Complex Reaction Mechanisms Research Articles

Microfluidic Technology: Uncovering the Mechanisms of Nanocrystal Nucleation and Growth.

The controlled and reproducible formation of colloidal semiconductor nanocrystals (or quantum dots) is of central importance in nanoscale science and technology. The tunable size- and shape-dependent properties of such materials make them ideal candidates for the development of efficient and low-cost displays, solar cells, light-emitting devices, and catalysts. The formidable difficulties associated with the macroscale preparation of semiconductor nanocrystals (possessing bespoke optical and chemical properties) result from the fact that underlying reaction mechanisms are complex and that the reactive environment is difficult to control. Automated microfluidic reactors coupled with monitoring systems and optimization algorithms aim to elucidate complex reaction mechanisms that govern both nucleation and growth of nanocrystals. Such platforms are ideally suited for the efficient optimization of reaction parameters, assuring the reproducible synthesis of nanocrystals with user-defined properties. This Account aims to inform the nanomaterials community about how microfluidic technologies can supplement flask experimentation for the ensemble investigation of formation mechanisms and design of semiconductor nanocrystals. We present selected studies outlining the preparation of quantum dots using microfluidic systems with integrated analytics. Such microfluidic reaction systems leverage the ability to extract real-time information regarding optical, structural, and compositional characteristics of quantum dots during nucleation and growth stages. The Account further highlights our recent research activities focused on the development and application of droplet-based microfluidics with integrated optical detection systems for the efficient and rapid screening of reaction conditions and a better understanding of the mechanisms of quantum dot synthesis. We describe the features and operation of fully automated microfluidic reactors and their subsequent application to high-throughput parametric screening of metal chalcogenides (CdSe, PbS, PbSe, CdSeTe), ternary and core/shell heavy metal-free quantum dots (CuInS2, CuInS2/ZnS), and all-inorganic perovskite nanocrystals (CsPbX3, X = Cl, Br, I) syntheses. Critically, concurrent absorption and photoluminescence measurements on millisecond to second time scales allow the extraction of basic parameters governing nanocrystal formation. Moreover, experimental data obtained from such microfluidic platforms can be directly supported by theoretical models of nucleation and growth. To this end, we also describe the use of metamodeling algorithms able to accurately predict optimized conditions of CdSe synthesis using a minimal number of sample parameters. Importantly, we discuss future challenges that must be addressed before microfluidic technologies are in a position to be widely adopted for the on-demand formation of nanocrystals. From a technology perspective, these challenges include the development of novel engineering platforms for the formation of complex architectures, the integration of monitoring systems able to harvest photophysical and structural information, the incorporation of continuous purification systems, and the application of optimization algorithms to multicomponent quantum dot systems.

Reactivity Patterns of (Protonated) Compound II and Compound I of Cytochrome P450: Which is the Better Oxidant?

The cytochromes P450 are versatile enzymes in human physiology that perform substrate hydroxylation reactions extremely efficiently. In this work, we present results of a computational study on the reactivity patterns of Compound I, Compound II, and protonated Compound II with model substrates, and we address the question of which of these compounds is the most effective oxidant? All calculations, regardless of the substrate, implicated that Compound I is the superior oxidant of the three. However, Compound II and protonated Compound II were found to react with free energies of activation that are only a few kcal mol-1 higher in energy than those obtained with Compound I. Therefore, Compound II and protonated Compound II should be able to react with aliphatic groups with moderate C-H bond strengths. We have analysed all results in detail and have given electronic, thermochemical, valence bond, and molecular orbital rationalizations on the reactivity differences and explained experimental product distributions. Overall, the findings implied that alternative oxidants could operate alongside Compound I in complex reaction mechanisms of enzymatic and synthetic iron porphyrinoid complexes.

Suitability of a radical based method for assessment of jet fuel antioxidant capacity and projected storage stability

Jet fuel stability is an important fuel property for management of fuel in storage. A method for predicting jet fuel stability, based on the colourimetric reaction of the stable free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) has been examined in this work. A range of jet fuels of known and unknown refining history, including a range of antioxidant compounds have been examined using the DPPH assay with mixed results. It was found that the DPPH technique responds predictably with a range of phenolic species however was less predictable with jet fuels of unknown provenance. Little to no correlation was observed when comparing unknown fuels storage stabilities based on oxidative and peroxide formation tests against the DPPH assay, however a good correlation was observed for fuels of known refining history for these same storage stability tests. The wide variation of results may in part be due to the very complex reaction mechanisms of DPPH with species in the fuels and due to the inherent chemical complexity of middle distillate fuels that may have undergone ageing reactions in storage prior testing with the DPPH assay.

Computational applications in organic electrochemistry

Two features of particular interest in organic electrochemistry are the reduction and/or oxidation potentials of organic substances and the structures of electrochemically generated intermediates. The development of accurate quantum chemical methods for determining the in recent years, together with continued improvements in computing technology with which to implement such methods, has resulted in opportunities to solve problems in chemistry that cannot be solved by experiment. Two major areas which have benefited most by the availability of computational technology are improved methods for (a) prediction of redox potentials of organic species and (b) determination of the structure of electrochemically generated ion pairs. The ability to predict the reduction or oxidation potential of an unknown substance can be of value in several ways: in planning for carrying out a reaction with a mediator of electron transfer, one can compute the redox potentials of likely compounds to choose the best candidate before setting out on what might be a lengthy synthetic program and when trying to analyze a complex reaction mechanism, one can compute the redox potentials of likely short-lived intermediates. The structures of electrogenerated intermediates can offer new insights into the binding between charged species.

On the mechanism of tool crater wear during titanium alloy machining

Tool crater wear is a major cause of accelerated tool failure during the machining of titanium alloy components. This paper presents a static diffusion couple method which replicates the complex reaction mechanisms occurring at the tool-chip interface during machining and is validated with dynamic turning trials. The diffusion couple tests are realised via the use of a bespoke vacuum compression rig. The rig consists of two graphite rods which secure the sample under a clamping load at the required temperature and pressure. Titanium alloys bearing a high molybdenum content are found to be less susceptible to TiC formation at the tool-workpiece interface. A TiC layer during machining coats and protects the tool from excessive crater wear therefore increasing tool life. Strong agreement was found between the diffusion couple technique and turning trial. This low cost test strategy will be used to design machinable titanium alloys and more compatible tools.

Single-Molecule Fluorescence Energy Transfer Assays for the Characterization of Reaction Pathways of miRNA-Argonaute Complex.

Argonaute proteins are key components of the microRNA-induced silencing complexes (miRISCs) that mediate the posttranscriptional gene silencing of microRNAs and small interfering RNA (siRNAs). The complex reaction mechanism of miRISC is expected to be characterized by tracing the reaction pathways of miRISC at the single-molecule level in real time. In this chapter, we describe single-molecule fluorescence resonance energy transfer (FRET) assays to observe the target binding and reaction pathways of miRISC composed of a recombinant Argonaute and a small RNA.

Thermal Joining of Highly Conductive Bonds by Using Reactive Al-Ni Nanofoils

In view of the demand for electrified long-range vehicles, the performance of traction batteries has to be improved. To achieve a high power density, the battery cells must be interconnected with low electrical resistance within the joints. The joining process has to fulfill specific requirements, as the battery cells may only be exposed to very low mechanical and thermal impacts. Joining by using reactive aluminum-nickel nanofoils represents an innovative technology meeting the abovementioned requirements. These foils are multilayered systems consisting of several hundred alternating monolayers of aluminum and nickel, each with a thickness in the nanometer range. Their unique ability is that they can react with temperatures up to 1500 °C for a duration of a few milliseconds upon external ignition. This thermal reaction energy serves as a heat source in the joining process, melting the materials in the interfacial surface. Subsequently, the joining partners solidify and form an adhesive bond when compressed properly. However, these advantageous characteristics are contrasted by complex reaction mechanisms and an unknown interaction of the process parameters. For the industrial application of the joining technology, the requirements for initiating the exothermic reaction must be known. Therefore, the process window and the mechanism of ignition have to be scrutinized. For this purpose, an experimental test setup was developed to generate and monitor short circuit currents to ignite the nanofoils. The amperage as well as the layer composition of the nanofoils were varied within a parameter study. Two independent process windows for a stable ignition were identified for all analyzed nanofoils.

Instantaneous Functionalization of Chemically Etched Silicon Nanocrystal Surfaces

AbstractRemarkable advances in surface hydrosilylation reactions of C=C and C=O bonds on hydride‐terminated silicon have revolutionized silicon surface functionalization. However, existing methods for functionalizing hydride‐terminated Si nanocrystals (H‐SiNCs) require long reaction times and elevated temperatures. Herein, we report a room‐temperature method for functionalizing H‐SiNC surfaces within seconds by stripping outermost atoms on H‐SiNC surfaces with xenon difluoride (XeF2). Detailed analysis of the reaction byproducts by in situ NMR spectroscopy and GC‐MS provided unprecedented insight into NC surface composition and reactivity as well as the complex reaction mechanism of XeF2 activated hydrosilylation.

Instantaneous Functionalization of Chemically Etched Silicon Nanocrystal Surfaces.

Remarkable advances in surface hydrosilylation reactions of C=C and C=O bonds on hydride-terminated silicon have revolutionized silicon surface functionalization. However, existing methods for functionalizing hydride-terminated Si nanocrystals (H-SiNCs) require long reaction times and elevated temperatures. Herein, we report a room-temperature method for functionalizing H-SiNC surfaces within seconds by stripping outermost atoms on H-SiNC surfaces with xenon difluoride (XeF2 ). Detailed analysis of the reaction byproducts by in situ NMR spectroscopy and GC-MS provided unprecedented insight into NC surface composition and reactivity as well as the complex reaction mechanism of XeF2 activated hydrosilylation.

Real-time analysis of RAG complex activity in V(D)J recombination

Single-molecule FRET (smFRET) and single-molecule colocalization (smCL) assays have allowed us to observe the recombination-activating gene (RAG) complex reaction mechanism in real time. Our smFRET data have revealed distinct bending modes at recombination signal sequence (RSS)-conserved regions before nicking and synapsis. We show that high mobility group box 1 (HMGB1) acts as a cofactor in stabilizing conformational changes at the 12RSS heptamer and increasing RAG1/2 binding affinity for 23RSS. Using smCL analysis, we have quantitatively measured RAG1/2 dwell time on 12RSS, 23RSS, and non-RSS DNA, confirming a strict RSS molecular specificity that was enhanced in the presence of a partner RSS in solution. Our studies also provide single-molecule determination of rate constants that were previously only possible by indirect methods, allowing us to conclude that RAG binding, bending, and synapsis precede catalysis. Our real-time analysis offers insight into the requirements for RSS-RSS pairing, architecture of the synaptic complex, and dynamics of the paired RSS substrates. We show that the synaptic complex is extremely stable and that heptamer regions of the 12RSS and 23RSS substrates in the synaptic complex are closely associated in a stable conformational state, whereas nonamer regions are perpendicular. Our data provide an enhanced and comprehensive mechanistic description of the structural dynamics and associated enzyme kinetics of variable, diversity, and joining [V(D)J] recombination.

Optimizing Binding Energies of Key Intermediates for CO2 Hydrogenation to Methanol over Oxide-Supported Copper.

Rational optimization of catalytic performance has been one of the major challenges in catalysis. Here we report a bottom-up study on the ability of TiO2 and ZrO2 to optimize the CO2 conversion to methanol on Cu, using combined density functional theory (DFT) calculations, kinetic Monte Carlo (KMC) simulations, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements, and steady-state flow reactor tests. The theoretical results from DFT and KMC agree with in situ DRIFTS measurements, showing that both TiO2 and ZrO2 help to promote methanol synthesis on Cu via carboxyl intermediates and the reverse water-gas-shift (RWGS) pathway; the formate intermediates, on the other hand, likely act as a spectator eventually. The origin of the superior promoting effect of ZrO2 is associated with the fine-tuning capability of reduced Zr(3+) at the interface, being able to bind the key reaction intermediates, e.g. *CO2, *CO, *HCO, and *H2CO, moderately to facilitate methanol formation. This study demonstrates the importance of synergy between theory and experiments to elucidate the complex reaction mechanisms of CO2 hydrogenation for the realization of a better catalyst by design.

Siderophores in Cloud Waters and Potential Impact on Atmospheric Chemistry: Photoreactivity of Iron Complexes under Sun-Simulated Conditions.

In the present work, the photoreactivity of a mixture of iron(III)–pyoverdin (Fe(III)–Pyo) complexes was investigated under simulated cloud conditions. Pyoverdins are expected to complex ferric ions naturally present in cloudwater, thus modifying their availability and photoreactivity. The spectroscopic properties and photoreactivity of Fe(III)-Pyo were investigated, with particular attention to their fate under solar irradiation, also studied through simulations. The photolysis of the Fe(III)–Pyo complex leads to the generation of Fe(II), with rates of formation (RFe(II)f) of 6.98 and 3.96 × 10–9 M s–1 at pH 4.0 and 6.0, respectively. Interestingly, acetate formation was observed during the iron-complex photolysis, suggesting that fragmentation can occur after the ligand-to-metal charge transfer (LMCT) via a complex reaction mechanism. Moreover, photogenerated Fe(II) represent an important source of hydroxyl radical via the Fenton reaction in cloudwater. This reactivity might be relevant for the estimation of the rates of formation and steady-state concentrations of the hydroxyl radical by cloud chemistry models and for organic matter speciation in the cloud aqueous phase. In fact, the conventional models, which describe the iron photoreactivity in terms of iron–aqua and oxalate complexes, are not in accordance with our results.

Artificial Force Induced Reaction Method for Systematic Determination of Complex Reaction Mechanisms.

Nowadays, computational studies are very important for the elucidation of reaction mechanisms and selectivity of complex reactions. However, traditional computational methods usually require an estimated reaction path, mainly driven by limited experimental implications, intuition, and assumptions of stationary points. However, the artificial force induced reaction (AFIR) method in the global reaction route mapping (GRRM) strategy can be used for unbiased and automatic reaction path searches for complex reactions. In this account, we highlight applications of the AFIR method to a variety of reactions (organic, organometallic, enzymatic, and photochemical) of complex molecular systems. In addition, the AFIR method has been successfully used to rationalise the origin of stereo- and regioselectivity. The AFIR method can be applied from small to large molecular systems, and will be a very useful tool for the study of complex molecular problems in many areas of chemistry, biology, and material sciences.

A study of the planarity of the pyrrolone fragment in 2-isopropyl-2,3-dihydro-1H-isoindol-1-one.

Orthophthalaldehyde (o-phthalaldehyde, OPA) is an aromatic dialdehyde bearing two electron-withdrawing carbonyl groups. The reactions of OPA with primary amines are broadly applied for the synthesis of important heterocyclic compounds with biological relevance. A number of such reactions have been investigated recently and several structures of condensation products have been reported, however, the complex reaction mechanism is still not fully understood and comprises concurrent as well as consecutive reactions. The reaction products depend on the primary amine which reacts with OPA, the reaction environment (solvent) and the proportion of the reactants. The title molecule, C11H13NO, the product of the reaction of OPA with isopropylamine, contains a five-membered pyrrole C4N ring with a carbonyl substituent, which forms part of the isoindolinone unit. Though this pyrrole ring contains one C atom in the sp(3)-hybridized state, it is fairly planar. The title molecule has been compared with similar structures retrieved from the Cambridge Structural Database in order to study this phenomenon. The planarity of this fragment has been explained by the presence of partially delocalized C-C, C-N and C-O bonds, and by an inner angle in the planar pentagonal ring (∼108°), which is close to the ideal tetrahedral value for the sp(3)-hybridized state of the constituent C atom. Due to this propitious angle, this C atom can be present in states intermediate between sp(3)- and sp(2)-hybridized in different structures, while still maintaining the planarity of the ring. There are only weak intermolecular C-H...O hydrogen bonds and C-H...π-electron ring interactions in the structure. In particular, it is the pyrrole ring which is involved in these interactions.

Kinetic Study of Low Density Polyethylene Using Thermogravimetric Analysis, Part 2: Isothermal Study

Thermal behavior of plastic materials can be improved by knowing thermal degradation kinetics for the optimal design and operation of pyrolysis process. In continuity of part 1 of the present study, Thermogravimetric analysis has been used for isothermal kinetic study of waste Low-Density Polyethylene (LDPE) pyrolysis under Argon atmosphere at different temperatures 420, 430, 440, 450, 460 and 470 °C. These isotherms were chosen after determining the range of thermal degradation of LDPE using Thermogravimetric analysis in non-isothermal conditions. This work aims to analyze the applicability of simples kinetic models to the prediction of the thermal behavior of the material, taking into account the complex chain reaction mechanism. Because of this complex reaction mechanism, the reaction order of pyrolysis of LDPE was determined. Freidman method has been used to obtain the activation energy and the frequency factor from isothermal experiments and compare them with the values obtained from non-isothermal experiments obtained in part 1 and with the results obtained by other authors.

Nanostructured CeO2 as catalysts for different AOPs based in the application of ozone and simulated solar radiation

Two CeO2 catalysts with different morphology (nanocubes and nanorods) were synthetized by hydrothermal treatment, characterized by TEM, XRD, N2 adsorption-desorption isotherms, XPS and DR-UV–vis spectroscopy and examined for different AOPs (photocatalytic oxidation, catalytic and photocatalytic ozonation) using simulated solar radiation (λ>300nm) or visible radiation (λ>390nm) from a Xe lamp, and DEET as target compound. Neither significant DEET removal nor mineralization was achieved by solar photocatalysis and the catalysts did not show an important beneficial effect during catalytic ozonation. The best results were obtained in photocatalytic ozonation process being nanorods more active under visible radiation according to its lower band gap energy whereas nanocubes presented the highest intrinsic photocatalytic activity under visible and solar radiation. Photocatalytic ozonation of DEET with CeO2 catalysts proceeds through a complex reaction mechanism that involves at least ozone-direct reaction that triggers a radical chain mechanism, indirect ozone reactions, ozone photolytic and photocatalytic decomposition and hydrogen peroxide formation-decomposition.

Kinetics and mechanism of olefin catalytic hydroalumination by organoaluminum compounds

The complex reaction mechanism of α-olefin catalytic hydroalumination by alkylalanes is investigated via mathematical modeling that involves plotting the kinetic models for the individual reactions that make up a complex system and a separate study of their principles. Kinetic parameters of olefin catalytic hydroalumination are estimated. Activation energies of the possible steps of the schemes of complex reaction mechanisms are compared and possible reaction pathways are determined.

Charge-Transfer Complex of p-Aminodiphenylamine with Maleic Anhydride: Spectroscopic, Electrochemical, and Physical Properties.

A new charge-transfer complex and the amide formed by the interaction between the electron donor of the p-aminodiphenylamine and the electron acceptor of maleic anhydride are investigated by spectroscopic methods. The amidation reaction is caused by proton and charge transfer between the maleic anhydride and p-aminodiphenylamine molecules. The Benesi-Hildebrand equation is used to determine the formation constant, the molar extinction coefficient and the standard Gibbs free energy of the complex by using UV/Vis spectroscopy. To reveal the electronic and spectroscopic properties of these molecules, theoretical computations are performed on the structures of maleic anhydride, p-aminodiphenylamine and the conformers of their charge-transfer complex. The charge-transfer complex and amidation reaction mechanism are also confirmed by IR and NMR spectroscopy and HRMS. The nature of the maleic anhydride-p-aminodiphenylamine complex is characterized by cyclic voltammetry, thermogravimetric analysis, XRD and SEM. Solid microribbons of this complex show higher thermal stability than p-aminodiphenylamine.

Reaction Dynamics of O((3)P) + Propyne: I. Primary Products, Branching Ratios, and Role of Intersystem Crossing from Crossed Molecular Beam Experiments.

We performed synergic experimental/theoretical studies on the mechanism of the O((3)P) + propyne reaction by combining crossed molecular beams experiments with mass-spectrometric detection and time-of-flight analysis at 9.2 kcal/mol collision energy (Ec) with ab initio electronic structure calculations at a high level of theory of the relevant triplet and singlet potential energy surfaces (PESs) and statistical calculations of branching ratios (BRs) taking into account intersystem crossing (ISC). In this paper (I) we report the results of the experimental investigation, while the accompanying paper (II) shows results of the theoretical investigation with comparison to experimental results. By exploiting soft electron ionization detection to suppress/mitigate the effects of the dissociative ionization of reactants, products, and background gases, product angular and velocity distributions at different charge-to-mass ratios were measured. From the laboratory data angular and translational energy distributions in the center-of-mass system were obtained for the five competing most important product channels, and product BRs were derived. The reactive interaction of O((3)P) with propyne under single collision conditions is mainly leading to the rupture of the three-carbon atom chain, with production of the radical products methylketenyl + atomic hydrogen (BR = 0.04), methyl + ketenyl (BR = 0.10), and vinyl + formyl (BR = 0.11) and the molecular products ethylidene/ethylene + carbon monoxide (BR = 0.74) and propandienal + molecular hydrogen (BR = 0.01). Because some of the products can only be formed via ISC from the entrance triplet to the low-lying singlet PES, we infer from their BRs an amount of ISC larger than 80%. This value is dramatically large when compared to the negligible ISC reported for the O((3)P) reaction with the simplest alkyne, acetylene. At the same time, it is much larger than that (∼20%) recently observed in the related reaction of the three-carbon atom alkene, O((3)P) + propene at a comparable Ec. This poses the question of how important it is to consider nonadiabatic effects and their dependence on molecular structure for this kind of combustion reactions. The prevalence of the CH3 over the H displacement channels is not explained by invoking a preference for the addition on the methyl-substituted acetylenic carbon atom, but rather it is believed to be due to the different tendencies of the two addition triplet intermediates CH3CCHO (preferentially leading to H elimination) and CH3COCH (preferentially leading to CH3 elimination) to undergo ISC to the underlying singlet PES. It is concluded that the main coproduct of the CO forming channel is singlet ethylidene ((1)CH3CH) rather than ground-state ethylene. By comparing the derived BRs with those very recently derived from kinetics studies at room temperature and 4 Torr we obtained information on how BRs vary with collision energy. The extent of ISC is estimated to remain essentially constant (∼85%) with increasing Ec from ∼1 to ∼10 kcal/mol. The present experimental results shed light on the mechanism of the title reaction at energies comparable to those involved in combustion and, when compared with the results from the statistical calculations on the ab initio coupled PESs (see accompanying paper II), lead to an in-depth understanding of the rather complex reaction mechanism of O + propyne. The overall results are expected to contribute to the development of more refined models of hydrocarbon combustion.

Surrogate Models for Online Monitoring and Process Troubleshooting of NBR Emulsion Copolymerization

Chemical processes with complex reaction mechanisms generally lead to dynamic models which, while beneficial for predicting and capturing the detailed process behavior, are not readily amenable for direct use in online applications related to process operation, optimisation, control, and troubleshooting. Surrogate models can help overcome this problem. In this research article, the first part focuses on obtaining surrogate models for emulsion copolymerization of nitrile butadiene rubber (NBR), which is usually produced in a train of continuous stirred tank reactors. The predictions and/or profiles for several performance characteristics such as conversion, number of polymer particles, copolymer composition, and weight-average molecular weight, obtained using surrogate models are compared with those obtained using the detailed mechanistic model. In the second part of this article, optimal flow profiles based on dynamic optimisation using the surrogate models are obtained for the production of NBR emulsions with the objective of minimising the off-specification product generated during grade transitions.