Standard Entropy Of Activation Research Articles

Tailoring Spinel with Perovskite: Sr Substitution in the Perovskite Phase of CoFe2O4–LaCoO3 Nanocomposite for Oxygen Evolution and Methanol Oxidation in Alkaline Medium

CoFe2O4–La1−xSrxCoO3 (x = 0.2, 0.6, 0.8) composites have been synthesized using a two‐step, low‐temperature wet chemical approach that combines coprecipitation and sol–gel techniques. The composite materials have been physicochemically analyzed using Fourier‐transform infrared spectroscopy, X‐ray diffraction, and scanning electron microscope. The electrocatalytic properties of the composite materials have been assessed in terms of their performance in oxygen evolution reaction (OER) and methanol oxidation reaction (MOR) in alkaline medium. Cyclic voltammetry has been used for analyzing the redox behavior of the materials in 1 M KOH solution. The electrocatalytic activity of the materials has been assessed using anodic polarization curves on Ni substrate and the CoFe2O4–La0.2Sr0.8CoO3 film electrode demonstrates excellent activity, with current densities of 11.1 mA cm−2 for OER and 47.4 mA cm−2 for MOR at 650 mV. Also, the CoFe2O4–La0.2Sr0.8CoO3 film electrode has the maximum specific activity of 1.1 × 103 mA cm−2 g−1 at 650 mV toward OER. The electrodes’ stability has been assessed through chronoamperometric experiments and additional insight into the enhancement of electrocatalytic activity gained through the electrochemical surface area estimations using electrochemical impedance spectroscopy. From chronoamperometric experiment, it has been observed that the CoFe2O4–La0.2Sr0.8CoO3 film electrode attains stability within 114 s with a current density of 108.7 mA cm−2. Furthermore, the thermodynamic parameters, including the standard enthalpy of activation (ΔH°#), standard entropy of activation (ΔS°#), and standard electrochemical energy of activation (), have been estimated by anodic polarization curve recorded in KOH as well as KOH with CH3OH solutions at various temperatures.

Optical, electrical and thermal investigations on brucinium di-hydrogen citrate tri-hydrate single crystal: An optimistic tool for microelectronics, OPO and OLED applications

Highly capable nonlinear optical brucinium di-hydrogen citrate tri-hydrate (BDHCTH) single crystal under organic material category has been grown-up successfully by slow evaporation method. Optical, electrical, and thermal investigations were made on title crystal to discover the suitability of fabrication of an optical as well as electrical device for the first time. Crystal structure analysis confirms that, the BDHCTH crystal belongs to triclinic crystal system with non-centro symmetric space group P1. The SLDT value of BDHCTH crystal was found to be 4.22 GW/cm2. Second harmonic generation efficiency of the BDHCTH compound is 1.08 times superior to the reference sample KDP. An attribution of single and sharp peak with lesser full width half maximum value confirms the presence of very fewer defects in the grown crystal. An electric response of the title compound shows the normal dielectric behavior. The piezoelectric charge co-efficient (d33) was found to be 4.5 p C/N. Photo luminescence study confirms that the synthesized sample possess violet, blue and red emission behaviors. Presences of various functional groups in the title sample were identified by FT-IR study. Thermal stability of the compound BDHCTH was found to be 135 °C. Kinetic parameters such as activation energy, Arrhenius factor, standard entropy of activation, standard enthalpy of activation, standard Gibb's energy of activation (ΔG*) and induction time of the grown sample were found to be 161.6396 k J mol−1, 7.4854 s−1, -230.8278 J K mol−1, 154.854 k J mol−1, 249.0314 k J mol−1 and 52 days respectively. At the end of an optical, electrical and thermal analysis, we concluded that the grown crystal is an optimistic tool for microelectronics, OPO and OLED applications due to existence of good SHG value, lesser defect and red emission behavior.

Capability of TiO2 and Fe3O4 nanoparticles loaded onto Algae (Scendesmus sp.) as a novel bio-magnetic photocatalyst to degration of Red195 dye in the sonophotocatalytic treatment process under ultrasonic/UVA irradiation

In this study, the magnetic photocatalyst Scendesmus/Fe3O4/TiO2 was synthesized, and its sonophotocatalytic properties in relation to the degradation of the Red195 dye were evaluated. Particles were characterized using a scanning electron microscope (SEM), Fourier's transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), and a vibrating-sample magnetometer (VSM). At a pH of 5, a photocatalyst dosage of 100 mg, an initial R195 concentration of 100 mg/l, an ultrasound power of 38W, and an exposure time of 20 min, the maximum Red195 removal efficiency (100%) was achieved. After five cycles of recycling, the composite's sonophotocatalytic degradation stability for R195 remains above 95%. Experiments on scavenging indicate that electrons (h+) and hydroxyls (OH-) are indispensable decomposition agents. The removal of R195 by Scendesmus/Fe3O4/TiO2 is consistent with the pseudo-first-order kinetic, Freundlich, and Henderson's isotherm models, as determined by kinetic and isotherm investigations. The negative activation enthalpy of the standard (ΔH°) illuminates the exothermic adsorption mechanism. The increase in standard Gibbs activation free energy (ΔG°) with increasing temperature reveals the process is not spontaneous. As indicated by the negative value of the standard entropy of activation (ΔS°), activation of the reactants resulted in a loss of freedom.

Strontium Substituted SmNiO3: Novel Electrode Materials for Alkaline Water Electrolysis

Sr-substituted SmNiO3 perovskite-type oxides have been investigated for their electrocatalytic properties towards oxygen evolution reaction (OER) in alkaline medium. Materials were obtained by using low temperature malic acid sol-gel route. To know the redox behaviour, electrocatalytic activity and thermodynamic parameters of oxides, cyclic voltammetry (CV) and anodic polarization curve (Tafel plot) were recorded in 1 M KOH at 25 ºC. X-ray diffraction (XRD) study indicates the formation of almost pure perovskite phase of the material. A pair of redox peaks was observed (anodic; Epa = 494±12 mV and corresponding cathodic; Epc = 360±4 mV) in the potential region 0.0-0.7 V prior to onset of OER. As observed in the case of La-based perovskite oxides, Sr-substitutions in the SmNiO3 also enhance the electrocatalytic properties of the material. However, Sm-based oxides showed least electrocatalytic activity as compared to La-based oxides. The estimated values of Tafel slope and reaction order indicate that each oxide electrode, except SmNiO3, follows similar mechanistic path towards OER. Standard entropy of activation (DS˚#), standard enthalpy of activation (DH˚#) and standard electrochemical energy of activation (DHel˚#) was determined by recording the anodic polarization curve in 1M KOH at different temperatures.

Kinetics of Electron Transfer Reaction between Co(II) and Chlorate Ions: Experimental and Modeling Study

This research article reports original experimental and modeling detail of kinetics of the electron transfer reaction between Co(II) and chlorate ions in acetic acid solution. Design of experiment methodology has been employed to elucidate the effects of temperature and initial concentrations of reactants on the rate of reaction. Levenberg-Marquardt method has been used to fit processed kinetic data (temperatures, initial concentrations of reactants, and concentrations and rates of production of Co(III)) on to various possible rate equations. This algorithm provides a proficient mean for compensating the capricious effects of the experimental process variables and results in the maximum likelihood estimates of the kinetic parameters. The most significant rate law has been selected, on the basis of statistical analyses of the residuals between the predicted and experimental rates. The analyses suggest that the intrinsic rate of reaction is proportional to first power of chlorate concentration but for Co(II) the order is fractional (0.7455 ≈ and#190;). The effect of temperature on the observed rate constant (precision = 0.02 %) is excellently described by the Arrhenius and Eyring equations and the sluggish nature of the reaction is clearly manifested by the high energy (andgt; 93 kJ/mol), negative entropy (-28.5286 J/mol-K) and very small equilibrium constant of activation. Further fairly negative standard entropy of activation shows there is usually considerable rearrangement of energy among various degrees of freedom during the formation of activated complex and proposes an associative mechanism for formation of the activated complex. This research is performed to develop a kinetic model for the electron transfer reaction between Co(II) and chlorate ion. As a result, a redox couple of Co(II)/Co(III) has been formed which is used as a potent oxidation catalyst in chemical industries.

Kinetics of Electron Transfer Reaction between Co(II) and Chlorate Ions: Experimental and Modeling Study

This research article reports original experimental and modeling detail of kinetics of the electron transfer reaction between Co(II) and chlorate ions in acetic acid solution. Design of experiment methodology has been employed to elucidate the effects of temperature and initial concentrations of reactants on the rate of reaction. Levenberg-Marquardt method has been used to fit processed kinetic data (temperatures, initial concentrations of reactants, and concentrations and rates of production of Co(III)) on to various possible rate equations. This algorithm provides a proficient mean for compensating the capricious effects of the experimental process variables and results in the maximum likelihood estimates of the kinetic parameters. The most significant rate law has been selected, on the basis of statistical analyses of the residuals between the predicted and experimental rates. The analyses suggest that the intrinsic rate of reaction is proportional to first power of chlorate concentration but for Co(II) the order is fractional (0.7455 ≈ and#190;). The effect of temperature on the observed rate constant (precision = 0.02 %) is excellently described by the Arrhenius and Eyring equations and the sluggish nature of the reaction is clearly manifested by the high energy (andgt; 93 kJ/mol), negative entropy (-28.5286 J/mol-K) and very small equilibrium constant of activation. Further fairly negative standard entropy of activation shows there is usually considerable rearrangement of energy among various degrees of freedom during the formation of activated complex and proposes an associative mechanism for formation of the activated complex. This research is performed to develop a kinetic model for the electron transfer reaction between Co(II) and chlorate ion. As a result, a redox couple of Co(II)/Co(III) has been formed which is used as a potent oxidation catalyst in chemical industries.

Activated complex theory is a classical theory suitable for food science with appropriate use.

Activated complex theory (ACT), apart from Van 't Hoff equation, has long been applying as an alternative tool to connect the kinetics (reaction rate constant, k) and thermodynamics parameters (including standard enthalpy of activation, △H++; standard entropy of activation, △S++; standard Gibbs free energy of activation, △G++). The study mainly focuses on ACT application in food systems, especially oil and fruit juice processing. Considering there are several improper calculations or mistakes often found in papers published recently in 2014-2019, three considerations are presented when applying the ACT, including 1) Understand that the reaction should be a single chemical elementary step; 2) Ensure that the units used should be consistent; 3) Effectively analyze the kinetics and thermodynamic parameters by choosing proper temperatures. This study is expected to further improve the understanding and correct application of this well-known theory in future work.

Extracting meaningful standard enthalpies and entropies of activation for surface reactions from kinetic rates

While analyses based on the Arrhenius kinetic model have been successfully applied since its introduction, the Arrhenius model neglects to describe pre-exponential factor in a scientifically meaningful fashion. Since the 1930s, transition-state theory (TST), has met with success in interpreting the pre-exponential factor’s value, allowing a standard entropy of activation to be estimated. However, analyses based on TST’s assumptions have been applied inconsistently in the literature, particularly in corrosion science, leading to difficulty in comparison of standard entropy of activations from different studies. In this work, the foundational principles of TST and standard states are discussed and a standard method to apply TST in analyzing rates of surface reactions is recommended. When full details are not available, reacting species’ concentrations should be normalized to the concentration of active surface sites. For corrosion reactions, conversion relationships are given to convert from units of corrosion rate to surface reaction rate, consistent with TST. This method is dubbed a surface reactant equi-density approximation. Application of this standard to reported data results in adjustments of the standard entropy of activation between − 65 and +50 J/mol K and brings reported entropies into a narrower range of values.

New, Amino Acid Based Zwitterionic Polymers as Promising Corrosion Inhibitors of Mild Steel in 1 M HCl

The zwitterionic monomers, N,N’-diallylamino propanephosphonate and amino acid residual N,N’-diallyl-l-methionine hydrochloride were synthesized, with excellent yields. These monomers were utilized in the preparation of zwitterionic homo and co-cyclopolymers 5–7 in aqueous solution using 2,2′-azobis (2-methylpropionamidine) dihydrochloride as an initiator. The polymers were characterized by FT-IR, NMR, and TGA. The performance of these synthesized polymers on mild steel in acidic solution was investigated by gravimetric method, Tafel extrapolation, linear polarization resistance, and electrochemical impedance spectroscopy. At 313 K, the maximum inhibition efficiencies of corrosion inhibitors 5–7 at 4.50 × 10−4 mol L−1 were found to be 85.2%, 83.3%, and 99.5%, respectively. The inhibition efficiencies obtained from gravimetric weight loss, potentiodynamic polarization, and electrochemical impedance spectroscopy measurements were in good agreement. Different adsorption isotherms were also explored to find the best fit, and found to obey Langmuir adsorption isotherm. The thermodynamic parameters, such as activation energy (Ea), standard enthalpy of activation (ΔH*), standard entropy of activation (ΔS*), adsorption–desorption equilibrium constant (Kads), and standard free energy of adsorption (ΔGoads), were determined. Electrochemical data indicated that the zwitterionic copolymer 7 acts as a mixed type inhibitor under the influence of anodic control. The surface morphology of mild steel corrosion was evaluated without and with corrosion inhibitors by AFM, SEM-EDX, and XPS, which confirmed the adsorption of inhibitor molecules on the metal surface.

Isothermal kinetics of exchange of water absorbed in calcium alginate hydrogel with ethanol

The isothermal kinetics of the exchange of absorbed water in calcium alginate hydrogels with ethanol was investigated. The isothermal kinetic curves were measured within the temperature range from 297 to 316 K. By the application of differential isoconversional methods, it was found that the process of exchange of water absorbed in calcium alginate hydrogel with ethanol has a unique rate determining step (a single step process). Using the model fitting method, it was determined that the kinetics of the exchange can be mathematically described with the kinetic model of the first-order chemical reaction. The values of the kinetic parameters of water exchange: the rate constants (km), energy of activation (Ea = 18.0 kJ?mol?1), and the pre-exponential factor (ln[A/min?1] = 4.0) were calculated. By using Eyring's equation, the values of thermodynamics parameters: standard enthalpy of activation (?H*), standard entropy of activation (?S*), and Gibbs energy of activation (?G*) were calculated. It was found that the rate of rearrangements of the bound water around hydrophilic groups of a polymeric network has a dominant effect on the kinetics of the exchange of the absorbed water with ethanol. The mechanism of the exchange of adsorbed water with ethanol was suggested.

Evidence of positive co-operativity in the micellar catalysis electron transfer reaction

Microenvironments consisting of surfactants are capable of either inhibiting or catalyzing the reaction rate depending on the specific interaction between surfactant and the reactant species. In this context, micellar catalyzed electron transfer reaction between methylene blue (MB) and ascorbic acid (AA) have been studied in presence of three cationic surfactants (TTAB, CTAB & CPC) at different temperatures under pseudo-first order condition. The kinetic data have been analyzed with different models (i.e., pseudo-phase model, Piszkiewicz's model and Raghavan-Srinivasan's model). The micellar binding constant evaluated from the three models are in good agreement with each other. Out of three surfactants, the lower standard enthalpy of activation (∆‡H°) and higher negative standard entropy of activation (∆‡S°) of CPC have been ascribed to the planar structure of the pyridinium head group which provides less steric hindrance to the attacking ascorbate ions. The formation of catalytic sub micellar aggregates between dye and micelle in the reaction medium in a ratio of 1:1 and 1:2 have been well justified by TEM measurements and the result of positive co-operative values. The catalytic activity of surfactants as well as micellar binding constant increases in the following order: CPC>CTAB>TTAB.

Isothermal kinetics of C60 polyhydroxylation in a two-phase system in the presence of tetrabutylammonium hydroxide

The isothermal kinetics of fullerene (C60) polyhydroxylation with sodium hydroxide in the presence of phase-transfer catalyst tetrabutylammonium hydroxide (TBAH) in a two-phase system was investigated. A spectrophotometric method for the determination of the degree of C60 polyhydroxylation was developed. The isothermal kinetic curves of C60 polyhydroxylation were measured in the temperature range from 300 to 328 K. The dependence of effective activation energy on the degree of polyhydroxylation was calculated by using the isoconversional method. The kinetics of polyhydroxylation is mathematically described successfully with the kinetic model of first order chemical reaction. The values of the rate constants, activation energy and pre-exponential factor of polyhydroxylation were calculated. The thermodynamic parameters of activated complex formation for C60 polyhydroxylation [standard enthalpy of activation (∆H*), standard entropy of activation (∆S*) and standard free Gibbs energy of activation (∆G*)] were calculated by using Eyring equation. The effect of stirring speed on the reaction rate constants of polyhydroxylation in the presence of the two types of phase transfer catalysts, TBAH and cetyltrimethylammonium bromide (CTAB) was determined. The reaction mechanism of phase-transfer catalyst and activation of C60 molecule for the reaction of polyhydroxylation was suggested.

Effect of Co-substitution on the electrocatalytic properties of Ni1.5Fe1.5O4 for oxygen evolution in alkaline solutions

Some ternary ferrites having composition CoxNi1.5-xFe1.5O4 (0.0 ≤ x ≤ 1.25) have been synthesized by NH4OH co-precipitation method at 11.5 pH. Materials, so obtained, were tested for their electrocatalytic properties towards oxygen evolution reaction (OER) in the form film on Ni-support in alkaline solution. The study showed that the electrocatalytic properties the material increased with partial substitution of Co for Ni in the base oxide (Ni1.5Fe1.5O4). The value being highest with 0.5 mol Co-substitution. At E = 850 mV vs Hg/HgO in 1M KOH at 25 ºC, the electrode showed apparent current density 206.2 mA cm-2, which is about 3 times higher than the base oxide. The Tafel slope values were ranged between 46-82 mV decade-1. A pair of redox peak, an anodic (EPa = 522±28 mV) and corresponding cathodic (EPc= 356±9 mV), was observed in the cyclic voltammetry (CV) study of the material. The thermodynamic parameters namely, standard apparent electrochemical enthalpy of activation ( ), standard enthalpy of activation ( ) and standard entropy of activation ( ) for the oxygen evolution reaction (OER) have also been determined by recording anodic polarization curve in 1M KOH. The value of was observed to be almost similar with each oxide electrode. The values were highly negative and ranged between ~ -165 and ~ -207 J deg-1 mol-1. Phase and morphology of materials have been investigated by using physical techniques X-ray diffraction, infrared spectroscopy (IR) and scanning electron microscope (SEM), respectively.

Deceleration the hydrolysis reaction of ethyl acetate ester by β-cyclodextrin in basic medium: transition state analog

Electrical conductivity measurements were used to study the hydrolysis reaction of dilute aqueous solution of ethyl acetate with the presence β-cyclodextrin (β-CD) within a concentration range between 0.00 and 0.00750 M in basic medium at 25.0 °C. 0.00265 M of β-CD was chosen as typical concentration for studying the same reaction at different temperatures between 21.0 and 35.0 °C. Two different values of activation energy for the hydrolysis reaction of free ethyl acetate and ethyl acetate/β-CD complex were evaluated, and their values are 46.3 and 62.0 kJ mol−1 respectively. The standard Gibbs energy of activation (∆‡Go), standard enthalpy of activation (∆‡Ho), and standard entropy of activation (∆‡So) for the two different cases were evaluated. β-CD plays a notable role in retarding the rate of hydrolysis of ethyl acetate in basic medium.

The Kinetics of the Cis-to-Trans Thermal Isomerization of 4-Anilino-4’- Nitroazobenzene are Highly Influenced by Solvent Polarity

The thermally stable trans isomers of donor-acceptor substituted azobenzene dyes can be easily transformed into unstable cis-form by light. Although azobenzene and its derivatives have been extensively investigated over the years, many aspects of the isomerization reaction kinetics are still unclear or controversial at best. Here, we investigate the kinetics of the thermal cis-trans isomerization of 4-anilino-4'-nitroazobenzene (4A4NAB) in six solvents of different polarities by means of flash photolysis. Using the transition state theory, we were able to determine a number of thermodynamic quantities including the standard enthalpy of activation Δ‡H0, the standard entropy of activation Δ‡S0, the Gibbs free energy of activation Δ‡G0, and the quasi-equilibrium constant K‡ of the isomerization reaction. Higher rates of isomerization and lower activation energies were observed in polar media, consistent with the formation of a charged transition state complex and the presence of solute−solvent interactions such as intermolecular hydrogen bonding. The data suggest that the cis-trans thermal isomerization of 4A4NAB is highly influenced by solvent polarity and the isomerization mechanism is likely to proceed via rotation about the azo double bond.

Electrocatalytic Properties of Egg-white Sol-gel Derived MnxFe3-xO4 (0 ≤ x ≤ 1.5) for Alkaline Water Electrolysis

Some binary ferrites having molecular formula, MnxFe3-xO4 (0 ≤ x ≤ 1.5) have been synthesized by a simple and cost effective egg white sol-gel route. In each preparation, nitrates of constituent metal were taken as starting materials and egg white as gelling agent. The material, thus obtained was altered in the form of film electrode on the pre-treated Ni- support using an oxide-slurry painting tech-nique and investigated as electrocatalysts for the oxygen evolution reaction in KOH solutions. The study showed that Mn-substitution in the base oxide matrix influences the electrocatalytic activity of the material significantly and found to be greatest with 0.5 mol Mn-substitution. The reaction order has been found to be approximately unity with respect to OH¯ concentration. The Tafel slope values at low potential ranged between ~ 52 and ~ 65 mVdecade-1. The thermodynamic parameters namely, standard apparent electrochemical enthalpy of activa-tion (), standard enthalpy of activation () and standard entropy of activation () for the oxygen evolution reac-tion (OER) have also been determined. It has been observed that and values decreased with Mn-substitution in the Fe3O4 lattice. This decrease inandwas found to be greatest with 0.5 mol Mn. Thevalues were also found to be highly negative varying between ~ - 185 and – 211 J deg-1 mol-1.

Kinetics of basic hydrolysis of tris(1,10‐phenanthroline)iron(II) in macromolecular assemblies of CTAB

AbstractThe kinetics of basic hydrolysis of tris(1,10‐phenanthroline)iron(II) has been carried out in aqueous, N‐cetyl‐N,N,N‐trimethyl ammonium bromide (CTAB) micellar, and CTAB reverse micellar media by UV–visible spectroscopy system. The reaction follows the overall second‐order kinetics; first order in each Fe(II) complex and the base (−OH). CTAB micelles catalyze the reaction rate through the adsorption of the Fe(II) complex and the hydroxyl ions on the micellar surface. In the reverse micellar medium, interesting physicochemical features are observed. Being ionic nature of reactants, both the reactants prefer to stay and react inside the water pool in place of the hydrophobic environment. The rate increases with w, that is, the size of the water pool, attains a maximum value at w = 8.33, and then decreases. But the rate increases as the concentration of surfactant increases at fixed w values. For a better explanation of the kinetic data, the activation parameters, standard enthalpy of activation (Δ‡H°), standard entropy of activation (Δ‡S°), and energy of activation (Ea) were determined. All kinetic data corroborate the proposed mechanism. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 579–589, 2011

Kinetic and mechanistic studies on the Pd-Cl cleavage of dichloro- [1-alkyl-2-(naphthylazo)imidazole]palladium(II) complexes by 2,2′-bipyridine

The interaction of 2,2′-bipyridine with dichloro-[1-alkyl-2-(α-naphthylazo) imidazole]palladium(II) [Pd(α-NaiR′)Cl2, 1] and dichloro-[1-alkyl-2-(β-naphthylazo) imidazole] palladium(II) [Pd(β-NaiR′)Cl2, 2] complexes {where alkyl R′ = Me (a), Et (b) or Bz (c)} in acetonitrile medium to yield [{1-alkyl-2-(α-naphthylazo) imidazole} (bipyridine)] palladium(II)diperchlorate (3a, 3b, 3c) and [{1-alkyl-2-(β-naphthylazo) imidazole}(bipyridine)] palladium(II) diperchlorate (4a, 4b, 4c) was studied. The products were characterized by microanalytical data and spectroscopic techniques (FT-IR, UV–vis, and NMR). The nucleophilic substitution reaction shows a first-order dependence of rate on each of the Pd(II) complex and 2,2′-bipyridine. Addition of LiCl to the reaction decreases the rate of reaction and has proved the cleavage of Pd–Cl bond at the rate-determining step. Thermodynamic parameters standard enthalpy of activation (Δ‡H°), and standard entropy of activation (Δ‡S°) were determined. The second-order rate constant k2 corroborates with the experimental Δ‡H° values. The negative values of Δ‡S° indicate that the reaction proceeds through an associative inner sphere mechanism. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 43: 130–140, 2011

Kinetics and mechanistic studies of the interaction of thiosulfate with cis-diaqua- bis[1-alkyl-2-(arylazo)imidazole]ruthenium(II) complexes

The nucleophilic substitution reaction of S 2O 3 2− with [Ru(HaaiR′) 2(OH 2) 2](ClO 4) 2 ( 1) [HaaiR′ = 1-alkyl-2-(phenylazo)imidazole] and [Ru(ClaaiR′) 2(OH 2) 2](ClO 4) 2 ( 2) [ClaaiR′ = 1-alkyl-2-(chlorophenylazo)imidazole] [where R′ = Me( a), Et( b) or Bz( c)] in acetonitrile–water (50% v/v) medium to yield Na 2[Ru(HaaiR′) 2(S 2O 3) 2] ( 3a, 3b or 3c) and Na 2[Ru(ClaaiR′) 2(S 2O 3) 2] ( 4a, 4b or 4c) has been studied. The products were characterized by microanalytical data and spectroscopic techniques (UV–Vis, NMR and mass spectroscopy). The reaction proceeds in two consecutive steps (A → B → C); each step follows first order kinetics with respect to each complex and S 2O 3 2−, and the first step second order rate constant ( k′ 2) is greater than the second step one ( k″ 2). An increase in the π-acidity of the ligand increases the rate. Thermodynamic parameters, the standard enthalpy of activation (Δ ‡ H 0) and the standard entropy of activation (Δ ‡ S 0), have been calculated for both steps using the Eyring equation from variable temperature kinetic studies. The low Δ ‡ H 0 and large negative Δ ‡ S 0 values indicate an associative mode of activation for both aqua ligand substitution processes.

Mechanism of interaction of DNA bases with dichloro‐[1‐alkyl‐2‐(naphthylazo)imidazole]palladium(II) complexes: A cytosine case

AbstractInteraction of cytosine (C) with dichloro‐[1‐alkyl‐2‐(α‐naphthylazo)imidazole]‐palladium(II) [Pd(α‐NaiR)Cl2, 1] and dichloro‐[1‐alkyl‐2‐(β‐naphthylazo)imidazole]‐palladium(II) [Pd(β‐NaiR)Cl2, 2] complexes {where alkyl R = Me (a), Et (b) or Bz (c)} in acetonitrile (MeCN)‐water (50% v/v) medium to yield [{1‐alkyl‐2‐(α‐naphthylazo)‐imidazole}bis(cytosine)]palladium(II)dichloride (3a, 3b, 3c) and [{1‐alkyl‐2‐(β‐naphthylazo)‐imidazole}bis(cytosine)]palladium(II)dichloride (4a, 4b, 4c) was studied. The products were characterized by microanalytical data and spectroscopic techniques (FT‐IR, UV–vis, and NMR). The reaction kinetics show first‐order dependence of the rate on each of the concentration of Pd(II) complex and C. External addition of Cl− ion (LiCl) did not influence this nucleophilic substitution rate process and has proved the cleavage of first PdCl bond is the rate‐determining step. Thermodynamic parameters standard enthalpy of activation (Δ‡Ho) and standard entropy of activation (Δ‡So) were determined from variable temperature kinetic studies. The negative values of Δ‡So indicate that the reaction proceeds through an associative inner sphere mechanism. The magnitude of the second‐order rate constant k2 increases in the following order: Pd(NaiEt)Cl2 (b) < Pd(NaiMe)Cl2 (a) < Pd(NaiBz)Cl2 (c) as well as Pd(α‐NaiR)Cl2 (1) < Pd(β‐NaiR)Cl2 (2), which corroborates with the experimental Δ‡Ho values. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 463–472, 2009