpret and design experimental investigations of changes in particle size and chemical composition in response to condensation, evaporation, and chemical reaction. For the condensational growth of water droplets, our kinetic model results provide a direct link between laboratory observations and molecular dynamic simulations, confirming that the accommodation coefficient of water at 270 K is close to unity. Literature data on the evaporation of dioctyl phthalate as a function of particle size and time can be reproduced, and the model results suggest that changes in the experimental conditions like aerosol particle concentration and chamber geometry may influence the evaporation kinetics and can be optimized for eðcient probing of specific physical effects and parameters. With regard to oxidative aging of organic aerosol particles, we illustrate how the formation and evaporation of volatile reaction products like nonanal can cause a decrease in the size of oleic acid particles exposed to ozone.

Kinetic multi-layer model of gas-particle interactions in aerosols and clouds (KM-GAP): linking condensation, evaporation and chemical reactions of organics, oxidants and water

We present a novel kinetic multi-layer model for gas-particle interactions in aerosols and clouds (KMGAP) that treats explicitly all steps of mass transport and chemical reaction of semi-volatile species partitioning between gas phase, particle surface and particle bulk. KMGAP is based on the PRA model framework (P¨oschl-Rudich- Ammann, 2007), and it includes gas phase diffusion, reversible adsorption, surface reactions, bulk diffusion and reaction, as well as condensation, evaporation and heat transfer. The size change of atmospheric particles and the temporal evolution and spatial profile of the concentration of individual chemical species can be modeled along with gas uptake and accommodation coefficients. Depending on the complexity of the investigated system and the computational constraints, unlimited numbers of semi-volatile species, chemical reactions, and physical processes can be treated, and the model shall help to bridge gaps in the understanding and quantification of multiphase chemistry and microphysics in atmospheric aerosols and clouds. In this study we demonstrate how KM-GAP can be used to analyze, interpret and design experimental investigations of changes in particle size and chemical composition in response to condensation, evaporation, and chemical reaction. For the condensational growth of water droplets, our kinetic model results provide a direct link between laboratory observations and molecular dynamic simulations, confirming that the accommodation coefficient of water at 270K is close to unity (Winkler et al., 2006). Literature data on the evaporation of dioctyl phthalate as a function of particle size and time can be reproduced, and the model results suggest that changes in the experimental conditions like aerosol particle concentration and chamber geometry may influence the evaporation kinetics and can be optimized for efficient probing of specific physical effects and parameters. With regard to oxidative aging of organic aerosol particles, we illustrate how the formation and evaporation of volatile reaction products like nonanal can cause a decrease in the size of oleic acid particles exposed to ozone.

The reaction between silylene and ammonia: some gas-phase kinetic and quantum chemical studies

Time-resolved kinetic studies of the reaction of silylene, SiH2, generated by 193 nm laser flash photolysis of silacyclopent-3-ene, have been carried out in the presence of ammonia, NH3. Second order kinetics were observed. The reaction was studied in the gas phase at 10 Torr total pressure in SF6 bath gas at each of the three temperatures, 299, 340 and 400 K. The second order rate constants (laser pulse energy of 60 mJ/pulse) fitted the Arrhenius equation: log(k/cm3 molecule-1 s-1) = (-10.37 ± 0.17) + (0.36 ± 1.12 kJ mol-1)/RTln10 Experiments at other pressures showed that these rate constants were unaffected by pressure in the range 10-100 Torr, but showed small decreases in value at 3 and 1 Torr. There was also a weak intensity dependence, with rate constants decreasing at laser pulse energies of 30 mJ/pulse. Ab initio calculations at the G3 level of theory, show that SiH2 + NH3 should form an initial adduct (donor-acceptor complex), but that energy barriers are too great for further reaction of the adduct. This implies that SiH2 + NH3 should be a pressure dependent association reaction. The experimental data are inconsistent with this and we conclude that SiH2 decays are better explained by reaction of SiH2 with the amino radical, NH2, formed by photodissociation of NH3 at 193 nm. The mechanism of this previously unstudied reaction is discussed.

Ozonolysis of methyl oleate monolayers at the air–water interface: oxidation kinetics, reaction products and atmospheric implications

Ozonolysis of methyl oleate monolayers at the air–water interface results in surprisingly rapid loss of material through cleavage of the C[double bond, length as m-dash]C bond and evaporation/dissolution of reaction products. We determine using neutron reflectometry a rate coefficient of (5.7 ± 0.9) × 10−10 cm2 molecule−1 s−1 and an uptake coefficient of [similar]3 × 10−5 for the oxidation of a methyl ester monolayer: the atmospheric lifetime is [similar]10 min. We obtained direct experimental evidence that <2% of organic material remains at the surface on atmospheric timescales. Therefore known long atmospheric residence times of unsaturated fatty acids suggest that these molecules cannot be present at the interface throughout their ageing cycle, i.e. the reported atmospheric longevity is likely to be attributed to presence in the bulk and viscosity-limited reactive loss. Possible reaction products were characterized by ellipsometry and uncertainties in the atmospheric fate of organic surfactants such as oleic acid and its methyl ester are discussed. Our results suggest that a minor change to the structure of the molecule (fatty acid vs. its methyl ester) considerably impacts on reactivity and fate of the organic film.

Thiol and Sulfenic Acid Oxidation of AhpE, the One-Cysteine Peroxiredoxin from Mycobacterium tuberculosis: Kinetics, Acidity Constants, and Conformational Dynamics

Drug resistance and virulence of Mycobacterium tuberculosis are partially related to the pathogen`s antioxidant systems. Peroxide detoxification in this bacterium is achieved by the heme-containing catalase peroxidase and different two-cysteine peroxiredoxins. M. tuberculosis genome also codifies for a putative one-cysteine peroxiredoxin, alkyl hydroperoxide reductase E (MtAhpE). Its expression was previously demonstrated at a transcriptional level, and the crystallographic structure of the recombinant protein was resolved under reduced and oxidized states. Herein, we report that the conformation of MtAhpE changed depending on its single cysteine redox state, as reflected by different tryptophan fluorescence properties and changes in quaternary structure. Dynamics of fluorescence changes, complemented by competition kinetic assays, were used to perform protein functional studies. MtAhE reduced peroxynitrite 2 orders of magnitude faster than hydrogen peroxide (1.9 x 10(7) M(-1) s(-1) vs 8.2 x 10(4) M(-1) s(-1) at pH 7.4 and 25 degrees C, respectively). The latter also caused cysteine overoxidation to sulfinic acid, but at much slower rate constant (40 M(-1) s(-1)). The pK(a) of the thiol in the reduced enzyme was 5.2, more than one unit lower than that of the sulfenic acid in the oxidized enzyme. The pH profile of hydrogen peroxide-mediated thiol and sulfenic acid oxidations indicated thiolate and sulfenate as the reacting species. The formation of sulfenic acid as well as the catalytic peroxidase activity of MtAhpE was demonstrated using the artificial reducing substrate thionitrobenzoate. Taken together, our results indicate that MtAhpE is a relevant component in the antioxidant repertoire of M. tuberculosis probably involved in peroxide and specially peroxynitrite detoxification.

Xyloglucan nano-aggregates: Physico-chemical characterisation in buffer solution and potential application as a carrier for camptothecin, an anti-cancer drug

In this work, native xyloglucan was extracted from Tamarindus indica seeds (XGT), and its properties in phosphate buffer solution (PBS) were evaluated in comparison with a commercial tamarind kernel powder (TKP). The physico-chemical characteristics of the polysaccharides such as molar mass, critical concentration and intrinsic viscosity were determined. Furthermore, using spectroscopic and microscopy techniques, it was observed that the XGs tested can be considered macromolecules able to aggregate as nano-entities of 60-140 nm. The XGT tended to an ordered and compact spherical conformation determined by the Huggins constant, circular dichroism, atomic force microscopy and transmission electron microscopy. After the determination of the properties in PBS the XGs, at concentrations of 25% above their critical aggregation concentration, were used to encapsulate camptothecin, an anti-cancer drug. The XGT sample showed an encapsulation efficiency of 42% and first-order drug delivery kinetics. These results demonstrated the importance of knowledge of the physico-chemical properties of polysaccharides, for example, to better conduct their biotechnological applications as drug carriers. (C) 2010 Elsevier Ltd. All rights reserved.

Reliable and fast sensor for in vitro evaluation of solar protection factor based on the photobleaching kinetics of a nanocrystalline TiO(2)/dye UV-dosimeter

A reliable and fast sensor for in vitro evaluation of solar protection factors (SPFs) of cosmetic products, based on the photobleaching kinetics of a nanocrystalline TiO(2)/dye UV-dosimeter, has been devised. The accuracy, robustness and suitability of the new device was demonstrated by the excellent matching of the predicted and the in vivo results up to SPF 70, for four standard samples analyzed in blind. These results strongly suggest that our device can be useful for routine SPF evaluation in laboratories devoted to the development or production of cosmetic formulations, since the conventional in vitro methods tend to exhibit unacceptably high errors above SPF similar to 30 and the conventional in vivo methods tend to be expensive and exceedingly time consuming. (C) 2011 Elsevier B.V. All rights reserved.

Effects of milk supplementation with skim milk powder, whey protein concentrate and sodium caseinate on acidification kinetics, rheological properties and structure of nonfat stirred yogurt

Milk supplementation with milk proteins in four different levels was used to investigate the effect on acidification and textural properties of yogurt. Commercial skim milk powder was diluted in distilled water, and the supplements were added to give different enriched-milk bases; these were heat treated at 90 degrees C for 5 min. These mixtures were incubated with the bacterial cultures for fermentation in a water bath, at 42 degrees C, until pH 4.50 was reached. Chemical changes during fermentation were followed by measuring the pH. Protein concentration measurements, microbial counts of Lactobacillus bulgaricus and Streptococcus thermophilus, and textural properties (G`, G ``, yield stress and firmness) were determined after 24 h of storage at 4 degrees C. Yogurt made with milk supplemented with sodium caseinate resulted in significant properties changes, which were decrease in fermentation time, and increase in yield stress, storage modulus, and firmness, with increases in supplement level. Microstructure also differed from that of yogurt produced with milk supplemented with skim milk powder or sodium caseinate. (C) 2009 Elsevier Ltd. All rights reserved.

Inactivation kinetics of polyphenol oxidase and peroxidase in green coconut water by microwave processing

The inactivation kinetics of enzymes polyphenol oxidase (PPO) and peroxidase (POD) was studied for the batch (discontinuous) microwave treatment of green coconut water. Inactivation of commercial PPO and POD added to sterile coconut water was also investigated. The complete time-temperature profiles of the experimental runs were used for determination of the kinetic parameters D-value and z-value: PPO (D(92.20 degrees C) = 52 s and z = 17.6 degrees C); POD (D(92.92 degrees C) = 16 s and z = 11.5 degrees C); PPO/sterile coconut water: (D(84.45 degrees C) = 43 s and z = 39.5 degrees C) and POD/sterile coconut water: (D(86.54 degrees C) = 20 s and z = 19.3 degrees C). All data were well fitted by a first order kinetic model. The enzymes naturally present in coconut water showed a higher resistance when compared to those added to the sterilized medium or other simulated solutions reported in the literature. The thermal inactivation of PPO and POD during microwave processing of green coconut water was significantly faster in comparison with conventional processes reported in the literature. (C) 2008 Elsevier Ltd. All rights reserved.

Studies on the kinetics of ascorbate oxidation at a ruthenium oxide hexacyanoferrate modified electrode towards the detection at microenvironments

The electrocatalytic oxidation of ascorbate on a ruthenium oxide hexacyanoferrate (RuOHCF) glassy carbon (GC) modified electrode was investigated at pH 6.9 by using rotating disc electrode (RDE) voltammetry. The influence of the systematic variation of rotation rate, film thickness, ascorbate concentration and the electrode potential indicated that the rate of cross-chemical reaction between Ru(III) centres immobilized into the film and ascorbate controls the overall process. The kinetic regime may be classified as a Sk `` mechanism and the second order rate constant for the surface electrocatalytic reaction was found to be 1.56 x 10(-3) mol(-1) L-1 s(-1) cm. A carbon fibre microelectrode modified with the RuOHCF film was successfully used as an amperometric sensor to monitor the ascorbate diffusion in a simulated microenvironment experiment. (C) 2008 Elsevier B.V. All rights reserved.