Homolytic Dissociation Research Articles

Study of Sterically Crowded Alkanes: Assessment of Non-Empirical Density Functionals Including Double-Hybrid (Cost-Effective) Methods.

We theoretically study here the homolytic dissociation reactions of sterically crowded alkanes of increasing size, carrying three different (bulky) substituents such as tert-butyl, adamantyl, and [1.1.1]propellanyl, employing a family of parameter-free functionals ranging from semi-local, to hybrid and double-hybrid models. The study is complemented with the interaction between a pair of HC(CH3)3 molecules at repulsive and attractive regions, as an example of a system composed by a pair of weakly bound sterically crowded alkanes. We also assessed the effect of incorporating reliable dispersion corrections (i. e., D4 or NL) for all the functionals assessed, as well as the use of a tailored basis set (DH-SVPD) for non-covalent interactions, which provides the best trade-off between accuracy and computational cost for a seemingly extended applications to branched or crowded systems. Overall, the PBE-QIDH/DH-SVPD and r2SCAN-QIDH/DH-SVPD methods represent an excellent compromise providing relatively low, and thus very competitive, errors at a fraction of the cost of other quantum-chemical methods in use.

Dissociative Electron Transfer to N‐(Arylthio)phthalimides: Factors Affecting the Formation and Dissociation of Radical Anions

AbstractA series of chemically and biologically relevant N‐(arylthio)phthalimides is synthesized and its electrochemical reduction investigated. The results of the electrochemical study are analysed through application of Savéant's dissociative electron transfer theory and with the assistance of theoretical calculations. Reduction of all investigated N‐(arylthio)phthalimides leads to the corresponding radical anions, followed by the ejection of the phthalimide anion resulting from the dissociation of the N−S chemical bond. This stepwise electron transfer mechanism is evidenced by the electrochemical data which allow determination of the standard reduction potentials of all compounds. In the formation of the N‐(arylthio)phthalimide radical anions, the theoretical calculations show that the incoming electron, is hosted by the phthalimidyl group indicating a homolytic dissociation mechanism. The dissociation rate constants of the radical anions are deduced and show that upon changing the substituent from the MeO, to Me, H, F, Cl and Br the dissociation becomes faster. A good correlation with the Hammett substituent constants is also obtained. Application of Savéant's theory through determination of the associated driving force and the intrinsic barrier energies help rationalize the observed results and show that the change in the dissociation rate constant depends mainly on the standard reduction potential of the parent compounds.

Homolytic H2 dissociation for enhanced hydrogenation catalysis on oxides

The limited surface coverage and activity of active hydrides on oxide surfaces pose challenges for efficient hydrogenation reactions. Herein, we quantitatively distinguish the long-puzzling homolytic dissociation of hydrogen from the heterolytic pathway on Ga2O3, that is useful for enhancing hydrogenation ability of oxides. By combining transient kinetic analysis with infrared and mass spectroscopies, we identify the catalytic role of coordinatively unsaturated Ga3+ in homolytic H2 dissociation, which is formed in-situ during the initial heterolytic dissociation. This site facilitates easy hydrogen dissociation at low temperatures, resulting in a high hydride coverage on Ga2O3 (H/surface Ga3+ ratio of 1.6 and H/OH ratio of 5.6). The effectiveness of homolytic dissociation is governed by the Ga-Ga distance, which is strongly influenced by the initial coordination of Ga3+. Consequently, by tuning the coordination of active Ga3+ species as well as the coverage and activity of hydrides, we achieve enhanced hydrogenation of CO2 to CO, methanol or light olefins by 4-6 times.

Theoretical mechanistic insights on the thermal and acid-catalyzed rearrangements of N-methyl-N-nitroanilines.

The thermal and acid-catalyzed rearrangement mechanisms of N-methyl-N-nitroanilines were theoretically investigated via density functional theory (DFT) calculations for all possible proposed mechanisms. The results indicate that the thermal rearrangement of N-methyl-N-nitroanilines undergoes a radical pair complex mechanism through the homolysis of their N-N bond to generate a radical pair complex and the recombination of the radical pairs followed by aromatization. For the acid-catalyzed rearrangements, N-methyl-N-nitroanilines are first protonated on the nitrogen atom of their aniline moiety and then generate protonated N-methyl-O-nitroso-N-phenylhydroxylamines through a three-membered spirocyclic oxadiaziridine transition state. The N-protonated N-methyl-O-nitroso-N-phenylhydroxylamines favor homolytic dissociation to generate N-methylaniline cationic radical and nitrogen dioxide complexes, which further combine together and aromatize to afford protonated N-methyl-o-nitroanilines and N-methyl-p-nitroanilines, respectively. The radical pair complexes are more stable than the corresponding solvent-caged radical pairs. The thermal rearrangements require higher activation energy than the corresponding acid-catalyzed rearrangements.

Surface hydrophobic MIL-100(Fe) MOFs to boost methane oxidation with nearly total selectivity to C1 oxygenates under mild conditions

Direct methane conversion into value-added liquid oxygenates is a holy-grail reaction. We modified MIL-100(Fe) MOFs materials with highly dispersed hydrophobic polydimethysiloxane nanoparticles to significantly enhance their water resistance. The C1 oxygenates demonstrated an impressive yield and selectivity of 83.13 mmol g cat.−1h−1 and 100 %, respectively, while achieving a turnover frequency of 332.5 h−1 for oxygenates. The results of spectroscopy characterization and theoretical calculation demonstrate that, during the hydrophobic modification process of the catalyst surface, the loss of some ligands leads to the formation of coordination unsaturated active sites (Fe(II) CUS). The Fe(II) CUS active sites can efficiently catalyze the cleavage of H2O2 to produce Fe(IV) = O active species with high Lewis acidity, those promote homolytic dissociation of methane C-H bond with a low energy barrier. This study expands the application of hydrophilic MOFs materials with coordinatively unsaturated metal sites in the heterogeneous catalytic reactions of liquid phase.

Theoretical Evaluation of Oleocanthal Reactive Centers.

Decarboxymethyl ligstroside aglycone (Oleocanthal) is an essential component of olive oil. It is therefore interesting to study its metabolism in the human body. In order to find the best possible starting point for this metabolism, a theoretical study was carried out using DFT calculations and docking studies. The DFT, B3LYP/6-311++G** and the PCM solvation model calculations were used to study the initial process of Oleocanthal metabolism by the CYP1A2 enzyme. Structures of radicals formed by homolytic dissociation of hydrogen atoms from the Oleocanthal structure were obtained and their properties were studied. Several parameters such as HOMO and LUMO energy gaps, Bond Dissociation Energy (BDE), hardness, and spin density of possible Oleocanthal radicals were taken into account. Docking of Oleocanthal into an enzyme binding pocket was also performed to locate the most probably metabolic site. Detailed analysis of the theoretical results allows the determination of the most likely reaction sites in Oleocanthal. The mode of binding of Oleocanthal to the CYP1A2 enzyme was also predicted. The results of the molecular docking studies are in agreement with the calculated quantum parameters. The theoretical predictions were compared with experimental data available in the scientific literature. A high correlation between theoretical calculations and experimental data was observed. The most likely site of Oleocanthal metabolism was identified. The results of our research support the usefulness of theoretical calculations in predicting metabolic pathways.

Atomization-Induced High Intrinsic Activity of a Biocompatible MgAl-LDH Supported Ru Single-Atom Nanozyme for Efficient Radicals Scavenging.

Developing efficient nanozymes to mimic natural enzymes for scavenging reactive radicals remains a significant challenge owing to the insufficient activity of conventional nanozymes. Herein, we report a novel Ru single-atom nanozyme (SAE), featuring atomically dispersed Ru atoms on a biocompatible MgAl-layered double hydroxide (Ru1 /LDH). The prepared Ru1 /LDH SAE shows high intrinsic peroxidase (POD)-like catalytic activity, which outperforms the Ru nanoclusters (NCs) nanozyme by a factor of 20 and surpasses most SAEs. The density functional theory calculations reveal that the high intrinsic POD-like activity of Ru1 /LDH can be attributed to a heterolytic path of H2 O2 dissociation on the single Ru sites, which requires lower free energy (0.43 eV) compared to the homolytic path dissociation on Ru NC (0.63 eV). In addition, the Ru1 /LDH SAE shows excellent multiple free radicals scavenging ability, including superoxide anion radical (O2 ⋅- ), hydroxyl radical (⋅OH), nitric oxide radical (NO⋅) and 2, 2-diphenyl-1-picrylhydrazyl radical (DPPH⋅). Given the advantages of Ru1 /LDH with high enzymatic activities, biosafety, and ease to scale up, it paves the way for exploring SAEs in the practical biological immunity system.

Atomization‐Induced High Intrinsic Activity of a Biocompatible MgAl‐LDH Supported Ru Single‐Atom Nanozyme for Efficient Radicals Scavenging

AbstractDeveloping efficient nanozymes to mimic natural enzymes for scavenging reactive radicals remains a significant challenge owing to the insufficient activity of conventional nanozymes. Herein, we report a novel Ru single‐atom nanozyme (SAE), featuring atomically dispersed Ru atoms on a biocompatible MgAl‐layered double hydroxide (Ru1/LDH). The prepared Ru1/LDH SAE shows high intrinsic peroxidase (POD)‐like catalytic activity, which outperforms the Ru nanoclusters (NCs) nanozyme by a factor of 20 and surpasses most SAEs. The density functional theory calculations reveal that the high intrinsic POD‐like activity of Ru1/LDH can be attributed to a heterolytic path of H2O2 dissociation on the single Ru sites, which requires lower free energy (0.43 eV) compared to the homolytic path dissociation on Ru NC (0.63 eV). In addition, the Ru1/LDH SAE shows excellent multiple free radicals scavenging ability, including superoxide anion radical (O2⋅−), hydroxyl radical (⋅OH), nitric oxide radical (NO⋅) and 2, 2‐diphenyl‐1‐picrylhydrazyl radical (DPPH⋅). Given the advantages of Ru1/LDH with high enzymatic activities, biosafety, and ease to scale up, it paves the way for exploring SAEs in the practical biological immunity system.

A Systematic Exploration of B-F Bond Dissociation Enthalpies of Fluoroborane-Type Molecules at the CCSD(T)/CBS Level.

Fluoroborane-type molecules (R1R2B-F) are of interest in synthetic chemistry, but to date, apart from a handful of small species (such as H2BF, HBF2, and BF3), little is known concerning the effect of substituents in governing the strength of the B-F bonds of such species toward homolytic dissociation in the gas phase. In this study, we have calculated the bond dissociation enthalpies (BDEs) of thirty unique B-F bonds at the CCSD(T)/CBS level using the high-level W1w thermochemical protocol. The B-F bonds in all species considered are very strong, ranging from 545.9 kJ mol-1 in (H2B)2B-F to 729.2 kJ mol-1 HBF2. Nevertheless, these BDEs still vary over a wide range of 183.3 kJ mol-1. The structural properties that affect the BDEs are examined in detail, and the homolytic BDEs are rationalized based on molecule stabilization enthalpies and radical stabilization enthalpies. Since polar B-F bonds may represent a challenging test case for density functional theory (DFT) methods, we proceed to examine the performance of a wide range of DFT methods across the rungs of Jacob's Ladder for their ability to compute B-F BDEs. We find that only a handful of DFT methods can reproduce the CCSD(T)/CBS BDEs with mean absolute deviations (MADs) below the threshold of chemical accuracy (i.e., with average deviations below 4.2 kJ mol-1). The only functionals capable of achieving this feat were (MADs given in parentheses): ωB97M-V (4.0), BMK (3.5), DSD-BLYP (3.8), and DSD-PBEB95 (1.8 kJ mol-1).

Особенности межмолекулярного взаимодействия в нефтяных дисперсных системах

Oil, gas condensates, oil products present a complex colloidal-dispersed system which often demonstrates the abnormally changing properties when external conditions change. Mixing the petroleum products can be accompanied by a non-linear behavior accompanied by synergistic and antagonistic effects. Understanding of the oil and oil products as oil dispersed systems, the specific features of intermolecular interaction largely clarify their behavior, changes in properties, chemistry and mechanism of reactions occurring in them. Petroleum systems are polydisperse, in which the components can coexist in different aggregate states, and the size of the dispersed phase can vary over a wide range. They consist of diverse compounds that differ in properties, structure, shapes and sizes of molecules. Due to the variety of components that make up oil disperse systems the intermolecular interactions determine such a feature of the behavior of oil systems as the phenomenon of self-organization and structuring, which manifest themselves when external influences change and are sensitive to them. They are characterized by the absence of charge and a minimum of charge-polarization interactions of molecules, and intermolecular interactions are largely determined by the presence of paramagnetic molecules. The uncompensated spin of macromolecular compounds due to the steric obstacles, a homolytic dissociation, and the presence of microelement compounds ensure the paramagnetism of petroleum dispersed systems. This leads to developing the stable associative combinations and the formation of complex structural units capable of redistributing components and layers between phases under the influence of external effects. Comprehensive analysis and unanimity of views on the physical and chemical interactions of the components of oil systems leading to a change in their structure, open up fundamentally new opportunities for intensifying processes in the practice of production, transportation and processing of hydrocarbon raw materials and the use of petroleum products, and also allow predicting the behavior of oil systems in processes which they are participating.

Unveiling the Surface Structure of ZnO Nanorods and H2 Activation Mechanisms with 17O NMR Spectroscopy.

ZnO plays a very important role in many catalytic processes involving H2, yet the details on their interactions and H2 activation mechanism are still missing, owing to the lack of a characterization method that provides resolution at the atomic scale and follows the fate of oxide surface species. Here, we apply 17O solid-state NMR spectroscopy in combination with DFT calculations to unravel the surface structure of ZnO nanorods and explore the H2 activation process. We show that six different types of oxygen ions in the surface and subsurface of ZnO can be distinguished. H2 undergoes heterolytic dissociation on three-coordinated surface zinc and oxygen ions, while the formed hydride species migrate to nearby oxygen species, generating a second hydroxyl site. When oxygen vacancies are present, homolytic dissociation of H2 occurs and zinc hydride species form from the vacancies. Reaction mechanisms on oxide surfaces can be explored in a similar manner.

Photolysis of fungicide triadimefon: A combined experimental and theoretical investigation on homolytic C[sbnd]O and C[sbnd]N bonds dissociation mechanisms

Among the most frequently used fungicides, triazole has been widely applied in agriculture on many crops. Extensive usage and low effective utilization result in the widespread occurrence of this kind of chemicals in the environment. Known as an important decomposition pathway in aquatic environment, photochemical transformation is believed to significantly affect the fate of organic contaminants. Herein, we reported the detailed photodegradation mechanism of a typical triazole, triadimefon (TDF), based on a combined experimental and theoretical strategy. Our results reveled that TDF could undergo a relative fast photodegradation with a pseudo-first-order rate constant of 0.14 h−1. Photoproducts identification using high-resolution mass spectrometry (HRMS) analyses indicated two different photolysis pathways of TDF, namely, the homolytic CO and CN bonds dissociation processes. Density functional theory (DFT) and time-dependent DFT (TDDFT) methods were employed to explore these two processes. Theoretical results showed that the CO bond dissociation could occur in either the S1 or T1 state, with the energy barrier being 9.0 and 10.7 kcal/mol, respectively, while the CN bond dissociation was only possible to occur in the T1 state, with a higher energy barrier of 24.3 kcal/mol. In addition, spin-orbit coupling calculations indicated that the triplet state could be effectively populated through S1 → T3 intersystem crossing. The present study highlighted the potential application of DFT and TDDFT methods as stable methods in photochemical study.

Machine Learning to Predict Homolytic Dissociation Energies of C-H Bonds: Calibration of DFT-based Models with Experimental Data.

Random Forest (RF) QSPR models were developed with a data set of homolytic bond dissociation energies (BDE) previously calculated by B3LYP/6-311++G(d,p)//DFTB for 2263 sp3C-H covalent bonds. The best set of attributes consisted in 114 descriptors of the carbon atom (counts of atom types in 5 spheres around the kernel atom and ring descriptors). The optimized model predicted the DFT-calculated BDE of an independent test set of 224 bonds with MAE=2.86 kcal/mol. A new data set of 409 bonds from the iBonD database (http://ibond.nankai.edu.cn) was predicted by the RF with a modest MAE (5.36 kcal/mol) but a relatively high R2 (0.75) against experimental energies. A prediction scheme was explored that corrects the RF prediction with the average deviation observed for the k nearest neighbours (KNN) in an additional memory of experimental data. The corrected predictions achieved MAE=2.22 kcal/mol for an independent test set of 145 bonds and the corresponding experimental bond energies.

Sulfone-Decorated Conjugated Organic Polymers Activate Oxygen for Photocatalytic Methane Conversion.

Oxidizing CH4 into liquid products with O2 under mild conditions still mainly relies on metal catalysis. We prepared a series of sulfone-modified conjugated organic polymers and found that the catalyst with proper SVI content (0.10) could drive O2 →H2 O2 →⋅OH to oxidize CH4 into CH3 OH and HCOOH directly and efficiently at room temperature under light irradiation. Experimental results showed that after 4 h reaction, decomposition rate and residual amounts of H2 O2 were 81.21 % and 4.83 mmol gcat -1 respectively, and CH4 conversion rate was 22.81 %. Mechanism studies revealed that illumination could induce the homolytic dissociation of S=O bonds on catalyst to produce oxygen and sulfur radicals, where the ⋅O could adsorb and activate CH4 , and the ⋅S could supply electrons for 1 O2 to generate H2 O2 and then for decomposing the H2 O2 into ⋅OH timely to oxidize CH4 . This research provided a novel organic catalysis approach for oxygen activation and utilization.

Sulfone‐Decorated Conjugated Organic Polymers Activate Oxygen for Photocatalytic Methane Conversion

AbstractOxidizing CH4 into liquid products with O2 under mild conditions still mainly relies on metal catalysis. We prepared a series of sulfone‐modified conjugated organic polymers and found that the catalyst with proper SVI content (0.10) could drive O2→H2O2→⋅OH to oxidize CH4 into CH3OH and HCOOH directly and efficiently at room temperature under light irradiation. Experimental results showed that after 4 h reaction, decomposition rate and residual amounts of H2O2 were 81.21 % and 4.83 mmol gcat−1 respectively, and CH4 conversion rate was 22.81 %. Mechanism studies revealed that illumination could induce the homolytic dissociation of S=O bonds on catalyst to produce oxygen and sulfur radicals, where the ⋅O could adsorb and activate CH4, and the ⋅S could supply electrons for 1O2 to generate H2O2 and then for decomposing the H2O2 into ⋅OH timely to oxidize CH4. This research provided a novel organic catalysis approach for oxygen activation and utilization.

Heterolytic Dissociation of H2 in Heterogeneous Catalysis

H2 dissociation plays crucial roles in catalytic hydrogenation reactions and hydrogen storage. Metal-nanoparticle-based heterogeneous catalysts often dissociate H2 via a homolytic pathway. The heterolytic H2 dissociation pathway was identified in several heterogeneous catalytic systems, including single-atom catalysts, metal–support interfaces, and bulk metal oxides/sulfides/nitrides/phosphides. The active site structures of these heterogeneous catalysts resemble homogeneous catalysts where the metal centers (Lewis acids) are coordinated with O/S/N/P atoms (Lewis bases). These Lewis acid–base pairs dissociate H2 molecules heterolytically into proton–hydride pairs, which favor the hydrogenation of polar functional groups in unsaturated hydrocarbons. In this review, we summarize the common structural features of heterogeneous, homogeneous, and enzyme catalysts in the heterolytic dissociation of H2. The active sites, Lewis acid–base pairs, are discussed throughout this review. The energy barriers and kinetic contributions of heterolytic and homolytic H2 dissociation pathways in heterogeneous catalytic systems are discussed. The spectroscopic evidence of the heterolytic H2 dissociation pathways is critically reviewed.

Cu/O Frustrated Lewis Pairs on Cu Doped CeO2(111) for Acetylene Hydrogenation: A First-Principles Study

In this work, the H2 dissociation and acetylene hydrogenation on Cu doped CeO2(111) were studied using density functional theory calculations. The results indicated that Cu doping promotes the formation of oxygen vacancy (Ov) which creates Cu/O and Ce/O frustrated Lewis pairs (FLPs). With the help of Cu/O FLP, H2 dissociation can firstly proceed via a heterolytic mechanism to produce Cu-H and O-H by overcoming a barrier of 0.40 eV. The H on Cu can facilely migrate to a nearby oxygen to form another O-H species with a barrier of 0.43 eV. The rate-determining barrier is lower than that for homolytic dissociation of H2 which produces two O-H species. C2H2 hydrogenation can proceed with a rate-determining barrier of 1.00 eV at the presence of Cu-H and O-H species., While C2H2 can be catalyzed by two O-H groups with a rate-determining barrier of 1.06 eV, which is significantly lower than that (2.86 eV) of C2H2 hydrogenated by O-H groups on the bare CeO2(111), showing the high activity of Cu doped CeO2(111) for acetylene hydrogenation. In addition, the rate-determining barrier of C2H4 further hydrogenated by two O-H groups is 1.53 eV, much higher than its desorption energy (0.72 eV), suggesting the high selectivity of Cu doped CeO2(111) for C2H2 partial hydrogenation. This provides new insights to develop effective hydrogenation catalysts based on metal oxide.

Isolated [B2(CN)6]2-: Small Yet Exceptionally Stable Nonmetal Dianion.

We report the observation of a small, yet remarkably stable, metal-free hexacyanodiborate dianion [B2(CN)6]2- in the gas phase. Negative ion photoelectron spectroscopy (NIPES) was employed to measure its spectra at multiple laser wavelengths, yielding a 1.9 eV electron binding energy (EBE) ─a remarkably high value of electronic stability and a ∼2.60 eV repulsive Coulomb barrier (RCB) for electron detachment. This rationalizes the observation of this dianion, although homolytic charge-separation dissociation into two [B(CN)3]•- is energetically favorable. Quantum chemical calculations demonstrate a D3d staggered conformation for both the dianion and radical monoanion, and the calculated EBE and RCB match the experimental values well. The simulated density of states spectrum reproduces all measured electronic transitions, while the simulated vibrational progressions for the ground state transition cover a much narrower EBE range compared to the experimental band, indicating appreciable auto-photodetachment via electronically excited dianion resonances.

Global Natural Orbital Functional: Towards the Complete Description of the Electron Correlation.

The current work presents a natural orbital functional (NOF) for electronic systems with any spin value independent of the external potential being considered, that is, a global NOF (GNOF). It is based on a new two-index reconstruction of the two-particle reduced density matrix for spin multiplets. The emergent functional describes the complete intrapair electron correlation, and the correlation between orbitals that make up both the pairs and the individual electrons. The interorbital correlation is composed of static and dynamic terms. The concept of dynamic part of the occupation numbers is introduced. To evaluate the accuracy achieved with the GNOF, calculation of a variety of properties is presented. They include the total energies and energy differences between the ground state and the lowest-lying excited state with different spin of atoms from H to Ne, ionization potentials of the first-row transition-metal atoms (Sc-Zn), and the total energies of a selected set of 55molecular systems in different spin states. The GNOF is also applied to the homolytic dissociation of selected diatomic molecules in different spin states and to the rotation barrier of ethylene, both paradigmatic cases of systems with significant multiconfigurational character. The values obtained agree with those reported at high level of theory and experimental data.

Acyclic Diene Metathesis (ADMET) Polymerization of 2,2,6,6‐Tetramethylpiperidine‐1‐sulfanyl (TEMPS) Dimers

Inside Cover: Marrying acyclic diene metathesis polymerization and BiTEMPS chemistry offers a simple method to prepare a tailor-made polymer with precise control over the primary structure. Pivotally, the macromolecule, decorated with sulfur-nitrogen bonds in each repeating unit, exhibits light-responsive reversible homolytic dissociation of disulfide linkage, where the specific treatment (e.g., anaerobic condition) is not required. The latter allows reorganization of the polymeric architecture on exposure to the stimulus, and this molecular-based response is foreseen to be translated to the macroscopic scale. This work is found in article number 2100118 by Hatice Mutlu and co-workers.