Experimental Rate Constants Research Articles

Increased Dissociation of Adamantanamines in Influenza A M2 S31N with Partial Block by Rimantadine

The ubiquitous mutation from serine (WT) to asparagine at residue 31 (S31N) in the influenza A M2 channel renders it insensitive to amantadine (AMT) and rimantadine (RMT) block, but it is unknown whether the inhibition results from weak binding or incomplete block. Two-electrode voltage clamp (TEVC) of transfected Xenopus oocytes revealed that the M2 S31N channel is essentially fully blocked by AMT at 10 mM, demonstrating that, albeit weak, AMT binding in a channel results in complete block of its proton current. In contrast, RMT achieves only a modest degree of block in the M2 S31N channel at 1 mM, with very little increase in block at 10 mM, indicating that the RMT binding site in the channel saturates with only modest block. From exponential curve fits to families of proton current wash-in and wash-out traces, the association rate constant (k1) is somewhat decreased for both AMT and RMT in the S31N, but the dissociation rate constant (k2) is dramatically increased compared with WT. The potentials of mean force (PMF) from adaptive biasing force (ABF) molecular dynamics simulations predict that rate constants should be exquisitely sensitive to the charge state of the His37 selectivity filter of M2. With one exception out of eight cases, predictions from the simulations with one and three charged side chains bracket the experimental rate constants, as expected for the acidic bath used in the TEVC assay. From simulations, the weak binding can be accounted for by changes in the potentials of mean force, but the partial block by RMT remains unexplained.

Gold Cementation Kinetic Features Using Dendritic Zinc Powders

Gold cementation kinetic features from cyanide solutions using zinc powders of various origins were studied. The first powder was obtained by distillation and is currently used in the vast majority of industries (traditional powder). The second powder was obtained by electrochemical reduction from alkaline solution (electrolytic powder). The main distinguishing feature of these powders is specific surface area. This indicator for the electrolytic powder is 2.6 times higher than for the traditional one (3.02 and 1.16 m2/g, respectively), which is associated with dendritic form of electrolytic powder. For cementation studies, a solution with a gold content of 50.8 μmol/dm3 and 0.04 mol/dm3 sodium cyanide (NaCN) was taken. Studies have shown the presence of a depassivation period associated with oxide film dissolution on the powder surface and overcoming diffusion difficulties. The depassivation period of a traditional powder (10–15 seconds) exceeds the depassivation period of an electrolytic powder (5–8 seconds). Values of cementation reaction experimental rate constants are determined for the process using traditional and electrolytic powders at different ratios of zinc and gold masses in solution. Experimental reaction rate constants for electrolytic powder under the studied conditions are 1.3–1.6 times higher than the experimental rate constants for traditional powder. Zinc oxidation rates are compared under these conditions for various ratios m(Zn)/m(Au). Powder dissolution rates referred to specific surface area (taken in initial period) practically do not differ. The electrolytic powder’s absolute dissolution rate at the initial time is almost two times greater. As powder reacts with an alkaline solution, absolute dissolution rates of electrolytic and traditional powders are equalized.

Kinetic features of gold cementation using dendritic zinc powders

The study covers kinetic features of gold cementation from cyanide solutions using two zinc powders of various origins. The first one was obtained by distillation and is currently applied in gold cementation from cyanide solutions (traditional powder). The second one was obtained by electrochemical reduction from the alkaline solution (experimental, electrolytic powder). The main distinguishing feature of these cementation powders is their specific surface area. This indicator for the electrolytic powder is 2.6 times higher than for the traditional one (3.02 m2/g and 1.16 m2/g, respectively) due to its dendritic form. The studies used a solution with a gold content of 50.8 μmol/dm3and a sodium cyanide content of 0.04 mol/dm3NaCN was taken. Cementation revealed a powder depassivation period associated with oxide film dissolution and overcoming diffusion difficulties. The traditional powder depassivation period (10–15 sec) exceeds that of the electrolytic powder (5–8 sec). Experimental rate constants of the cementation reaction were determined for the process involving both powders under study at different ratios of zinc and gold masses in the solution. Experimental reaction rate constants for the electrolytic powder under the studied conditions were 1.3– 1.6 times higher than that for the traditional powder. It was found that oxidation rates of zinc powders compared at different ratios of zinc and gold masses are virtually the same. At the same time, absolute rates of electrolytic powder dissolution in the initial period are nearly 2 times greater. As the powder reacts with the alkaline solution, absolute dissolution rates of electrolytic and traditional powders are equalized.

Kinetic prediction of reverse intersystem crossing in organic donor\u2013acceptor molecules

Reverse intersystem crossing (RISC), the uphill spin-flip process from a triplet to a singlet excited state, plays a key role in a wide range of photochemical applications. Understanding and predicting the kinetics of such processes in vastly different molecular structures would facilitate the rational material design. Here, we demonstrate a theoretical expression that successfully reproduces experimental RISC rate constants ranging over five orders of magnitude in twenty different molecules. We show that the spin flip occurs across the singlet–triplet crossing seam involving a higher-lying triplet excited state where the semi-classical Marcus parabola is no longer valid. The present model explains the counterintuitive substitution effects of bromine on the RISC rate constants of previously unknown molecules, providing a predictive tool for material design.

A photochemical study of the triplet excited state of pyrene-4,5-dione and pyrene-4,5,9,10-tetrone derivatives

The present study details the use of nanosecond laser transient absorption spectroscopy to investigate the reactivity of the triplet excited states of pyrene-4,5-dione and pyrene-4,5,9,10-tetrone and the respective 2,7-di-t-butyl derivatives. The quinone triplet excited states were quenched by 2-propanol and 1,4-cyclohexadiene with quenching rate constants (kq) of the order 107 L mol−1 s−1, as well as by phenols, amines, and N-acetyl tryptophan methyl ester (model biological compounds) with near diffusion limit experimental rate constants (kq > 109 L mol−1 s−1) and very small kinetic isotopic effects for phenolic H/D abstraction. Hydrogen abstraction from the quenchers gave rise to the corresponding radical pairs. The intermediates were spectroscopically characterized. The quinones can also act as type II photosensitizers as they can generate singlet oxygen with quantum yields of up to 0.90. Finally, steady-state photolysis of the compounds demonstrated that pyrene-4,5-diones undergo a one-step reduction whilst pyrene-4,5,9,10-tetrones are reduced in two sequential stages. Both the diones and the tetrones are reduced to the respective aromatic catechols. Notably, the changes are reversible indicating that the compounds can act as photo-redox catalysts for reactions involving hydrogen atom abstraction (HAT).

Conclusive determination of ethynyl radical hydrogen abstraction energetics and kinetics*

The ethynyl radical (C 2 H) is ubiquitous, appearing in a variety of environments from interstellar space to oxy-acetylene flames. Under these diverse conditions, ethynyl has a high affinity to abstract hydrogen atoms from nearby molecules. In this study, C 2 H + HX → C 2 H 2 + X , X = CH 3 , NH 2 , OH, F, PH 2 , SH, Cl, C 2 H, CN, and NC reactions have been examined at a rigorously high level of theory. Various additive corrections have been appended to the coupled-cluster framework to converge the relative enthalpy of stationary points on the C 2 H + HX surfaces to subchemical accuracy (≤0.5 kcal mol − 1 ). The barriers for the abstraction of a hydrogen from NH 3 , PH 3 , and H 2 S are submerged below the relative enthalpy of their reactant asymptotes by 2.75, 2.48, and 1.39 kcal mol − 1 , respectively. ions from H 2 , CH 4 , H 2 O, and HCl are predicted to have moderate barriers to the abstraction of 2.64, 0.77, 3.82, and 2.19 kcal mol − 1 , respectively, suggesting these reactions will be significant to various systems at moderate to high temperatures. Accurate kinetics were obtained for the C 2 H + H 2 , CH 4 , NH 3 , and H 2 O reactions over a wide range of temperatures with excellent agreement to present experimental rate constants.

Integration of machine learning with phase field method to model the electromigration induced Cu6Sn5 IMC growth at anode side Cu/Sn interface

Currently, in the era of big data and 5G communication technology, electromigration has become a serious reliability issue for the miniaturized solder joints used in microelectronic devices. Since the effective charge number (Z*) is considered as the driving force for electromigration, the lack of accurate experimental values for Z* poses severe challenges for the simulation-aided design of electronic materials. In this work, a data-driven framework is developed to predict the Z* values of Cu and Sn species at the anode based LIQUID, Cu6Sn5 intermetallic compound (IMC) and FCC phases for the binary Cu-Sn system undergoing electromigration at 523.15 K. The growth rate constants (kem) of the anode IMC at several magnitudes of applied low current density (j = 1 × 106 to 10 × 106 A/m2) are extracted from simulations based on a 1D multi-phase field model. A neural network employing Z* and j as input features, whereas utilizing these computed kem data as the expected output is trained. The results of the neural network analysis are optimized with experimental growth rate constants to estimate the effective charge numbers. For a negligible increase in temperature at low j values, effective charge numbers of all phases are found to increase with current density and the increase is much more pronounced for the IMC phase. The predicted values of effective charge numbers Z* are then utilized in a 2D simulation to observe the anode IMC grain growth and electrical resistance changes in the multi-phase system. As the work consists of the aspects of experiments, theory, computation, and machine learning, it can be called the four paradigms approach for the study of electromigration in Pb-free solder. Such a combination of multiple paradigms of materials design can be problem-solving for any future research scenario that is marked by uncertainties regarding the determination of material properties.

A conceptual DFT analysis of the plausible mechanism of some pericyclic reactions

Out of several pericyclic reactions, Diels-Alder (DA) reaction is one of the most widely used synthetic processes. In the present work, several models and methodologies have been used to determine and to analyze the plausible mechanism of some representative DA cycloaddition reactions. A comparison between the dual descriptor and the bond reactivity indices corresponding to the natural bond orbital of the reagents is included, which provides a complete description of the plausible reaction mechanism. In the next step, two very recent models are used to determine the local electronic density transfer and redistribution between the reactants involved. The description of the local electronic density transfer has been made in two stages; first, the variation in the net charge on the atoms is obtained, and then, the electronic density transfer between the natural bond orbitals is calculated. The values obtained using the two models are correlated with the experimental rate constants of the reactions. Finally, the natural bond orbitals are obtained at several steps along the reaction path and the variation in their partial occupation is compared with the corresponding electron density transfer among these orbitals. Furthermore, frontier molecular orbital (FMO) approach has been employed to understand the more feasible way of interaction between the DA pair. Relative electrophilicity descriptors like net electrophilicity (∆ω±), net reactivity index (NRI, Δ $$ {\omega}_R^{\pm } $$ ), and electrophilicity difference (∆ω) between DA pairs have also been employed to describe the studied reaction mechanisms especially whether they follow non-polar-concerted/polar-stepwise pathway along with their classification in terms of normal or inverse electron demand. Furthermore, adaptive natural density partitioning method (AdNDP) and energy decomposition analyses (EDA) in conjunction with natural orbital for chemical valence (NOCV) have been made use of in order to analyze the actual bonding situation in the transition state (TS).

Kinetics of the reaction between a lignin model compound and chlorine dioxide

Abstract In this experimental study using 5,5′-diallyl-2,2′-dihydroxy-3,3′-dimethoxybiphenyl (DDD, a laboratory-synthesized biphenyl lignin model compound), the activity, kinetics and mechanisms in aqueous solution were investigated for the delignification of ClO2. The influence of the process operating conditions (initial pH and temperature) on the reaction pathway and product distribution were studied as well. The DDD instant concentration was calculated by a special differential ultraviolet–visible (UV–vis) method to exclude the effect caused by newly generated products. Kinetic parameters for the reaction in aqueous solution were determined for DDD and ClO2 with a kinetic model by regression analysis. The reaction with excess ClO2 at a pH of 3.32 was monitored to elucidate the reaction kinetics. The reaction was proven to be first-order with respect to DDD. The overall kinetic order of the reaction obeys the characteristics of a second-order kinetic reaction at lower pH values, but the fit to a second-order rate law is gradually lost with an increase in the pH. Under the experimental conditions first-order rate constants (k), second-order rate constants (K) and reaction stoichiometries were affected by the initial pH and temperature. Gas chromatography-mass spectrometry (GCMS), UV–vis spectroscopy and an AOX analyzer were employed to characterize the newly generated products. The activation energy was calculated to be 141.58 kJ mol−1 by the Arrhenius equation. The potential reaction pathway of one of the newly generated product ((1Z,3Z,5Z,7Z)-2,7-diallylocta-1,3,5,7-tetraene-1,4,5,8-tetracarboxylate detected by GCMS) is proposed.

Rethinking the description of water product in polyatomic OH/OD + XH (X ≡ D, Br, NH2 and GeH3) reactions: theory/experimental comparison

Davis’ group and Setser’s group using different experimental techniques measured water vibrational distribution for OH/OD + XH → H2O/HOD + X hydrogen abstraction reactions (X ≡ D, Br, NH2, GeH3). Theoretically, this issue has been studied by several groups: different potential energy surfaces (PESs) have been developed for each system, and quasi-classical trajectory (QCT) and quantum mechanics (QM) calculations have been performed. However, important experimental/theoretical controversies still exist. In the present work, we have revisited and performed new kinetics and dynamics calculations, comparing the theoretical and experimental results on the same footing. In general, theoretical results reproduce reasonably well experimental rate constants and total vibrational energy released to water, ~ 50–60%, although they underestimate water bending excitation. In the present work, we analyse different causes of this theory/experiment discrepancy and propose different mechanisms to explain the water bending excitation for diatom–diatom and polyatomic systems. Finally, we reflect on the ability of QCT and QM calculations to provide reasonable (although not quantitative) predictions for polyatomic reactions and observe that even using full-dimensional QM calculations on very accurate PESs, agreement with the experiment is far from that reached in the case of triatomic systems.

Kinetics of the Light-Oxygen Effect in Aqueous Solutions of Proteins

Molecular mechanisms of the participation of proteins in reactive oxygen species (ROS) generation resulting from the singlet-oxygen effect upon laser exposure in solutions of various proteins are considered. The kinetic equations describing ROS generation are derived, and their approximate general solution is found in the analytical form. By comparison of the found kinetic dependences with experimental data, orders and rate constants of corresponding chemical reactions are determined. An analysis of the results makes it possible to consider that fundamental mechanisms of ROS formation reactions in solutions of various proteins are identical.

Quantum chemical and kinetic study of the CCl2 + HCl → CHCl3 insertion reaction

The CCl2 + HCl → CHCl3 (1) insertion reaction has been investigated by ab initio molecular orbital and kinetic calculations. The geometries of reactants, products and transition state were optimized with a large number of density functional theory (DFT) formulations combined with the 6-311++G(3df,3pd) basis set. The reaction barriers calculated with G4(MP2)//DFT and G4//DFT theories of 1.16 ± 0.27 and 0.61 ± 0.26 kcal mol−1 are consistent with the barrier height of ΔE0# = 1.54 ± 0.30 kcal mol−1 estimated from the room temperature experimental rate constant. Calculated enthalpies of reaction of −53.97 ± 0.09 (G4(MP2)//DFT), −55.27 ± 0.09 (G4//DFT) and −56.52 ± 3.90 kcal mol−1 (DFT) agree well with experimental values. The obtained rate constants over the 300–2000 K temperature range at the high and low pressure limit can be expressed as k1,∞(G4(MP2)//DFT) = (4.8 ± 1.8) × 10−14 (T/300 K)2.12±0.19 and k1,∞(G4//DFT) = (9.8 ± 3.4) × 10−14 (T/300 K)1.77±0.16 in cm3 molecule−1 s−1, and k1,0 = [HCl] 3.40 × 10−27 (T/300 K)−6.57 exp(−2218 K/T) cm3 molecule−1 s−1. Falloff curves for the intermediate pressure range obtained for the reactions (1, −1) with these rate constants and a derived central broadening factor of Fcent = 0.16 + 0.84 exp(−T/331 K) + exp(−7860 K/T) were compared with reported experimental rate data.

Correcting Rate Constants from Anharmonic Molecular Dynamics for Quantum Effects.

Anharmonicity can greatly affect rate constants. One or even several orders of magnitude of deviation are found for obtaining rate constants using the standard rigid-rotor harmonic-oscillator model. In turn, reactive molecular dynamics (MD) simulations are a powerful way to explore chemical reaction networks and calculate rate constants from the fully anharmonic potential energy surface. However, the classical nature of the dynamics and the required numerical efficiency of the force field limit the accuracy of the resulting kinetics. We combine the best of both worlds by presenting an approximation that pairs anharmonic information intrinsic to classical MD with high-accuracy energies and frequencies from quantum-mechanical electronic structure calculations. The proposed scheme is applied to hydrogen abstractions in the methane system, which allows for the benchmarking of rate constants corrected by our approach against experimental rate constants. This comparison reveals a standard deviation of factor 2.6. Two archetypes of possible failure are identified in the course of a detailed investigation of the CH3• + H• → CH22• + H2 reaction. From this follows the application range of the method, within which the method shows a standard deviation of factor 2.1. The computational efficiency and beneficial scaling of the method allow for application to larger systems, as shown for hydrogen abstraction from 2-butanone by HO2•.

Quantum Effects on the D + H3+ → H2D+ + H Deuteration Reaction and Isotopic Variants.

The title reaction and its isotopic variants are studied using quasi-classical trajectory (QCT) (without taking into account corrections to account for the possible zero point energy breakdown) and ring polymer molecular dynamics (RPMD) methods with a full dimensional and accurate potential energy surface which presents an exchange barrier of approximately 0.144 eV. The QCT rate constant increases when the temperature decreases from 1500 to 10 K. On the contrary, the RPMD rate constant decreases with decreasing temperature, in semiquantitative agreement with recent experimental results. The present RPMD results are in between the thermal and translational experimental rate constants, extracted from the measured data to eliminate the initial vibrational excitation of H3+, obtained in an arc discharge. The difference between the present RPMD results and experimental values is attributed to the possible existence of non thermal vibrational excitation of H3+, not completely removed by the semiempirical model used for the analysis of the experimental results. Also, it is found that, below 200 K, the RPMD trajectories are trapped, forming long-lived collision complexes, with lifetimes longer than 1 ns. These collision complexes can fragment by either redissociating back to reactants or react to products, in the two cases tunneling through the centrifugal and reaction barriers, respectively. The contribution of the formation of the complex to the total deuteration rate should be calculated with more accurate quantum methods, as has been found recently for reactions of larger systems, and the present four atoms system is a good candidate to benchmark the adequacy of RPMD method at temperatures below 100 K.

Unimolecular HBr and HF Elimination Reactions of Vibrationally Excited C2H5CH2Br and C2D5CHFBr: Identification of the 1,1-HBr Elimination Reaction from C2D5CHFBr and Search for the C2D5(F)C:HBr Adduct.

Chemical activation experiments and computational methods have been used to study the unimolecular reactions of C2H5CH2Br and C2D5CHFBr with 90 and 93 kcal mol-1 of vibrational energy, respectively. The four-centered elimination reactions of HBr and DBr are the dominant reactions; however, 2,1-DF, 1,1-HBr, and 1,1-HF reactions are also observed from C2D5CHFBr. The main focus was to search for the role of the C2D5(F)C:HBr adduct in the 1,1-HBr elimination for comparison with carbene adducts in 1,1-HX(Y) elimination from RCHXY (X,Y = Cl and F) molecules. Models of transition states and molecules from electronic structure calculations were used in statistical calculations of the rate constants to assign threshold energies for each reaction based on the experimental rate constants. The threshold energy for 2,1-HBr elimination from 1-bromopropane is 50 kcal mol-1, which is in basic agreement with thermal activation experiments. Comparison of the 2,1-DBr and 2,1-HBr rate constants permits discussion of the kinetic isotope effects and the effect of F atom substitution on the threshold energy for 2,1-HBr elimination. Although CD3CD═CDF from 1,1-HBr elimination of C2D5CHFBr followed by D atom migration is an experimentally observed product, dissociation of the C2D5(F)C:HBr adduct may be the rate-limiting step rather than crossing the barrier associated with the transition state for 1,1-HBr elimination. The calculated dissociation energies of C2H5(X)C:HF adducts are 9.9, 9.3, and 9.0 kcal mol-1 for X = F, Cl, and Br, and the values for C2H5(F)C:HX are 9.9, 6.4, and ∼4.9 kcal mol-1.

Aqueous Chlorination Kinetics of Cyclic Alkenes-Is HOCl the Only Chlorinating Agent that Matters?

Although Cl2 and Cl2O have been recognized as highly reactive constituents of free available chlorine (FAC), robust rate constants for Cl2 and Cl2O remain scarce in the environmental literature. In this work, we explored the chlorination kinetics of three structurally related alkenes (α-ionone, β-ionone, and dehydro-β-ionone), a class of compounds whose reactivities with Cl2 and Cl2O have not been previously investigated. Second-order rate constants for Cl2, Cl2O, and HOCl were computed from experimental rate constants obtained at various pH values, [Cl-], and [FAC]. Our results show that while HOCl is the predominant chlorinating agent for the most reactive alkene, Cl2 and Cl2O can dominate the chlorination kinetics of the less reactive alkenes at high [Cl-] and high [FAC], respectively. The tradeoff between overall reactivity with FAC and selectivity for Cl2 and Cl2O previously observed for aromatic compounds also applies to the alkenes examined. In laboratory experiments in which high [FAC] may be used, omission of Cl2O in data modeling could yield second-order rate constants of dubious validity. In chlorinating real waters with elevated [Cl-], formation of Cl2 may enhance the formation kinetics of chlorinated disinfection byproducts (DBPs) and exacerbate the burden of DBP control for water utilities.

Experimental and Theoretical Study of the O(1D) + HD Reaction.

This work addresses the kinetics and dynamics of the gas-phase reaction between O(1D) and HD molecules down to low temperature. Here, measurements were performed by using a supersonic flow (Laval nozzle) reactor coupled with pulsed laser photolysis for O(1D) production and pulsed-laser-induced fluorescence for O(1D) detection to obtain rate constants over the 50-300 K range. Additionally, temperature-dependent branching ratios (OD + H/OH + D) were obtained experimentally by comparison of the H/D atom atom yields with those of a reference reaction. In parallel, theoretical rate constants and branching ratios were calculated by using three different techniques; mean potential phase space theory (MPPST), the statistical quantum mechanical method (SQM), and ring polymer molecular dynamics (RPMD). Although the agreement between experimental and theoretical rate constants is reasonably good, with differences not exceeding 30% over the entire temperature range, the theoretical branching ratios derived by the MPPST and SQM methods are as much as 50% larger than the experimental ones. These results are presented in the context of earlier work, while the possible origins of the discrepancies between experiment and theory are discussed.

Olefin hydroformylation and kinetic studies using mono- and trinuclear N,O-chelate rhodium(I)-aryl ether precatalysts

Rh(I)-salicylaldimine-triazole mononuclear (6) and trinuclear (7) complexes based on an aryl-ether scaffold were investigated as precatalysts in the rhodium-catalysed hydroformylation of the terminal olefin, 1-octene, and internal olefins, 7-tetradecene and 4-octene. The complexes generally show good catalytic activity in the hydroformylation of 1-octene at temperatures ranging from 75 °C to 95 °C and syngas pressures of 20 to 40 bar. The precatalysts have excellent stability and could be reused several times using organic solvent nanofiltration under optimum conditions of 85 °C and 40 bar. Kinetic studies using catalyst precursor 6 were investigated by evaluating the effect of temperature, syngas total pressure and catalyst loading on the rate of hydroformylation. The activation energy for the hydroformylation of 1-octene was calculated to be 62 kJ mol−1 and the experimental rate constants were found to be in good agreement with the predicted rate data obtained using a modified fundamental mechanism-based rate model.

Mechanistically Guided Predictive Models for Ligand and Initiator Effects in Copper-Catalyzed Atom Transfer Radical Polymerization (Cu-ATRP).

Copper-catalyzed atom transfer radical polymerization (Cu-ATRP) is one of the most widely used controlled radical polymerization techniques. Notwithstanding the extensive mechanistic studies in the literature, the transition states of the activation/deactivation of the growing polymer chain, a key equilibrium in Cu-ATRP, have not been investigated computationally. Therefore, the understanding of the origin of ligand and initiator effects on the rates of activation/deactivation is still limited. Here, we present the first computational analysis of Cu-ATRP activation transition states to reveal factors that affect the rates of activation and deactivation. The Br atom transfer between the polymer chain and the Cu catalyst occurs through an unusual bent geometry that involves pronounced interactions between the polymer chain end and the ancillary ligand on the Cu catalyst. Therefore, the rates of activation/deactivation are determined by both the electronic properties of the Cu catalyst and the ligand-initiator steric repulsions. In addition, our calculations revealed the important role of ligand backbone flexibility on the activation. These theoretical analyses led to the identification of three chemically meaningful descriptors, namely HOMO energy of the catalyst ( EHOMO), percent buried volume ( Vbur%), and distortion energy of the catalyst (Δ Edist), to describe the electronic, steric, and flexibility effects on reactivity, respectively. A robust and simple predictive model for ligand effect on reactivity is thereby established by correlating these three descriptors with experimental activation rate constants using multivariate linear regression. Validation using a structurally diverse set of ligands revealed the average error is less than ±2 kcal/mol compared to the experimentally derived activation energies. The same approach was also applied to develop a predictive model for reactivity of different alkyl halide initiators using R-X bond dissociation energy (BDE) and Cu-X halogenophilicity as descriptors.

Development of Prediction Models on Base-Catalyzed Hydrolysis Kinetics of Phthalate Esters with Density Functional Theory Calculation.

Many phthalate esters (PAEs) are chemicals of high production volume and of toxicological concern. The second-order rate constant for base-catalyzed hydrolysis ( kB) is a key parameter for assessing environmental persistence of PAEs. However, the kB values for most PAEs are lacking, and the experimental determination of kB encounters various difficulties. Herein, density functional theory (DFT) methods were selected by comparing empirical kB values of five PAEs and five carboxylic acid esters with the DFT-calculated ones. Results indicate that PAEs with cyclic side chains are more vulnerable to base-catalyzed hydrolysis than PAEs with linear alkyl side chains, followed by PAEs with branched alkyl side chains. By combining experimental and DFT-calculated second-order rate constants for base-catalyzed hydrolysis of one side chain in PAEs ( kB_sidechain), quantitative structure-activity relationship models were developed. The models can differentiate PAEs with the departure of the leaving group (or the nucleophilic attack of OH-) as the rate-determining step in the hydrolysis and estimate kB values, which provides a promising way to predict hydrolysis kinetics of PAEs. The half-lives of the investigated PAEs were calculated and vary from 0.001 h to 558 years (pH = 7∼9), further illustrating the necessity of prediction models for hydrolysis kinetics in assessing the environmental persistence of chemicals.