Theoretical investigation into the low-temperature oxidation of ethylbenzene

The classical topic on the oxidation of alkylbenzene has been revisited via performing accurate theoretical calculations to address the salient features for the initial oxidation of ethylbenzene. Potential energy surfaces are mapped out for all possible reactions in the systems of (1-phenylethyl + O₂ and 2-phenylethyl + O₂). Reaction rate constants at the high-pressure limit are calculated for all possible reactions in these two systems. Direct H abstraction from 1-phenylethyl radical by oxygen molecule appears to be an important route for the formation of styrene from the oxidation of ethylbenzene. Concerted elimination of HO₂ is predicted to contribute significantly the production of styrene from system of 2-phenylethyl + O₂; especially at the atmospheric pressure and intermediate temperatures. Formation of the other major experimental product, benzaldehyde, is attributed to the unimolecular decomposition of C₆H₅CH₂(O)CH₃ rather than to unimolecular isomerisation of the two initial peroxy adducts. Kinetic and mechanistic data presented herein are instrumental for better understanding of the oxidative decomposition of ethylbenzene, i.e., major constituents of commonly formulated fuel surrogates.     

Radio-frequency measurements of UNiX compounds (X=Al, Ga, Ge) in high magnetic fields

We performed radio-frequency (RF) skin-depth measurements of antiferromagnetic UNiX compounds (X=Al, Ga, Ge) in magnetic fields up to 60 T and at temperatures between 1.4 to ~60 K. Magnetic fields are applied along different crystallographic directions and RF penetration-depth was measured using a tunnel-diode oscillator (TDO) circuit. The sample is coupled to the inductive element of a TDO resonant tank circuit, and the shift in the resonant frequency Δf of the circuit is measured. The UNiX compounds exhibit field-induced magnetic transitions at low temperatures, and those transitions are accompanied by a drastic change in Δf. The results of our skin-depth measurements were compared with previously published B–T phase diagrams for these three compounds.     

Theoretical study of reaction pathways of dibenzofuran and dibenzo-p-dioxin under reducing conditions

A density functional theory (DFT) study was carried out to investigate possible reactions of dibenzofuran (DF) and dibenzo-p-dioxin (DD) in a reducing environment. Reaction energies, barrier heights, and molecular parameters for reactants, intermediates, products, and transition states have been generated for a wide range of possible reactions. It was found that C-O beta-scission in DF incurs a very large energy barrier (107 kcal/mol at 0 K), which is just 3 kcal/mol less than the direct H fission from C-H in DF to form dibenzofuranyl radicals. It was found that DF allows direct H addition to C1-C4 and C6-C9 as well as addition of two H atoms from a hydrogen molecule at sites 1 and 9 of DF. A bimolecular reaction of DF with H or H2 is found to have a significantly lower barrier than unimolecular decomposition through C-O beta-scission. An explanation for the predominance of polychlorinated dibenzofurans (PCDF) over polychlorinated dibenzo-p-dioxins (PCDD) in municipal waste pyrolysis is presented in the view of the facile conversion of DD into DF through ipso-addition at the four C sites of the two C-O-C central bonds in DD.     

Superconducting pairs with extreme uniaxial anisotropy in URu2Si2

We report magnetic field orientation-dependent measurements of the superconducting upper critical field in high quality single crystals of URu(2)Si(2) and find the effective g factor estimated from the Pauli limit to agree remarkably well with that found in quantum oscillation experiments, both quantitatively and in the extreme anisotropy (≈10(3)) of the spin susceptibility. Rather than a strictly itinerant or purely local f-electron picture being applicable, the latter suggests the quasiparticles subject to pairing in URu(2)Si(2) to be "composite heavy fermions" formed from bound states between conduction electrons and local moments with a protected Ising behavior. Non-Kramers doublet local magnetic degrees of freedom suggested by the extreme anisotropy favor a local pairing mechanism.     

The gas-phase ozonolysis reaction of methylbutenol: A mechanistic study

The gas-phase ozonolysis reaction of methylbutenol through the Criegee mechanism is investigated. The initial reaction leads to a primary ozonide (POZ) formation with barriers in the range of 10–28 kJ mol⁻¹. The formation of 2-hydroxy-2-methyl-propanal (HMP) and formaldehyde-oxide is more favorable, by 10 kJ mol⁻¹, than the syn-CI and formaldehyde. The unimolecular dissociation of the more stable syn-CI via 1,5-H transfer into an epoxide is more favored than the epoxide and ³O₂ formation. The ester channel led to the formation of the acetone and formic acid favorably from the anti-CI. The hydration of the anti-CI with H₂O and (H₂O)₂ is significantly barrierless with a higher plausibility to the latter, and thus they may lead to the formation of peroxides and ultimately OH radicals, as well as airborne particulate matter. Reaction of anti-CI with water dimers enhances its atmospheric reactivity by a factor of 28 in reference to water monomers.     

Effect of Fe2O3 nanoparticles on combustion of coal surrogate (Anisole): Enhanced ignition and formation of persistent free radicals

This contribution explores the effect of nanoparticles of iron (III) oxide (Fe₂O₃) on the combustion of coal surrogate, i.e., anisole, identifying the changes in ignition features as well as the occurrence of persistent organic pollutants in the initiation channels. The method applies packed-bed reactor coupled with Fourier transform infrared (FTIR) spectroscopy to quantitate the ignition temperature under typical fuel-rich conditions, in-situ electron paramagnetic resonance (EPR) to elucidate the formation of environmentally-persistent free radicals (EPFR), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to monitor the chemisorption of organic substrates on the nanoparticles, as well as X-ray diffraction for particles characterisation (PXRD). We employ cluster-based quantum mechanical calculation to map the reaction pathway within the scope of the density functional theory. The results of Fe₂O₃-mediated combustion of anisole depict an excessive reduction in ignition temperature from 500?°C around 220?°C at λ?=?0.8. As confirmed both from EPR and DRIFTS measurements, the chemisorption of anisole on α-Fe₂O₃ surfaces follows the direct dissociation of the O–CH₃ (and OCH₂–H), leading to the formation of surface-bound phenoxy radicals at temperatures as low as 25?°C and incurring an estimated energy barrier of E_a?=?18?kJ mol^?1 and a preexponential factor of A?=?2.7?×?10¹² M^?1 s^?1. This insight applies to free-radical chain reactions that induce spontaneous fires of coal, as coal comprises ferric oxide nanoparticles, and equally to coexistence of aromatic fuels with thermodynamically reactive Fe₂O₃ surface, e.g., in fly ash, at the cooled-down tail of combustion stacks.     

Theoretical study on the thermodynamic properties and self-decomposition of methylbenzenediol isomers

Alkylated hydroxylated aromatics are major constituents of various types of fuels, including biomass and low-rank coal. In this study, thermochemical parameters are obtained for the various isomeric forms of methylbenzenediol isomers in terms of their enthalpies of formation, entropies, and heat capacities. Isodesmic work reactions are used in quantum chemical computations of the reaction enthalpies for O-H and H?C-H bond fissions and the formation of phenoxy- and benzyl-type radicals. A reaction potential energy on the singlet-state surface surface is mapped out for the unimolecular decomposition of the 3-methylbenzene-1,2-diol isomer. According to the calculated high pressure-limit reaction rate constants, concerted hydrogen molecule elimination from the methyl group and the hydroxyl group, in addition to intermolecular H migration from the hydroxyl group, dominates the unimolecular decomposition at low to intermediate temperatures (T ≤ 1200 K). At higher temperatures, O-H bond fission and concerted water elimination are expected to become the sole decomposition pathways.     

Adsorption of chlorophenol on the Cu(1 1 1) surface: A first-principles density functional theory study

The interaction between a 2-chlorophenol (C₆H₄OHCl) molecule and the Cu(1 1 1) surface has been investigated using density functional theory as an initial step in gaining a better understanding of the catalyzed formation of dioxin compounds on a clean copper surface. The 2-chlorophenol molecule is found to form several weakly bonded, horizontally and vertically oriented configurations. Dissociative modes have also been investigated. For the latter, the formation of phenyl and benzyne fragments is found to be more energetically favourable than the formation of 2-chlorophenoxy radicals.     

Rate constants for hydrogen abstraction reactions by the hydroperoxyl radical from methanol, ethenol, acetaldehyde, toluene, and phenol

An important step in the initial oxidation of hydrocarbons at low to intermediate temperatures is the abstraction of H by hydroperoxyl radical (HO(2)). In this study, we calculate energy profiles for the sequence: reactant + HO(2) → [complex of reactants] → transition state → [complex of products] → product + H(2)O(2) for methanol, ethenol (i.e., C(2)H(3)OH), acetaldehyde, toluene, and phenol. Rate constants are provided in the simple Arrhenius form. Reasonable agreement was obtained with the limited literature data available for acetaldehyde and toluene. Addition of HO(2) to the various distinct sites in phenol is investigated. Direct abstraction of the hydroxyl H was found to dominate over HO(2) addition to the ring. The results presented herein should be useful in modeling the lower temperature oxidation of the five compounds considered, especially at low temperature where the HO(2) is expected to exist at reactive levels.     

Theoretical study on the unimolecular decomposition of thiophenol

The potential energy surface for the unimolecular decomposition of thiophenol (C(6)H(5)SH) is mapped out at two theoretical levels; BB1K/GTlarge and QCISD(T)/6-311+G(2d,p)//MP2/6-31G(d,p). Calculated reaction rate constants at the high pressure limit indicate that the major initial channel is the formation of C(6)H(6)S at all temperatures. Above 1000 K, the contribution from direct fission of the S-H bond becomes important. Other decomposition channels, including expulsion of H(2) and H(2)S are of negligible importance. The formation of C(6)H(6)S is predicted to be strong-pressure dependent above 900 K. Further decomposition of C(6)H(6)S produces CS and C(5)H(6). Overall, despite the significant difference in bond dissociation, i.e., 8-9 kcal/mol between the S-H bond in thiophenol and the O-H bond in phenol, H migration at the ortho position in the two molecules represents the most accessible initial channel.