The effect of a radical scavenger on the propagation of flames in an exothermic-endothermic system

The propagation of a premixed laminar flame supported by an exothermic chemical reaction under adiabatic conditions but subject to inhibition through parallel endothermic chemical processes is considered. These consist of the endothermic decomposition of an inhibitor W leading to the formation of a ‘radical scavenger’ S, which acts as a catalyst for the removal of active radicals X through an additional termination step. The heat loss through the endothermic reaction and the action of the radical scavenger, represented by the parameters α and ρ, both have a strong quenching effect on wave propagation. The dependence of the flame velocity c on α and ρ is determined by numerical integration of the flame equations for a range of values of the other parameters. The (ρ ,c) curve can have at least one turning point, the (α,c) curve can be monotone or it can have one or three turning points, depending on the values of the parameters β, representing the rate at which inhibitor is consumed, μ, the ratio of the activation energies of the reactants and the Lewis numbers. The additional feature caused by the scavenger is that the (α, c) curve has a turning point for any (μ, β) parameter pair if ρ is sufficiently large. A new feature of the model is that, for non-zero values of ρ, there can be four solutions below critical values of α. This behaviour is confirmed by a high activation energy analysis, which also reveals some additional features of the flame structure resulting from the presence of the radical scavenger.AMS subject classification: 80A25, 35K57, 35B32     

Kinetic study of adsorption of some biocompounds at the oil/water interface

The adsorption kinetics of some local anesthetics, like dibucaine and tetracaine, and of stearic acid from bulk solutions at the oil/water interface was studied by using the pendent drop and ring methods. The anesthetics were dissolved in aqueous solutions (pH 2), and the fatty acid was dissolved in benzene, each biocompound at several different concentrations in bulk solutions. Kinetic equations for Langmuir mechanism of adsorption at oil/water interface were tested. The kinetic analysis shows that Langmuir kinetic approach describes the dynamic interfacial pressures within the limits of the experimental errors over a wide range of time and for different surfactant concentrations in bulk solutions. It is also concluded that this approach allows the calculation of the ratio of the adsorption and desorption rate constants of these biocompounds at the oil/water interface. Obtained results are in substantial agreement with earlier reported data for the surfactant adsorption as, well as with their molecular structure.     

Adsorption kinetics of some carotenoids at the oil/water interface

The kinetic analysis of the adsorption of two carotenoids (i.e., ethyl ester of β-apo-8′-carotenoic acid and β-carotene, all trans-isomers) from n-hexane solutions at the oil/water interface is presented for several carotenoid concentrations in the oil phase. A new kinetic approach is developed and it addresses the diffusion adsorption associated with a reversible interfacial reaction, which describes the reorientation of surfactant molecules between two conformations. This approach leads to a general analytical expression that contains four physical parameters and describes with high accuracy the experimental dynamic interfacial tensions for the two carotenoids, which independently adsorb from n-hexane phase to the n-hexane/water interface. The calculations give the characteristic times for the carotenoid adsorption at the oil/water interface in terms of diffusion relaxation and kinetic relaxation times. The results explain the long time effects on the adsorption of these carotenoids at the oil/water interface. The data are in substantial agreement with the molecular structure of these carotenoids and with the earlier data recorded for cholesterol adsorption at the n-heptane/water interface. Based on these findings, we propose a molecular mechanism for the interfacial transformation of carotenoid molecules at a hydrophobic/hydrophilic interface.     

Thermal behavior of different cocoa powder varieties and their physicochemical,phytochemical and microbiological characteristics

Four cocoa powder varieties processed in different European countries (Germany, Poland, Romania and Bulgaria) were subjected to physicochemical, phytochemical and microbiological analysis. The cocoa powders were extensively characterized by recording their pH and titratable acidity, respectively, the polyphenols and also the methylxantine derivatives content (theobromine and caffeine). The cocoa powders pH ranged between 5.37 and 8.23, while the titratable acidity was 3.2–4.3 miliequivalent (100 g)^?1 of cocoa powder. Their total polyphenols content ranged between 0.986?÷?2.003 g GAE/(100 g)^?1. The methylxanthine derivatives (theobromine and caffeine) were analyzed by the HPLC method and ranges of 0.992–1.174% for theobromine and 0.096–0.369% for caffeine were obtained. Thermal analysis (TG–DTA) combined with mass spectrometry (MS) elucidated the decomposition processes and the volatile substances (CO, CO₂, H₂O, NO, theobromine, caffeine). The thermal analysis revealed transformations in the cocoa powders composition: drying and water loss; decomposition of pectic polysaccharides; lipids, amino acids and proteins, crystalline phase transformations and carbonizations. The microbiological analysis tested the degree of preservation of the cocoa powders across time, specifically immediately after unwrapping and after 14 days.

     

Evidence of AlII Radical Addition to Benzene

Electrophilic Al^III species have long dominated the aluminum reactivity towards arenes. Recently, nucleophilic low-valent Al^I aluminyl anions have showcased oxidative additions towards arenes C−C and/or C−H bonds. Herein, we communicate compelling evidence of an Al^II radical addition reaction to the benzene ring. The electron reduction of a ligand stabilized precursor with KC₈ in benzene furnishes a double addition to the benzene ring instead of a C−H bond activation, producing the corresponding cyclohexa-1,3(orl,4)-dienes as Birch-type reduction product. X-ray crystallographic analysis, EPR spectroscopy, and DFT results suggest this reactivity proceeds through a stable Al^II radical intermediate, whose stability is a consequence of a rigid scaffold in combination with strong steric protection.     

A Modular Access to Divinyldiphosphenes with a Strikingly Small HOMO–LUMO Energy Gap

The olefinic C−H bond functionalization of (NHC)CHPh (NHC=IPr=C{(NAr)CH}₂ 1 ; SIPr=C{(NAr)CH₂}₂ 2 ; Ar=2,6-iPr₂C₆H₃), derived from classical N-heterocyclic carbenes (NHCs), with PCl₃ affords the dichlorovinylphosphanes {(NHC)C(Ph)}PCl₂ (NHC=IPr 3 , SIPr 4 ). Two-electron reduction of 3 and 4 with magnesium leads to the formation of the divinyldiphosphenes [{(NHC)C(Ph)}P]₂ (NHC=IPr 5 , SIPr 6 ) as crystalline solids. Unlike literature-known diphosphenes, which are mostly yellow or orange, 5 is a green whereas 6 is a purple solid. Although the P=P bond lengths of 5 (2.062(1)) and 6 (2.055(1) Å) are comparable to those of the known diphosphenes (2.02–2.08 Å), the C−P bond lengths of 5 (1.785(1)) and 6 (1.797(1) Å) are, however, considerably shorter than a C −P single bond length (1.85 Å), indicating a considerable π-conjugation between C=C and P=P moieties. The HOMO–LUMO energy gap for 5 (4.15) and 6 (4.52 eV) is strikingly small and thus the narrowest among the diphosphenes (>4.93 eV) reported as yet. Consequently, 5 readily undergoes P=P bond cleavage at room temperature on treatment with sulfur to form the unique dithiophosphorane {(IPr)C(Ph)}P(S)₂ 7 . Interestingly, reaction of 5 with selenium gives the selenadiphosphirane [{(IPr)C(Ph)}P]₂Se 8 with an intact P−P bond.     

Instability,Rupture and Fluctuations in Thin Liquid Films: Theory and Computations

Journal of Statistical Physics - Thin liquid films are ubiquitous in natural phenomena and technological applications. They have been extensively studied via deterministic hydrodynamic equations,...     

The [B3(NN)3]+ and [B3(CO)3]+ Complexes Featuring the Smallest π‐Aromatic Species B3+

We report the spectroscopic identification of the [B₃(NN)₃]⁺ and [B₃(CO)₃]⁺ complexes, which feature the smallest π‐aromatic system B₃⁺. A quantum chemical bonding analysis shows that the adducts are mainly stabilized by L→[B₃L₂]⁺ σ‐donation.     

Comparison of Hydrogen and Gold Bonding in [XHX]−, [XAuX]−, and Isoelectronic [NgHNg]+, [NgAuNg]+ (X=Halogen,Ng=Noble Gas)

Quantum chemical calculations at the MP2/aug‐cc‐pVTZ and CCSD(T)/aug‐cc‐pVTZ levels have been carried out for the title compounds. The electronic structures were analyzed with a variety of charge and energy partitioning methods. All molecules possess linear equilibrium structures with D_∞h symmetry. The total bond dissociation energies (BDEs) of the strongly bonded halogen anions [XHX]^? and [XAuX]^? decrease from [FHF]^? to [IHI]^? and from [FAuF]^? to [IAuI]^?. The BDEs of the noble gas compounds [NgHNg]⁺ and [NgAuNg]⁺ become larger for the heavier atoms. The central hydrogen and gold atoms carry partial positive charges in the cations and even in the anions, except for [IAuI]^?, in which case the gold atom has a small negative charge of ?0.03 e. The molecular electrostatic potentials reveal that the regions of the most positive or negative charges may not agree with the partial charges of the atoms, because the spatial distribution of the electronic charge needs to be considered. The bonding analysis with the QTAIM method suggests a significant covalent character for the hydrogen bonds to the noble gas atoms in [NgHNg]⁺ and to the halogen atoms in [XHX]^?. The covalent character of the bonding in the gold systems [NgAuNg]⁺ and [XAuX]^? is smaller than in the hydrogen compound. The energy decomposition analysis suggests that the lighter hydrogen systems possess dative bonds X^?→H⁺←X^? or Ng→H⁺←Ng while the heavier homologues exhibit electron sharing through two‐electron, three‐center bonds. Dative bonds X^?→Au⁺←X^? and Ng→Au⁺←Ng are also diagnosed for the lighter gold systems, but the heavier compounds possess electron‐shared bonds.