Dissociation Gibbs Free Energy Research Articles

Experimental and theoretical study on the thermal stability and pyrolysis mechanism of octamethyltrisiloxane (MDM) with lubricating oil

In actual organic Rankine cycle systems (ORCs), the inclusion of lubricating oil in the working fluid is nearly unavoidable. Under the condition of high-temperature operation, the presence of lubricating oil can potentially exacerbate the thermal stability of the working fluid. The impact of polyol ester (POE) and mineral lubricant on the thermal stability of octadethyltrisiloxane (MDM) is studied. Results indicate that both POE and mineral lubricant contribute to the decomposition of MDM to a certain extent. Specifically, at a 5.0% mass fraction of POE and mineral lubricant, MDM remains its stable at 230 °C and 210 ℃, respectively. However, as the mass fraction of lubricant increases, the decomposition of MDM intensifies, with mineral lubricant exhibiting a more pronounced effect compared to POE lubricant. Regarding pyrolysis products, the presence of lubricants leads to an increase in CH4, C2H6 and C2H4 gases. Furthermore, the addition of lubricant promotes the formation of siloxane oligomers, particularly MD5M to MD9M, and enhances the production of unidentified products. Compared to MDM, both POE and mineral lubricant molecules exhibit lower bond dissociation Gibbs free energies. The free radicals generated from lubricant decomposition are prone to hydrogen abstraction reactions with MDM to promote MDM decomposition. Moreover, ReaxFF simulation results demonstrate that lubricant molecules also enhanced the activation of MDM and increased its susceptibility to Si-C bond cleavage. Following the removal of the methyl group, the Si-O bonds in the radicals are more likely to break, leading to the formation of additional siloxane oligomers through Si-O bond recombination. Kinetic analysis reveals that the apparent activation energy of MDM decomposition drops from 134.4 kJ∙mol−1 to 110.8 kJ∙mol−1 as the mass fraction of n-hexadecane (model compounds for mineral lubricant) rises from 0% to 10.30%.

Screening of Organic Small Molecule Excipients on Ternary Solid Dispersions Based on Miscibility and Hydrogen Bonding Analysis: Experiments and Molecular Simulation.

The preparation of solid dispersions by mixing insoluble drugs with polymers is the main way to improve the aqueous solubility of drugs. The introduction of organic small molecule excipients into binary solid dispersions is expected to further enhance drug solubility by regulating intermolecular hydrogen bonding within the system at the microscopic level. In this study, we used carbamazepine (CBZ) as the target drug and polyvinylpyrrolidone as the solid dispersion matrix and screened the third component from 13 organic small molecules with good miscibility in the solid dispersion based on the principle of similarity of solubility parameters. The hydrogen bonding parameters and dissociation Gibbs free energy of the 13 organic small molecule-CBZ dimer were calculated by quantum mechanical simulation, and the tryptophan (Try) was identified as the optimal third component of organic small molecule. The migration of CBZ in binary and ternary systems was also analyzed by molecular dynamics simulation. On this theoretical basis, the corresponding solid dispersions were prepared, characterized, and tested for solubility analysis, which verified that the drug solubility was stronger for the system with the addition of polar fractions and the Try was indeed the best third component of organic small molecule compound, which was consistent with the simulation predictions. This screening method may provide theoretical guidance for drug modification design and clinical studies.

Urchin-Like Structured MoO2 /Mo3 P/Mo2 C Triple-Interface Heterojunction Encapsulated within Nitrogen-Doped Carbon for Enhanced Hydrogen Evolution Reaction.

The development of highly efficient and cost-effective hydrogen evolution reaction (HER) catalysts is highly desirable to efficiently promote the HER process, especially under alkaline condition. Herein, a polyoxometalates-organic-complex-induced carbonization method is developed to construct MoO2 /Mo3 P/Mo2 C triple-interface heterojunction encapsulated into nitrogen-doped carbon with urchin-like structure using ammonium phosphomolybdate and dopamine. Furthermore, the mass ratio of dopamine and ammonium phosphomolybdate is found critical for the successful formation of such triple-interface heterojunction. Theoretical calculation results demonstrate that such triple-interface heterojunctions possess thermodynamically favorable water dissociation Gibbs free energy (ΔGH2O ) of -1.28eV and hydrogen adsorption Gibbs free energy (ΔGH* ) of -0.41eV due to the synergistic effect of Mo2 C and Mo3 P as water dissociation site and H* adsorption/desorption sites during the HER process in comparison to the corresponding single components. Notably, the optimal heterostructures exhibit the highest HER activity with the low overpotential of 69mV at the current density of 10mA cm-2 and a small Tafel slope of 60.4mV dec-1 as well as good long-term stability for 125h. Such remarkable results have been theoretically and experimentally proven to be due to the synergistic effect between the unique heterostructures and the encapsulated nitrogen-doped carbon.

Mechanisms of Csp3-H functionalization of acetonitrile or acetone with coumarins: A DFT investigation

The mechanisms for the cross-dehydrogenative coupling (CDC) between coumarin and acetonitrile (or acetone) have been investigated with M06-L-D3 method, and 6−31+G(d,p) basis set for nonmetal elements and pseudopotential basis set of SDD for potassium atom. SMD model was applied to simulate the solvent effect. The computational results show that tert-Butyl peroxybenzoate (TBPB) is the best oxidant with the lowest bond dissociation Gibbs free energy (BDE=11.2 kcal/mol in the solvent of acetonitrile) and could be easily disintegrated into two radicals (PhCOO and t-BuO). In the first and second reactions, 2H-chromen-2-one reacted with acetontrile and acetone, respectively. Firstly, the Csp3-H activations of acetontrile and acetone could be achieved by PhCOO and t-BuO radicals; then cross-dehydrogenative coupling reactions converted 2H-chromen-2-one into final products P1 and P2; Gibbs free energy surfaces of these two reactions suggest that paths a4 and b4 would be the favorable paths with lower Gibbs energy barriers and releasing much energy. While in the third reaction, acetonitrile reacted with (E)-4-phenylbut-3-en-2-one via six possible paths, and path c4 could be the favorable one. When radical scavenger BHT (butylated hydroxytoluene) was added into the first reaction, only fourth product P4 can be yielded. The Gibbs free energy surface analysis of fourth reaction shows that BHT is much active than acetonitrile to go through the Csp3-H activation and path d2 is the favorable, which can agree with the experimental results. The BDE, localized orbital locator (LOL) isosurfaces, Laplacian bond order (LBO), electron density of bond critical point (ρBCP) and electron spin density isosurface graphs can be used to analyze the structure and reveal the reaction substances.

Computational Evaluation of the Oxidative Cleavage of Triazine Derivatives for Electrosynthesis

Computational chemistry was used to study the N–C bond dissociation Gibbs free energies (298 K, acetonitrile) for a test set of 54 triazine derivatives, comprising combinations of 6 triazinyl radic...

The influence of the cationic carbenes on the initiation kinetics of ruthenium-based metathesis catalysts; a DFT study.

Cationic carbenes are a relatively new and rare group of ancillary ligands, which have shown their superior activity in a number of challenging catalytic reactions. In ruthenium-based metathesis catalysis they are often used as ammonium tags, to provide water-soluble, environment-friendly catalysts. In this work we performed computational studies on three cationic carbenes with the formal positive charge located at different distances from the carbene carbon. We show that the predicted initiation rates of Grubbs, indenylidene, and Hoveyda–Grubbs-like complexes incorporating these carbenes show little variance and are similar to initiation rates of standard Grubbs, indenylidene, and Hoveyda–Grubbs catalysts. In all investigated cases the partial charge of the carbene carbon atom is similar, resulting in comparable Ccarbene–Ru bond strengths and Ru–P/O dissociation Gibbs free energies.

DFT study of nitrogen monoxide adsorption and dissociation on Rh[sbnd]Cu nano clusters

Adsorption and dissociation of NO on RhxCu4-x(x = 0–4) nano clusters were investigated using density functional theory. Adsorption energy, total charge on NO, NO bond length, and NO vibrational frequency for various modes of NO adsorption were analyzed. Adsorption from the nitrogen end of NO on the Rh atom(s) of the clusters are more favored and adsorption energies are in the −1.02 eV to −2.59 eV range. NO binds stronger to Rh-Cu mixed clusters compared to pure Cu4 cluster, so NO bond is significantly weakened upon adsorption on the former. NO binding to more atoms of the clusters results in a corresponding decrease of the NO vibrational frequency. Dissociation of NO was also investigated for a proposed mechanism by calculation of dissociation Gibbs free energy. It is observed that, dissociation process on Cu4 cluster is energetically unfavorable and increasing the amount of Rh in the clusters favors the dissociation of NO.

Theoretical insights into the mechanism of redox switch in heat shock protein Hsp33.

Heat shock protein 33 (Hsp33) is activated in the presence of H2O2 by a very interesting redox switch based on a tetra-coordinated zinc-cysteine complex present in the fully reduced and inactive protein form. The oxidation of this zinc center by H2O2 induces formation of two S-S bridges and the zinc release followed by the protein unfolding. We report here a theoretical study of the step-by-step sequence of the overall process starting with the oxidation of the first cysteine residue and ending with the zinc release. Each reaction step is characterized by its Gibbs free energy barrier (∆G (‡)). It is predicted that the first reaction step consists in the oxidation of Cys263 by H2O2 which is by far the most reactive cysteine (∆G (‡) = 15.4 kcal mol(-1)). The next two reaction steps are the formation of the first S-S bridge between Cys263 and Cys266 (∆G (‡) = 13.6 kcal mol(-1)) and the oxidation of Cys231 by H2O2 (∆G (‡) = 20.4 kcal mol(-1)). It is then shown that the formation of the second S-S bridge (Cys231-Cys233) before the zinc release is most unlikely (∆G (‡) = 34.8 kcal mol(-1)). Instead, the release of zinc just after the oxidation of the third cysteine (Cys231) is shown to be thermodynamically (dissociation Gibbs free energy ∆G d = 6.0 kcal mol(-1)) and kinetically (reaction rate constant k d ≈ 10(6) s(-1)) favored. This result is in good agreement with the experimental data on the oxidation mechanism of Hsp33 zinc center available to date.

Fragmentation, structure, and energetics of small sodium formate clusters: Evidence for strong influence of entropic effects

The behavior of small cationic sodium formate clusters [NaOOCH]nNa+ was studied using a combination of experimental and theoretical approaches. Energy-dependent tandem mass spectra were measured using a triple quadrupole and an ion trap type instrument, while structure and energetics were obtained from density functional theory calculations. Analysis of the relative abundances and fragmentation patterns indicate the existence of certain unstable cluster sizes, like the tetramer [NaOOCH]4Na+. This observation is in contrast to alkali halides, where this species has enhanced stability. Surprisingly, this ‘anti-magic’ sodium formate cluster is shown to be compact, energetically low-lying by quantum chemical calculations. The apparent contradiction can be explained by taking into account entropy effects as calculated dissociation Gibbs free energy values show remarkably improved agreement with experimental trends. Fragmentation of these clusters is a good example of reactions that are predominantly directed by entropy and not by energy constraints.

Calculation of the Gibbs free energy of solvation and dissociation of HCl in water via Monte Carlo simulations and continuum solvation models

The Gibbs free energy of solvation and dissociation of hydrogen chloride in water is calculated through a combined molecular simulation/quantum chemical approach at four temperatures between T = 300 and 450 K. The Gibbs free energy is first decomposed into the sum of two components: the Gibbs free energy of transfer of molecular HCl from the vapor to the aqueous liquid phase and the standard-state Gibbs free energy of acid dissociation of HCl in aqueous solution. The former quantity is calculated using Gibbs ensemble Monte Carlo simulations using either Kohn-Sham density functional theory or a molecular mechanics force field to determine the system's potential energy. The latter Gibbs free energy contribution is computed using a continuum solvation model utilizing either experimental reference data or micro-solvated clusters. The predicted combined solvation and dissociation Gibbs free energies agree very well with available experimental data.

Hydride Affinity Scale of Various Substituted Arylcarbeniums in Acetonitrile

Combined with the integral equation formalism polarized continuum model (IEFPCM), the hydride affinities of 96 various acylcarbenium ions in the gas phase and CH(3)CN were estimated by using the B3LYP/6-31+G(d)//B3LYP/6-31+G(d), B3LYP/6-311++G(2df,2p)//B3LYP/6-31+G(d), and BLYP/6-311++G(2df,2p)//B3LYP/6-31+G(d) methods for the first time. The results show that the combination of the BLYP/6-311++G(2df,2p)//B3LYP/6-31+G(d) method and IEFPCM could successfully predict the hydride affinities of arylcarbeniums in MeCN with a precision of about 3 kcal/mol. On the basis of the calculated results from the BLYP method, it can be found that the hydride affinity scale of the 96 arylcarbeniums in MeCN ranges from -130.76 kcal/mol for NO(2)-PhCH(+)-CN to -63.02 kcal/mol for p-(Me)(2)N-PhCH(+)-N(Me)(2), suggesting most of the arylcarbeniums are good hydride acceptors. Examination of the effect of the number of phenyl rings attached to the carbeniums on the hydride affinities shows that the increase of the hydride affinities takes place linearly with increasing number of benzene rings in the arylcarbeniums. Analyzing the effect of the substituents on the hydride affinities of arylcarbeniums indicates that electron-donating groups decrease the hydride affinities and electron-withdrawing groups show the opposite effect. The hydride affinities of arylcarbeniums are linearly dependent on the sum of the Hammett substituent parameters σ(p)(+). Inspection of the correlation of the solution-phase hydride affinities with gas-phase hydride affinities and aqueous-phase pK(R)(+) values reveals a remarkably good correspondence of ΔG(H(-)A)(R(+)) with both the gas-phase relative hydride affinities only if the α substituents X have no large electron-donating or -withdrawing properties and the pK(R)(+) values even though the media are dramatically different. The solution-phase hydride affinities also have a linear relationship with the electrophilicity parameter E, and this dependence can certainly serve as one of the most effective ways to estimate the new E values from ΔG(H(-)A)(R(+)) or vice versa. Combining the hydride affinities and the reduction potentials of the arylcarbeniums, we obtained the bond homolytic dissociation Gibbs free energy changes of the C-H bonds in the corresponding hydride adducts in acetonitrile, ΔG(HD)(RH), and found that the effects of the substituent on ΔG(HD)(RH) are very small. Simple thermodynamic analytic platforms for the three C-H cleavage modes were constructed. It is evident that the present work would be helpful in understanding the nature of the stabilities of the carbeniums and mechanisms of the hydride transfers between carbeniums and other hydride donors.

Thermodynamic Estimate of pKa Values of the Carboxylic Acids in Aqueous Solution with the Density Functional Theory

AbstractThe 96 pKa values of 85 carboxylic acids in aqueous solution were calculated with the density functional theory method at the level of B3LYP/6‐31+G(d,p) and the polarizable continuum model (PCM) was used to describe the solvent. In the calculations of pKa values, the dissociation Gibbs free energies were directly calculated using carboxylic acid dissociation reactions in aqueous solution, i. e., no thermodynamic cycle was employed, which is different from the previous literatures. A highly significant correlation of R2=0.95 with a standard deviation (SD) of 0.36 between the experimental pKa values and the calculated dissociation Gibbs free energies [ΔG(calc.)] was found. The slope of pKa vs. (G(calc.)/(20303RT) is only 47.6% of the theoretically expected value, which implies that the ΔG(calc.) value from the theoretical calculation is larger than the actual one for all 85 carboxylic acids studied. Thus, by adding the 0.476 scaling‐factor into the slope, we can derive a reliably procedure that can reproduce the experimental pKa values of carboxylic acids. The pKa values furnished by this procedure are in good agreement with the experimental results for carboxylic acids in aqueous solution.

A Promising Method for Phosphinidene Generation: Complexes of Phosphinidenes with N‐Donor ligands

1,3,2-diazaphospholenes and related compounds can formally be regarded as complexes of phosphinidenes (R-P) with 1,4-diazabutadienes. The dissociation Gibbs free energies of these "complexes" were calculated by using density functional theory (B3LYP/3-21G(*) and B3LYP/6-311+G**). The dissociation Gibbs free energies show systematic dependence on the phosphorus substituent as well as on the stability of the N-donor ligand formed as a byproduct. The thermodynamics and kinetics of the dissociations were thoroughly examined. The results allow us to conclude that novel routes of phosphinidene generation can be developed.