Radical Decomposition Research Articles

Energy characteristics of chemical bonds and radical reorganizations

Based on relationships between the mean thermochemical bond energies (MTBEs), bond dissociation energies, and enthalpies of atomization, a method have been proposed for calculation of MTBEs and the energies of reorganization of molecular fragments into radicals (radical reorganization energies) in the course of a monomolecular radical decomposition reaction. The method makes it possible to calculate the radical reorganization energies and MTBEs of organic and organometallic compounds, including polyradicals and aromatic compounds.

Opposite effects of a singlet oxygen quencher on photochemical degradation of dicyano-substituted poly(phenylenevinylenes) with different side chains

RO-diCN-PPV and C8-diCN-PPV, poly(1,4-phenylene-1,2-dicyanovinylene) with alkoxy and octyl side chains, have recently been shown to photodegrade via a singlet oxygen mechanism, and RO-diCN-PPV is seven times more stable. To improve photostability, 1,4-diazabicyclo[2.2.2]octane (DABCO), a singlet oxygen quencher, was used as a dopant. To our surprise, DABCO exhibited opposite effects on their photodegradation. With 15 mol% DABCO, degradation rate of C8-diCN-PPV decreased by 65%, while that of RO-diCN-PPV increased by 246%. The DABCO content in C8-diCN-PPV film remained unchanged during 20 min of illumination, but mostly disappeared in RO-diCN-PPV in only 5 min due to decomposition. IR and MW analysis results suggest that DABCO slowed down degradation of C8-diCN-PPV without altering the mechanism, but accelerated RO-diCN-PPV photodegradation by initiating a radical process. C8-diCN-PPV's HOMO energy is lower than that of DABCO by 1.78 eV, a gap too wide for efficient electron transfer to happen. On the other hand, the HOMO of RO-diCN-PPV is only lower by 1.14 eV, allowing DABCO to donate electron to photoexcited RO-diCN-PPV to initiate a radical process that damaged the polymer and destroyed DABCO itself. It was also found that, in RO-diCN-PPV, radical decomposition takes very different paths from those of RO-PPVs and produce very different products.

Optimal classification for the diagnosis of duchenne muscular dystrophy images using support vector machines.

This study aimed to investigate the optimal support vector machines (SVM)-based classifier of duchenne muscular dystrophy (DMD) magnetic resonance imaging (MRI) images. T1-weighted (T1W) and T2-weighted (T2W) images of the 15 boys with DMD and 15 normal controls were obtained. Textural features of the images were extracted and wavelet decomposed, and then, principal features were selected. Scale transform was then performed for MRI images. Afterward, SVM-based classifiers of MRI images were analyzed based on the radical basis function and decomposition levels. The cost (C) parameter and kernel parameter [Formula: see text] were used for classification. Then, the optimal SVM-based classifier, expressed as [Formula: see text]), was identified by performance evaluation (sensitivity, specificity and accuracy). Eight of 12 textural features were selected as principal features (eigenvalues [Formula: see text]). The 16 SVM-based classifiers were obtained using combination of (C, [Formula: see text]), and those with lower C and [Formula: see text] values showed higher performances, especially classifier of [Formula: see text]). The SVM-based classifiers of T1W images showed higher performance than T1W images at the same decomposition level. The T1W images in classifier of [Formula: see text]) at level 2 decomposition showed the highest performance of all, and its overall correct sensitivity, specificity, and accuracy reached 96.9, 97.3, and 97.1%, respectively. The T1W images in SVM-based classifier [Formula: see text] at level 2 decomposition showed the highest performance of all, demonstrating that it was the optimal classification for the diagnosis of DMD.

Products and Mechanism of the Reaction of 1-Pentadecene with NO3 Radicals and the Effect of a -ONO2 Group on Alkoxy Radical Decomposition.

The linear C15 alkene, 1-pentadecene, was reacted with NO3 radicals in a Teflon environmental chamber and yields of secondary organic aerosol (SOA) and particulate β-hydroxynitrates, β-carbonylnitrates, and organic peroxides (β-nitrooxyhydroperoxides + dinitrooxyperoxides) were quantified using a variety of methods. Reaction occurs almost solely by addition of NO3 to the C═C double bond and measured yields of β-hydroxynitrate isomers indicate that 92% of addition occurs at the terminal carbon. Molar yields of reaction products determined from measurements, a proposed reaction mechanism, and mass-balance considerations were 0.065 for β-hydroxynitrates (0.060 and 0.005 for 1-nitrooxy-2-hydroxypentadecane and 1-hydroxy-2-nitrooxypentadecane isomers), 0.102 for β-carbonylnitrates, 0.017 for organic peroxides, 0.232 for β-nitrooxyalkoxy radical isomerization products, and 0.584 for tetradecanal and formaldehyde, the volatile C14 and C1 products of β-nitrooxyalkoxy radical decomposition. Branching ratios for decomposition and isomerization of β-nitrooxyalkoxy radicals were 0.716 and 0.284 and should be similar for other linear 1-alkenes ≥ C6 whose alkyl chains are long enough to allow for isomerization to occur. These branching ratios have not been measured previously, and they differ significantly from those estimated using structure-activity relationships, which predict >99% isomerization. It appears that the presence of a -ONO2 group adjacent to an alkoxy radical site greatly enhances the rate of decomposition relative to isomerization, which is otherwise negligible, and that the effect is similar to that of a -OH group. The results provide insight into the effects of molecular structure on mechanisms of oxidation of volatile organic compounds and should be useful for improving structure-activity relationships that are widely used to predict the fate of these compounds in the atmosphere and for modeling SOA formation and aging.

Ultraviolet Photodissociation Dynamics of the Allyl Radical via the B̃(2)A1(3s), C̃(2)B2(3py), and Ẽ(2)B1(3px) Electronic Excited States.

Ultraviolet (UV) photodissociation dynamics of jet-cooled allyl radical via the B̃(2)A1(3s), C̃(2)B2(3py), and Ẽ(2)B1(3px) electronically excited states are studied at the photolysis wavelengths from 249 to 216 nm using high-n Rydberg atom time-of-flight (HRTOF) and resonance-enhanced multiphoton ionization (REMPI) techniques. The photofragment yield (PFY) spectra of the H atom products are measured using both allyl chloride and 1,5-hexadiene as precursors of the allyl radical and show a broad peak centered near 228 nm, whereas the previous UV absorption spectra of the allyl radical peak around 222 nm. This difference suggests that, in addition to the H + C3H4 product channel, another dissociation channel (likely CH3 + C2H2) becomes significant with increasing excitation energy. The product translational energy release of the H + C3H4 products is modest, with the P(ET) distributions peaking near 8.5 kcal/mol and the fraction of the average translational energy in the total excess energy, ⟨fT⟩, in the range 0.22-0.18 from 249 to 216 nm. The P(ET)'s are consistent with production of H + allene and H + propyne, as suggested by previous experimental and theoretical studies. The angular distributions of the H atom products are isotropic, with the anisotropy parameter β ≈ 0. The H atom dissociation rate constant from the pump-probe study gives a lower limit of 1 × 10(8)/s. The dissociation mechanism is consistent with unimolecular decomposition of the hot allyl radical on the ground electronic state after internal conversion of the electronically excited state.

Conductivity Modulation in Polymer Electrolytes and their Composites due to Ion-Beam Irradiation

Polymers are a class of materials widely used in different fields of applications. With imminent applications of polymers, the study of radiation induced changes in polymers has become an obvious scientific demand. The bombardment by ion beam radiations has become one of the most promising techniques in present day polymer research. When the polymers are irradiated, a variety of physical and chemical changes takes place due to energy deposition of the radiation in the polymer matrix. Scissoring, cross-linking, recombination, radical decomposition, etc. are some of the interesting changes that are obvious in polymers. The modification in polymer properties by irradiation depends mainly on the nature of radiation and the type of polymer used.Polymer electrolytes are obtained by modifying polymers by doping, complexing, or other chemical processes. In general, they suffer from low conductivity due to high crystallinity of the matrix. The effect of radiation on polymer electrolyte is expected to alter their crystalline nature vis-a-vis electrical properties. This review article shall elaborate modifications in the physical and chemical properties of polymer electrolytes and their composites. The variations in properties have been explored on PEO based polymer electrolyte and correlated with the parameters responsible for such changes. Also a comparison with different types of the polymers irradiated with a wide range of ion beams has been established.

Effect of molecular structure of nitroalkanes on the C-NO2 bond strength and activation energy of the gas-phase radical decomposition according to the quantum-chemical simulation

The non-empirical (MP2 and CCSD), composite (G4), and DFT (B3LYP, CAM-B3LYP, B98, and wB97XD) methods with different basis sets have been employed to compute the formation enthalpies of a series of C1–C4 nitroalkanes and the hydrocarbon radicals formed via the C-N bond rupture as well as the C–N bond dissociation energies. For the series of isomeric nitroalkanes, the effects of the nature of the participating carbon atom (primary, secondary, or tertiary) and the size of adjacent alkyl group on the C-N bond strength have been analyzed. The reasons for different thermochemical and kinetic estimates of the C-N bond dissociation energy in nitroalkanes have been discussed.

Experimental Study on Ethane Ignition Delay Times and Evaluation of Chemical Kinetic Models

The ignition delay times of ethane were measured using a high-pressure shock tube at different pressures (p = 1.2, 5.0, and 20.0 atm) and equivalence ratios (ϕ = 0.5, 1.0, and 2.0) with different argon diluent ratios. Correlations of the measured ignition delay times were provided. The measurements were compared to calculations from several representative chemical kinetic models to evaluate their performances. Results showed that Aramco Mech 1.3 could well reproduce the measured ignition delay times over a wide range, while GRI Mech 3.0 significantly overpredicted the measurements at stoichiometric and high equivalence ratio. To find out the reasons for the differences and similarities of the mechanisms on calculating the ignition delay time of ethane, sensitivity analysis and reaction pathway analysis were conducted. It is observed that the mechanisms have similar pathway for ethane consumption, while they have significant differences for ethyl radical decomposition reactions. Results also indicated that the incompleteness of the C2H5 + O2 reaction channels and the underestimation of the rate constant of reaction C2H4 + H (+M) = C2H5 (+M) might be responsible for the overestimation of GRI Mech 3.0 on ethane ignition. The mechanisms studied give similar prediction at a high equivalence ratio because reactions with a significant difference on rate constants do not show a high sensitivity coefficient at the condition.

Novel Reaction Mechanisms Pathways in the Electron Induced Decomposition of Solid Nitromethane (CH3NO2) and D3-Nitromethane (CD3NO2)

Icy films of nitromethane (CH3NO2) together with D3-nitromethane (CD3NO2) were exposed to ionizing radiation to investigate the mechanisms in the decomposition of (D3)-nitromethane. The radiation induced modification of the ices was followed during the radiation exposure in situ via infrared spectroscopy (condensed phase) and via temperature-programmed desorption (TPD) exploiting reflectron time-of-flight mass spectrometry joined with single photon ionization of the subliming molecules at 10.49 eV (gas phase). The infrared spectroscopic and kinetics studies reveal the synthesis of methyl nitrite (CH3ONO) via isomerization of nitromethane (CH3NO2) as well as the presence of the molecular (formaldehyde (H2CO), nitrosylhydride (HNO)) and radical decomposition pathways (methoxy radical (CH3O), nitrogen monoxide (NO)) accounting for about 85% of all products formed. Here, TPD studies exploiting reflectron time-of-flight mass spectroscopy together with single photon ionization exposed three classes of complex m...

Nitric Oxide Is Reduced to HNO by Proton-Coupled Nucleophilic Attack by Ascorbate, Tyrosine, and Other Alcohols. A New Route to HNO in Biological Media?

The role of NO in biology is well established. However, an increasing body of evidence suggests that azanone (HNO), could also be involved in biological processes, some of which are attributed to NO. In this context, one of the most important and yet unanswered questions is whether and how HNO is produced in vivo. A possible route concerns the chemical or enzymatic reduction of NO. In the present work, we have taken advantage of a selective HNO sensing method, to show that NO is reduced to HNO by biologically relevant alcohols with moderate reducing capacity, such as ascorbate or tyrosine. The proposed mechanism involves a nucleophilic attack to NO by the alcohol, coupled to a proton transfer (PCNA: proton-coupled nucleophilic attack) and a subsequent decomposition of the so-produced radical to yield HNO and an alkoxyl radical.

Ab Initio Chemical Kinetics for the CH3 + O((3)P) Reaction and Related Isomerization-Decomposition of CH3O and CH2OH Radicals.

The kinetics and mechanism of the CH3 + O reaction and related isomerization-decomposition of CH3O and CH2OH radicals have been studied by ab initio molecular orbital theory based on the CCSD(T)/aug-cc-pVTZ//CCSD/aug-cc-pVTZ, CCSD/aug-cc-pVDZ, and G2M//B3LYP/6-311+G(3df,2p) levels of theory. The predicted potential energy surface of the CH3 + O reaction shows that the CHO + H2 products can be directly generated from CH3O by the TS3 → LM1 → TS7 → LM2 → TS4 path, in which both LM1 and LM2 are very loose and TS7 is roaming-like. The result for the CH2O + H reaction shows that there are three low-energy barrier processes including CH2O + H → CHO + H2 via H-abstraction and CH2O + H → CH2OH and CH2O + H → CH3O by addition reactions. The predicted enthalpies of formation of the CH2OH and CH3O radicals at 0 K are in good agreement with available experimental data. Furthermore, the rate constants for the forward and some key reverse reactions have been predicted at 200-3000 K under various pressures. Based on the new reaction pathway for CH3 + O, the rate constants for the CH2O + H and CHO + H2 reactions were predicted with the microcanonical variational transition-state/Rice-Ramsperger-Kassel-Marcus (VTST/RRKM) theory. The predicted total and individual product branching ratios (i.e., CO versus CH2O) are in good agreement with experimental data. The rate constant for the hydrogen abstraction reaction of CH2O + H has been calculated by the canonical variational transition-state theory with quantum tunneling and small-curvature corrections to be k(CH2O + H → CHO + H2) = 2.28 × 10(-19) T(2.65) exp(-766.5/T) cm(3) molecule(-1) s(-1) for the 200-3000 K temperature range. The rate constants for the addition giving CH3O and CH2OH and the decomposition of the two radicals have been calculated by the microcanonical RRKM theory with the time-dependent master equation solution of the multiple quantum well system in the 200-3000 K temperature range at 1 Torr to 100 atm. The predicted rate constants are in good agreement with most of the available data.

The thermal decomposition of the benzyl radical in a heated micro-reactor. I. Experimental findings.

The pyrolysis of the benzyl radical has been studied in a set of heated micro-reactors. A combination of photoionization mass spectrometry (PIMS) and matrix isolation infrared (IR) spectroscopy has been used to identify the decomposition products. Both benzyl bromide and ethyl benzene have been used as precursors of the parent species, C6H5CH2, as well as a set of isotopically labeled radicals: C6H5CD2, C6D5CH2, and C6H5 (13)CH2. The combination of PIMS and IR spectroscopy has been used to identify the earliest pyrolysis products from benzyl radical as: C5H4=C=CH2, H atom, C5H4-C ≡ CH, C5H5, HCCCH2, and HC ≡ CH. Pyrolysis of the C6H5CD2, C6D5CH2, and C6H5 (13)CH2 benzyl radicals produces a set of methyl radicals, cyclopentadienyl radicals, and benzynes that are not predicted by a fulvenallene pathway. Explicit PIMS searches for the cycloheptatrienyl radical were unsuccessful, there is no evidence for the isomerization of benzyl and cycloheptatrienyl radicals: C6H5CH2⇋C7H7. These labeling studies suggest that there must be other thermal decomposition routes for the C6H5CH2 radical that differ from the fulvenallene pathway.

Kinetic and mechanistic investigations of the thermal decomposition of methyl-substituted cycloalkyl radicals

We perform systemic theoretical investigations on the thermal decomposition of 2-Me-cyclobutyl, 2-Me-cyclopentyl and 2-Me-cyclohexyl radicals at CBS-QB3 and CCSD(T) levels.

Thermal degradation kinetics and decomposition mechanism of polyesters based on 2,5-furandicarboxylic acid and low molecular weight aliphatic diols

In the present work three novel alipharomatic polyesters, namely poly(ethylene 2,5-furandicarboxylate) (PEF), poly(propylene 2,5-furandicarboxylate) (PPF), and poly(butylene 2,5-furandicarboxylate) (PBF) have been prepared by applying the two-stage melt polycondensation method. The interest for polyesters prepared from renewable resources has increased recently, since they can be synthesized using monomers, like furfural or hydroxymethylfurfural and aliphatic diols. A systematic investigation of the thermal stability and decomposition kinetics of furanoate polyesters was performed using thermogravimetric analysis (TGA) and pyrolysis-gas chromatography/mass spectroscopy (Py–GC/MS). From TGA curves and activation energies it was found that PBF is less stable thermally, than the other two polyesters. The thorough studies of evolving decomposition compounds along with the isoconversional and model-fitting analysis of mass loss data led to the proposal of a decomposition mechanism for every polyester. The decomposition mechanism of 2,5-furandicarboxylate polyesters was evaluated with Py–GC/MS and from the identified decomposition products it was found that the decomposition of these polyesters is taking place mainly via β-hydrogen bond scission and in lower extent with α-hydrogen scission. 2,5-Furandicarboxylic acid, furoic acid, allyl- and diallyl-compounds are produced in the first case while aldehydes in the second. Radical decomposition also takes place producing carbonyl compounds.

Performance of methanol kinetic mechanisms at oxy-fuel conditions

Methanol premixed flames were studied under oxy-fuel conditions for the first time. Laminar burning velocities were measured with the heat flux method at atmospheric pressure for unburnt gas temperatures of 308–358K within a stoichiometric range of ϕ=0.8–1.5. A linear relationship between temperature and laminar burning velocity on a log–log scale was observed. The experimental results are discussed by comparison to modeling results from three kinetic mechanisms. All models gave an overprediction of the laminar burning velocity. It was demonstrated that implementation of recently advised rate constants for reactions of methanol with O2, HO2, H and OH, together with modification of the third-body efficiency for H2O in the decomposition of the formyl radical, significantly improves model performance both for methanol combustion in air and at oxy-fuel conditions.

Theoretical investigation of the hydrogen shift reactions in peroxy radicals derived from the atmospheric decomposition of 3-methyl-3-buten-1-ol (MBO331)

The hydroxy peroxy radical derived from the oxidation of 3-methyl-3-buten-1-ol (MBO331), can undergo four different hydrogen shift (H-shift) reactions. We have compared optimized geometries, barrier heights and reaction rate constants obtained with five different DFT functionals (BLYP, B3LYP, BHandHLYP, wB97X-D and M06-2X) with the aug-cc-pVTZ basis set. We found that the single-point CCSD(T)-F12A/VDZ-F12 energies calculated at the different DFT geometries had very similar barrier heights. The wB97X-D, M06-2X and CCSD(T)-F12A/VDZ-F12 barrier heights are comparable. The atmospheric decomposition of the MBO331 peroxy radical was found to undergo a 1,5-CH H-shift reaction with a reaction rate constant of about 1s−1.

Studies of the electrochemical behavior of moniliformin mycotoxin and its sensitive determination at pretreated glassy carbon electrodes in a non-aqueous medium

The electrochemical oxidation of moniliformin (MON) mycotoxin in acetonitrile (ACN)+0.1moldm−3 (C4H9)4NPF6 at electrochemically pretreated glassy carbon electrodes (EPGCE) and fiber disk carbon ultramicroelectrodes (FCUME) is studied for the first time. Electrochemical techniques used were cyclic (CV), convolution and square wave (SWV) voltammetries as well as controlled potential electrolysis. Experimental results allowed inferring a complex electro-oxidation mechanism with adsorption of reactant and homogeneous chemical steps coupled to the electron transfer reaction.The UV–Vis spectroscopy was used as a non-electrochemical technique in order to follow the disappearance/appearance of the reactant/reaction product/s during controlled potential electrolysis.The most probable electro-oxidation mechanism proposed on the base of digital simulation results of cyclic voltammograms is one of the CEC type. Thermodynamic and kinetics parameters were calculated from the fitting of experimental cyclic voltammograms, while the diffusion parameter was calculated from convoluted cyclic voltammograms. We propose that reaction products obtained are produced through the decomposition of the MON radical generated from the electrochemical oxidation of MON anion.On the other hand, the MON quantitative determination was performed using adsorptive SWV at EPGCE. An extremely low detection limit of 1×10−11moldm−3 (1.2ppt) for a signal to noise ratio of 3:1 was calculated.

Experimental Study of the Reaction of Isopropyl Nitrate with OH Radicals: Kinetics and Products

ABSTRACTThe kinetics of the reaction of isopropyl nitrate (IPN) with OH radicals has been studied using a low‐pressure flow tube reactor coupled with a quadrupole mass spectrometer: OH + (CH3)2CHONO2 → products (2). The rate constant of the title reaction was determined using both the absolute method, monitoring the kinetics of OH radicals consumption in excess of IPN, and the relative rate method using the reaction of OH with Br2 as reference one and following HOBr formation. As a result of the absolute and relative measurements, the overall rate coefficients, k2 = (6.6 ± 1.2) × 10−13 exp(–(233 ± 56)/) was determined at a pressure of 1 Torr of helium over the temperature range 268–355 K. Acetone, resulting from H‐atom abstraction from the tertiary C–H bond of IPN followed by 2‐nitroxy‐2‐propyl radical decomposition, was found to be a major reaction product with the yield of 0.82 ± 0.13, independent of temperature in the range 277–355 K.

Photoionization mass spectrometric measurements of initial reaction pathways in low-temperature oxidation of 2,5-dimethylhexane.

Product formation from R + O2 reactions relevant to low-temperature autoignition chemistry was studied for 2,5-dimethylhexane, a symmetrically branched octane isomer, at 550 and 650 K using Cl-atom initiated oxidation and multiplexed photoionization mass spectrometry (MPIMS). Interpretation of time- and photon-energy-resolved mass spectra led to three specific results important to characterizing the initial oxidation steps: (1) quantified isomer-resolved branching ratios for HO2 + alkene channels; (2) 2,2,5,5-tetramethyltetrahydrofuran is formed in substantial yield from addition of O2 to tertiary 2,5-dimethylhex-2-yl followed by isomerization of the resulting ROO adduct to tertiary hydroperoxyalkyl (QOOH) and exhibits a positive dependence on temperature over the range covered leading to a higher flux relative to aggregate cyclic ether yield. The higher relative flux is explained by a 1,5-hydrogen atom shift reaction that converts the initial primary alkyl radical (2,5-dimethylhex-1-yl) to the tertiary alkyl radical 2,5-dimethylhex-2-yl, providing an additional source of tertiary alkyl radicals. Quantum-chemical and master-equation calculations of the unimolecular decomposition of the primary alkyl radical reveal that isomerization to the tertiary alkyl radical is the most favorable pathway, and is favored over O2-addition at 650 K under the conditions herein. The isomerization pathway to tertiary alkyl radicals therefore contributes an additional mechanism to 2,2,5,5-tetramethyltetrahydrofuran formation; (3) carbonyl species (acetone, propanal, and methylpropanal) consistent with β-scission of QOOH radicals were formed in significant yield, indicating unimolecular QOOH decomposition into carbonyl + alkene + OH.

Theoretical investigation on the atmospheric fate of CF3C(O)OCH 2O radical: alpha-ester rearrangement vs oxidation at 298 K.

A theoretical study on the mechanism of the thermal decomposition of CF(3)C(O)OCH(2)O radical is presented for the first time. Geometry optimization and frequency calculations were performed at the MPWB1K/6-31 + G(d, p) level of theory and energetic information further refined by calculating the energy of the species using G2(MP2) theory. Three plausible decomposition pathways including α-ester rearrangement, reaction with O(2) and thermal decomposition (C-O bond scission) were considered in detail. Our results reveal that reaction with O(2) is the dominant path for the decomposition of CF(3)C(O)OCH(2)O radical in the atmosphere, involving the lowest energy barrier, which is in accord with experimental findings. Our theoretical results also suggest that α-ester rearrangement leading to the formation of trifluoroacetic acid TFA makes a negligible contribution to decomposition of the title alkoxy radical. The thermal rate constants for the above decomposition pathways were evaluated using canonical transition state theory (CTST) at 298 K.