Radical-Pair Formation in Hydrocarbon (Aut)Oxidation

The reaction profiles for the uni- and bimolecular decomposition of benzyl hydroperoxide have been studied in the context of initiation reactions for the (aut)oxidation of hydrocarbons. The unimolecular dissociation of benzyl hydroperoxide was found to proceed through the formation of a hydrogen-bonded radical-pair minimum located +181 kJ mol⁻¹ above the hydroperoxide substrate and around 15 kJ mol⁻¹ below the separated radical products. The reaction of toluene with benzyl hydroperoxide proceeds such that O−O bond homolysis is coupled with a C−H bond abstraction event in a single kinetic step. The enthalpic barrier of this molecule-induced radical formation (MIRF) process is significantly lower than that of the unimolecular O−O bond cleavage. The same type of reaction is also possible in the self-reaction between two benzyl hydroperoxide molecules forming benzyloxyl and hydroxyl radical pairs along with benzaldehyde and water as co-products. In the product complexes formed in these MIRF reactions, both radicals connect to a centrally placed water molecule through hydrogen-bonding interactions.     

Methyl methacrylate as solvent for the thermal decomposition of the cyclic molecule pinacolone diperoxide: Toward the polymerization process

The decomposition rate constant (k_d) of pinacolone diperoxide (PDP, 3,6‐diterbutyl‐3,6‐dimethyl‐1,2,4,5‐tetraoxacyclohexane) in methyl methacrylate (MMA) is determined by the kinetic study of its thermal decomposition at temperatures from 110 °C to 140 °C. The calculated k_d values for PDP are higher than the corresponding values previously determined and reported for diethyl ketone triperoxide (DEKTP, 3,3,6,6,9,9‐hexaethyl‐1,2,4,5,7,8‐hexaoxacyclononane), for example, at 140 °C the k_d for PDP is 75.4 × 10^?5 s^?1, while for DEKTP, it is 50.6 × 10^?5 s^?1. The difference in the k_d between 130 °C and 140 °C indicates that the decomposition mechanism, sequential and/or concerted, is a function of temperature. The conformations of both initiators justify the higher k_d for PDP in MMA than DEKTP, where one single conformer is found for PDP, whereas 212 conformers are found for DEKTP. Bulk polymerization of MMA using PDP as the initiator reveals also the presence of an induction period, such as in DEKTP case. This work provides mechanistic insights into the interactions among the bifunctional cyclic peroxide PDP and the MMA monomer and their influence on the polymerization kinetics. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 997–1007     

Acid‐Stable Peroxoniobophosphate Clusters To Make Patterned Films

Two new peroxoniobophosphate clusters were isolated as tetramethylammonium (TMA) salts having the stoichiometries: TMA₅[HNb₄P₂O₁₄(O₂)₄]?9 H₂O and TMA₃[H₇Nb₆P₄O₂₄(O₂)₆]?7 H₂O. The former is stable over the pH range: 3<pH<12 and the latter is stable only below pH 3. These two molecules interconvert as a function of solution pH. The [H₇Nb₆P₄O₂₄(O₂)₆]^3? cluster can be used to fabricate patterned niobium phosphate films by electron‐beam lithography after solution deposition.     

Structure and conformational flexibility of hexamethyldiperoxacyclononane

3,3,6,6,9,9-Hexamethyl-1,2,4,5-tetraoxacyclononane, one of the most thermally stable organic peroxides, has been studied by X-ray structural analysis. Possible conformations of this compound have been calculated by the molecular mechanics method.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 110–112, January, 1995.     

Chemiluminescence of oxygen-free acetone solutions of dimethyldioxirane

The decomposition of dimethyldioxirane (1) in oxygen-free acetone solutions (46°C) is accompanied by chemiluminescence (CL) in the visible spectral region. The emitter of CL is triplet-excited methyl acetate (2 ^*(T)). For the decomposition of solutions of1 in acetone and deuterated dimethyldioxirane in acetone-D₆ the decay of CL follows the first-order kinetics, and the kinetic isotope effect is observed. Two mechanisms of the formation of2 ^*(T) are discussed: (a) chain-radical process and (b) isomerization of1 to2. Dedicated to Prof. W. Adam on the occasion of his 60th birthday. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 477–479, March, 1997.     

Heterogeneous decomposition ofp-methoxy- andp-methyl-dibenzoyl peroxides by a montmorillonite amine intercalate

A kinetic study of the heterogeneous thermal decomposition ofp-MeO-DBP andp-Me-DBP catalysed by the montmorillonite benzidine intercalate has been made. The kinetics of decomposition were observed to follow a three-halves order reaction with respect to the peroxides. The activation parameters in heterogeneous media have lower values compared with the corresponding values in homogeneous ones. The relationships between H ^* and S ^* were plotted and show two converging straight lines which indicate that both electron donating and withdrawing substituents on the peroxide enhance the rate of decomposition under these conditions.     

Characterization of selected organic compound classes in secondary organic aerosol from biogenic VOCs by HPLC/MS<Superscript><Emphasis Type="Italic">n</Emphasis></Superscript>

The formation of secondary organic aerosols (SOA) has been investigated intensively during the last two decades in numerous field and laboratory studies and a general understanding exists about the major particle-phase products. However, recent studies show that several new product classes, such as esters, peroxides or organosulfates, also have to be considered in order to understand the detailed chemical mechanisms leading to SOA as well as to predict the aerosol mass loadings. For the identification and quantification of these three compound classes as well as for carboxylic SOA compounds, liquid chromatography (LC)/mass spectrometry (MS) is the most appropriate analytical method. In this article we try to summarize briefly the work that has been done for the determination of SOA-related carboxylic acids and we present new LC/tandem MS results on the characterization of esters, peroxides and organosulfates. In contrast to earlier work, the mass-spectrometric characterization of the individual compounds is always based on the comparison with authentic reference compounds.     

Trajectory surface hopping molecular dynamics on Chemiluminescence of cyclic peroxides

Chemiluminescence (CL) of peroxides is one of the most highly sensitive and most useful analytical techniques. Although the mechanisms of CL were studied experimentally and theoretically in the past decades, the chemiexcitation that a ground-state specie being excited from its electronic ground state to yield excited-state products by a chemical reaction is still not completely understood. Direct dynamics simulation on CL reaction which takes into account the full complexity of the relevant potential energy surface characterizes nonadiabatic processes involved in chemiexcitation and could provide access not only to the available reaction channels but also to statistical quantities such as reaction times and quantum yields. In the last decade, the trajectory surface hopping (TSH) molecular dynamics (MD) which is one of the mixed quantum-classical approaches is hence adopted to simulate the nonadiabatic process in CL of cyclic peroxides. In this article, the basic principle of TSH-MD and the successful applications on the CL reactions were shortly review.