Quantum chemical study of mechanisms of dissociation and isomerization reactions in some molecules and radicals of astrophysical significance: Cyanides and related molecules

A theoretical study of the mechanism of photodecomposition in carbonyl cyanide, diethynyl ketone, acetyl cyanide and formyl cyanide has been conducted using density functional and MP2 theories. A complete analysis of the electronic spectra of these molecules in terms of nature, energy and intensity of electronic transitions has been provided by time-dependent density functional theory. Mixing coefficients and main configurations of the electronic states have been used to identify the states leading to the photodecomposition process. While the Rydberg state ¹(n,3s) is involved in the dissociation of formyl cyanide and acetyl cyanide, the π*_CC/π*_CN states are involved in the case of carbonyl cyanide and diethynyl ketone. In all cases, however, stepwise decomposition process is preferred over the concerted reaction process. Based on potential energy curves for bond dissociation and the transition state and IRC studies, it is found that besides the direct dissociation of carbonyl cyanide, a photoisomerization process through a non-planar transition state may also occur resulting in the formation of a stable and planar isomer CNC(O)CN. A complete vibrational analysis of the higher energy isomer has been conducted and several new fundamental bands are predicted. Some of the earlier experimental results on the photodecomposition mechanism and energies of photofragments in carbonyl cyanide and acetyl cyanide have been rationalized.     

A Quantum-chemical study of the mechanism of formation of styrene polymerization centers under initiation by the ferrocene—benzoyl peroxide system

The mechanism of formation of free polystyrene growth radicals and radicals bound to complexes with ferrocene when polymerization was performed in the presence of the ferrocene—benzoyl peroxide initiating system was studied by quantum-chemical calculations with the use of the PRIRODA program and the PBE/3z method. The calculated structure of the charge transfer complex between ferrocene and benzoyl peroxide was in agreement with the experimental data. The calculated heat effects of possible complex decomposition reactions showed that radical formation easily occurred in the presence of the monomer only. The most favorable energetically scheme of the formation of the active centers of the complex-radical polymerization of styrene, including the intermediate formation of the ferrocene—benzoyloxy radical complex, was suggested.     

Oxidative decomposition mechanisms of lithium peroxide clusters: an Ab Initio study

Oxidative decomposition of solid lithium peroxide is an important part of the charging process in a Li-O₂ battery. In this paper, we investigate oxidative decomposition mechanisms of lithium peroxide clusters as molecular models for solid lithium peroxide using density functional methods to understand charging processes in advanced energy storage systems. Most calculations are done using a (Li₂O₂)₄ cluster with similar results obtained from a larger (Li₂O₂)₁₆ cluster. Reaction pathways of the clusters involving different sequences of oxidation, oxygen evolution, lithium cation removal, and spin excitation are investigated. The computations suggest that certain oxidative decomposition routes may not have dependence on the oxygen evolution or Li-ion removal kinetics due to the exothermicity of oxygen removal and Li⁺ removal (by solvent) upon oxidation. The computed charge potentials evaluated using a tetramer model indicates that it is possible to have low overcharge potential provided there exists a good electronic conductivity to facilitate the oxidative decomposition. Finally, oxidation potentials of a series of Li_xO_y clusters are investigated to assess their dependence on stoichiometry and how the local site from which the electrons are being removed affects the charge potentials.     

Concerted SN2 mechanism for the hydrolysis of acid chlorides: comparisons of reactivities calculated by the density functional theory with experimental data

DFT computations have been performed in acetone and water solvents in order to investigate the mechanism of hydrolysis of acid chlorides. Acetyl chloride and chloroacetyl chloride hydrolyze via concerted, one‐step S_N2 mechanism, with the attack of water at the sp² hybridized carbon atom of the C?O group, and the transition state (TS) has distorted tetrahedral geometry. Solvent molecules act as general base and general acid catalysts. The TS of chloroacetyl chloride is tighter and less polar than the TS of acetyl chloride. The structure of the S_N2 TS for the hydrolysis of benzoyl chlorides changes with the substituents and the solvent. Tight and loose TSs are formed for substrates bearing electron withdrawing (e‐w) and electron donating (e‐d) groups, respectively. In acetone, only the e‐w effect of the substituents increase the reactivity of the substrates, and the change of the structure of the TSs with the substituents is small. In water, polar and very loose TSs are formed in the reactions of benzoyl chlorides bearing e‐d substituents, and the rate enhancing effect of both e‐d and e‐w groups can be computed at higher level of theory. Calculated reactivities and the changes of the structure of the TSs with substituents and solvent are in accordance with the results of kinetic studies. In S_N2 nucleophilic substitutions late/early TSs are formed if the attacking reagent is poorer/better nucleophile than the leaving group, and loose/tight TSs are formed for substrates bearing e‐d/e‐w substituents and in protic/aprotic solvents. Copyright © 2010 John Wiley & Sons, Ltd.     

Production mechanism of active species on the oxidative bromination following perhydrolase activity

Hypobromous acid and molecular bromine have been described as the active species involved in the oxidative bromination using perhydrolase, which catalyzes the reaction from acetic acid and hydrogen peroxide to peracetic acid (AcOOH). However, the brominating activity of them in a chemical model system was lower than that of the active species produced by the spontaneous reaction between AcOOH and Br^?. Consequently, acetyl hypobromite (AcOBr) was suggested as new active species on the bromination by detection of the decarboxylation in the reaction between AcOOH and Br^? and the strong brominating power with some tolerance against H₂O₂. Its production mechanism was explained as the ionic reaction involving the protonated intermediate of AcOOH by kinetic analysis. Copyright © 2015 John Wiley & Sons, Ltd.     

Understanding the mechanism of DNA deactivation in ion therapy of cancer cells: hydrogen peroxide action*

Changes in the medium of biological cells under ion beam irradiation has been considered as a possible cause of cell function disruption in the living body. The interaction of hydrogen peroxide, a long-lived molecular product of water radiolysis, with active sites of DNA macromolecule was studied, and the formation of stable DNA-peroxide complexes was considered. The phosphate groups of the macromolecule backbone were picked out among the atomic groups of DNA double helix as a probable target for interaction with hydrogen peroxide molecules. Complexes consisting of combinations including: the DNA phosphate group, H₂O₂ and H₂O molecules, and Na⁺ counterion, were considered. The counterions have been taken into consideration insofar as under the natural conditions they neutralise DNA sugar-phosphate backbone. The energy of the complexes have been determined by considering the electrostatic and the Van der Waals interactions within the framework of atom-atom potential functions. As a result, the stability of various configurations of molecular complexes was estimated. It was shown that DNA phosphate groups and counterions can form stable complexes with hydrogen peroxide molecules, which are as stable as the complexes with water molecules. It has been demonstrated that the formation of stable complexes of H₂O₂-Na⁺-PO₄ ^- may be detected experimentally by observing specific vibrations in the low-frequency Raman spectra. The interaction of H₂O₂ molecule with phosphate group of the double helix backbone can disrupt DNA biological function and induce the deactivation of the cell genetic apparatus. Thus, the production of hydrogen peroxide molecules in the nucleus of living cells can be considered as an additional mechanism by which high-energy ion beams destroy tumour cells during ion beam therapy.     

Microwave-assisted synthesis of carbon-supported carbides catalysts for hydrous hydrazine decomposition

Microwave-assisted synthesis of carbon-supported Mo₂C and WC nanomaterials was studied. Two different routes were utilized to prepare MoO₃ (WO₃) - C precursors that were then subjected to microwave irradiation in an inert atmosphere. The effect of synthesis conditions, such as irradiation time and gas environment, was investigated. The structure and formation mechanism of the carbide phases were explored. As-synthesized nanomaterials exhibited catalytic activity for hydrous hydrazine (N₂H₄·H₂O) decomposition at 30–70 ^°C. It was shown that the catalyst activity significantly increases if microwave irradiation is applied during the decomposition process. Such conditions permit complete conversion of hydrazine to ammonia and nitrogen within minutes. This effect can be attributed to the unique nanostructure of the catalysts that includes microwave absorbing carbon and active carbide constituents.     

High time resolution spectroscopy studies of the explosive decomposition of silver azide

We have analyzed the existing assumptions about the mechanism of the explosive decomposition of the heavy metal azides. We present here a study of the kinetics of the explosive decomposition of silver azide, initiated by a laser pulse. New effects are identified and studied, including a pre-explosion conductivity and luminescence. On the basis of our results, we conclude that the reaction proceeds by a chain reaction mechanism. In terms of band theory, we propose a model of the primary chain step of the reaction as the creation of a quasilocalized state deep in the valence band with two holes localized at a defect (N₃ ⁰), with impact ionization occurring as they “float up”. State University, Kemerov. Translated from Ivvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 162–175, November, 1996.     

On combustion enhancement mechanisms in the case of electrical-discharge-excited oxygen molecules

Combustion intensification mechanisms in a supersonic flow of a hydrogen-oxygen mixture behind the oblique shock wave front are investigated for the case when vibrations and the a ¹Δ_g and b ¹Σ _g ⁺ electron states of a O₂ molecule are excited by an electrical discharge. The presence of vibrationally excited and electronically excited O₂ molecules in the oxygen plasma allows intensification of the chain mechanism in the H₂-O₂ mixture even if the energy put into O₂ molecules in the discharge is low. Excitation of O₂ molecules is several tens of times more efficient for acceleration of oxygen-hydrogen mixture combustion than mere heating of the gas by an electrical discharge. In addition, low-temperature inflammation of the mixture with electrical-discharge-excited O₂ molecules makes it possible to raise the efficiency of conversion of the reactant chemical energy to heat compared with the conventional way of combustion initiation by heating.     

Synthesis and Spectral Characterization of Methyl‐2‐pyridyl Ketone Benzoyl Hydrazone and Its Complexes with Rare Earth Nitrates

Abstract

The hydrazone ligand, methyl‐2‐pyridyl ketone benzoyl hydrazone (L), and its complexes with rare earth nitrates have been synthesized. These new complexes with the general formula of Ln(L)₂(NO₃)₃ · nH₂O (where Ln=La, n=5.5; Ce, Pr, n=5; Nd, Eu, n=4) were characterized by mass spectra, elemental analysis, IR spectra, thermal analysis, UV spectra, molar conductivity, and luminescent spectra. All the complexes are stable in air. The results show that the lanthanide ions in each complex are coordinated through oxygen and nitrogen atoms of the ligand, the oxygen atoms of the nitrate, and coordinated water molecules. The amide‐oxygen atoms of L coordinate to the Ln ions in its keto‐form. Tentative structures for the complexes have been proposed.     

The effect of oxygen–oxygen repulsion in the production of water from hydrogen–oxygen reaction on the surface: A computer simulation study

The formation of water from hydrogen–oxygen reaction on a metal surface is of immense importance due to the technological reasons. This reaction has been studied via a thermal mechanism on a Pt single crystal surface where the two molecules, H₂ and O₂, have been adsorbed dissociatively in atomic form. The reaction takes place between the adsorbed atoms through an intermediate OH radical. We have studied this reaction via a thermal (Langmuir–Hinshelwood mechanism) as well as a non-thermal mechanism (precursor mechanism) by the Monte Carlo computer simulations. In this study, we have applied a novel approach based upon the experimental observations that the dissociated oxygen atoms do not sit next to one another on a catalytic surface. Some interesting results like the shifting of the phase transition points, the broadening of the reaction window width and the elimination of the second-order phase transition in the non-thermal reaction mechanism are obtained by considering various possibilities of the reaction scheme. The phase diagrams as well as the snapshots of the surface covered with the reacting species are presented.     

The repetitive microwave discharge as a catalyst for a chemical reaction

Results are presented from studies on plasma catalysis of the decomposition of methane into hydrogen and carbon in a repetitive microwave discharge. The dependence of the propagation velocity of a plasma channel on the sort of gas is determined; from this dependence, a preliminary conclusion can be drawn about both the mechanism for the development of the discharge and the ion composition of the discharge plasma. The measurements of the electron temperature in the discharge show that the rate at which active particles are produced is high enough to explain the acceleration of a chemical reaction by chain processes with the participation of these particles.     

Single-molecule Detection of Reactive Oxygen Species: Application to Photocatalytic Reactions

In this review, we have focused on the oxidation reactions of single dye molecules by reactive oxygen species (ROS). The methodologies for the single-molecule detection of ROS, such as hydroxyl radical (HO^•), singlet oxygen (O₂(a¹Δ_g)), and hydrogen peroxide (H₂O₂), have been introduced together with examples. In particular, a successful application using the single-molecule fluorescence technique for the investigation of the TiO₂ photocatalytic oxidation reactions is demonstrated in detail.     

Chemisorption and decomposition reactions of oxygen-containing organic molecules on clean Pd surfaces studied by UV photoemission

We have used uv photoeinission (primarily at a photon energy hv = 40.8 eV) to study chemisorption and decomposition reactions of small oxygen-containing organic molecules on clean polycrystalline Pd surfaces at 120 and 300 K. These molecules include methanol (CH₃OH), dimethyl ether (CH₃OCH₃) formaldehyde (H₂CO), acetaldehyde [H(CH₃)CO], and acetone [(CH₃)₂CO]. Chemisorption bonding of these molecules to the Pd surface occurs primarily through the lone-pair orbitais associated with the oxygen atoms, as evidenced by chemical bonding shifts of these orbitais toward larger electron binding energy relative to the other adsorbate valence orbitals. At 300 K all the molecules studied decompose on the surface, resulting in chemisorbed CO. Since chemisorbed (as well as condensed) phases of some of these molecules (CH₃OH and H(CH₃)CO) are observed at low temperature, the decomposition to CO is a thermally-activated reaction. The observed orbital shifts associated with chemisorption bonding are used to make rough estimates of interaction strengths and chemisorption bond energies (within the framework of Mulliken's theory of electron donor-acceptor complexes as applied to chemisorption by Grimley). The resulting heats of chemisorption are consistent with the observed surface reactions.     

Controlling oxygen vacancies and properties of ZnO

Intrinsic defects in semiconductors play crucial roles on their electrical and optical properties. In this article, we report on a facile method to control concentration of oxygen vacancies inside ZnO nanostructures and related physical properties based on adjustment of thermal transformation conditions from ZnO₂ to ZnO, including annealing atmosphere and temperature. ZnO₂ spheres assembled with nanoparticles were formed through the reaction between zinc nitrate and hydrogen peroxide. Significantly, it was found that the adopted temperature and atmosphere have remarkable impact on the concentration of oxygen vacancies, which was revealed by the variations of featured Raman scattering peaks at 584 cm⁻¹. Furthermore, with the increase of oxygen vacancies inside ZnO, the optical band-gap was found to red-shift 350 meV and the room-temperature ferromagnetism became stronger up to 1.6 emu/mg. The defect formation and evolution were discussed according to the chemical equilibrium of decomposition reaction under special local heating environment. This work demonstrated that ZnO₂ decomposition is an effective process to control the defect states inside ZnO and related properties.     

Electromagnetic properties and microwave absorption properties of BaTiO3-carbonyl iron composite in S and C bands

BaTiO₃ powders are prepared by sol-gel method. The carbonyl iron powder is prepared via thermal decomposition of iron pentacarbonyl. Then BaTiO₃-carbonyl iron composite with different mixture ratios was prepared using the as-prepared material. The structure, morphology, and properties of the composites are characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction, scanning electron microscopy (SEM), and a network analyzer. The complex permittivity and reflection loss of the composites have been measured at different microwave frequencies in S- and C-bands employing vector network analyzer model PNA 3629D vector. The effect of the mass ratio of BaTiO₃/carbonyl iron on the microwave loss properties of the composites is investigated. A possible microwave absorbing mechanism of BaTiO₃-carbonyl iron composite has been proposed. The BaTiO₃-carbonyl iron composite can find applications in suppression of electromagnetic interference, and reduction of radar signature.     

An XPS study of the water adsorption on Cu(110)

The adsorption of H₂O on clean and oxygen precovered Cu(110) is studied at temperatures between 90 and 300 K by XPS. On the oxygen precovered surface three O(1s) emission lines are detected at lower temperature. They originate from adsorbed atomic oxygen, from OH groups, and from H₂O molecules. For an oxygen coverage of half a monolayer, XPS indicates that during H₂O decomposition the preadsorbed oxygen does not directly participate in the OH formation. After water adsorption on the clean surface three O(1s) emission lines are found, which are due to H₂O molecules, “disturbed” H₂O molecules, and OH groups. The XPS results are directly correlated with information about the adsorbates obtained by UPS. Coverages are determined from the XPS spectra for the detected species.     

Born-Oppenheimer breakdown effects and hyperfine structure in the rotational spectra of SbF and SbCl

Pure rotational spectra have been measured for the ground electronic states of SbF and SbCl. The molecules were prepared by laser ablation of Sb metal in the presence of SF₆ or Cl₂, respectively. Their spectra were measured with a cavity pulsed jet Fourier transform microwave spectrometer. Although both molecules have two unpaired electrons, they are subject to Hund’s coupling case (c), and have X₁0⁺ ground states. The spectra have been interpreted with the formalism of ¹Σ⁺ molecules. For both molecules spectra of several isotopomers have been measured in the ground and first excited vibrational states. Large hyperfine splittings attributable to both nuclear quadrupole coupling and nuclear spin-rotation coupling have been observed. A Dunham-type analysis has produced unusually large Born-Oppenheimer breakdown parameters, which are interpreted in terms of the electronic structures of the molecules.     

Announcement

The He(I) and He(II) photoelectron spectra of benzoyl chloride have been subjected to a relative band intensity analysis. The spectral assignment, which is supported by theoretical ionization cross-sections calculated from a GAUSSIAN 70 wavefunction, illustrates the use of He(II) radiation for molecules containing heavy atoms. The low ionization potentials of benzoyl chloride are described in molecular orbital terms as π < π < n_o < n_C? ～ ⁿ_Cl.     

Effective detection of benzoyl peroxide in flour based on parameter selection of Raman hyperspectral system

Penetration depth and spatial resolution of Raman hyperspectral imaging system were studied for effective detection of benzoyl peroxide in flour. The determinations of parameters were achieved by using the single-band background-correct image of a benzoyl peroxide Raman characteristic band and a simple threshold method. The selected parameters were used to detect mixture samples with different concentrations. Percentage of detected benzoyl peroxide pixels was positively correlated to its concentration. The result shows that parameters selected in this study are effective for the detection of benzoyl peroxide additive in flour and can be used for quantitative analysis in the future.