Determination Of Rate Constants Research Articles

Alizarin as a potential protector of proteins against damage caused by hydroperoxyl radical

Alizarin is a natural anthraquinone molecule with moderate antioxidative capacity. Some earlier investigations indicated that it can inhibit osteosarcoma and breast carcinoma cell proliferation by inhibiting of phosphorylation process of ERK protein (extracellular signal-regulated kinases). Several mechanisms of deactivation of one of the most reactive oxygen species, hydroperoxyl radical, by alizarin are estimated: hydrogen atom abstraction (HAA), radical adduct formation (RAF), and single electron transfer (SET). The plausibility of those mechanisms is estimated using density functional theory. The obtained results indicated HAA as the only thermodynamically plausible mechanism. For that purpose, two possible mechanistic pathways for hydrogen atom abstraction are studied in detail: hydrogen atom transfer (HAT) and proton-coupled electron transfer (PCET). Water and benzene are used as models of solvents with opposite polarity. To examine the difference between HAT and PCET is used kinetical approach based on the Transition state theory (TST) and determined rate constants (k). Important data used for a distinction between HAT and PCET mechanisms are obtained by applying the Quantum Theory of Atoms in Molecules (QTAIM), and by the analysis of single occupied molecular orbitals (SOMOs) in transition states for two examined mechanisms. The molecular docking analysis and molecular dynamic are used to predict the most probable positions of binding of alizarin to the sequence of ApoB-100 protein, a protein component of plasma low-density lipoproteins (LDL). It is found that alizarin links the nitrated polypeptide forming the π-π interactions with the amino acids Phenylalanine and Nitrotyrosine. The ability of alizarin to scavenge hydroperoxyl radical when it is in a sandwich structure between the polypeptide and radical species, as the operative reaction mechanism, is not significantly changed concerning its antioxidant capacity in the absence of polypeptide. Therefore, alizarin can protect the polypeptide from harmful hydroperoxyl radical attack, positioning itself between the polypeptide chain and the reactive oxygen species.

Electrospray Mass Spectrometry to Study Combinatorial iClick Reactions and Multiplexed Kinetics of [Ru(N3)(N∧N)(terpy)]PF6 with Alkynes of Different Steric and Electronic Demand.

In a combinatorial approach, a family of ruthenium(II) azido complexes [Ru(N3)(N∧N)(terpy)]PF6 with terpy = 2,2':6',2″-terpyridine and N∧N as a bidentate chelator derived from 2,2'-biypridine and its 4,4'-disubstituted derivatives, 2,2'-bipyrimidine, and 1,10-phenanthroline were reacted with different internal and terminal alkynes to give access to a total of 7 × 7 = 49 triazolato complexes in a room-temperature catalyst-free iClick reaction. The reactants were mixed in a repurposed high-performance liquid chromatography (HPLC) autosampler, and the reaction progress was monitored by direct injection into an electrospray mass spectrometer. The ratio of the peak intensities of [Ru(N3)(N∧N)(terpy)]+ and [Ru(triazolato)(N∧N)(terpy)]+ was converted to a colored heat map for facile visual inspection of the conversion ratio. By automated multiple injections of the reaction mixture in fixed time intervals and plotting peak intensities over reaction time, pseudo-first-order rate constants were easily determined. Finally, nonoverlapping isotope patterns of the azido starting materials and triazolato products enabled multiplexed parallel determination of rate constants for four different ruthenium(II) azido complexes from a single sample vial, thereby reducing experiment time by 75%.

Proton transfer kinetics of transition metal hydride complexes and implications for fuel-forming reactions.

Proton transfer reactions involving transition metal hydride complexes are prevalent in a number of catalytic fuel-forming reactions, where the proton transfer kinetics to or from the metal center can have significant impacts on the efficiency, selectivity, and stability associated with the catalytic cycle. This review correlates the often slow proton transfer rate constants of transition metal hydride complexes to their electronic and structural descriptors and provides perspective on how to exploit these parameters to control proton transfer kinetics to and from the metal center. A toolbox of techniques for experimental determination of proton transfer rate constants is discussed, and case studies where proton transfer rate constant determination informs fuel-forming reactions are highlighted. Opportunities for extending proton transfer kinetic measurements to additional systems are presented, and the importance of synergizing the thermodynamics and kinetics of proton transfer involving transition metal hydride complexes is emphasized.

Accurate detection of enzymatic degradation processes of gelatin–alginate microcapsule by 1H NMR spectroscopy: Probing biodegradation mechanism and kinetics

With an increase in the severity of environmental pollution caused by microbeads, the development of biodegradable microcapsules that can be applied in diverse fields has attracted significant attention. The degradation processes are directly related to biodegradable microcapsule creation with high stability and persistence. In this study, biodegradable microcapsules are synthesized via a complex coacervation approach using gelatin and alginate as the capsule main wall materials; additionally, enzyme-induced decomposition mechanisms are proposed by observing spectral changes in proton nuclear magnetic resonance (1H NMR) analyses. Additional analytical techniques confirm the chemical structure, morphology, and size distribution of the synthesized capsules; these uniform spherical microcapsules are 20–30 μm in size and possess a smooth surface. In addition to characterization, the microcapsules were exposed to targeted enzymes to investigate enzymatic effects using short-term and long-term degradation kinetics. Close inspection reveals that determination of the degradation rate constant of the major components in the capsule is feasible, and suggests two types of 4-stage degradation mechanisms that are enzyme-specific. These investigations demonstrate that capsule degradation can be explored in detail using 1H NMR spectroscopy to provide a viable strategy for monitoring degradation properties in the development of new biodegradable polymers.

Square wave voltammetry as a powerful tool for studying multi-electron molecular catalysts

• SWV of homogeneous two-electron molecular catalysis is theoretically investigated. • The rigorous theory applies to wide mechanistic diversity and SWV parameters. • SWV signal can enable the determination of catalytic pathways and rate constants. • SWV components are remarkably valuable to elucidate the catalyst active form. • Accurate guidelines and data analysis protocols are given. A comprehensive theoretical investigation of the square wave voltammetry (SWV) of the homogeneous two-electron molecular catalysis of electrochemical reactions is carried out. The SWV signal is studied as a function of the pulse time, the SW amplitude and the mechanistic features (catalytic rate constants, substrate concentration, formal potentials), also paying attention to the SW components that are demonstrated to be remarkably helpful in the identification of the catalyst active form. Thus, the ability of the SWV technique for the elucidation of the catalytic species and pathways, and for the determination of the corresponding rate constants is assessed, establishing accurate and valuable diagnosis criteria and procedures for quantitative data analysis based on mathematical expressions that are rigorous and easy-to-implement, and that consider the influence of diffusion on the response.

Overlooked roles of Cl2O and Cl2 in micropollutant abatement and DBP formation by chlorination

This study investigated the roles of diverse free available chlorine (FAC) species including HOCl/OCl−, H2OCl+, Cl2O, and Cl2 in the degradation of micropollutants. The degradation of 5 micropollutants was significantly affected by pH, FAC dosage, and chloride (Cl−) concentration. The reaction orders in FAC (n) of 5 micropollutants (acetaminophen, carbamazepine, naproxen, gemfibrozil, and mecoprop) ranged from 1.4 ± 0.2 to 2.1 ± 0.3 at pH 3 − 5, evidencing the importance of Cl2O and Cl2 for micropollutant abatement. A simplified method for the determination of second-order rate constants (k) of specific FAC species with micropollutants was developed. Herein, the k for neutral/dissociated forms of 5 micropollutants with Cl2 and Cl2O were determined in the ranges of 9.3 (± 0.2) × 102 ∼ 2.9 (± 0.2) × 109 M−1 s−1 and 1.8 (± 0.1) × 104 ∼ 3.7 (± 0.6) × 109 M−1 s−1, respectively. They were 4 − 7 orders of magnitude higher than those of HOCl, whereas those of OCl− and H2OCl+ were negligible. By using kinetic modeling, Cl2 was more important under acidic conditions and higher Cl− levels with contributions of 37.9 – 99.2% at pH 5 in pure water. Cl2O played a dominant role in micropollutant degradation in pure water (56.4 − 87.3%) under neutral conditions. Furthermore, both Cl2 and Cl2O played vital roles in the formation of disinfection byproducts (DBPs) during chlorination of carbamazepine and natural organic matter. This study highlights the overlooked roles of Cl2O and Cl2 in micropollutant abatement and DBP formation during chlorination.

A study on attempt for determination of permeation kinetics of coumarin at lipid bilayer of liposomes by using capillary electrophoresis with moment analysis theory

It was tried to develop a moment analysis method for the determination of lipid membrane permeability. The first absolute and second central moments of elution peaks measured by liposome electrokinetic chromatography (LEKC) are analyzed by using moment equations. As a concrete example, elution peak profiles of coumarin in a LEKC system, in which liposomes consisting of 1-palmitoyl-2-oleoyl-sn‑glycero-3-phosphocholine (POPC) and phosphatidylserine (PS) are used as a pseudo-stationary phase, were analyzed. It seems that lipid membrane permeability of coumarin across the lipid bilayer of POPC/PS liposomes was measured by the moment analysis method because previous permeability measurements using parallel artificial membrane permeability assay (PAMPA) and Caco-2 cells indicated that coumarin is permeable across lipid bilayer. However, it was also pointed out that the moment analysis method with LEKC is not effective for the determination of lipid membrane permeability and that it provides information about adsorption/desorption kinetics at lipid bilayer of liposomes. Therefore, different moment equations were also developed for the determination of adsorption/desorption rate constants of coumarin from the LEKC data. It was demonstrated that permeation rate constants at lipid bilayer or adsorption/desorption rate constants can be determined from the LEKC data on the basis of moment analysis theory for the mass transfer phenomena of coumarin at the lipid bilayer of POPC/PS liposomes. Mass transfer kinetics of solutes at lipid bilayer should be determined under the conditions that liposomes originally be because they are self-assembling and dynamic systems formed through weak interactions between phospholipid monomers. The moment analysis method using LEKC is effective for the experimental determination of the mass transfer rate constants at the lipid bilayer of liposomes because neither immobilization nor chemical modification of liposomes is necessary when LEKC data are measured. It is expected that the results of this study contribute to the dissemination of an opportunity for the determination of permeation rate constants or adsorption/desorption rate constants at the lipid bilayer of liposomes to many researchers because capillary electrophoresis is widespread.

Multiconformer transition state theory rate constant and branching ratios for the OH‐initiated reaction of CH3OCF2CHF2 and its primary product, HC(O)OCF2CHF2

AbstractObtaining rate constants and branching ratios for the atmospheric degradation of organic compounds is crucial for their environmental assessment and for a deeper understanding of their atmospheric chemistry and reactivity. The theoretical prediction of such quantities is a challenging endeavor that grows with conformational complexity. In this work, we have theoretically calculated the rate constants and branching ratios for the tropospheric OH‐initiated oxidation reactions of the hydrofluoroether and its primary product, the fluorinated ester. The calculations were performed with a cost‐effective multiconformer transition state theory protocol employing the recently developed constrained transition state randomization method. The calculated rate constant lies within the error bars of the recommended experimental value, while the calculated rate coefficient of represents, to our knowledge, the first determination of this reaction's rate constant. The results provide new insight and elucidate the main mechanistic issues discussed in recent experimental work while they also encourage the use of this protocol in investigations of the tropospheric reactivity of other fluorinated organic compounds.

Rate constant determination in the reaction of gold ion with carbonyl using single-atom chemistry in gas phase

The reactions of single gold ion Au+ with CO and H2O molecules in gas phase were studied by experiment and calculation. A reaction tube was connected to the end of the jet to destroy the supersonic state of the molecular beam and enhance the collisions between the laser-sputtered metal ions with gas molecules, so as to ensure the equilibrium reaction conditions. The ionic products were analyzed by a time-of-flight mass spectrometer. The collision temperature in reaction tube was calibrated to 409 K by the reaction of Au+ ion and H2O molecules. The contents of ionic products Au(CO)+ and Au(CO)2+ in the reaction of Au+ with CO molecules were monitored under different concentrations of CO in precursor gas. The combination reaction rate constants of single Au + ion sequentially with two CO molecules were determined to be k1 = 1.1 (2) × 10−12 cm3s−1 and k2 = 4.5 (5) × 10−10 cm3s−1. The geometric structures and binding energies of the ionic products, as well as the pathway and rate constants of the reactions were predicted by a high-level ab initial calculation. The binding mode of Au+ and CO and the formation of molecular orbital were discussed.

Kinetic modelling of the bromine-ammonia system: Formation and decomposition of bromamines

Bromamines i.e. monobromamine (NH2Br), dibromamine (NHBr2), and tribromamine (NBr3) can be formed during oxidative treatment of waters containing bromide and ammonia. The formation and decomposition of bromamines in aqueous solution was investigated and a comprehensive kinetic model of the bromine-ammonia system was developed at 23 ± 1 °C. Determination of rate constants and model validation were primarily performed at pH 8.0 – 8.3 for subsequent application to seawater disinfection. The rate constant of NHBr2 self-decomposition was determined by second-order rate law linearization with k9 = 5.5 (± 0.8) M−1s−1 at pH 8.10. The rate constant of NBr3 self-decomposition increased proportionately to the concentration of hydroxide ions (OH−) according to the equation k10 = 4.4 (± 0.1) × 107. [OH−] over the pH range 6.0 – 8.5, which gave k10 = 56 (± 1) M−1s−1 at pH 8.10. The rate constants of NHBr2 and NBr3 formation were obtained by fitting model-predicted data to the experimental results and were found to be k3 = 2.3 (± 0.2) × 104M−1s−1 and k5 = 4.0 (± 0.6) × 103M−1s−1, respectively at pH 8.10. NBr3 was also found to react with NHBr2 with k11 = 3.4 (± 0.2) × 103M−1s−1 at pH 8.10. A kinetic model was proposed based on these experimental rate constants and literature values, which provided a good prediction of bromamines formation and decomposition for various initial bromine and ammonia concentrations. The kinetic model was also used to accurately predict the total oxidant concentration and the speciation of bromamines during breakpoint bromination. This study provides kinetic data to model more complex oxidative systems such as seawater chlorination in the presence of ammonia.

Interrogating the chemical mechanism for carbon dioxide (CO2) reactivity in aqueous tertiary amine solutions: Can a simple parallel hydration reaction mechanism work?

AbstractIn this study, the kinetics of CO2 absorption into tertiary amine methyldiethanolamine (MDEA) was studied intensively using both experimental and theoretical approaches. The experimental work measured the kinetics of CO2(aq) react with MDEA over a range of concentrations including dilute (0.5–10.0 mM) and concentrated (0.5–2.5 M) solutions via stopped‐flow spectrophotometric at 25.0°C. The resulting kinetic data has been theoretically evaluated using three chemical reaction mechanism variants including the complete base catalysis mechanism, the simplified base catalysis mechanism, and the parallel CO2 hydration mechanism. Results showed that the simplified base catalysis mechanism in the absence of the kinetic pathways for CO2 via hydroxide and water is insufficient to predict the measured kinetic data while the complete base catalysis mechanism and parallel hydration mechanisms can accurately predict the kinetics of CO2 absorption into MDEA solutions. In particular, the experimental data of reversibility of the hydration reactions at low pH was in poor agreement with the simplified model predicted results. Given the simplicity of the parallel hydration mechanism with no requirement for determination of rate constants of the base catalysis reaction unlike the complete base catalysis mechanism, the parallel hydration mechanism can be considered the simplest and preferred mechanism describing the reaction between tertiary amine MDEA and CO2. © 2022 Society of Chemical Industry and John Wiley & Sons, Ltd.

Pitfalls of quantifying intersystem crossing rates in singlet-fission chromophore solutions

Singlet fission (SF), a process that produces two triplet excitons from one singlet exciton, has attracted recent interest for its potential to circumvent the detailed-balance efficiency limit of single-junction solar cells. For the potential of SF to be fully realized, accurate assignment and quantification of SF is necessary. Intersystem crossing (ISC) is another process of singlet to triplet conversion that is important to distinguish from SF to avoid either over- or under-estimation of SF triplet production. Here, we quantify an upper bound on the rate of ISC in two commonly studied SF chromophores, TIPS–pentacene and TIPS–tetracene, by using transient absorption spectroscopy of solutions of varying concentrations in toluene. We show that SF in solutions of these acenes has previously been misidentified as ISC, and vice versa. By determining a bimolecular SF rate constant in concentrated solutions in which SF dominates over ISC, we distinguish triplet formation due to SF from triplet formation due to ISC and show that the characteristic time scale of ISC must be longer than 325 ns in TIPS–pentacene, while it must be longer than 118 ns in TIPS–tetracene. We additionally note that no excimer formation is observed in the relatively dilute (up to 8 mM) solutions studied here, indicating that previous excimer formation observed at much higher concentrations may be partially due to aggregate formation. This work highlights that an accurate quantification of ISC is crucial as it leads to accurate determination of SF rate constants and yields.

Generation and Reactivity of 1-Imidocarbenium Cations in the Friedel-Crafts-type Reaction.

Herein, we discuss the formation and reactivity of 1-imidocarbenium cations in Friedel-Crafts-type reaction between 1-imidoalkylphosphonium salts and arenes. The observed weakening of Cα-P+ bond is described qualitatively and quantitatively. The determination of rate constants and activation energies of Cα-P+ bond cleavage enabled systematic reactivity investigations of a series of phosphonium salts with different structures. Finally, the application scope for the imidoalkylation of aromatic hydrocarbons was explored. The results confirm that the generated 1-imidocarbenium cations are reactive enough to alkylate strongly activated, less-activated, or even inactivated aromatic compounds.

Determination of rate constants for conformational changes of RNA helicases by single-molecule FRET TIRF microscopy.

RNA helicases couple nucleotide-driven conformational changes to the unwinding of RNA duplexes. Interaction partners can regulate helicase activity by altering the rate constants of these conformational changes. Single-molecule FRET experiments on donor/acceptor-labeled, immobilized molecules are ideally suited to monitor conformational changes in real time and to extract rate constants for these processes. This article provides guidance on how to design, perform, and analyze single-molecule FRET experiments by TIRF microscopy. It covers the theoretical background of FRET and single-molecule TIRF microscopy, the considerations to prepare proteins of interest for donor/acceptor labeling and surface immobilization, and the principles and procedures of data analysis, including image analysis and the determination of FRET time traces, the extraction of rate constants from FRET time traces, and the general conclusions that can be drawn from these data. A case study, using the DEAD-box protein eIF4A as an example, highlights how single-molecule FRET studies have been instrumental in understanding the role of conformational changes for duplex unwinding and for the regulation of helicase activities. Selected examples illustrate which conclusions can be drawn from the kinetic data obtained, highlight possible pitfalls in data analysis and interpretation, and outline how kinetic models can be related to functionally relevant states.

Antiradical activity, base-catalyzed hydrolysis and partition coefficients of some surfactants

Antiradical activity and some environmental characteristics such as biodegradability and bioavailability of several coconut oil derived surfactants have been studied by photometry and UV–Vis absorption spectroscopy. The effects of polar group size and type were revealed using non-ionic surfactants cocamide monoethanolamine, cocamide diethanolamine, amphoteric surfactant cocamidopropyl betaine, and anionic surfactants sodium lauryl sulfate and sodium alfa olefin sulfonate. Free radical scavenging properties were examined by determination of rate constant of the reaction with hydroxyl radicals. To calculate the rate constant the competitive reaction between hydroxyl radicals and p-nitroso-N,N-dimethyl aniline was investigated. The results show pronounced antioxidant activity of surfactants. To confirm the environmentally friendly character of mentioned surfactants the biodegradability and accumulation were studied using base-catalyzed hydrolysis and partition coefficient between n-octanol/water layers. The obtained results show that type of head group and the carbon chain structure have major effect on these properties.

Evaluation of thermodynamic consistency of kinetic parameters in cyclic enzyme-catalyzed reaction networks

It is shown by analysis of a cyclic enzymatic network that the presence of zero net flux/current cycles impose strong thermodynamic constraints on the values of the kinetic parameters. Thermodynamic analysis is carried out for the four-node pyramidal reaction network involving the conversion of 7,8-­dihydrofolate (H2F) and NADPH in the presence of H+ to 5,6,7,8-­tetrahydrofolate (H4F) and NADP+ catalyzed by the universal enzyme dihydrofolate reductase (DHFR). The complete set of empty reaction routes in the DHFR pathway is identified, equations for the thermodynamic constraints imposed on the rate constants of the various elementary steps are written down, and the thermodynamic consistency of the experimentally determined rate constants is evaluated. The visual approach based on the theory of graphs can be employed to test the thermodynamic consistency of kinetic parameters of general cyclic enzyme-catalyzed reaction networks.

Identification of Overpotentials in Vanadium Redox Flow Battery with Reference Electrodes and Determination of Apparent Electrochemical Rate Constants

Vanadium redox flow battery (VRFB) is an amateur technology that passed pilot stages in fulfilling the stationary applications at the energy storage market with a high share of renewable power generation. The commercial applications of VRFBs require the optimization of their internal losses and developing the optimal control strategies for in-grid operation. This calls for a deep understanding of the internal VRFB processes, which greatly benefit from both the mathematical modeling and experimental testing for the model validation and refinement.However, the available experimental data for VRFBs is usually limited by open-circuit voltage (OCV) and potential (OCP), charge/discharge voltage profiles at constant load current, full cell polarization curves at the fixed state of charge (SOC), and internal stack ohmic resistances. This also complicates the decoupling and accessing the voltage loss sources, with the prevailing ones usually classified as ohmic, concentration, and activation overpotentials. As such, Aaron et al. (1) showed that decoupling of overpotentials at positive and negative half-cell requires the half-cell potentials measurements with reference electrodes (RE) in addition to full cell data. Different configurations of RE incorporation (2–4) were suggested with a limited agreement with full cell experimental data. The alternative approach with extrapolation of parameters presented in the literature is also challenging due to their strong dependence on an experimental protocol (e.g., the electrode material and its pre-treatment procedure, a membrane, and a setup geometry). As an illustration, the comprehensive review Roznyatsovskaya et al. (5) highlights the four orders of magnitude difference in reported surface area-normalized rate constants. Therefore, the development of a self-consistent experimental setup is crucial for understanding and decoupling internal losses for flow batteries. Combined with VRFB modeling, this may extend accurate predictions of its operation at extreme SOCs and load currents, where voltage losses hamper the efficiency of the battery with the dominating ones usually attributed to concentration (mass-transport in solutions) and activation (sluggish kinetics of reactions on electrodes) overpotentials.Herein, we developed the experimental setup adopting a 5cm2 single flow cell equipped with reference electrodes at positive and negative sides. The experiments were performed in symmetric and full cell designs. The polarization curves were measured at different SOCs [10 – 90 %], electrolyte flow rates [5-30 ml min-1], and load currents. The additional effort was made to compare the results of the experiments in symmetric and full cell designs and extract the apparent kinetic rate constants for positive and negative half-cells. The latter was performed within the 0D modeling approach, which is based on general mass and charge conservation principles. The key feature of the model is the approach for the determination of the integral mass-transfer coefficient on the electrode-electrolyte interface that allows describing non-linear polarization behaviour at high current densities. The effective thermodynamically consistent single-step reaction mechanisms were considered at both half cells, thus decreasing the number of free kinetic parameters. Overall, the proposed approach demonstrated the growing impact of the membrane losses at high current densities. The extracted apparent rate constants under conditions of a real flow battery provide important insights for developing reliable and efficient VRFBs applicable for energy storage systems.1 D. Aaron, Z. Tang, A. B. Papandrew and T. A. Zawodzinski, J. Appl. Electrochem., 2011, 41, 1175–1182.2 H. Lim, J. S. Yi and D. Lee, J. Power Sources, 2019, 422, 65–72.3 S. Ressel, A. Laube, S. Fischer, A. Chica, T. Flower and T. Struckmann, J. Power Sources, 2017, 355, 199–205.4 M. Cecchetti, A. Casalegno and M. Zago, J. Power Sources, 2018, 400, 218–224.5 N. Roznyatovskaya, J. Noack, K. Pinkwart and J. Tübke, Curr. Opin. Electrochem., 2020, 19, 42–48.

Review on solid-state anaerobic digestion of lignocellulosic biomass and organic solid waste.

Sustainable management of organic solid wastes especially the municipal solid waste (MSW) is essential for the realization of various sustainable development goals (SDGs). Resource recovery centric waste processing technologies generate valorizable products to meet the operations and maintenance (O&M) costs while reducing the GHG emissions. Solid-state anaerobic digestion (SSAD) of organic solid wastes is a biomethanation process performed at a relatively higher total solids (TS) loading in the range of 10-45%. SSAD overcomes various limitations posed by conventional anaerobic slurry digesters such as higher degradable matter per unit volume of the bioreactor resulting in a smaller footprint, low freshwater consumption, low wastewater generation, simple upstream and downstream processes, relatively lower operation, and maintenance costs. This review elucidates the recent developments and critical assessment of different aspects of SSAD, such as bioreactor design, operational strategy, process performances, mass balance, microbial ecology, applications, and mathematical models. A critical assessment revealed that the operating scale of SSAD varies between 1000 and 100,000 ts/year at organic loading rate (OLR) of 2-15g volatile solids (VS)/L·day. The SSAD experiences process failures due to the formation of volatile fatty acids (VFAs), biogas pockets and clogging of the digestate outlet. Acclimatization of microbes accelerates the startup phase, steady-state performances, and the enrichment of syntrophic microbes with 10-50 times greater population of cellulolytic and xylanolytic microbes in thermophilic SSAD over mesophilic SSAD. Experimental limitations in the accurate determination of rate constants and the oversimplification of biochemical reactions result in an inaccurate prediction by the models.

Combined Experimental and Computational Kinetics Studies for the Atmospherically Important BrHg Radical Reacting with NO and O2.

We report on the first experimental determination of the absolute rate constant of the reaction of BrHg + NO in N2 bath gas using a laser photolysis-laser-induced fluorescence (LP-LIF) system. The rate constant of the reaction of BrHg + NO is determined to be 7.0-0.9+1.3 × 10-12 cm3 molecule-1 s-1 over 50-700 Torr and 315-353 K. The absence of a pressure or temperature dependence suggests that this reaction leads mainly to mercury reduction (Hg + BrNO) rather than mercury oxidation (BrHgNO). Our theoretical calculations using NEVPT2 energies on density functional theory (DFT) geometries are consistent with a barrierless reaction to form Hg + BrNO. The equilibrium constants and the rate constants of the reaction BrHg + O2 ⇌ BrHgOO are computed theoretically because they are too low to be measured in the LP-LIF system. Molecular oxygen quenches the LIF signal of BrHg with a large rate constant of (1.7 ± 0.1) × 10-10 cm3 molecule-1 s-1. Thus, different techniques that capture the absolute [BrHg(X̃)] would be advantageous for kinetics studies of BrHg reactions in the presence of O2. The computed equilibrium constant suggests a non-negligible upper limit of the fraction of BrHg stored as BrHgOO (up to 0.5) at low-temperature conditions, e.g., in the upper troposphere and in polar winters at ground level. Preliminary results indicate that BrHgOO behaves like HOO or organic peroxy radicals in reactions with atmospheric radicals.

Photochromic Performance of Donor‐Acceptor Stenhouse Adducts in Polymer Binders and Solution

AbstractDonor‐acceptor Stenhouse adducts (DASAs) are an exciting new class of negative photochromic compounds, offering high colouration and rapid photo‐switching speeds, however the relationship between binder properties and photo‐switching behaviour of DASAs in films is poorly understood. The determination and analysis of rate constants of seven DASAs in five polymer binders showed a complex relationship between the glass transition temperature (Tg) of the binder, the solubility parameters of the matrix and the push‐pull nature of the donor and acceptor moieties. The rate constants of photo‐switching steps that require large degrees of molecular motion such as the actinic and thermal carbon‐carbon double bond rotations (kbleach and k−1) were found to be highly dependent on the Tg of the polymer binder, with higher Tg binders restricting these processes, resulting in slower switching rates and impacting changes in colour intensity. Correlation of rate constants with the Hansen solubility parameter for solvents and binders revealed strong relationships between the sum of the polarity and hydrogen bonding parameters (δP+δH) and the actinic and thermal carbon‐carbon double bond rotations (kbleach and k−1). This is attributed to changes in the zwitterionic resonance contributions of the triene with increases in these two solubility parameters.