Bond Fission Research Articles

Reactions of products from thermal degradation of PVC with nanoclusters of α-Fe2O3 (hematite)

Polyvinyl chloride (PVC) plastics constitute a large fraction of buildings, packaging and electronic devices, whereas, the annual emission electric arc furnace dust (EAFD) from steel manufacturing operations has recently peaked at nearly 6Mt. Co-pyrolysis of PVC with EAFD currently represents a focal abatement technology for both categories of pollutants. However, despite of several experimental investigations; the mechanisms underlying interaction between EAFD and PVC remain largely speculative. Herein, we examine theoretically reactions of major products from thermal degradation of PVC with nanoclusters of iron (III) oxide, α-Fe2O3 (hematite) as a representative model for the various metal oxides in EAFD. The facile nature for the H-Cl bond fission over hematite is in line with experimental findings, pointing out to formation of iron chlorides from pyrolysis of Fe2O3-PVC mixtures. Interaction of selected chlorinated C1-C3 cuts with the hematite structure preferentially proceeds via a dissociative adsorption pathway. Results from this study shall be instrumental to understand, on a precise molecular basis, fixation of halogens on transitional metal oxides; a viable thermal recycling approach for polymeric materials laden with halogenated constituents.

Theory-guided access to efficient photodegradation of the simplest perfluorocarboxylic acid: Trifluoroacetic acid

The photodegradation approaches of perfluorocarboxylic acids have attracted considerable attention and have been developed extensively. However, the reaction channels along which the perfluorocarboxylic acid molecules dissociate remain to be deciphered by means of the quantum chemical method at the electronically excited state level of theory until now. Here we report the photodissociation mechanism of the simplest perfluorocarboxylic acid, trifluoroacetic acid, using the complete active space self-consistent field (CASSCF) and the multi-configurational second-order perturbation (CASPT2) methods. The CC and CO α bond fission channels were both taken into account. Based on the constructed potential energy surfaces, it is concluded that the CC α bond fission, which would probably account for further degradations and mineralizations, may mainly take place in the triplet manifolds via intersystem crossing from the S1 state. Thus, taking the computational results of the simple member of perfluorocarboxylic acids as a rational clue, strategies to enhance intersystem crossing process efficiencies of the photodegradation of perfluorocarboxylic acids can be developed.

ReaxFF molecular dynamics simulation of pyrolysis and combustion of pyridine

The reactive molecular dynamics (ReaxFF MD) simulation was applied to investigate the initial reaction mechanism of pyrolysis and combustion of pyridine. By analyzing the dynamic change with time of intermediates and products formed during the simulation process, the underlying detailed chemical reactions of pyrolysis and combustion of pyridine for a variety of reaction conditions (temperature, pressure/density and heating rate) were revealed. We found that a raise in temperature, pressure/density and heating rate shortens the initiation time and accelerates the pyrolysis of pyridines, producing more species and numbers of products. Specifically, the free radicals of H and pyridyl were generated from the pyridine molecules at the early stage of pyrolysis simulation, and prominent products observed during these decompositions were H2, C2H2 and HCN, indicating the pyrolysis of pyridine is a chain process initiated principally by CH bond fission. And we investigated the dynamic transformation pathways of nitrogen during the pyrolysis process. As the pyrolysis temperature and pressure/density increase, the nitrogen transformation process from HCN and CN to NH3 and N2 will be promoted. In addition, for the simulation of the oxidation of pyridines, we found that increasing temperature promotes the combustion of pyridine but only has a limited effect on the oxidation process when it reached a higher range, and the initiation of oxidation is the unimolecular CH bond fission and bimolecular reaction with O2 to form the pyridyl radical, which can further react with oxygenated species to form the pyridoxy radical. The simulation results agreed reasonably well with previous experimental studies, implying that this paper offers a new and promising approach to systematically study the detailed and dynamic chemical reactions of thermal decomposition of nitrogen compounds.

Cyclometalation via Carbon–Fluorine Bond Activation Induced by Silver Particles

On treatment with either freshly prepared silver nanoparticles or silver particles mechanochemically generated by stirring silver(I) oxide in dichloromethane, the complex [Cp*IrCl2(κC-MeNC3H2NCH2C6F5)] underwent a rapid and clean cyclometalation involving carbon—fluorine bond fission to afford the product [Cp*IrCl(κC2-MeNC3H2NCH2C6F4)].

Variational Effect and Anharmonic Torsion on Kinetic Modeling for Initiation Reaction of Dimethyl Ether Combustion.

The reaction of dimethyl ether (DME) with molecular oxygen has been considered to be the dominant initiation pathway for DME combustion compared to the C-O bond fission. This work presents a detailed mechanism and kinetics investigation for the O2 + DME reaction with theoretical approaches. Using the CCSD(T)/6-311+G(2df,2pd) potential energy surface with the M06-2X/MG3S gradient, Hessian, and geometries, rate constants are evaluated by multistructural canonical variational transition-state theory (MS-CVT) including contributions from hindered rotation and multidimensional tunneling over the temperature range 200-2800 K. The CCSD(T) and QCISD(T) with 6-311+G(2df,2pd) calculations predict a barrier of 190-194 kJ mol-1 for the O2 + DME reaction based on the optimized structures at various levels. It is proposed that there exists a weakly interacting adducts on the product side with subsequent dissociation to the separate HO2 and CH3OCH2 radicals. Torsions in transition state are found to be significantly coupled to generate four conformations whose contributions do influence the rate constant predictions. Variational effects are observed to be significant at high temperatures, while tunneling effect quickly becomes insignificant with temperature. Finally, four-parameter Arrhenius expression 9.14 × 1013(T/300)-3.15 exp[-184.52(T + 110.23)/(T2 + 110.232)] cm3 mol-1 s-1 describes the temperature dependence of MS-CVT rate constants with small-curvature tunneling correction.

A mechanistic study of nitrite reduction on iron(ii) complexes of methylated N-confused porphyrins.

Proton delivery to the prosthetic group is a crucial step to sustain the activity of nitrite reductase. An iron N-confused porphyrin (NCP) complex, which is capable of relaying protons from the outer pyrrolic nitrogen (Nout-H) of the inverted pyrrole ring to the axial coordinated ligand, has been demonstrated to facilitate facile nitrite reduction. Time-dependent FTIR studies on the reaction between [FeII(HCTPPMe)Br] (1) and a nitrite anion revealed a two-step process involving conversion of the starting complex 1 to an {Fe(NO)}7 intermediate, [Fe(CTPPMe)(NO)] (5), before the detection of [Fe(CTPPCH2)(NO)] (3), an {Fe(NO)}6 end product. Moreover, spectroscopic data confirm that Nout-H on the NCP core is indispensable to the proceeding of the nitrite reduction reaction. Mass spectra have detected the coordination of a nitrite to the iron center while DFT theoretical calculations suggest that subsequent intramolecular proton transfer to a nitro group to form [Fe(CTPPMe)(HNO2)] (6a) evokes a homolytic N-OH bond fission on axial nitrous acid due to an enhanced π-back-bonding to produce an {Fe(NO)}7 intermediate and to release a hydroxyl radical. The subsequent oxidation of an {Fe(NO)}7 intermediate by the hydroxyl radical gave the final product, {Fe(NO)}6 [Fe(CTPPCH2)(NO)] (3). This study illustrates a proton assisted small molecule activation on the iron N-confused porphyrin coordination sphere and provides complemental insights into the mechanism of enzymatic nitrite reduction reactions.

Redox Reactions of Pentaammine Cobalt (III) Complexes of α-Hydroxy Acids by Nicotinium Dichromate in Micealar Medium

The kinetics of one electron transfer mode seems to be unavailable for NDC with Cobalt (III) bound and unbound complexes of α- hydroxy acids in surfactant medium, NDC oxidizes Cobalt (III) bound and unbound α hydroxy acids. It rules out the synchronous C-C bond fission and electron transfer to Cobalt (III) centre. Oxidation of above complexes increases with increase in temperature. The increase in the rate is observed with increase in the concentration of the surfactant. The added DDAC enhances the rate of oxidation of a reaction much more than ALS. Similar trends have been observed in lactato, glycolato and Mandelato Co (III) complexes

Ultrafast α -CC bond cleavage of acetone upon excitation to 3p and 3d Rydberg states by femtosecond time-resolved photoelectron imaging.

The radiationless electronic relaxation and α -CC bond fission dynamics of jet-cooled acetone in the S1 (nπ*) state and in high-lying 3p and 3d Rydberg states have been investigated by femtosecond time-resolved mass spectrometry and photoelectron imaging. The S1 state was accessed by absorption of a UV pump photon at selected wavelengths between λ = 320 and 250 nm. The observed acetone mass signals and the S1 photoelectron band decayed on sub-picosecond time scales, consistent with a recently proposed ultrafast structural relaxation of the molecules in the S1 state away from the Franck-Condon probe window. No direct signatures could be observed by the experiments for CC dissociation on the S1 potential energy hypersurface in up to 1 ns. The observed acetyl mass signals at all pump wavelengths turned out to be associated with absorption by the molecules of one or more additional pump and/or probe photons. In particular, absorption of a second UV pump photon by the S1 (nπ*) state was found to populate a series of high-lying states belonging to the n = 3 Rydberg manifold. The respective transitions are favored by much larger cross sections compared to the S1 ← S0 transition. The characteristic energies revealed by the photoelectron images allowed for assignments to the 3p and 3dyz states. At two-photon excitation energies higher than 8.1 eV, an ultrafast reaction pathway for breaking the α -CC bond in 50-90 fs via the 3dyz Rydberg state and the elusive ππ* state was observed, explaining the formation of acetyl radicals after femtosecond laser excitation of acetone at these wavelengths.

Kinetics of oxidative cracking of n‐hexane to olefins over VOx/Ce‐Al2O3 under gas phase oxygen‐free environment

The kinetics of oxidative cracking of n‐hexane to olefins using lattice oxygen of VOx/Ce‐Al2O3 is investigated. The TPR/TPO analysis shows a consistent reducibility (79%) of VOx/Ce‐Al2O3 in repeated redox cycles. The total acidity of the sample is found to be 0.54 mmol/g with 22% are strong acid sites that favors olefin selectivity. The oxidative cracking of n‐hexane in a fluidized CREC Riser simulator gives approximately 60% olefin selectivity at 30% n‐hexane conversion. A kinetic model is developed considering (1) cracking, (2) oxidative dehydrogenation (ODH), and (3) catalyst deactivations. The proposed cracking mechanism considers adsorption, C–H and C–C bond fission and desorption as elementary steps and implemented by pseudo steady state hypothesis. A Langmuir‐Hinshelwood mechanism is found to represent the ODH reactions. The developed model fits the experimental data with favorable statistical indicators. The estimated specific reaction rate constants are also found to be consistent with the product selectivity data. © 2016 American Institute of Chemical Engineers AIChE J, 63: 130–138, 2017

Theoretical study on the metabolic mechanisms of levmepromazine by cytochrome P450.

Levomepromazine, an "older" typical neuroleptic, is widely applied in psychiatry for the treatment of schizophrenia. The biotransformation of Levomepromazine remains elusive up to now, but found to result in the formation of different derivatives that may contribute to the therapeutic and/or side-effects of the parent drug. The present work aims to resolve the metabolic details of Levomepromazine catalyzed by cytochrome P450, an important heme-containing enzyme superfamily, based on DFT calculation. Two main metabolic pathways have been addressed, S-oxidation and N-demethylation. The mechanistic conclusions have revealed a stepwise transfer of two electrons mechanism in S-oxidation reaction. N-demethylation is a two-step reaction, including the rate-determining N-methyl hydroxylation which proceeds via the single electron transfer (SET) mechanism and the subsequent C-N bond fission through a water-assisted enzymatic proton-transfer process. N-demethylation is more feasible than S-oxidation due to its lower activation energy and N-desmethyllevomepromazine therefore is the most plausible metabolite of Levomepromazine. Each metabolic pathway proceeds in a spin-selective manner (SSM) mechanism, predominately via the LS state of Cpd I. Our observations are in good accordance with the experimental results, which can provide some general implications for the metabolic mechanism of Levomepromazine-like drugs. Graphical abstract The metabolic mechanisms of levmepromazine by cytochrome P450.

Kinetics of the gas-phase thermal decomposition of 3-chloropropene

The pyrolysis of 3-Chloropropene was studied in a static system in the temperature range 683–750K and pressure 45–92Torr. For the overall reaction the Arrhenius parameters were observed as, Ea=183.2±5.6kJmol−1, and log10 (A, s−1)=9.49±0.41. Two mechanisms, dehydrochlorination molecular elimination and CCl bond fission are discussed in detail. To interpret the fall-off behaviour, RRKM/ME calculations were adopted and the pressure dependent rate constants were calculated at collision efficiency of 0.5. From the pressure dependence study and RRKM calculations, it can be concluded that we are at the high-pressure limit.

Exergonic and Spontaneous Production of Radicals through Beryllium Bonds

AbstractHigh‐level ab initio calculations show that the formation of radicals, by the homolytic bond fission of Y−R (Y=F, OH, NH2; R=CH3, NH2, OH, F, SiH3, PH2, SH, Cl, NO) bonds is dramatically favored by the association of the molecule with BeX2 (X=H and Cl) derivatives. This finding is a consequence of two concomitant effects, the significant activation of the Y−R bond after the formation of the beryllium bond, and the huge stabilization of the F. (OH., NH2.) radical upon BeX2 attachment. In those cases where R is an electronegative group, the formation of the radicals is not only exergonic, but spontaneous.

Exergonic and Spontaneous Production of Radicals through Beryllium Bonds.

High-level ab initio calculations show that the formation of radicals, by the homolytic bond fission of Y-R (Y=F, OH, NH2 ; R=CH3 , NH2 , OH, F, SiH3 , PH2 , SH, Cl, NO) bonds is dramatically favored by the association of the molecule with BeX2 (X=H and Cl) derivatives. This finding is a consequence of two concomitant effects, the significant activation of the Y-R bond after the formation of the beryllium bond, and the huge stabilization of the F(.) (OH(.) , NH2 (.) ) radical upon BeX2 attachment. In those cases where R is an electronegative group, the formation of the radicals is not only exergonic, but spontaneous.

Bond Fission and Non-Radiative Decay in Iridium(III) Complexes.

We investigate the role of metal-ligand bond fission in the nonradiative decay of excited states in iridium(III) complexes with applications in blue organic light-emitting diodes (OLEDs). We report density functional theory (DFT) calculations of the potential energy surfaces upon lengthening an iridium-nitrogen (Ir-N) bond. In all cases we find that for bond lengths comparable to those of the ground state the lowest energy state is a triplet with significant metal-to-ligand change transfer character ((3)MLCT). But, as the Ir-N bond is lengthened there is a sudden transition to a regime where the lowest excited state is a triplet with significant metal centered character ((3)MC). Time-dependent DFT relativistic calculations including spin-orbit coupling perturbatively show that the radiative decay rate from the (3)MC state is orders of magnitude slower than that from the (3)MLCT state. The calculated barrier height between the (3)MLCT and (3)MC regimes is clearly correlated with previously measured nonradiative decay rates, suggesting that thermal population of the (3)MC state is the dominant nonradiative decay process at ambient temperature. In particular, fluorination both drives the emission of these complexes to a deeper blue color and lowers the (3)MLCT-(3)MC barrier. If the Ir-N bond is shortened in the (3)MC state another N atom is pushed away from the Ir, resulting in the breaking of this bond, suggesting that once the Ir-N bond breaks the damage to the complex is permanent-this will have important implications for the lifetimes of devices using this type of complex as the active material. The consequences of these results for the design of more efficient blue phosphors for OLED applications are discussed.

ChemInform Abstract: A Very Peculiar Family of N‐Heterocyclic Phosphines: Unusual Structures and the Unique Reactivity of 1,3,2‐Diazaphospholenes

AbstractReview: 79 refs.

Control of Nonadiabatic Passage through a Conical Intersection by a Dynamic Resonance.

Nonadiabatic processes, dominated by dynamic passage of reactive fluxes through conical intersections (CIs), are considered to be appealing means for manipulating reaction paths, particularly via initial vibrational preparation. Nevertheless, obtaining direct experimental evidence of whether specific-mode excitation affects the passage at the CI is challenging, requiring well-resolved time- or frequency-domain experiments. Here promotion of methylamine-d2 (CH3ND2) molecules to spectral-resolved rovibronic states on the excited S1 potential energy surface, coupled to sensitive D photofragment probing, allowed us to follow the N-D bond fission dynamics. The branching ratios between slow and fast D photofragments and the internal energies of the CH3ND(X̃) photofragments confirm correlated anomalies for predissociation initiated from specific rovibronic states. These anomalies reflect the existence of a dynamic resonance that strongly depends on the energy of the initially excited rovibronic states, the evolving vibrational mode on the repulsive S1 part during N-D bond elongation, and the manipulated passage through the CI that leads to CH3ND radicals excited with C-N-D bending. This resonance plays an important role in the bifurcation dynamics at the CI and can be foreseen to exist in other photoinitiated processes and to control their outcome.

The Onset of H + Ketene Products from Vinoxy Radicals Prepared by Photodissociation of Chloroacetaldehyde at 157 nm.

We investigate the unimolecular dissociation of the vinoxy radical (CH2CHO) prepared with high internal energy imparted from the photodissociation of chloroacetaldehyde (CH2ClCHO) at 157 nm. Using a velocity map imaging apparatus, we measured the speed distribution of the recoiling chlorine atoms, Cl((2)P3/2) and Cl((2)P1/2), and derived from this the resulting distribution of kinetic energy, P(ET), imparted to the Cl + vinoxy fragments upon dissociation. Using conservation of energy, the distribution of kinetic energy was used to determine the total internal energy distribution in the radical. The P(ET) derived for the C-Cl bond fission presented in this work suggests the vinoxy radicals are mostly formed in the à state. We also took ion images at m/z = 42 and m/z = 15 to characterize the branching between the unimolecular dissociation channels of the vinoxy radical to H + ketene and methyl + CO products. Our results show a marked change in the branching ratio between the two channels from the previous study on the photodissociation of chloroacetaldehyde at 193 nm by Miller et al. (J. Chem. Phys., 2004, 121, 1830) in that the production of ketene is now favored over the production of methyl. To help analyze the data, we developed a model for the branching between the two channels that takes into account how the change in rotational energy en route to the products affects the vibrational energy available to surmount the barriers to the channels. The model predicts the portion of the C-Cl bond fission P(ET) that produces dissociative vinoxy radicals, then predicts the branching ratio between the H + ketene and CH3 + CO product channels at each ET. The model uses Rice-Ramsperger-Kassel-Marcus rate constants at the correct sums and densities of vibrational states while accounting for angular momentum conservation. We find that the predicted portion of the P(ET) that produces H + ketene products best fits the experimental portion (that we derive by taking advantage of conservation of momentum) if we use a barrier height for the H + ketene channel that is 4.0 ± 0.5 kcal/mol higher than the isomerization barrier en route to CH3 + CO products. Using the G4 computed isomerization barrier of 40.6 kcal/mol, this gives an experimentally determined barrier to the H + ketene channel of 44.6 kcal/mol. From these calculations, we also predict the branching ratio between the H + ketene and methyl + CO channels to be ∼2.1:1.

Thermal Decomposition of 3-Bromopropene. A Theoretical Kinetic Investigation.

A detailed kinetic study of the gas-phase thermal decomposition of 3-bromopropene over wide temperature and pressure ranges was performed. Quantum chemical calculations employing the density functional theory methods B3LYP, BMK, and M06-2X and the CBS-QB3 and G4 ab initio composite models provide the relevant part of the potential energy surfaces and the molecular properties of the species involved in the CH2═CH-CH2Br → CH2═C═CH2 + HBr (1) and CH2═CH-CH2Br → CH2═CH-CH2 + Br (2) reaction channels. Transition-state theory and unimolecular reaction rate theory calculations show that the simple bond fission reaction ( 2 ) is the predominant decomposition channel and that all reported experimental studies are very close to the high-pressure limit of this process. Over the 500-1400 K range a rate constant for the primary dissociation of k2,∞ = 4.8 × 10(14) exp(-55.0 kcal mol(-1)/RT) s(-1) is predicted at the G4 level. The calculated k1,∞ values lie between 50 to 260 times smaller. A value of 10.6 ± 1.5 kcal mol(-1) for the standard enthalpy of formation of 3-bromopropene at 298 K was estimated from G4 thermochemical calculations.

Elucidating the two and three-body fragmentation channels on isotopically labeled nitrous oxide by a two-color asymmetric laser field

We report the interaction of isotopically labeled nitrous oxide (15N14N16O) time-of-flight mass spectroscopy. The phase dependence of two-body fragmentation channels for linear and circular polarization reveals that the nitrogen–nitrogen bond fission channel is in accordance with the ‘selective ionization of oriented molecules’ mechanism, while the nitrogen–oxygen bond rupture involves an electron recollision mechanism. For the three-body-fragmentation channel resulting in 14N3+ ions it is demonstrated that it may proceed via a concerted and/or a sequential manner. The phase dependence of the sequential channel is indicative of the following dissociation route: 15N14N16O→15N14N+16O→15N+14N+16O.

Chain Length and Thermal Sensitivity of the Infrared Spectra of a Homologous Series of Anhydrous Silver(I)n-Alkanoates

The thermal and chain length sensitivity of the infrared spectra of some solid state anhydrous silver(I) salts (n-octanoate ton-eicosanoate, inclusive) are discussed. At ambient temperature, the tilted alkyl chains, anchored to the metal planes via chelating bidentate coordination to the silver ions, are crystallized in the fully extendedall-transconformation. Interestingly, though all compounds are crystallized in a monoclinic crystal system, their lateral chain packing,van der Waalseffects, and hence vibrational features are chain length-dependent. This is a direct result of electrostatic effects of the COO group in addition to vibrational coupling between CH2, CH3, and COO modes, an effect which varies significantly with chain length. Variable temperature infrared measurements indicate significant irreversible changes in the metal-carboxyl coordination sphere, most likely due to bond fission. For long chain adducts (nc>12), thermally induced crystal system switching, monoclinic to triclinic, indicates greater thermal sensitivity of their alkyl chains. During heating, the regions of the hydrocarbon chains, furthest from the COO, become increasingly molten and mobile as the stepwise melt advances towards the solid COO moieties. This solid-liquid melting behaviour is responsible for mesophase formation in metal carboxylate systems.