Mechanism of hydroxyl radical-induced breakdown of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-d)

Oxidative transformations by the hydroxyl radical are significant in advanced oxidation processes for the breakdown of organic pollutants, yet mechanistic details of the reactions are lacking. A combination of experimental and computational methods has been employed in this study to elucidate the reactivity of the hydroxyl radical with the widely used herbicide 2,4-D (2,4-dichlorophenoxyacetic acid). The experimental data on the reactivity of the hydroxyl radical in the degradation of the herbicide 2,4-D were obtained from gamma-radiolysis experiments with both (18)O-labeled and unlabeled water. These were complemented by computational studies of the (.)OH attack on 2,4-D and 2,4-DCP (2,4-dichlorophenol) in the gas phase and in solution. These studies firmly established the kinetically controlled attack ipso to the ether functionality as the main reaction pathway of (.)OH and 2,4-D, followed by homolytic elimination of the ether side chain. In addition, the majority of the early intermediates in the reaction between the hydroxyl radical and 2,4-DCP, the major intermediate, were identified experimentally. While the hydroxyl radical attacks 2,4-D by (.)OH-addition/elimination on the aromatic ring, the oxidative breakdown of 2,4-DCP occurs through (.)OH addition followed by either elimination of chlorine or formation of the ensuing dichlorophenoxyl radical.     

Hydroxyl radicals in ice: insights into local structure and dynamics

The hydroxyl radical and its reactivity within ice environments are crucial to many important atmospheric reactions. The associated molecular mechanisms are largely unknown due to challenges posed by direct experimental measurements and computational studies of this transient species. Here we report insights into the local structure and behaviour of the hydroxyl radical in bulk ice through an extensive study utilizing Car-Parrinello molecular dynamics simulations. Interstitial and in-lattice hydroxyl radicals in hexagonal ice were investigated at primarily 190 K. Our findings, utilizing both HCTH/120 and BLYP functionals, show that OH* can exhibit greater mobility than other ice defects (the trapping energy estimated to be only 0.09 eV). We observe the formation of a two-center three-electron hemibond structure between the hydroxyl radical and an in-lattice water molecule; while controversial, such a structure in ice may be amenable to experimental detection due to its relative stability. Our results show that interstitial water molecules can strongly influence the mobility of the hydroxyl radical in bulk ice through the displacement of the radical to an interstitial location. We also demonstrate that the H-transfer reaction from an interstitial water to the radical is a rare event in ice. Together, these results predict that the radical can be a reactive species in bulk ice, as both interstitial and in-lattice OH* can be available for reactions with other species. These microscopic insights should contribute to our understanding of the reactivity of OH* in ice and its implications to atmospheric reactions.     

Kinetic and product study of the gas-phase reactions of OH radicals, NO(3) radicals, and O(3) with (C(2)H(5)O)(2)P(S)CH(3) and (C(2)H(5)O)(3)PS

Rate constants for the reactions of OH radicals and NO3 radicals with O,O-diethyl methylphosphonothioate [(C(2)H(5)O)(2)P(S)CH(3); DEMPT] and O,O,O-triethyl phosphorothioate [(C(2)H(5)O)(3)PS; TEPT] have been measured using relative rate methods at atmospheric pressure of air over the temperature range 296-348 K for the OH radical reactions and at 296 +/- 2 K for the NO(3) radical reactions. At 296 +/- 2 K, the rate constants obtained for the OH radical reactions (in units of 10(-11) cm(3) molecule(-1) s(-1)) were 20.4 +/- 0.8 and 7.92 +/- 0.27 for DEMPT and TEPT, respectively, and those for the NO(3) radical reactions (in units of 10(-15) cm(3) molecule(-1) s(-1)) were 2.01 +/- 0.20 and 1.03 +/- 0.10, respectively. Upper limits to the rate constants for the reactions of O(3) with DEMPT and TEPT of <6 x 10(-20) cm(3) molecule(-1) s(-1) were determined in each case. Rate constants for the OH radical reactions, measured relative to k(OH + alpha-pinene) = 1.21 x 10(-11) e(436/T) cm(3) molecule(-1) s(-1), resulted in the Arrhenius expressions k(OH + DEMPT) = 1.08 x 10(-11) e(871+/-25)/T cm(3) molecule(-1) s(-1) and k(OH + TEPT) = 8.21 x 10(-13) e(1353+/-49)/T cm(3) molecule(-1) s(-1) over the temperature range 296-348 K, where the indicated errors are two least-squares standard deviations and do not include the uncertainties in the reference rate constant. Diethyl methylphosphonate was identified and quantified from the OH radical and NO(3) radical reactions with DEMPT, with formation yields of 21 +/- 4%, independent of temperature, from the OH radical reaction and 62 +/- 11% from the NO(3) radical reaction at 296 +/- 2 K. Similarly, triethyl phosphate was identified and quantified from the OH radical and NO(3) radical reactions with TEPT, with formation yields of 56 +/- 9%, independent of temperature, from the OH radical reaction and 78 +/- 15% from the NO(3) radical reaction at 296 +/- 2 K.     

A computational study of the reactions of a beta-diketiminatoaluminium(I) complex with the hydrogen atom and the electron

A computational study has been performed to examine the reactions of a model beta-diketiminatoaluminium (I) complex with the hydrogen atom and with the electron. It was found that the hydrogen atom adds to the metal centre exothermically (DeltaH(rxn)=-202 kJ mol(-1)), and the spin density in the resulting radical resides entirely on the beta-diketiminato ligand. The spin density of the corresponding radical anion is very similar to the H-adduct.     

A computational methodology for accurate predictions of rate constants in solution: Application to the assessment of primary antioxidant activity

The accurate prediction of rate constants for chemical reactions in solution, using computational methods, is a challenging task. In this work, a computational protocol designed to be a reliable tool in the study of radical‐molecule reactions in solution is presented. It is referred to as quantum mechanics‐based test for overall free radical scavenging activity (QM‐ORSA) because it is mainly intended to provide a universal and quantitative way of evaluating the free radical scavenging activity of chemical compounds. That is, its primary antioxidant activity. However, it can also be successfully applied to obtain accurate kinetic data for other chemical reactions in solution. The QM‐ORSA protocol has been validated by comparison with experimental results, and its uncertainties have been proven to be no larger than those arising from experiments. Further applications of QM‐ORSA are expected to contribute increasing the kinetic data for free radical‐molecule reactions relevant to oxidative stress, which is currently rather scarce. © 2013 Wiley Periodicals, Inc.     

Theoretical study of the conversion of sulfonyl precursors into chains of poly(p-phenylene vinylene)

The elimination and side reactions involved in the thermal conversion of sulfonyl precursor chains into poly(p-phenylene vinylene) (PPV) have been studied in detail, using Density Functional theory, along with the MPW1K functional. The performance of the MPW1K functional for describing radical dissociation and internal conversion reactions of sulfonyl precursors has been assessed against the results of benchmark CCSD(T) calculations. Enthalpies as well as entropies are calculated at different temperatures at the level of the rigid rotor-harmonic oscillator approximation. Entropy effects on internal elimination reactions are very limited. In sharp contrast, at the temperatures under which the conversion is usually performed (550 K), entropy contributions to the activation energies are found to be very significant and to strongly favor direct radical dissociations of the precursors. Further radical side reactions following an E(i) conversion through an alkyl substituent may also significantly contribute to the formation of sp(3) defects and/or cross-linked structures in the polymer-an advantageous feature for the making of materials with improved photoluminescence efficiencies.     

A combined theoretical and experimental study of efficient and fast titanocene-catalyzed 3-exo cyclizations

The mechanism of titanocene mediated 3-exo cyclizations was investigated by a combined theoretical and experimental study. A gradient corrected density functional theory (DFT) method has been scaled against titanocene dichloride, the parent butenyl radical, and in bond dissociation energy (BDE) calculations. The BP86 method using density fitting, and a basis set of triple-zeta quality emerged as a highly reliable tool for studying titanocene mediated radical reactions. The computational results revealed important kinetic and thermodynamic features of cyclopropane formation. Surprisingly, the beta-titanoxy radicals, the first intermediates of our investigations, were demonstrated to possess essentially the same thermodynamic stabilization as the corresponding alkyl radicals by comparison of the calculated BDEs. In contrast to suggestions for samarium mediated reactions, the cyclization was shown to be thermodynamically favorable in agreement with earlier kinetic studies. It was established that stereoselectivity of the cyclization is governed by the stability of the intermediates and thus the trans disubstituted products are formed preferentially. The observed ratios of products are in good to excellent agreement with the DFT results. By a combination of computational and experimental results, it was also shown that for the completion of the overall cyclopropane formation the efficiency of the trapping of the cyclopropylcarbinyl radicals is decisive.     

Kinetic Monte Carlo model of charge transport in hematite (alpha-Fe(2)O(3))

The mobility of electrons injected into iron oxide minerals via abiotic and biotic electron transfer processes is one of the key factors that control the reductive dissolution of such minerals. Building upon our previous work on the computational modeling of elementary electron transfer reactions in iron oxide minerals using ab initio electronic structure calculations and parametrized molecular dynamics simulations, we have developed and implemented a kinetic Monte Carlo model of charge transport in hematite that integrates previous findings. The model aims to simulate the interplay between electron transfer processes for extended periods of time in lattices of increasing complexity. The electron transfer reactions considered here involve the IIIII valence interchange between nearest-neighbor iron atoms via a small polaron hopping mechanism. The temperature dependence and anisotropic behavior of the electrical conductivity as predicted by our model are in good agreement with experimental data on hematite single crystals. In addition, we characterize the effect of electron polaron concentration and that of a range of defects on the electron mobility. Interaction potentials between electron polarons and fixed defects (iron substitution by divalent, tetravalent, and isovalent ions and iron and oxygen vacancies) are determined from atomistic simulations, based on the same model used to derive the electron transfer parameters, and show little deviation from the Coulombic interaction energy. Integration of the interaction potentials in the kinetic Monte Carlo simulations allows the electron polaron diffusion coefficient and density and residence time around defect sites to be determined as a function of polaron concentration in the presence of repulsive and attractive defects. The decrease in diffusion coefficient with polaron concentration follows a logarithmic function up to the highest concentration considered, i.e., approximately 2% of iron(III) sites, whereas the presence of repulsive defects has a linear effect on the electron polaron diffusion. Attractive defects are found to significantly affect electron polaron diffusion at low polaron to defect ratios due to trapping on nanosecond to microsecond time scales. This work indicates that electrons can diffuse away from the initial site of interfacial electron transfer at a rate that is consistent with measured electrical conductivities, but that the presence of certain kinds of defects will severely limit the mobility of donated electrons.     

Curing theory of Af- Ag type free radical polymerization (III)

Evaluation of defects in the polymer network is important to characterize the polymer materials, in which there always exist the defects that affect the physical and chemical properties of polymer network. Taking A_f- A_g type nonlinear free radical polymerization as an example, one type of defects called dangling loops in the gel network is investigated by means of the statistical theory of polymeric reactions. The number of dangling loops and the probability of its formation are obtained by analyzing the polymer network structure in detail.     

Kinetics and mechanism of oxidation of quinols by tris(1,10-phenanthroline) iron(III) complexes in aqueous acidic perchlorate media

The kinetics of oxidation of several substituted quinols by a series of Tris(1,10-phenanthroline)iron(III) complexes has been investigated with a stopped-flow technique at 6.0 and 20.0°C. The reactions were found to be first order on both reactants and independent of acidity. The second-order specific rate constants were strongly dependent on free energy of reaction. An interpretation of the mechanism in the light of Marcus theory has been developed. The first electron abstraction with semiquinone radical formation has been suggested as the rate-determining step, and on this basis, intrinsic parameters of the reactions have been derived. A good agreement was found between experimental and computed data.     

Selective alkane C-H-bond functionalizations utilizing oxidative single-electron transfer and organocatalysis

Alkane C-H-bond functionalization methods not utilizing metal-catalysis are discussed based on experimental and computational data, beginning with molecule-induced homolysis (reactions of alkanes with dioxiranes). Electrophilic reactions are elaborated next with an emphasis on mechanistic details that reveal that many so-called electrophilic C-H or C-C-bond insertions can be rationalized by electron-transfer reactions (inner sphere, H-coupled, and outer sphere). Finally, radical functionalizations utilizing carbon-centered (relatively stable) radicals generated under organocatalytic (phase-transfer catalysis, PTC) conditions are presented as valuable alternatives to other radical-chain alkane functionalizations. The remarkable chemo- and regioselectivities of these PTC radical reactions and the tolerance of high strain in certain aliphatic hydrocarbons make them particularly useful for laboratory-scale halogenations, in particular, iodinations of unactivated alkane C-H-bonds.     

Computational study on reaction mechanisms and kinetics of RNCN (R = H, F, Cl, Br, CH3) radicals with NO

We carried out a computational study of radical reactions of RNCN (R = H, F, Cl, Br, CH(3)) + NO to investigate how the substitution can influence their corresponding energy barriers and rate coefficients. The preferable reactive sites of RNCN radicals with various substituents are calculated by employing the Fukui functions and hard-and-soft acid-and-base theory, which were generally proved to be successful in the prediction and interpretation of regioselectivity in various types of electrophilic and nucleophilic reactions. Our calculated results clearly show that if the substituted RNCN radical has electron-donating substituent (for R = CH(3)), its corresponding barrier heights for transition states will be substantially decreased. The possible explanations of the observed increase and/or decrease in the energy barriers for the varied substituted RNCN radicals are also analyzed in this article.     

Curing theory of Af- Ag type free radical polymerization (III)

Evaluation of defects in the polymer network is important to characterize the polymer materials, in which there always exist the defects that affect the physical and chemical properties of polymer network. Taking A_f- A_g type nonlinear free radical polymerization as an example, one type of defects called dangling loops in the gel network is investigated by means of the statistical theory of polymeric reactions. The number of dangling loops and the probability of its formation are obtained by analyzing the polymer network structure in detail.     

Reactivity of (eta(6)-arene)tricarbonylchromium complexes toward additions of anions, cations, and radicals.

A computational and experimental study of additions of electrophiles, nucleophiles, and radicals to tricarbonylchromium-complexed arenes is reported. Competition between addition to a complexed arene and addition to a noncomplexed arene was tested using 1,1-dideuterio-1-iodo-2-((phenyl)tricarbonylchromium)-2-phenylethane. Reactions under anionic and cationic conditions give exclusive formation of 1,1-dideuterio-1-((phenyl)tricarbonylchromium)-2-phenylethane arising from addition to the complexed arene. Radical conditions (SmI(2)) afford two isomeric products, reflecting a 2:1 preference for radical addition to the noncomplexed arene. In contrast, intermolecular radical addition competition experiments employing ketyl radical addition to benzene and (benzene)tricarbonylchromium show that addition to the complexed aromatic ring is faster than attack on the noncomplexed species by a factor of at least 100,000. Density functional theory calculations using the B3LYP method, employing a LANL2DZ basis set for geometry optimizations and a DZVP2+ basis set for energy calculations, for all three reactive intermediates showed that tricarbonylchromium stabilizes all three types of intermediates. The computational results for anionic addition agree well with established chemistry and provide structural and energetic details as reference points for comparison with the other reactive intermediates. Intermolecular radical addition leads to exclusive reaction on the complexed arene ring as predicted by the computations. The intramolecular radical reaction involves initial addition to the complexed arene ring followed by an equilibrium leading to the observed product distribution due to a high-energy barrier for homolytic cleavage of an exo bond in the intermediate cyclohexadienyl radical complex. Mechanisms are explored for electrophilic addition to complexed arenes. The calculations strongly favor a pathway in which the cation initially adds to the metal center rather than to the arene ring.     

Investigating the Mechanism of Palladium‐Catalyzed Radical Oxidative C(sp3)−H Carbonylation: A DFT Study

Palladium (Pd)‐catalyzed radical oxidative C?H carbonylation of alkanes is a useful method for functionalizing hydrocarbons, but there is still a lack of understanding of the mechanism, which restricts the application of this reaction. In this work, density functional theory (DFT) calculations were carried out to study the mechanism for a Pd‐catalyzed radical esterification reaction. Two plausible reaction pathways have been proposed and validated by DFT calculations. The computational results reveal that the generated alkyl radical prefers to add to the carbon monoxide (CO) molecule to form a carbonyl radical before bonding with the Pd species. Radical addition onto Pd followed by CO migratory insertion was unfavorable owing to the high energy barrier of the migratory insertion step. The regioselectivity of the C(sp³)?H carbonylation was also investigated by DFT. The results show that the regioselectivity is controlled by both the bond dissociation energy of the reacting C?H bond and the stability of the corresponding generated carbon radical. Competitive side reactions also affected the yield and regioselectivity owing to the rapid consumption of the stable radical intermediate.     

A Nucleophilic Gold(III) Carbene Complex

The first Au^III carbene complex was prepared by reacting a geminal dianion with a (P,C) cyclometalated Au^III precursor. Its structure and bonding situation have been thoroughly investigated by experimental and computational means. The presence of a high‐energy highest occupied molecular orbital (HOMO) centered at the carbene center suggests nucleophilic character for the Au^III carbene complex. This unprecedented feature was confirmed by reactions with two electrophiles (PhNCS and CS₂), resulting in two types of C=C coupling reactions.     

Philicity of Acetonyl and Benzoyl Radicals: A Comparative Experimental and Computational Study

In this work, the reactivities of acetonyl and benzoyl radicals in aromatic substitution and addition reactions have been compared in an experimental and computational study. The results show that acetonyl is more electrophilic than benzoyl, which is rather nucleophilic. A Hammett plot analysis of the addition reactions of the two radicals to substituted styrenes clearly support the nucleophilicity of benzoyl, but in the case of acetonyl, no satisfactory linear correlation with a single substituent-related parameter was found. Computational calculations helped to rationalize this effect, and a good linear correlation was found with a combination of polar parameters (σ⁺) and the radical stabilization energies of the formed intermediates. Based on the calculated philicity indices for benzoyl and acetonyl, a quantitative comparison of these two radicals with many other reported radicals is possible, which may help to predict the reactivities of other aromatic radical substitution reactions.     

Predicted reaction rates of H(x)N(y)O(z) intermediates in the oxidation of hydroxylamine by aqueous nitric acid

This work reports computed rate coefficients of 90 reactions important in the autocatalytic oxidation of hydroxylamine in aqueous nitric acid. Rate coefficients were calculated using four approaches: Smoluchowski (Stokes-Einstein) diffusion, a solution-phase incarnation of transition state theory based on quantum chemistry calculations, simple Marcus theory for electron-transfer reactions, and a variational TST approach for dissociative isomerization reactions that occur in the solvent cage. Available experimental data were used to test the accuracy of the computations. There were significant discrepancies between the computed and experimental values for some key parameters, indicating a need for improvements in computational methodology. Nonetheless, the 90-reaction mechanism showed the ability to reproduce many of the trends seen in experimental studies of this very complicated kinetic system. This work highlights reactions that may govern the system evolution and branching behavior critical to the stability of the system. We hope that this analysis will guide experimental investigations to reduce the uncertainties in the critical rate coefficients and thermochemistry, allowing an unambiguous determination of the dominant reaction pathways in the system. Advances in efficient and accurate solvation models that effectively separate entropic and enthalpic contributions will most directly benefit solution-phase modeling efforts. Methods for more accurately estimating activity coefficients, including at infinite dilution in multicomponent mixtures, are needed for modeling high ionic strength aqueous systems. A detailed derivation of the solution-phase equilibrium and transition state theory rate expressions in solution is included in the Supporting Information.     

Competitive reactions of organophosphorus radicals on coke surfaces

The efficacy of organophosphorus radicals as anticoking agents was subjected to a computational study in which a representative set of radicals derived from industrially relevant organophosphorus additives was used to explore competitive reaction pathways on the graphene-like coke surface formed during thermal cracking. The aim was to investigate the nature of the competing reactions of different organophosphorus radicals on coke surfaces, and elucidate their mode of attack and inhibiting effect on the forming coke layer by use of contemporary computational methods. Density functional calculations on benzene and a larger polyaromatic hydrocarbon, namely, ovalene, showed that organophosphorus radicals have a high propensity to add to the periphery of the coke surface, inhibiting methyl radical induced hydrogen abstraction, which is known to be a key step in coke growth. Low addition barriers reported for a phosphatidyl radical suggest competitive aptitude against coke formation. Moreover, organophosphorus additives bearing aromatic substituents, which were shown to interact with the coke surface through dispersive π-π stacking interactions, are suggested to play a nontrivial role in hindering further stacking among coke surfaces. This may be the underlying rationale behind experimental observation of softer coke in the presence of organophosphorus radicals. The ultimate goal is to provide information that will be useful in building single-event microkinetic models. This study presents pertinent information on potential reactions that could be taken up in these models.     

Quantum Chemistry: A Powerful Tool in Polymer Reaction Engineering

Summary: The potentials of computational techniques based on quantum mechanics, to support and complement the experimental analysis, are examined. Mechanisms and reaction paths involved in the free radical polymerization of widely used monomers are studied through a computational approach based on Density Functional Theory (DFT). First, the attention is focused on the initiation kinetics in order to evaluate the role of the initiators in the polymerization process. Methyl acrylate, methyl methacrylate, acrylonitrile, and styrene homopolymerization using different initiators are studied. Then, propagation kinetics is investigated. In particular, the propagation kinetic rate constants for different kinds of acrylates, methacrylates and acetates are calculated and compared with experimental data reported in the literature. The same computational approach is applied to the study of secondary reactions (backbiting, beta-scission) occurring during free radical polymerizations. Finally, the same methodologies are applied to copolymer systems, with emphasis on the evaluation of the role of penultimate effect. The copolymers vinyl acetate/methyl methacrylate and styrene/methyl methacrylate are investigated as system characterized by weak and strong penultimate effect, respectively.