Kinetics of the Gas Phase Reactions of the Criegee Intermediate CH2OO with O3 and IO

The kinetics of the gas phase reactions of the Criegee intermediate CH2OO with O3 and IO have been studied at 296 K and 300 Torr through simultaneous measurements of CH2OO, the CH2OO precursor (CH2I2), O3, and IO using flash photolysis of CH2I2/O2/O3/N2 mixtures at 248 nm coupled to time-resolved broadband UV absorption spectroscopy. Experiments were performed under pseudo-first-order conditions with respect to O3, with the rate coefficients for reactions of CH2OO with O3 and IO obtained by fitting to the observed decays of CH2OO using a model constrained to the measured concentrations of IO. Fits were performed globally, with the ratio between the initial concentration of O3 and the average concentration of IO varied in the range 30 to 700, and gave kCH2OO+O3 = (3.6 ± 0.8) × 10-13 cm3 molecule-1 s-1 and kCH2OO+IO = (7.6 ± 1.4) × 10-11 cm3 molecule-1 s-1 (where the errors are at the 2σ level). The magnitude of kCH2OO+O3 has a significant effect on the steady state concentration of CH2OO in chamber studies. Atmospheric implications of the results are discussed

The Nature of the Sodium Dodecylsulfate Micellar Pseudophase as Studied by Reaction Kinetics

The nature of the rate-retarding effects of anionic micelles of sodium dodecyl sulfate (SDS) on the water-catalyzed hydrolysis of a series of substituted 1-benzoyl-1,2,4-triazoles (1a–f) has been studied. We show that medium effects in the micellar Stern region of SDS can be reproduced by simple aqueous model solutions containing small-molecule mimics for the surfactant headgroups and tails, namely sodium methyl sulfate (NMS) and 1-propanol, in line with our previous kinetic studies for cationic surfactants (Buurma et al. J. Org. Chem. 2004, 69, 3899−3906). We have improved our mathematical description leading to the model solution, which has made the identification of appropriate model solutions more efficient. For the Stern region of SDS, the model solution consists of a mixture of 35.3 mol dm–3 H2O, corresponding to an effective water concentration of 37.0 mol dm–3, 3.5 mol dm–3 sodium methylsulfate (NMS) mimicking the SDS headgroups, and 1.8 mol dm–3 1-propanol mimicking the backfolding hydrophobic tails. This model solution quantitatively reproduces the rate-retarding effects of SDS micelles found for the hydrolytic probes 1a–f. In addition, the model solution accurately predicts the micropolarity of the micellar Stern region as reported by the ET(30) solvatochromic probe. The model solution also allows the separation of the individual contributions of local water concentration (water activity), polarity and hydrophobic interactions, ionic strength and ionic interactions, and local charge to the observed local medium effects. For all of our hydrolytic probes, the dominant rate-retarding effect is caused by interactions with the surfactant headgroups, whereas the local polarity as reported by the solvatochromic ET(30) probe and the Hammett ρ value for hydrolysis of 1a–f in the Stern region of SDS micelles is mainly the result of interactions with the hydrophobic surfactant tails. Our results indicate that both a mimic for the surfactant tails (NMS) and a mimic for the surfactant headgroups (1-propanol) are required in a model solution for the micellar pseudophase to reproduce all medium effects experienced by a variety of different probes

Reactivity in organised assemblies

This report reviews the 2008 literature on reactivity in organised assemblies. The report is subdivided in sections discussing (1) reactivity in micelles, including medium effects and compartmentalisation, enzymatic catalysis in micellar media and metallomicellar catalysis, (2) reactivity in vesicles, (3) reactivity in emulsions, (4) reactions in assemblies resulting from dynamic combinatorial chemistry and template approaches, (5) DNA-based approaches to influence reactivity including catalysis by DNA, DNA-templated synthesis and DNA-based asymmetric synthesis, (6) reactivity in nanoparticle-immobilised assemblies and (7) modulation of nanoparticle catalysis through the use of thermoresponsive polymers. We have chosen to interpret “organised assemblies” in a rather more liberal way than in previous years.1–3 now including assemblies such as those involving DNA as well as constructions resulting from dynamic combinatorial self assembly which were turned into covalent “non dynamic assemblies”. In all cases, however, (self) organisation and (self) assembly are crucial to the results discussed. Related to the liberal interpretation of organised assemblies, this report does not (and cannot) aim to present an exhaustive review

Reactivity in organised assemblies

This report reviews the 2009 literature on reactivity in organised assemblies, with virtually all examples taken from literature covering aqueous systems. The report is broadly organised along the lines that we set out last year, but with the addition of a “miscellaneous” section containing two papers that we did not want to leave out of this report. Nevertheless, as always, we cannot and will not claim that this report provides an exhaustive review. Several main themes covered by this report were reviewed in 2009; Prins and Scrimin reviewed covalent capture which they defined as relying on “the combined use of covalent and noncovalent synthesis by taking advantage of their complementarity”, Peyralans and Otto reviewed recent highlights in systems chemistry, metallomicellar catalysis was reviewed by Tecillaet al. and one of us reviewed kinetic medium effects on organic reactions in aqueous colloidal solutions

The Nature of the Sodium Dodecylsulfate Micellar Pseudophase as Studied by Reaction Kinetics

The nature of the rate-retarding effects of anionic micelles of sodium dodecyl sulfate (SDS) on the water-catalyzed hydrolysis of a series of substituted 1-benzoyl-1,2,4-triazoles (<b>1a</b>–<b>f</b>) has been studied. We show that medium effects in the micellar Stern region of SDS can be reproduced by simple aqueous model solutions containing small-molecule mimics for the surfactant headgroups and tails, namely sodium methyl sulfate (NMS) and 1-propanol, in line with our previous kinetic studies for cationic surfactants (Buurma et al. J. Org. Chem. 2004, 69, 3899−3906). We have improved our mathematical description leading to the model solution, which has made the identification of appropriate model solutions more efficient. For the Stern region of SDS, the model solution consists of a mixture of 35.3 mol dm^–3 H₂O, corresponding to an effective water concentration of 37.0 mol dm^–3, 3.5 mol dm^–3 sodium methylsulfate (NMS) mimicking the SDS headgroups, and 1.8 mol dm^–3 1-propanol mimicking the backfolding hydrophobic tails. This model solution quantitatively reproduces the rate-retarding effects of SDS micelles found for the hydrolytic probes <b>1a</b>–<b>f</b>. In addition, the model solution accurately predicts the micropolarity of the micellar Stern region as reported by the E_T(30) solvatochromic probe. The model solution also allows the separation of the individual contributions of local water concentration (water activity), polarity and hydrophobic interactions, ionic strength and ionic interactions, and local charge to the observed local medium effects. For all of our hydrolytic probes, the dominant rate-retarding effect is caused by interactions with the surfactant headgroups, whereas the local polarity as reported by the solvatochromic E_T(30) probe and the Hammett ρ value for hydrolysis of <b>1a</b>–<b>f</b> in the Stern region of SDS micelles is mainly the result of interactions with the hydrophobic surfactant tails. Our results indicate that both a mimic for the surfactant tails (NMS) and a mimic for the surfactant headgroups (1-propanol) are required in a model solution for the micellar pseudophase to reproduce all medium effects experienced by a variety of different probes

Branching Ratios in Reactions of OH Radicals with Methylamine, Dimethylamine, and Ethylamine

The branching ratios for the reaction of the OH radical with the primary and secondary alkylamines: methylamine (MA), dimethylamine (DMA), and ethylamine (EA), have been determined using the technique of pulsed laser photolysis–laser-induced fluorescence. Titration of the carbon-centered radical, formed following the initial OH abstraction, with oxygen to give HO₂ and an imine, followed by conversion of HO₂ to OH by reaction with NO, resulted in biexponential OH decay traces on a millisecond time scale. Analysis of the biexponential curves gave the HO₂ yield, which equaled the branching ratio for abstraction at αC–H position, <i>r</i>_αC–H. The technique was validated by reproducing known branching ratios for OH abstraction for methanol and ethanol. For the amines studied in this work (all at 298 K): <i>r</i>_αC–H,MA = 0.76 ± 0.08, <i>r</i>_αC–H,DMA = 0.59 ± 0.07, and <i>r</i>_αC–H,EA = 0.49 ± 0.06 where the errors are a combination in quadrature of statistical errors at the 2σ level and an estimated 10% systematic error. The branching ratios <i>r</i>_αC–H for OH reacting with (CH₃)₂NH and CH₃CH₂NH₂ are in agreement with those obtained for the OD reaction with (CH₃)₂ND (<i>d</i>-DMA) and CH₃CH₂ND₂ (<i>d</i>-EA): <i>r</i>_{αC–H,d‑DMA} = 0.71 ± 0.12 and <i>r</i>_{αC–H,d‑EA} = 0.54 ± 0.07. A master equation analysis (using the MESMER package) based on potential energy surfaces from G4 theory was used to demonstrate that the experimental determinations are unaffected by formation of stabilized peroxy radicals and to estimate atmospheric pressure yields. The branching ratio for imine formation through the reaction of O₂ with α carbon-centered radicals at 1 atm of N₂ are estimated as <i>r</i>_CH2NH2 = 0.79 ± 0.15, <i>r</i>_CH2NHCH3 = 0.72 ± 0.19, and <i>r</i>_CH3CHNH2 = 0.50 ± 0.18. The implications of this work on the potential formation of nitrosamines and nitramines are briefly discussed

Unimolecular Kinetics of Stabilized CH₃CHOO Criegee Intermediates: <i>syn</i>-CH₃CHOO Decomposition and <i>anti</i>-CH₃CHOO Isomerization

The kinetics of the unimolecular decomposition of the stabilized Criegee intermediate syn-CH3CHOO has been investigated at temperatures between 297 and 331 K and pressures between 12 and 300 Torr using laser flash photolysis of CH3CHI2/O2/N2 gas mixtures coupled with time-resolved broadband UV absorption spectroscopy. Fits to experimental results using the Master Equation Solver for Multi-Energy well Reactions (MESMER) indicate that the barrier height to decomposition is 67.2 ± 1.3 kJ mol–1 and that there is a strong tunneling component to the decomposition reaction under atmospheric conditions. At 298 K and 760 Torr, MESMER simulations indicate a rate coefficient of 150–81+176 s–1 when tunneling effects are included but only 5–2+3 s–1 when tunneling is not considered in the model. MESMER simulations were also performed for the unimolecular isomerization of the stabilized Criegee intermediate anti-CH3CHOO to methyldioxirane, indicating a rate coefficient of 54–21+34 s–1 at 298 K and 760 Torr, which is not impacted by tunneling effects. Expressions to describe the unimolecular kinetics of syn- and anti-CH3CHOO are provided for use in atmospheric models, and atmospheric implications are discussed