Factors determining the reactivity of the bromine atom toward haloalkanes

Experimental data concerning reactions of the bromine atoms with haloalkanes and carbonyl compounds (25 reactions) have been analyzed within the intersecting parabolas model. The following factors have an effect on the activation energy of these reactions: enthalpy of reaction, triplet repulsion, electronegativity of reaction center atoms, dipole–dipole and multidipole interactions of the reaction center with polar groups, and the interaction of π electrons with electrons of the reaction center. The increments characterizing the contribution from each factor to the activation energy of the reaction have been calculated. The increment ΔEμ, which characterizes the dipole–dipole interaction in the transition state, and the dipole moment of the polar group (μ) are correlated by the following empirical equation: ln(ΔE _μ/Σμ) = ?0.14 + 0.47(ΔE _μ/Σμ) ? 0.024(ΔE _μ/Σμ)².     

Reactivity of bromoalkanes in reactions of coordinated molecular decay

The results from experiments on reactions of the coordinated molecular decay of RBr bromoalkanes on olefin and HBr are analyzed using the model of intersecting parabolas (MIP). Kinetic parameters within the MIP are calculated from the experimental data, enabling calculation of the activation energies (E) and rate constants (k) of such reactions, based on the enthalphy of the reaction and the MIP algorithms. The factors affecting the E of the RBr decay reaction are established: the enthalphy of the reaction, triplet repulsion, the energy of radical R? stabilization, the presence of a π bond adjacent to the reaction center, and the dipole–dipole interaction of polar groups. The energy spectrum of the partial energies of activation is constructed for the reaction of coordinated molecular decay of RBr, and the E and k of inverse addition reactions are evaluated.     

Reactivity of fluoroalkanes in reactions of coordinated molecular decomposition

Experimental results on the coordinated molecular decomposition of RF fluoroalkanes to olefin and HF are analyzed using the model of intersecting parabolas (IPM). The kinetic parameters are calculated to allow estimates of the activation energy (E) and rate constant (k) of these reactions, based on enthalpy and IPM algorithms. Parameters E and k are found for the first time for eight RF decomposition reactions. The factors that affect activation energy E of RF decomposition (the enthalpy of the reaction, the electronegativity of the atoms of reaction centers, and the dipole–dipole interaction of polar groups) are determined. The values of E and k for reverse reactions of addition are estimated.     

Oxidation kinetics of crystal violet by potassium permanganate in acidic medium

The oxidation kinetics of crystal violet (a triphenylmethane dye) by potassium permanganate was focused in an acidic medium by the spectrophotometric method at 584 nm. The oxidation reaction of crystal violet by potassium permanganate is carried out in an acidic medium at different temperatures ranging within 298–318 K. The kinetic study was carried out to investigate the effect of the concentration, ionic strength and temperature. The reaction followed first order kinetics with respect to potassium permanganate and crystal violet and the overall rate of the reaction was found to be second order. Thermodynamic activation parameters like the activation energy (E_a), enthalpy change (ΔH*), free energy change (ΔG*), and entropy change (ΔS*) have also been evaluated.     

The energy and geometric characteristics of the transition state in reactions of RO<Stack><Subscript>2</Subscript><Superscript>•</Superscript></Stack> with carbonyl compound C-H bonds

The energy and geometry of the transition state in reactions of the ethyl peroxyl radical with ethane, ethanol (its α and β C-H bonds), acetone, butanone-2, and acetaldehyde were calculated by the density functional theory method. In all these reactions (except EtO_2/? + ethanol α C-H bond), the C…H…O reaction center has an almost linear configuration (φ = 176° ± 2°); polar interaction only influences the r ^≠ (C…O) interatomic bond. In the reaction of EtO_2/? with the ethanol α C-H bond, it is the O-H…O H-bond formed in the transition state that determines the configuration of the reaction center with the angle φ(C…H…O) = 160°. The results were used to estimate the r ^≠ (C…H) and r ^≠ (O…H) interatomic bonds in the transition state by the method of intersecting parabolas and the contribution of polar interaction to the activation energy of reactions between peroxyl radicals and aldehydes and ketones.     

Theoretical Study of Substituent Effect in Aryl Group Migration in (<Emphasis Type="Italic">para</Emphasis>-C<Subscript>6</Subscript>H<Subscript>4</Subscript>X)Mn(CO)5 Complexes

In this study, we report substituent effect on aryl group migration in (para-C₆H₄X)Mn(CO)₅ complexes using mpw1pw91 quantum chemical calculations. These calculations reveal good linear relationships between barrier energy (ΔE), activation energy (ΔH^?), activation free energy (ΔG^?) values and rate constants with Hammett constants of X-substituents. The occupancy values of Mn–CO_cis and Mn–C(O)-(para-C₆H₄X) bonds in reactant, transition state and product were calculated by Natural bond orbital (NBO) method.     

Polar effect and geometry of the transition state in the reactions of ozone with C-H bonds of polar molecules

A large body of experimental data on the reactions of ozone with C-H bonds of polar molecules in the liquid and gas phases is analyzed in the framework of the intersecting parabolas model. The reactions are considered as the abstraction reaction O₃ + RH → HOOO^. + R^.. The contribution from the polar effect to the activation energy of such reactions is calculated. This contribution is ?6.8 kJ/mol for the reactions of ozone with aliphatic alcohols, and is ?8.1, ?11.7, ?6.8, and ?2.2 kJ/mol for the reactions of ozone with ketones, ethers, 1,3-dioxolanes, and 1,3-dioxanes, respectively. The contribution is insignificant in the reactions of ozone with aldehydes. The interatomic distances in the transition state of these reactions r ^#(C…H) and r ^#(O…H) and the angle between the C…H and O…H bonds are calculated. For the reactions in polar solvents, the contribution from solvation to the activation energy is calculated. In most of the systems considered, this contribution is insignificant (from ?1 to ?3 kJ/mol). The reactions involving ozone are compared to the reactions of peroxy radicals with the same classes of compounds.     

The X<Superscript>−</Superscript>···benzohydrazide complexes: the interplay between anion-π and H-bond interactions

The compounds containing the benzohydrazide (BH) nucleus have a variety of biological activities because of various noncovalent intermolecular interactions. The interplay between anion-π and H-bond interactions, which can affect the activity of compounds, has been investigated in ten substituted BH exposed to the chloride ion using the quantum mechanical calculations. The total interaction energy is separated into the anion-π (ΔE _Aπ) and H-bond (ΔE _HB) contributions where both interactions are presented in the complexes. The electron-withdrawing substituents (EWSs) increase |ΔE _Aπ| and decrease |ΔE _HB|, while reversed changes are observed with the electron-donating substituents (EDSs). In addition, the total binding energy (ΔE) becomes more/less negative in the presence of EWSs/EDSs. The synergetic effects of mentioned interactions and substituent effects have also been investigated using the atoms in molecules (AIM), natural bond orbital (NBO) and molecular electrostatic potential (MEP) analyses. A good correlation is found between the energy data and the Hammett constants, the minimum of electrostatic potential (V _min) and the results of population analyses.     

Voltammetric determination of dihydroxybenzenes at a mechanically renewed electrode made from a graphite-epoxy composite

Differences of potentials of anodic and cathodic peaks (ΔE _p) are determined in cyclic voltammograms of dihydroxybenzene/p-benzoquinone redox systems at an electrode made of a graphite-epoxy composite in a wide pH range. The data obtained (ΔE _p = 29 ± 1 mV) are close to the thermodynamic values for two-electron reversible reactions. This indicates that the electrode mechanically renewed by cutting a 0.5-μm surface layer directly in a test solution exhibits a high activity in such electrochemical reactions. The potentials of anodic and cathodic peaks are proportional to the pH of the supporting electrolyte solution in the range from 1.0 to 9.0. A change of 58 ± 1 mV in E _p per unit pH for all isomers shows that the first stage of the oxidation of each dihydroxybenzene isomer involves one electron and is accompanied by the detachment of one hydrogen ion, that is, an intermediate oxidation product, semiquinone, is formed. Despite the closeness of the potentials of hydroquinone and pyrocatechol peaks (ΔE = 100 mV), a scheme is proposed for the selective voltammetric determination of dihydroxybenzene isomers in a 0.1 M HCl solution in hydroquinone-pyrocatechol, pyrocatechol-resorcinol, and hydroquinone-resorcinol binary mixtures. The concentrations of hydroquinone and pyrocatechol are found from cathodic peaks and that of resorcinol, from the anodic peak. The results are well reproducible and contain no systematic error.     

Substituted benzotriazoles as inhibitors of copper corrosion in borate buffer solutions

The adsorption of substituted 1,2,3-benzotriazoles (R-BTAs) onto copper is measured via ellipsometry in a pure borate buffer (pH 7.4) and satisfactorily described by Temkin’s isotherm. The adsorption free energy (?ΔG _a ⁰ ) values of these azoles are determined. The (?ΔG _a ⁰ ) values are found to rise as their hydrophobicity, characterized by the logarithm of the partition coefficient of a substituted BTA in a model octanol–water system (logP), grows. The minimum concentration sufficient for the spontaneous passivation of copper (C _min) and a shift in the potential of local copper depassivation with chlorides (E _pt) after an azole is added to the solution (i.e., ΔE = E _pt ⁱⁿ ? E _pt ^backgr characterizing the ability of its adsorption to stabilize passivation) are determined in the same solution containing a corrosion additive (0.01М NaCl) for each azole under study. Both criteria of the passivating properties of azoles (logC _min and ΔE) are shown to correlate linearly with logP, testifying to the role played by surface activity of this family of organic inhibitors in protecting copper in an aqueous solution.     

Reactivity of alkyl iodides in molecular decomposition reactions

The experimental results for the concerted molecular decomposition of alkyl iodides RI to olefin and HI were analyzed in terms of the intersecting parabolas model (IPM). The activation energies (E) and rate constants (k) of the earlier unstudied reactions of the concerted molecular decomposition of RI were calculated on the basis of the enthalpy of the reaction and IPM algorithms. The factors that influence on E of RI decay were established: the enthalpy of the reaction, the energy of stabilization of radical R*, the length and force constant of the C—I bond, and the size of the halogen atom. The values of E and k for the backward reactions of HI addition to olefins were estimated.     

Thermodynamic activation parameters of hindered rotation of the CF<Subscript>3</Subscript> group in the 4-nitrophenyltrifluoromethylsulfone radical anion in DMF

The thermodynamic activation parameters of hindered rotation of the CF₃ group in the 4-nitrophenyltrifluoromethylsulfone radical anion in DMF were determined from the temperature dependence of the EPR line widths and spin density distributions calculated by the U-B3LYP method in the 6-31+G* basis set. In the range 293 > T > 199 K, the activation energy of hindered rotation E _F depends on the temperature and changes in the range 9.67 < E _F < 18.95 kJ·mol^?1; the changes in the activation enthalpy and entropy are 7.23 < ΔH ^≠ < 17.30 kJ·mol^?1 and ?53.45 < ΔS ^≠ < ?11.37 J·(mol·K)^?1, respectively. Based on the suggested method for evaluating the inner product of the g tensor and the tensor of anisotropic hfi with the ¹⁴N nucleus for nitrobenzene radical anions in the liquid state we calculated the correlation time and determined the activation energy of rotational diffusion of the 4-nitrophenyltrifluoromethylsulfone radical anion in DMF, E _r = 20.175±0.54 kJ·mol^?1.     

Study of Fluorescence Quenching on Novel Coumarin Derivatives by Aniline in Different Solvents

The steady-state fluorescence quenching of novel coumarin derivatives; 4-(2, 6-dibromo-4-methyl-phenoxymethyl)-benzo[h]chromen-2-one [DMB] and 6-methoxy-4-p-tolyoxymethyl-chromen-2-one [TMC] has been studied in toluene, benzene, dioxane, acetonitrile and tetrahydrofuran [THF] using aniline as a quencher at room temperature with a view to understanding the role of diffusion in the quenching mechanism. The probability of quenching per encounter (p) is calculated in all the solvents. Further, an activation energy for quenching (E _a) was estimated using the values of p and the literature values of activation energy for diffusion (E _d). The magnitudes of these parameters indicate that the fluorescence quenching of these molecules by aniline is not solely due to the material diffusion but there is also a contribution of an activation energy.     

A comparison of energy changes accompanying growth processes by <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis>

Calculations are made using the equations Δ_r G = Δ_r H ? TΔ_r S and Δ_r X = Δ_r H ? Δ_r Q where Δ_r X represents the free energy change when the exchange of absorbed thermal energy with the environment is represented by Δ_r Q. The symbol Q has traditionally represented absorbed heat. However, here it is used specifically to represent the enthalpy listed in tabulations of thermodynamic properties as (H _T  ? H ₀) at T = 298.15 K, the reason being that for a given substance TS equals 2.0 Q for solid substances, with the difference being greater for liquids, and especially gases. Since Δ_r H can be measured, and is tangibly the same no matter what thermodynamics are used to describe a reaction equation, a change in the absorbed heat of a biochemical growth process system as represented by either Δ_r Q or TΔ_r S would be expected to result in a different calculated value for the free energy change. Calculations of changes in thermodynamic properties are made which accompany anabolism; the formation of anabolic, organic by-products; catabolism; metabolism; and their respective non-conservative reactions; for the growth of Saccharomyces cerevisiae using four growth process systems. The result is that there is only about a 1% difference in the average quantity of free energy conserved during growth using either Eq. 1 or 2. This is because although values of TΔ_r S and Δ_r Q can be markedly different when compared to one another, these differences are small when compared to the value for Δ_r G or Δ_r X.     

The thermodynamic characteristics of formation of organic molecule complexes with the magnesium ion in water: The results of quantum-chemical modeling

The Gibbs energy ΔG _b of formation of organic molecule complexes with the Mg²⁺ ion in water was calculated on the basis of a two-stage scheme for the complex formation reaction. The first stage is ligand transfer from infinity into the second coordination sphere of the Mg²⁺ ion, and the second stage is the dissociation of bonds between water molecules and the Mg²⁺ ion and the formation of bonds between the ligand and Mg²⁺. The contribution of the first reaction stage to ΔG _b was calculated on the assumption that the ligand was a solid body with a charge or dipole moment (if the ligand was neutral). The contribution of the second stage to ΔG _b was calculated using quantum-chemical modeling. The major contribution to ΔG _b was made by a change in the internal energy of the complex as a result of the dissociation/formation of coordination bonds and a change in the electric component of the Gibbs energy of interaction between the magnesium ion and molecule with water when they formed a complex. The contribution of the nonpolar component of complex interaction with water was comparatively small. Accurate calculations of the contribution of vibrational degrees of freedom to ΔG _b were also of importance.     

Synthesis and isoconversional kinetic study of the formation of LiNiPO<Subscript>4</Subscript> from Ni<Subscript>3</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript>·8H<Subscript>2</Subscript>O as a new precursor

The nanosized LiNiPO₄ was successfully synthesized by a solid-state reaction between the new Ni₃(PO₄)₂·8H₂O precursor and Li₃PO₄ at 700 °C in air atmosphere. The formation of LiNiPO₄ was generated via three thermal decomposition steps. The samples were characterized by Fourier transform infrared, X-ray diffraction, scanning electron microscopy, atomic absorption/atomic emission spectrophotometers, and thermogravimetric/differential thermal gravimetric/differential thermal analysis techniques. The activation energy (E_α) values of the three steps were calculated by Vyazovkin method and determined to be 90.39?±?5.79, 197.81?±?7.46, and 308.66?±?12.03 kJ mol^?1, respectively. The average E_α values from this method are very close to E_α from KAS method. The most probable mechanism functions g(α) of three steps were evaluated by using the masterplots method and found to be the F_1/3 (first step), F_3/2 (second step), and D₄ (final step), respectively. The pre-exponential factors (A) values of three steps were obtained based on the E_α and g(α). The kinetic triplet parameters of the formation of LiNiPO₄ from the new precursor are reported in the first time.     

Catalytic properties of copper chromite ferrites in water gas shift reaction and hydrogen oxidation

The catalytic properties of a series of copper chromite ferrite samples with the composition CuCr_2–xFe_xO₄ (where x = 0–2) and a spinel-type structure in reactions with reducing (water gas shift reaction, WGSR) and oxidizing (the oxidation of hydrogen) reaction atmospheres were studied. The samples were obtained by the thermal decomposition of mixed hydroxo compounds. The distribution of Cu²⁺ ions in the tetrahedral and octahedral crystallographic positions of spinel, which depends on the Cr³⁺/Fe³⁺ ratio, affects the apparent activation energy (E_a) in both of the reactions. In WGSR, E_a is ～33 kJ/mol for CuCr₂O₄, in which Cu²⁺ ions mainly occupy tetrahedral positions, whereas E_a ≈ 100 kJ/mol for CuFe₂O₄, in which Cu²⁺ ions mainly occupy octahedral positions. In the reaction of hydrogen oxidation, E_a is ～71 kJ/mol for CuCr₂O₄ or ～42 kJ/mol for CuFe₂O₄. The value of E_a for the mixed chromite ferrites changes with the replacement of chromium ions by iron ions and, hence, with a ratio between the amounts of copper ions in the tetrahedral and octahedral oxygen positions of spinel.     

Influence of the nature of solvent and substituents on the oxidation potential of 2,2,6,6-tetramethylpiperidine 1-oxyl derivatives

The influence of solvent (DMF, MeCN, and water) and R1, R2 substituent nature on the formal oxidation potential (E°´) of 4-R1,R2-2,2,6,6-tetramethylpiperidine 1-oxyls (1a—f) on a glass carbon electrode was studied by cyclic voltammetry. It was shown that for all the solvents the observed dependence had the form E°´ = ρ″σ″ + b, where σ″ is the substituent constant. The b values decreased with an increase of the solvent solvating ability, while the values ρ″ are similar for all the solvents, surpassing the corresponding values for nitroxyls of the imidazoline series with substituents at position 3, which can be interpreted as an abnormally strong influence of the substituent remote from the reaction center in 1a—f. The experimental values E°´ were linearly correlated with the reaction free energy values (ΔG) calculated by DFT B3LYP and MP2 for the gas phase contribution and by HF/PCM for the contribution of solvation effects. When applying the B3LYP and the HF/PCM approaches in combination, the dependence of ^E°´ on Δ^G for all the considered solvents was described by a linear correlation equation with a slope close to unity and a constant term which was close to the theoretical value of the absolute potential of the reference electrode used.