Dissociation Enthalpies Research Articles

Light-Initiated C–H Activation via Net Hydrogen Atom Transfer to a Molybdenum(VI) Dioxo

MoO2Cl2(bpy-tBu) (1) is shown to be a potent one-electron oxidant upon irradiation with 365 nm light in various solvents, while being a weak two-electron oxidant in the dark. Complex 1 is characterized to activate various types of C-H bonds photochemically, including allylic and benzylic positions as well as alkanes and aldehydes. In all of these oxidations, 1 ultimately forms a bimetallic Mo(V)/Mo(V) species with a μ-oxo ligand (2). Depending on the substrate, the major organic product is identified as either an oxygenated or a C-C coupled (homodimerized) compound along with a minor chlorinated species. The product selectivity is proposed to be dependent upon the relative values between the bond dissociation enthalpy (BDE) of a potentially new C-OH bond within the product versus the BDE of a Mo-OH motif within a Mo(V)O(OH) intermediate. Based on this, we can estimate the BDE for Mo-OH to be 83-93 kcal/mol. Mechanistic studies suggest that the C-H activation occurs via a net hydrogen atom transfer (HAT) from 1* occurring either asynchronously or via a stepwise electron-proton transfer (ET-PT) process. Complex 2 is further demonstrated to reform dioxo 1 in the presence of chemical oxidants.

Thermophysical property analysis of semi-clathrate hydrates of tetrabutyl-(ammonium/phosphonium) salts as phase change materials for cold storage

Hydrate has attracted extensive attention because of its adjustable phase change temperature and high cold storage density. In this paper, the thermophysical properties of semi-clathrate hydrates as the phase change materials (PCMs) formed by aqueous solutions of tetrabutyl-(ammonium/phosphonium) salts (TBAF, TBAC, TBAHSO4, and TBPB) with different concentrations (0.05∼0.44, mass fraction) were systematically studied, including formation/dissociation temperature, phase change enthalpy, cycle performance and hydrate number. The results showed that semi-clathrate hydrate had good cycle performance during cold storage and release. The randomness of hydrate formation temperature was much less than that of ice, which suggested the hydrate formation conditions were more controllable. With the increase of solution concentration, the apparent driving force for hydrate formation decreased, and the proportion of hydrate in total enthalpy increased from 5% to 100%. In addition, the adjustable range of phase change temperature of semi-clathrate hydrate formed by the solution above is 0.61∼27.9 °C, where ranged from high to low is TBAF, TBAC, TBPB, and TBAHSO4. The total phase change enthalpy was about 122∼302.6 kJ/kg. The range of the maximum, intermediate and minimum hydrate number of TBAHSO4 hydrate crystal were estimated to be 35∼38.7, 32∼35, and 24∼28.3, respectively, by dissociation temperature and enthalpy analysis using differential scanning calorimeter (DSC). This study could form the basis for energy saving in water-based cold storage process. According to the needs of different cold storage temperatures, different salt solutions can be selected as cold storage media, so as to achieve the purpose of energy saving.

Selective activation of four quasi-equivalent C–H bonds yields N-doped graphene nanoribbons with partial corannulene motifs

Selective C–H bond activation is one of the most challenging topics for organic reactions. The difficulties arise not only from the high C–H bond dissociation enthalpies but also the existence of multiple equivalent/quasi-equivalent reaction sites in organic molecules. Here, we successfully achieve the selective activation of four quasi-equivalent C–H bonds in a specially designed nitrogen-containing polycyclic hydrocarbon (N-PH). Density functional theory calculations reveal that the adsorption of N-PH on Ag(100) differentiates the activity of the four ortho C(sp3) atoms in the N-heterocycles into two groups, suggesting a selective dehydrogenation, which is demonstrated by sequential-annealing experiments of N-PH/Ag(100). Further annealing leads to the formation of N-doped graphene nanoribbons with partial corannulene motifs, realized by the C–H bond activation process. Our work provides a route of designing precursor molecules with ortho C(sp3) atom in an N-heterocycle to realize surface-induced selective dehydrogenation in quasi-equivalent sites.

The Influence of Syringic Acid and Erucic Acid on the Antioxidant Properties of Natural Rubber: Experimental and Molecular Simulation Investigations.

In this work, the influence of syringic acid (SA) and erucic acid (EA) on the oxidation resistance of natural rubber (NR) was investigated by combining experimental and computational methods. The antioxidant activities of SA and EA were predicted by calculating the enthalpy of bond dissociation (BDE), the anti-migration ability of antioxidants (AOs) in the rubber matrix by calculating the mean square displacement (MSD), and the effect of antioxidants on oxygen barrier properties of rubber materials by calculating the permeability coefficient (P). The predicted result is that EA has a better comprehensive performance than SA. The DPPH (2,2-diphenyl-1-picrylhydrazyl) test and mechanical properties test demonstrated the results predicted by the simulations. Both SA and EA can protect natural rubber, while EA has a better comprehensive effect.

(Digital Presentation) Phosphonium Salts As Potential Guest Substances for Ionic Semiclathrate Hydrates

Semiclathrate hydrates (SCHs) are ionic crystalline compounds consisting of an organic guest cation in several cages formed by hydrogen bonding of water1. SCHs have recently been considered for application to air conditioning systems and refrigeration systems since desirable equilibrium temperatures and the considerable decomposition enthalpies are observed in the solid-liquid equillibria2. Ionic guest substances of SCHs include quaternary onium compounds such as tetrabutylammonium2 and tetrabutylphosphonium based salts3. We report here the preparation and thermodynamic properties of SCHs based on quaternary phosphonium bromides containing an unsaturated carbon-carbon double bond, as shown in Fig. 1, in comparison with the conventional saturated phosphonium salt, such as tetrabutylphosphonium bromide (P4444-Br).The unsaturated phosphonium bromides (P444(1All)-Br and P444(2All)-Br) were synthesized by quaternarization reaction of tributylphosphine with alkenyl bromides in a nitrogen atmosphere. The aqueous solutions were prepared from 5 to 50 w/w %. After thorough stirring, the aqueous solutions were cooled in a freezer to form SCHs. The SCHs were immersed into a thermostatic bath at 268 K for at least 1 day. The temperature in the thermostatic bath was raised from 268 K at a rate of 0.1 K/step (1 step: more than 5 h). The equilibrium temperatures were defined as the temperature at which SCHs were completely dissolved. The ionic crystal structure was analyzed by a powder X-ray diffraction (PXRD) technique, the dissociation enthalpy was measured by a micro differential scanning calorimeter (μDSC).Fig. 2 depicts the solid-liquid phase equilibrium diagram representing the phase equilibrium temperatures of SCHs. The equilibrium temperature of P444(1All)-Br SCH was higher than P4444-Br SCH. These results might be due to the steric effect of alkenyl group. As shown in Fig. 3, the ionic crystal structure of both P444(1All)-Br and P444(All)-Br SCHs were orthorhombic. The dissociation enthalpy of P444(1All)-Br SCHs and P444(2All)-Br SCHs were 188 J/g and 199 J/g, respectively. The unsaturated phosphonium bromides would be regarded as potential guest substances used for latent heat storage materials in air conditioning systems.References M. Oshima, et al, J. Chem. Thermodynamics, 90, 277 (2015).H. Oyama, et al, Fluid Phase Equilibria, 234, 131 (2005).J. Shimada, et al, Chem. Eng. Sci., 236, 116514 (2021). Figure 1

A thermodynamic framework to identify apposite refrigerant former for hydrate-based applications

High latent heat storage capacity with naturally assisted salt rejection makes the clathrate compounds appropriate for applications towards load management and desalination processes. Adding to these energy savings are the ease of operations provided by water and the mild conditions at which the refrigerant hydrates are occurred. A direct comparison between these hydrates becomes unfeasible due to the scattered experimental data. Though thermodynamics can streamline this dispersed data, they are currently limited to being a proof of concept most accurately representing the experimental observations. We address this critical deficit of phase assessment and identify, from among R13, R14, R22, R23, R125, R134a and R152a, the most suitable hydrate former for the concerned application. An approach based on van der Waals and Platteeuw model is undertaken and the estimates are quantified in terms of percent average absolute relative deviations (% AARD). An average AARD of 1.75% and 2.68% is observed in pure and aqueous electrolytic phase of NaCl, KCl, CaCl2 and MgCl2, respectively. The model predictions are then estimated at temperature/salinity of 281 K/0 wt% and 284 K/3.5 wt%. Together with the qualitative assessment of the hydrate phase, viz, vapor pressure, compressibility and dissociation enthalpy, R152a refrigerant is observed to be the appropriate former for applications to both load management and desalination.

Rheological study of mixed cyclopentane + CO2 hydrate slurry in a dynamic loop for refrigeration systems

The use of CO2 hydrates as phase change materials is promising for secondary loop refrigeration, as these compounds present adjustable dissociation temperature that can be higher than 273.15 K and have high enthalpy of dissociation. In the present work, by using various promoters, cyclopentane (CP) in this study, the formation pressure of hydrates may be significantly lowered than CO2 hydrates. For their application as a secondary refrigerant, the rheological properties of mixed CP + CO2 hydrate slurry need to be controlled. The flow properties of CP and mixed CP + CO2 hydrate slurries were studied in a dynamic flow loop at different cyclopentane contents ranging from 3 wt% to 15 wt%, using a capillary viscometer based on Rabinowitch and Mooney equation. Rheological parameters (viscosity, flow behavior…) related to these two slurries were compared. The experimental results demonstrate that CP and mixed CP + CO2 hydrates slurries exhibit a shear thinning behavior and the viscosity values vary between 3 and 12 mPa.s-1 at [100–1000 s−1] for CP hydrates slurries and between 2 and 10 mPa s−1 at [100–1200 s−1] for CP + CO2 hydrates slurries. CP hydrate and CP + CO2 hydrate slurry viscosity is close to that of TBPB hydrate and lower than that of CO2 hydrate. Moreover, mixed CP + CO2 have shown a time-evolution of the rheological behavior.

Critical analysis of radical scavenging properties of atorvastatin in methanol recently estimated via density functional theory

In this communication we draw attention on serious flaws that plague recently reported antioxidant properties of atorvastatin (ATV) in methanol. First and foremost, we emphasize that the O-H bond dissociation energies (BDE) of about 400kcal/mol previously reported are completely wrong. Further, we present results refuting the previous claim that the proton affinity (PA) of ATV is smaller than that of the ascorbic acid. That unfounded claim relies on incorrect data for PA’s ascorbic acid (which we correct here) circulated in the literature. Further, we correct the values of the chemical reactivity indices (e.g., chemical hardness, electrophilicity index, electroaccepting and electrodonating powers), which were inadequately estimated previously via Kohn–Sham HOMO and LUMO energies. Finally, our updated values for O-H bond dissociation enthalpy (BDE = 91.4kcal/mol) and electron transfer enthalpy (ETE = 105.7kcal/mol) tentatively suggest that direct H-atom transfer (HAT) and sequential proton loss electron transfer (SPLET) may coexist.

Dioxygen Activation and Nδ,Nε-Dihydroxylation Mechanism Involved in the Formation of N-Nitrosourea Pharmacophore in Streptozotocin Catalyzed by Nonheme Diiron Enzyme SznF.

SznF is a nonheme diiron-dependent enzyme that catalyzes the critical N-nitrosation involved in the formation of the N-nitrosourea moiety in the pancreatic cancer drug streptozotocin. The N-nitrosation contains two successive N-hydroxylation and N-nitrosation steps, which are carried out by two separate active sites, namely, the central domain and cupin domain. Recently, the crystal structure of SznF was obtained, and the central domain was proved to contain a diiron cofactor to catalyze the N-hydroxylation. In this work, to gain insights into the O2 activation and the successive N-hydroxylation mechanism, on the basis of the high-resolution crystal structure, the enzyme-substrate complex models were constructed, and a series of combined QM/MM calculations were performed. Based on our calculations, the activation of O2 starts from the diiron(II,III)-superoxo (S) to generate the diiron(IV)-oxo species (Q) via a diiron(III,III)-peroxo (P)-like transition state or unstable intermediate (P'), and species P' can be described as a hybridization of diiron(IV)-oxo species and diiron(III,III)-peroxo (P) owing to the long distances of Fe1-Fe2 (4.22 Å) and O1-O2 (1.89 Å), which is different from those of other nonheme diiron enzymes. In the following hydroxylation of Nδ and Nε, the Nδ-hydroxylation was confirmed to occur first, agreeing with the experimental observations. Because the diiron(IV)-oxo species (Q) is responsible for hydroxylation, the reaction follows the H-abstraction/OH rebound mechanism, and the first abstraction occurs on the Nδ-H rather than Nε-H, which may be attributed to the different orientation of Fe(IV)-oxo relative to N-H as well as the bond dissociation enthalpies of two N-H bonds. The hydroxylation of N-methyl-L-arginine does not employ the diiron(III,III)-hydroperoxo (P″) to trigger the electrophilic attack of the guanidine to directly form the N-O bond, as previously suggested. In addition, our calculations also revealed that the direct attack of the Fe(IV)═O unit to the Nδ of the substrate corresponds to a higher barrier than that in the H-abstraction/OH rebound mechanism. These results may provide useful information for understanding the formation of the di-hydroxylation intermediate involved in the biosynthesis of N-nitrosation.

Exploring the pyrolysis chemistry of 1,3,5-trimethylcyclohexane with insight into fuel isomeric and multiple substitution effects

To reveal insights into the combustion mechanism of multiple alkyl substituent cycloparaffins, this work reports an experimental and modeling study of 1,3,5-trimethylcyclohexane (T135MCH) pyrolysis in an extended flow reactor at low and atmospheric pressures. More than 30 species were detected and quantified employing synchrotron vacuum ultraviolet photoionization molecular beam mass spectrometry, and a detailed kinetic model developed based on reaction classes and update kinetic data was validated against the measured species profiles with a reasonable agreement. The reaction flux analyses were performed to reveal the key pathways of the fuel decomposition, intermediates production and aromatics formation. For the primary decomposition, the branching ratios of reaction types show strong dependence on changes of pressures and temperatures, including unimolecular methyl elimination, unimolecular ring-opening isomerization and H-abstraction. Besides the direct dissociation channels, major intermediate hydrocarbons are formed via stepwise dehydrogenation, recombination with ĊH3 radical or “formally direct” chemically activated reactions triggered by Ḣ atom addition. Monocyclic aromatic hydrocarbons such as benzene and toluene can be produced by traditional H-abstraction/β-C-H scission sequence, cyclopentadiene-related pathways, or recombination mechanism from small linear products. The formations of indene and naphthalene are controlled by C5+C5 and C5+C4 mechanism respectively. The comparison work of species profiles combined with theoretical calculations of bond dissociation enthalpies (BDEs) was performed to reveal the multiple CH3-group substituent and isomeric effects of methylcyclohexane (MCH), 1,2,4-trimethylcyclohexane (T124MCH) and T135MCH on pyrolysis activity and ethylene/benzene formation. Besides the increased reaction active sites, the added CH3-group and ortho-substitution can both weaken the strength of CC and CH bonds, leading to the promoting decomposition activity. The different formation tendencies of products are caused by different BDEs, length of carbon skeleton, as well as complex fuel-specific pathways.

Experimental Hydrate Phase Equilibrium Data Relevant to Bitter Melon, Pineapple, and Grape Juice Concentration.

One of the major challenges experienced by the fruit juice industry is the steady rise in energy costs. Hence, it is of industrial interest to find possible environmentally friendly measures that reduce energy consumption while cost-effectively maintaining the quality of manufactured products. Hydrate-based juice concentration technology can be used to overcome this challenge. In the present work, experimental hydrate phase equilibrium conditions of three systems involving juices (system 1, CO2 + grape juice; system 2, CO2 + pineapple juice; system 3, CO2 + bitter melon juice) were measured using an isochoric pressure search method. The temperature and pressure ranges for reported experimental data were 272.6–282.3 K and 1.17–3.85 MPa, respectively. Results have shown that a decrease in water cut from 98.3 to 88.5 ± 2.53 wt % could shift the hydrate phase equilibrium conditions toward higher pressures and lower temperatures. This proved that all investigated juices exhibited inhibitory effects on gas hydrate formation. To properly assess the energy requirements for this novel technology, molar hydrate dissociation enthalpies were estimated using the Clausius–Clapeyron relations under different measurement conditions. Finally, it was established that a hydrate-based fruit juice concentration technology would be a credible alternative to existing commercial technologies, on the basis of the dehydration ratio of 57% obtained in the present study.

How Soft Is Secondary Electrospray Ionization?

Secondary electrospray ionization (SESI) mass spectrometry (MS) is a direct infusion technique often used for untargeted metabolomics, e.g., for online breath analysis. SESI is thought to be a soft ionization method, which is important to avoid interference from in-source fragments and to simplify compound annotation. In this work, benzylammonium ions, formed from volatile benzylamines, with known bond dissociation enthalpies were used as thermometer ions to investigate the internal energy distribution of ions that are produced by SESI. It is shown that SESI is softer than electrospray ionization (ESI), and therefore, SESI indeed qualifies as a soft ionization technique. However, we also found that the standard MS instrument settings used in the SESI community are relatively harsh. Proper soft tuning of the instrument is essential to fully benefit from the softness that SESI can provide. Moreover, there is evidence from in-source collision-induced dissociation (CID) experiments that analytes can be solvated in SESI under soft conditions, which supports a recently proposed SESI mechanism referred to as ligand switching.

Antioxidant and anticancer properties of plant‐based bioactive flavonoids cardamonin and alpinetin: A theoretical insight from •OOH antiradical and Cu (II) chelation mechanisms

AbstractA density functional theory (DFT) study aimed at a comparative investigation on the antioxidant mechanisms of cardamonin and alpinetin, two flavonoid bioactive components derived from plant sources with potential medicinal properties, has been performed through assessment of their •OOH scavenging ability in non‐polar medium and Cu (II) chelation ability in the absence and presence of explicit water molecules stabilising the metal cation. In addition, a comparison of the antioxidant properties of cardamonin with the moiety of phloroglucinol and acylated phloroglucinol has been performed to establish cardamonin's antioxidant potentiality among the phloroglucinol family of compounds. The results of the investigation reveal that alpinetin could be considered a better radical scavenger than cardamonin within the non‐polar lipid bilayer medium of biological systems. However, the role of both compounds as •OOH scavengers may be limited, in comparison to standard antioxidant molecules, such as α‐tocopherol, because of the fact that the bond dissociation enthalpy for the hydrogen atom abstraction mechanism has a positive value, which signifies that the radical scavenging mechanism is not a thermodynamically preferred reaction. Cardamonin is found also to be a weaker antioxidant molecule than phloroglucinol and acylated phloroglucinol moieties. In terms of metal chelation mechanism, cardamonin is a better Cu (II) chelator than alpinetin since it has a higher metal ion affinity than alpinetin upon coordination to the Cu (II) cation. The results obtained could also imply that cardamonin is a better agent in dealing with angiogenesis, tumour growth and metastatis stages of cancer than alpinetin, since the ability of chelating metal cation shown by cardamonin ascribes it a better role as an anticancer agent than alpinetin. Therefore, although alpinetin may prove to play an important role as a promising antioxidant and antiinflamatory agent than cardamonin, by scavenging radical species, cardamonin would prove more valuable in the control of various stages of cancer growth than alpinetin.

Efficient and Original Technique for the Optimization and Intensification of Membrane Processes Relating to the Treatment and Recovery of Discharges Loaded with the Emerging Pollutant Paracetamol

Paracetamol (acetaminophen) is one of the most commonly used pain relievers and fever reducers due to the high stability, solubility, and hydrophilicity; it is mainly detected in pharmaceutical releases. Its presence in aquatic systems is a serious ecological concern. The pharmaceutical industries are using several technologies for separation, extraction, and control such as membrane processes because of their numerous advantages. In the present work, a polymer inclusion membrane containing Tween 20 (TW20) as an extractive agent was fabricated for the treatment and recovery of effluents loaded with emerging pollutants (paracetamol). After characterization of the morphology and porosity, the membrane was adopted to carry out the extraction and recovery processes of paracetamol, assisted by activated carbon. Kinetic and thermodynamic models were applied to determine the values of permeability (P), initial flux (J0), association constant (Kass), and diffusion apparent coefficient (D*) parameters. Subsequently, activation parameters energy (Ea), association and dissociation enthalpies (ΔH#ass and ΔH#dis, respectively), and entropy (ΔS#) were measured. For this original technique, the effects of activated carbon on the evolution of processes carried out across the membrane have been investigated and indicate a significant optimization and intensification of the membrane performance and total paracetamol extraction rate. In addition, this technique is clean and does not affect the structure or the stability of the membrane.

Antioxidant Potential of Santowhite as Synthetic and Ascorbic Acid as Natural Polymer Additives.

A wide variety of additives are used to improve specific characteristics of the final polymeric product. Antioxidant additives (AAs) can prevent oxidative stress and thus the damage of polymeric materials. In this work, the antioxidant potential and thus the applicability of Santowhite (SW) as synthetic and ascorbic acid (Asc) as natural AAs were explored by using computational tools. Two density functional theory (DFT) methods, M05-2X and M06-2X, have been applied in combination with the 6-311++G(2d,2p) basis set in gas phase. Three antioxidant mechanisms have been considered: hydrogen atom transfer (HAT), single electron transfer-proton transfer (SET-PT), and sequential proton loss electron transfer (SPLET). Bond dissociation enthalpy (BDE), ionization potential (IP), proton dissociation enthalpy (PDE), proton affinity (PA), and electron transfer enthalpy (ETE) have been computed for each potential hydrogen donor site. The results indicate that the antioxidant potential of Asc is higher than SW. Furthermore, some of the C-H bonds, depending on their position in the structures, are potent radical scavengers, but O-H groups are more prone to donate H-atoms to free radicals. Nonetheless, both additives can be potentially applied to safeguard common polymers and prohibit oxidative stress-induced material deterioration.

Stability, atomic charges, bond‐order analysis, and the directionality of lone‐electron pairs on nitriles and isocyanides

AbstractThe carbon atoms in :CN and the nitrogen atoms in CN: are always the global electrostatic minima in the whole molecules of nitriles and isocyanides. The global molecular electrostatic potential minima (Vmin) values and the electrostatic potential isosurfaces higher than Vmin by 10 kcal/mol were investigated and visualized based on the optimized geometrical structures at B3LYP‐D3(BJ)/def2‐QZVP level for a series of nitriles and isocyanides with the selected 21 functional groups. The relative stability between RCN: and :CNR was quantified with isomerization energies and bond dissociation enthalpies by single‐point energy (SPE) calculations at B2PLYP‐D3(BJ)/cc‐pVTZ level. The bond dissociation enthalpies of RCN: were evaluated, and the standard molar enthalpies of formation for RCN: and R· were also calculated to estimate the bond dissociation enthalpies. The theoretical results show that the directionality of the lone pairs influences electrostatic distribution. The population analyses in terms of Hirshfeld and ADCH atomic charges and bond‐order analyses via Laplacian and fuzzy bond orders were also discussed in detail.

Thermodynamic and kinetic studies on antioxidant capacity of amentoflavone: a DFT (density functional theory) computational approach

Density functional theory (DFT) at the theoretical M06-2X/6-311G(d,p) level was used to assess thermodynamics and kinetics in the antioxidative action of amentoflavone (AF). The antioxidative HAT pathway (H-atom transfer) is assigned to this compound in gas, but the SPL-ET (sequential proton loss-electron transfer) is the main route in polar solvents methanol and water. In all four mediums gas, benzene, methanol, and water, 4‴-OH is the most active site in free radical quenching with the lowest BDE (bond dissociation enthalpy) values of 81.8–84.8 kcal/mol, as well as it exerted the PA (proton affinity) values of 29.8–33.0 kcal/mol in methanol and water. Regarding kinetics, when interacted with •OOH and •NO2 in gas and methanol, 4‴-OH group is also responsible for the lowest ΔG # values (Gibbs free energy of activation), and the highest rate constant K values. Acidic assessment also indicated that 4‴-OH is associated with the strongest acidity (the lowest pKa). Two favorable oriented 4‴-OH and 7-OH groups further exhibited antioxidative activity since they prevented metal ions Zn2+ and Fe2+ from participating in free radical producing processes, in which the most stable complex [FeAF(H2O)4] generated the lowest IE value of −206.2 kcal/mol, and E gap value of 3.491 kcal/mol, but the highest MIA values of 184.6 kcal/mol in methanol.

Quantum chemical calculations of formation enthalpies of cations and anions of ionic liquids

The development of advanced ionic-liquid-based applications requires accurate thermochemical data for both ionic liquids and their constituent ions. Herein, the formation enthalpies of alkylimidazolium cations ([CnMIm+], n = 2, 4, 6, 8, 10) and various anions (trifluoromethanesulfonate ([TfO−]), ethyl sulfate ([ES−]), dicyanamide ([DCA−]), and bis(trifluoromethane)sulfonylimide ([NTf2−])) in the ideal gas state were obtained by quantum chemical calculations using density functional theory (DFT: B3LYP, B98, M06, M06-2X) and composite (G4) thermochemical models in the framework of the isodesmic reaction approach. The enthalpies of heterolytic dissociation of neutral ion pairs (NIPs) for the [CnMIm+][X−] series (n = 2, 4, 6, 8, 10; X− = [TfO−], [ES−], [NTf2−], [DCA−], [BF4−], [PF6−], [Cl−], [Br−], [I−]) were derived using the obtained data and literature formation enthalpies (ΔfH°) of NIPs. Analysis of the thermochemical property trends revealed that the formation enthalpies of [CnMIm+] decrease linearly as the alkyl chain length increases. In addition, the use of the isodesmic reaction approach improved the reliability of the computed data. Furthermore, owing to its internal self-consistency, the G4 method was found to be the most appropriate for describing the thermodynamic quantities of the cations and anions of ionic liquids.

Fisetin and Robinetin antiradical activity under solvent effect: density functional theory study

The structural and antioxidant activity of two flavonols, namely, Fisetin and Robinetin, have been investigated employing the density functional theory (DFT) using B3LYP functional and 6-311++G (d, p) basis set. The calculations were performed in the gas phase and under the solvent effect of water, dimethylsulfoxide (DMSO), methanol, and benzene. The Hydrogen-Atom Transfer (HAT), single Electron Transfer Followed by Proton Transfer (SET-PT), and sequential Proton Loss Electron Transfer (SPLET) mechanisms were investigated to rationalize the radical scavenging capacities and to identify the favored antioxidant mechanism. Hence, the bond dissociation enthalpies (BDE) ionization potential (IP), IE, proton dissociation enthalpy (PDE), proton affinity (PA), and electron Transfer enthalpy (ETE) related to each mechanism were reported and discussed in function of the solvent effect. For both flavonols, the results showed that 4'-OH hydroxyl is the preferred active site following the trend 4'-OH > 3'-OH > 3-OH > (5'-OH) > 7-OH. Besides, the HAT mechanism is energetically the most favored pathway. The energetically favored solvents follow the trends water > DMSO > benzene > methanol and benzene > DMSO > methanol > water, for Fisetin and Robinetin, respectively.

The mechanism and rate constant of hydrogen transfer from solvent radicals to coal-based model compounds in direct coal liquefaction

In direct coal liquefaction, in addition to stabilizing coal radicals, the ability of solvent-derived cyclohexadienyl radicals to hydrolyze bridge bonds in coal is a part of their hydrogen-donating activity. In this work, the density functional theory calculations and transition state theory were employed quantitatively to analyze the rate constants of hydrogen transfer from solvent-derived cyclohexadienyl radicals to coal model compounds. Results indicated that polycyclic 9,10-dihydrophenanthrene, 4,5-dihydropyrene and 4,5,9,10-tetrahydropyrene radicals have higher hydrogenolysis ability, therefore they can promote the breakage of the Caryl–Calkyl bonds more effectively. There is no obvious correlation between the rate constants of hydrogen transfer from H-donors radicals to ipso-C of different coal model compounds and the dissociation enthalpies of Caryl–Calkyl bond. However, it can be found that model compounds with higher bond dissociation energies are more susceptible to hydrogenolysis by solvent-derived cyclohexadienyl radicals. Among the coal model compounds containing the same functional group, the coal model compounds with substituent groups directly attached to aromatic rings are more easily hydrolyzed by solvent-derived cyclohexadienyl radicals.