Gibbs Free Energy Of Activation Research Articles

Molecular Interactions in Solutions of 1,1-Bis(4-(2-oxopropoxy)phenyl)cyclohexane on Ultrasonic Velocity, Density, and Viscosimetric Data

The ultrasonic velocity, density, and viscosity are measured at various concentrations 0.1–0.01 mol dm–3 in the solutions of 1,1-bis(4-(2-oxopropoxy)phenyl)cyclohexane (BMAPC) in 1,4-dioxane (DO), ethyl acetate (EA), tetrahydrofuan (THF) at 298–313 K and at atmospheric pressure. Parameters: acoustical impedance, adiabatic compressibility, Rao’s molar sound function, Van der Waals constant, internal pressure, free volume, intermolecular free path length, classical absorption coefficient and viscous relaxation time, Gibbs free energy of activation, enthalpy of activation and entropy of activation were determined. Change of the parameters with T indicated existence of strong molecular interactions in solutions and further supported by positive values of solvation number. Gibbs free energy of activation decrease linearly with increasing C and T in the DO system while it decreases with C and increase with T in EA and THF. Enthalpy and entropy of activation were found to be slightly concentration dependent.

One-step preparation of reduced graphene oxide aerogel loaded with mesoporous copper ferrite nanocubes: A highly efficient catalyst in microwave-assisted Fenton reaction.

Heterogeneous Fenton reaction is an attractive method for degradation of organic pollutants due to its high efficiency and non-selectivity and it also causes no secondary pollution. However, low degradation rate and poor recyclability of the catalysts limit its applications for water purification. To overcome this, herein, copper ferrite/reduced graphene oxide (CF/rGO) aerogel was prepared by a one-step hydrothermal method, as a highly efficient catalyst for the microwave-assisted Fenton reaction (MAFR). Under optimal conditions (500 W of microwave power, 600 μL of H2O2, 15 mg of catalyst, and 30 mg/L of RhB), the degradation efficiency of CF/rGO aerogel at 1.0 min (95.7%) was higher than that of reference samples at 3.0 min. Thermodynamical study showed the activation energy, enthalpy change, entropy change, and Gibbs free energy change were 0.73 kJ/mol, -49.5 kJ/mol, -0.135 kJ/mol·K, and -6.8 kJ/mol, respectively, indicating that MAFR was an endothermic and non-spontaneous process.Radical trapping experiments showed that OH, O2-, and h+ played a combined role in RhB degradation. Besides high catalytic activity, CF/rGO aerogel also displayed good reusability, showing removal efficiency of 87.4% after 5 cycles. The high efficiency, good reusability, and simple process make CF/rGO aerogel a promising catalyst for wastewater treatment.

Conversion of C2–4 Carboxylic Acids to Hydrocarbons on HZSM-5: Effect of Carbon Chain Length

The conversion of C2–4 carboxylic (acetic, propionic, and n-butyric) acids on HZSM-5 was studied at 350 °C. The reaction proceeds through ketonization (to acetone, 3-pentanone, and 4-heptanone, respectively), aldol condensation, and tertiary reactions to aromatics-rich hydrocarbons. The product distribution is influenced by the carbon chain length of carboxylic acids due to the shape selectivity of ZSM-5. The turnover frequency (TOF) of ketonization of carboxylic acids decreases while the TOF of aldol condensation of corresponding ketones increases with increasing carbon chain length. Langmuir–Hinshelwood mechanism analysis shows that the true activation enthalpy of both ketonization and aldol condensation increases with increasing carbon chain length, due to the increased steric hindrance for the formation of a C–C bond. However, the true Gibbs free activation energy increases for ketonization but decreases for aldol condensation with increasing chain length, due to a greater contribution of activation e...

Exploring Hydrogen Evolution Accompanying Nitrogen Reduction on Biomimetic Nitrogenase Analogs: Can Fe-N xH yIntermediates Be Active Under Turnover Conditions?

Nitrogen reduction reaction (N2RR) carried out on biomimetic catalytic systems is considered to be a promising alternative for the traditional Haber-Bosch ammonia synthesis. Unfortunately, the selectivity of the currently known biomimetic catalysts is poor, as they also catalyze the unproductive hydrogen evolution reaction (HER). In the present computational study, we examine the HER activity of early N2RR intermediates in EP3 (E = B, Si) ligated single-site biomimetic iron complexes by calculating and comparing the activation Gibbs free energies of HER and N2RR elementary steps. We find that, in contrast to previous suggestions, early N2RR intermediates are not likely sources of HER under turnover conditions, as the barriers of the competing N2RR steps are significantly lower. Consequently, future research should focus on preventing other potential HER mechanisms, e.g., hydride formation, rather than accelerating the consumption of early N2RR intermediates as proposed earlier to design more efficient biomimetic catalysts.

Understanding the Reactivity of Mn-Oxo Porphyrins for Substrate Hydroxylation: Theoretical Predictions and Experimental Evidence Reconciled.

The Mn-oxo porphyrin (MnOP) mechanism for substrate hydroxylation is computationally studied with the aim to better understand reactivity in these systems. Theoretical studies suggest Mn(V)OP species to be very reactive intermediates with thermally accessible reaction barriers represented by low-spin/high-spin-crossover occurring in the Mn(V)OP oxidant, and kinetics for selected Mn(V)OP species indeed find high reactivity. On the other hand, MnOP complexes lead to modest yields in hydroxylation reactions of several different substrates, implying low rate constants and high reaction barriers. The resolution of this inconsistency is very important to understand the reactivity of Mn-oxo porphyrins and to improve the catalytic conditions. In this work we use the toluene hydroxylation by the Mn(V)OP(H2O)+ complex as a case study to gain deep insight into the reaction mechanism. Minimum energy crossing point (MECP) results on the H-abstraction process from toluene indicate a first crossover from a singlet to a triplet spin state of the Mn(V)OP(H2O)+ species with a thermally accessible barrier, followed by a very facile H-abstraction by the triplet complex. Issues concerning (i) the validation of the level of the density functional theory employed (BP86) to describe the singlet-triplet energy gap in the Mn(V)OP(H2O)+ system versus highly accurate DMRG-CASPT2/CC calculations, and (ii) the influence of the axial ligand (X = none, Cl-, CH3CN, OH-, and O2-) on MnOP reactivity, which models the different experimental conditions, are addressed. The ligand trans influence mainly controls the reactivity through the singlet-triplet energy gap modulation, with the porphyrin ruffling distortion also finely tuning it. Finally, a stepwise model for the H-abstraction process is proposed which allows a direct comparison between the calculated and experimentally measured Gibbs free activation energy barriers ( Zhang et al. J. Am. Chem. Soc. 2005 , 127 , 6573 - 6582 ). The low yields in catalysis are shown not to be due to low reactivity of Mn(V).

Synthesis and Thermal Properties of Three Energetic Ion Salts of 3,6-Bis[(1H-1,2,3,4-tetrazol-5-yl)-amino]-1,2,4,5-Tetrazine

Three energetic ion salts of 3,6-bis[(1H-1,2,3,4-tetrazol-5-yl)-amino]-1,2,4,5-tetrazine(BTATz), namely, methylamine salt(compound 1), ethylenediamine salt(compound 2), and diethylamine salt(compound 3), were synthesized and characterized by elemental analysis, Fourier transform infrared spectrometry, 1H NMR spectroscopy, and 13C NMR spectroscopy. The crystal structure of compound 1 was determined by single-crystal X-ray crystallography, and structural analysis revealed that it belonged to the monoclinic system with P21/c space group. In addition, the thermal behavior of the three compounds was studied by differential scanning calorimetry and thermogravimetry techniques. The thermal decomposition peak temperatures of the compounds were 574.89, 545.60, and 606.72 K, indicating that the three ion salts exhibited good thermal stability. The kinetic mechanism equations of the main decomposition process and the entropy of activation(ΔS≠), enthalpy of activation(ΔH≠), and Gibbs free energy of activation(ΔG≠) of the three compounds were also obtained. Moreover, the thermal safety of the compounds was evaluated by the values of the self-accelerated decomposition temperature(TSADT), thermal ignition temperature(TTIT), and critical temperature of thermal explosion(Tb). The results showed that all the compounds demonstrated good thermal safety, and the thermal safety of compound 3 was better than that of the others.

Equilibrium, Kinetics and Thermodynamics of Bovine Serum Albumin from Carbon Based Materials Obtained from Food Wastes

In this research, the adsorption of bovine serum albumin (BSA) onto activated carbon (AC) obtained from apple bark was carried out and the thermodynamic parameters of adsorption process were investigated. Besides, the functions involved in BSA attachment were examined by adsorption experiments on retention capacities for BSA at 298 K, pH of 7, ionic strength of 5.10−2 M, and initial concentration of 5.10−2 g L−1, respectively. The bovine serum albumin (BSA) adsorption experiment onto activated carbon (AC) indicated that the highest adsorption yield was achieved at pH 5.5. The BSA molecules at pH 5.5 are very stable and that pH value is close to isoelectronic point of BSA. The surface structural change of BSA and activated carbon was studied before and after the experiment using scanning electron microscopy (SEM) analysis and Fourier Transform Infrared Spectroscopy (FTIR). By the way, the thermodynamic functions such as Gibbs free energy (ΔG∗), activation energy (Ea), activation entalphy (ΔH∗), and activation entropy (ΔS∗) were calculated as − 66.17, 37.73, − 29.09 kJ mol−1, and + 124.42 J mol−1 K−1 for bovine serum albumin (BSA) adsorption, respectively. The adsorption of the process was investigated using Eyring and Arrhenius equations and the adsorption kinetic of BSA on AC was found to be coherent with the pseudo-second-order model.

The study of thermo-physical properties of binary liquid mixtures of isoamyl alcohol with amines at 298.15–308.15 K

ABSTRACTThe values of density (ρ), viscosity (η) and speed of sound (u) were measured for binary mixtures of 3-methyl-1-butanol (isoamyl alcohol) with n-butylamine (NBA), sec-butylamine (SBA), tert-butylamine (TBA), n-hexylamine (NHA) and n-octyl amine (NOA) over the entire range of mole fractions and at three temperatures (298.15, 303.15 and 308.15 K). From these measurements, the values of excess molar volume(VE), viscosity deviation (∆η), deviation in isentropic compressibility (∆ks), excess Gibbs free energy of activation of viscous flow (G*E) and Grungberg–Nissan interaction parameter (d′) have been calculated. The excess parameter deviations are fitted to Redlich–Kister-type polynomial equation using multi-parametric nonlinear regression analysis to estimate the binary coefficients and standard deviations. The results were discussed in terms of molecular interactions, dipole–dipole interactions and H-bond interactions between unlike molecules. Further, the effects of temperature and amine group in different amines were also studied for the binary mixtures.

Mechanistic, Energetic and Structural Studies of γ-Fe2O3 Nanoparticles Functionalized with Drug Artemisinin

Using density functional theory, three noncovalent interactions and mechanism of covalent functionalization of drug artemisinin onto γ-Fe2O3 nanoparticles have been investigated. Quantum molecular descriptors of noncovalent configurations were studied. It was specified that binding of drug artemisinin with γ-Fe2O3 nanoparticles is thermodynamically suitable. Hardness and the gap of energy between LUMO and HOMO of artemisinin drug are higher than those of noncovalent artemisinin-γ-Fe2O3 nanoparticle configurations, showing the reactivity of artemisinin increases in the presence of γ-Fe2O3 nanoparticles. Artemisinin can bond to γ-Fe2O3 nanoparticles through carbonyl group. The activation energies, the activation enthalpies and the activation Gibbs free energies of this reaction were calculated. The activation parameters and thermodynamic data indicate that this reaction is exothermic and spontaneous and can take place at room temperature. These results could be generalized to other similar drugs.

Physical-Chemical Properties of Binary Liquid Mixtures of Isoamyl Alcohol with Chloroethanes at 298–308 K

The density (ρ), viscosity (η), and speed of sound (u) were measured for binary mixtures of 3‑methyl-1-butanol(isoamylalcohol) with 1,2-dichloroethane (DCE), 1,1,2-trichloroethane (TCE), and 1,1,2,2-tetrachloroethane (TECE) over the entire range of mole fractions and at temperature 298.15, 303.15, and 308.15 K. From these measurements, the values of excess molar volume (VE), deviation in viscosity (∆η), deviation in isentropic compressibility (∆ks), excess Gibbs free energy of activation of viscous flow (G*E) and Grunberg−Nissan interaction parameter (d ') have been calculated. The excess parameter deviations have been fitted to Redlich−Kister type polynomial equation using multiparametric nonlinear regression analysis to estimate the binary coefficients and standard deviations. The results were discussed in terms of molecular interactions and dipole-dipole interactions. The effects of temperature and chlorine atoms number of investigated chloroethanes were also studied for the binary mixtures.

Quantum chemical study of the mechanism of the palladium-catalysed C−H acetoxylation of benzene

In this work, the mechanism of the palladium-catalysed acetoxylation of benzene C-H has been studied theoretically in detail. Based on experimental studies, a four-step mechanism for this reaction had been proposed, that is, C−H activation of benzene is the rate-determining step which forms an intermediate ( k1 pathway) which is subsequently oxidized to produce a high-valent Pd intermediate ( k2 pathway). Using quantum chemical calculations, all pathways were investigated, and the activation energy, activation enthalpy and activation Gibbs free energy for all steps were calculated and compared with each other. It was determined that the RDS proceeds through a square complex instead of a T-shaped complex. The activation energy related to the k2 pathway is higher than that of the RDS, and therefore, a new mechanism is proposed.

Volumetric and viscometric investigation of aqueous solution of [Bmim][BF4] from 288.15 to 318.15 K

Density and viscosity of aqueous [Bmim][BF4] were measured at the temperature range from 288.15 to 318.15K with a step of 5K under atmospheric pressure. All the concentrations of the aqueous solutions were controlled bellow 1.000mol·kg−1. The values of apparent molar volumes were obtained at different temperatures with the corresponding concentrations. According to Redlich-Mayer equation, the empirical parameters (SV and BV) and the limiting apparent molar volume at infinite dilution, ϕVi0, were acquired. The volume properties were also investigated using Pitzer's equation the result showed that Redlich-Mayer's and Pitzer's equation were mutually proved. Based on the equation of ϕVi0 and T, the limiting apparent molar expansibilities, Eϕ∞, were calculated and discussed. Through the Jones-Dole empirical equation, the viscosity coefficient A and B of aqueous [Bmim][BF4] were obtained. In addition, the standard molar activation Gibbs free energy of solute, Δμ2≠0, were obtained, and the activation entropy, ΔS2≠0, as well as activation enthalpy, ΔH2≠0, can be calculated by fitting Δμ2≠0 against T. Using a semi-empirical method the viscosity of aqueous [Bmim][BF4] was predicted and the result showed a good agreement with experimental values.

Comparing the performances of various density functionals for modelling the mechanisms and kinetics of bimolecular free radical reactions in aqueous solution.

We carried out an investigation of the performances of 18 density functionals (DFs) for modelling the mechanisms and kinetics of the aqueous phase reactions between the α-hydroxyisopropyl radical and 9 organic substrates. The primary goal was to evaluate the applicability of density functional theory specifically in conjunction with the polarizable continuum model (DFT/PCM) for a fully implicit description of the aqueous environment. Accordingly, the solute is augmented with the explicit molecule(s) of the water solvent only when it is confirmed that the water participates in the reaction mechanism directly and not just as a potential donor or acceptor of additional hydrogen bonds. The tested DFs are chosen by systematically ascending the Jacob's ladder of DFs with particular emphasis on the versatile Minnesota family. For most of the DFs we used the empirical corrections for the dispersion in accordance with Grimme's D3 or D3-BJ models. The optimum DFs are determined on the basis of the lowest mean absolute errors (MAEs) and the largest Pearson correlation coefficients (PCCs) with respect to the set of experimentally determined rate constants. The studied substrates are carbon tetrachloride (CCl4), chloroform (CHCl3), trichloroacetate (Cl3Ac-), chloral hydrate (ClH), iodoacetate (IAc-), iodoacetamide (IAm), 5-bromouracil (BrU), 5-nitrouracil (NO2U), and cysteamine (Cys+). The mechanisms that contribute dominantly to the observed rate constants are: chlorine abstractions for CCl4, CHCl3, Cl3Ac-, and ClH; proton-coupled electron transfer (PCET) for IAc-; water-assisted PCET and iodine abstraction for IAm; ortho-addition for BrU and NO2U; and hydrogen atom abstraction from the sulphur atom for Cys+. It is found that in the DFT/PCM setting climbing up the Jacob's ladder does not necessarily imply a systematically increasing accuracy. Thus, M06-D3 and PBE0-D3 exhibit the best performance according to the lowest MAEs (1.10 and 1.26 kcal mol-1 MAEs in the Gibbs free energies of activation), and M06-D3, M06-2X-D3 and MN15 according to the largest PCCs (0.95, 0.94, and 0.94). In a surprising contrast, the three tested double-hybrid DFs, B2PLYP-D3, DSD-PBEP86, and PBEQIDH, all exhibit comparatively large MAEs and poor PCCs, and therefore do not appear well-suited for use in the DFT/PCM framework.

Using Thermodynamics to Assess the Molecular Interactions of Tetrabutylphosphonium Carboxylate–Water Mixtures

Densities, ρ, viscosities, η, and enthalpies of mixing, , of binary [P4 4 4 4][CnCOO]–water mixtures (with n=1, 2 or 7) were determined at atmospheric pressure as a function of temperature. The excess, , apparent, , and partial, , molar volumes were deduced from experimental data, as well as fragilities, m*, and excess Gibbs free energies of activation of viscous flow, . exhibited predominantly negative deviation from ideality, with a minimum at approximately ~0.8 for all three systems, indicating strong hydrogen-bonding interactions. All three binary systems were found to be fragile, with [P4 4 4 4][C7COO] showing the smallest deviations in fragility with the addition of water. values of the systems were exothermic over the entire composition range, having the following trend: [P4 4 4 4][C2COO]>[P4 4 4 4][C7COO]>[P4 4 4 4][C1COO].

Removal of the commercial reactive dye Procion Blue MX-7RX from real textile wastewater using the synthesized Fe2O3 nanoparticles at different particle sizes as a source of Fenton's reagent.

The aim of the present study was to signify the role of the particle size of an iron source in the photo-Fenton system for textile dyeing wastewater oxidation. In this respect, a facile synthesis of Fe2O3 nanoparticles (Fe2O3 NPs) via a simple sol–gel route using FeCl3 with different molarities was investigated. The XRD patterns confirmed the formation of Fe2O3 nanoparticles. Furthermore, different molar concentrations of the FeCl3 precursor significantly influenced the size and shape of the nanoparticles. The treatment of wastewater effluents containing the reactive dye Procion Blue MX-7RX from a real textile dyeing facility, was investigated using the synthesized Fe2O3 NPs as a source of the Fenton's reagent photocatalyst. The reaction was initiated and enhanced using an artificial UV source for increasing the ˙OH radical yield. System parameters such as the initial dye load in wastewater, H2O2 and Fe2O3 NP concentrations, pH and the working temperature were investigated for process optimization. The quality of water in this investigation was examined by making measurements of chemical oxygen demand (COD), total suspended solids (TSS) and dye removal which decreased during the illumination time. The effects of different Fe2O3 NPs based on the varying precursor solution molarities namely F1, F3, F5 and F7 on the wastewater remediation were investigated, and the optimum Fe2O3 NPs were found to be F1, which exhibited the highest dye removal value of 83% and a chemical oxygen demand (COD) reduction of 88%. Furthermore, the oxidation kinetics of the Procion Blue dye were studied, and the data were well fitted with the second order kinetics. Finally, the thermodynamic parameters including the changes in the Gibbs free energy of activation, entropy of activation and enthalpy of activation for the Procion Blue oxidation with different Fenton's reagent sources under ultraviolet light illustrated that the reaction was non-spontaneous and endothermic.

Pectin hydrolysis in cashew apple juice by Aspergillus aculeatus URM4953 polygalacturonase covalently-immobilized on calcium alginate beads: A kinetic and thermodynamic study

The kinetics and thermodynamics of pectin hydrolysis in cashew apple juice by polygalacturonase (PG) from Aspergillus aculeatus URM4953 covalently-immobilized on calcium alginate beads were investigated. Immobilized-PG activity in cashew apple juice was the highest at 20 °C, showing a maximum hydrolysis rate of 58.2 mg/mL·min, a catalytic constant of 166.2 s−1 and an affinity constant of 113.0 mg/mL. Since the enzyme exhibited an allosteric behavior, the hydrolysis rate was modeled, with excellent accuracy, by the Hill Equation as function of pectin concentration. The Hill coefficient increased from 3 to 5 with increasing temperature from 20 to 50 °C, evidencing a positive cooperativity mechanism. The reaction activation energy and the standard enthalpy variation of enzyme unfolding were 80.3 and 16.6 kJ/mol, respectively. Consistently with the kinetic results, PG-catalyzed pectin hydrolysis proceeded with maximum spontaneity at 20 °C, showing activation Gibbs free energy, enthalpy and entropy of 59.3 kJ/mol, 77.9 kJ/mol and 63.4 J/mol·K, respectively. Immobilized PG was successful in the hydrolysis of cashew apple juice pectin, requiring a low temperature to act optimally.

Quantitative structure-activity relationship models for predicting reaction rate constants of organic contaminants with hydrated electrons and their mechanistic pathways

The hydrated electron (eaq−)-based reduction processes are promising for removing organic pollutants in water engineering systems. The reductive kinetics, especially the second order rate constants (keaq−) of eaq− with organic compounds, is important for evaluating and modeling the advanced reduction processes. In this study, the keaq−values for aliphatic compounds and phenyl-based compounds are, for the first time, modeled by the quantitative structure-activity relationship (QSAR) method. The structural features governing the reactivity of two classes of organic compounds toward eaq− were revealed, and the energy of the lowest unoccupied molecular orbital (ELUMO), one-electron reduction potential (ERED) and polarizability (α) were found to be the important molecular parameters in both two models. The built QSAR models provide robust predictive tools for estimating the removal of emerging pollutants using eaq− during wastewater treatment processes. Additionally, quantum chemical calculations were employed to probe into the mechanism and feasibility of the single electron transfer (SET) pathway in the eaq−-based reduction process. The thermodynamic investigation suggests that the compounds with electron-withdrawing groups tend to possess higher keaq− and lower Gibbs free energy (ΔGSET) and Gibbs free energies of activation (∆‡GSET∘) than the ones with electron-donating groups, indicating the SET process occurs more readily. It is also found that the refractory halogenated compounds can achieve dehalogenation via the SET pathway.

Kinetics of complex formation of Fe(III) with caffeic acid: Experimental and theoretical study.

Kinetic study on the complexation of caffeic acid with ferric chloride was performed in aqueous solution at pH 9.0. The complex was characterized by IR, UV-Vis spectroscopy and FE-SEM techniques. Kinetic data were obtained and used to model the reaction kinetics for the disappearance of the caffeic acid. The operational variables have been studied in the temperature range from 25 to 45 °C, initial iron and caffeic acid concentration from 0.6 mg/100 ml (ranging from 2.0 ml to 8 ml/100 ml). The complexation reaction was found to be a first-order with rate constants for k1 (formation) 4.7 × 10-3 s-1. Additionally, the effect of concentration and temperature on the complexation reaction was investigated. Apparent kinetic parameters of the complex formation of Fe(III) with caffeic acid were found as; pre-exponential factor A (-1.17 min-1), enthalpy of activation (ΔH°)# (-2188 J mol-1), entropy of activation, (ΔS°)# (-7.34 J mol-1 K-1) and Gibbs free energy of activation, (ΔG°)#(-0.68 J). The apparent activation energy of the complexation reaction was evaluated to be 289.29 J mol-1 which is consistent with the chemistry of Fe(III) with polyphenols which are supposed to mimic the interaction of Fe(III) with transferrin in biological media. Various theoretical parameters of the studied complex such as hardness, electronegativity, softness, total energy, dipole moment and point group symmetry were calculated employing Density functional theory (DFT) and found as 0.04465, 0.2130, 22.39, 0.5201 × 10-8 eV, 15.13 Debye, and C1, respectively.

Unexpected Routes of the Mutagenic Tautomerization of the T Nucleobase in the Classical A·T DNA Base Pairs: A QM/QTAIM Comprehensive View.

In this paper using quantum-mechanical (QM) calculations in combination with Bader's quantum theory of “Atoms in Molecules” (QTAIM) in the continuum with ε = 1, we have theoretically demonstrated for the first time that revealed recently highly-energetic conformers of the classical A·T DNA base pairs – Watson-Crick [A·T(wWC)], reverse Watson-Crick [A·T(wrWC)], Hoogsteen [A·T(wH)] and reverse Hoogsteen [A·T(wrH)] – act as intermediates of the intrapair mutagenic tautomerization of the T nucleobase owing to the novel tautomerisation pathways: A·T(wWC)↔A·T*(w⊥WC); A·T(wrWC)↔A·(w⊥rWC); A·T(wH)↔A·T*(w⊥H); A·T(wrH)↔A·(w⊥rH). All of them occur via the transition states as tight ion pairs (A+, protonated by the N6H2 amino group)·(T−, deprotonated by the N3H group) with quasi-orthogonal geometry, which are stabilized by the participation of the strong (A)N6+H···O4−/O2−(T) and (A)N6+H···N3−(T) H-bonds. Established tautomerizations proceed through a two-step mechanism of the protons moving in the opposite directions along the intermolecular H-bonds. Initially, proton moves from the N3H imino group of T to the N6H2 amino group of A and then subsequently from the protonated N6+H3 amino group of A to the O4/O2 oxygen atom of T, leading to the products – A·T*(w⊥WC), A·(w⊥rWC), A·T*(w⊥H), and A·(w⊥rH), which are substantially non-planar, conformationally-labile complexes. These mispairs are stabilized by the participation of the (A)N6H/N6H'···N3(T) and (T)O2H/O4H···N6(A) H-bonds, for which the pyramidalized amino group of A is their donor and acceptor. The Gibbs free energy of activation of these mutagenic tautomerizations lies in the range of 27.8–29.8 kcal·mol−1 at T = 298.15 K in the continuum with ε = 1.

Thermodynamic characterization of binary mixtures of poly(propylene glycol) 425 with toluene and o-, m- and p-xylenes

Density ρ, viscosity η and refractive index nD have been experimentally measured for four binary mixtures of poly(propylene glycol) of average molecular weight 425 + toluene/o-xylene/m-xylene/p-xylene over the temperature range T = (293.15–333.15) K with temperature step 5 K and at atmospheric pressure. From the experimental data excess molar volumes VE, deviations in refractive index ΔnD, viscosity deviations Δη and excess Gibbs free energy of activation of viscous flow ΔG*E were calculated and fitted with Redlich-Kister polynomial in order to analyze non-ideal behavior of the studied mixtures. From ΔG* values excess entropy ΔS*E and excess enthalpy of activation of viscous flow ΔH*E were also calculated. It was shown that specific weak, but numerous, attractive interactions and entropic effect, due to difference in size of the pure component molecules, are responsible for volumetric and viscosity behaviour of the studied systems. The viscosities of the studied systems were predicted with the UNIFAC-VISCO and ASOG-VISCO models and correlated with the McAllister equation.