Kinetics of alpha-hydroxy-alkylperoxyl radicals in oxidation processes. HO2*-initiated oxidation of ketones/aldehydes near the tropopause

A comparative theoretical study is presented on the formation and decomposition of alpha-hydroxy-alkylperoxyl radicals, Q(OH)OO* (Q = RR'C:), important intermediates in the oxidation of several classes of oxygenated organic compounds in atmospheric chemistry, combustion, and liquid-phase autoxidation of hydrocarbons. Detailed potential energy surfaces (PESs) were computed for the HOCH2O2* <==>HO2* + CH2O reaction and its analogues for the alkyl-substituted RCH(OH)OO* and R2C(OH)OO* and the cyclic cyclo-C6H10(OH)OO*. The state-of-the-art ab initio methods G3 and CBS-QB3 and a nearly converged G2M//B3LYP-DFT variant were found to give quasi-identical results. On the basis of the G2M//B3LYP-DFT PES, the kinetics of the approximately equal to 15 kcal/mol endothermal alpha-hydroxy-alkylperoxyl decompositions and of the reverse HO2*+ ketone/aldehyde reactions were evaluated using multiconformer transition state theory. The excellent agreement with the available experimental (kinetic) data validates our methodologies. Contrary to current views, HO2* is found to react as fast with ketones as with aldehydes. The high forward and reverse rates are shown to lead to a fast Q(OH)OO* <==>HO2* + carbonyl quasi-equilibrium. The sizable [Q(OH)OO*]/[carbonyl] ratios predicted for formaldehyde, acetone, and cyclo-hexanone at the low temperatures (below 220 K) of the earth's tropopause are shown to result in efficient removal of these carbonyls through fast subsequent Q(OH)OO* reactions with NO and HO2*. IMAGES model calculations indicate that at the tropical tropopause the HO2*-initiated oxidation of formaldehyde and acetone may account for 30% of the total removal of these major atmospheric carbonyls, thereby also substantially affecting the hydroxyl and hydroperoxyl radical budgets and contributing to the production of formic and acetic acids in the upper troposphere and lower stratosphere. On the other hand, an RRKM-master equation analysis shows that hot alpha-hydroxy-alkylperoxyls formed by the addition of O(2) to C(1)-, C(2)-, and C(3)-alpha-hydroxy-alkyl radicals will quasi-uniquely fragment to HO2* plus the carbonyl under all atmospheric conditions.     

Near-Field Diffractive Optical Pumping of a Laser Medium

A Nd:YAG rod active element placed inside a resonator was pumped by the second harmonic radiation (the wavelength =0.532 m) of a Q-switched Nd:YAG laser diffracted at a circular aperture. Various distributions of pumping intensity in the Fresnel (near-field) region along the direction of the pump beam diffraction were produced. Inversion profiles with maxima or minima at the resonator axis were formed in the active element, depending on its distance from the circular aperture. Gain-switched operation of the Nd:YAG rod laser at =1.064 m is reported at the fundamental mode and also at the TEM₀₁, TEM_01* and other higher-order modes for different positions of the active element in the near-field region. Applications of such diffractive optical pumping for the concentration of the pumping radiation into the active media and for laser beam profling are discussed. © 2004 The Optical Society of Japan     

Stochastic resonance to control diffusive motion in chemistry

This paper reports on a novel procedure to tune the effective diffusion coefficient of a field-sensitive reactant in the presence of a periodic external field. We investigate the motion of two negatively charged azo dyes interacting with alpha-cyclodextrin (alpha-CD) upon action of a periodic square wave electrical field. We show that the dyes exhibit an effective diffusion coefficient D(eff) that depends on the rate constants for dye complexation within alpha-CD, the period and the amplitude of the field. UV-vis absorption, gradient field (1)H NMR, and fluorescence correlation spectroscopy (FCS) after two photon excitation are used to evidence that D(eff) may be increased far beyond its intrinsic value when specific relations interpreted as a stochastic resonance are fulfilled. The present results may find useful applications in chemical kinetics as well as for molecular sorting.     

Probabilistic and fractal aspects of Lévy trees

We investigate the random continuous trees called Lévy trees, which are obtained as scaling limits of discrete Galton-Watson trees. We give a mathematically precise definition of these random trees as random variables taking values in the set of equivalence classes of compact rooted -trees, which is equipped with the Gromov-Hausdorff distance. To construct Lévy trees, we make use of the coding by the height process which was studied in detail in previous work. We then investigate various probabilistic properties of Lévy trees. In particular we establish a branching property analogous to the well-known property for Galton-Watson trees: Conditionally given the tree below level a, the subtrees originating from that level are distributed as the atoms of a Poisson point measure whose intensity involves a local time measure supported on the vertices at distance a from the root. We study regularity properties of local times in the space variable, and prove that the support of local time is the full level set, except for certain exceptional values of a corresponding to local extinctions. We also compute several fractal dimensions of Lévy trees, including Hausdorff and packing dimensions, in terms of lower and upper indices for the branching mechanism function which characterizes the distribution of the tree. We finally discuss some applications to super-Brownian motion with a general branching mechanism.     

An answer to the spiro versus ansa dilemma in cyclophosphazenes: Part I. N3P3Cl4[NH(CH2)3NH]

Structural investigation, mainly by mass spectrometry and X-ray crystallography, of the product obtained from the reaction of propylenediamine and N₃P₃Cl₆ shows it to have a monospiro structure. It is demonstrated that in this case data collected through indirect structural methods such as IR or NMR spectroscopy could not provide a priori the right answer.     

540—Evolution with time of the zero-current potential of a gold electrode in Escherichia coli cultures supplied with lipoic acid: Part I. Mathematical modeling and computer simulation

Zero-current potential measurements (gold electrode) are suitable for continuously following the oxidation—reduction reactions of exogenic lipoic acid during Escherichia coli bacterial growth. This paper relates to a mathematical modeling of the experimental time-courses of potential.First, an empirical mathematical relation was obtained in vitro (i.e. in a sterile culture broth) between the zero-current potential and the concentrations of electroactive species that coexist and prevail in vivo (i.e. during the cultures).Secondly, a system of simple kinetic equations was proposed to express the metabolic, physical or chemical processes responsible for the in vivo time evolutions of the concentrations of electroactive species from which the time evolution of the electrode potential during the cultures was obtained. Most of these equations have been standardized by direct measurements. Numerical values could be applied to the remaining parameters of the model by comparing the computer-simulated time-courses of potential with experimental potential—time signals.The model properly fitted the experimental reality. It substantiated a theoretical correlation between the time evolutions of potential and the reductive activity of cultures by means of growth parameters relative to the population of organisms and transport or consumption parameters relative to the bacterial cell.     

Etude des aldimines lithiees : III. Action des epoxydes et synthese d'amino-2 et d'hydroxy-2 tetrahydrofurannes

The reaction of lithiated aldimines, prepared from activated amides, with epoxides leads to good yields of 2-aminotetrahydrofurans (neutral hydrolysis) and 2-hydroxytetrahydrofurans (acid hydrolysis). Various epoxides were examined.