Entropy Of Activation Research Articles

Molecular-dynamic/DFT-electronic theoretical studies coupled with electrochemical investigations of the carrot pomace extract molecules inhibiting potency toward mild steel corrosion in 1 M HCl solution

The present work introduces the carrot pomace extract as sufficient material for controlling the mild steel aggression in 1 M HCl media. The Tafel electrochemical investigations evidenced that 400 ppm of the carrot pomace extract could control both cathodic and anodic reactions with 95% inhibition index and Electrochemical impedance spectroscopy (EIS) achievements displayed around 94% metal corrosion degree reduction after 5 h steel immersion in the acidic chloride-based media. According to the weight loss test, the assessments have displayed that 400 ppm of carrot pomace extract can control the metal corrosion by 97% and continuously descend until 87% in 55 °C. The atomic force microscope (AFM) results demonstrated that the surface average roughness (Sa) values were mitigated from 127 nm to 72 nm. The contact angle measurements revealed that the angle value raised to 97° after dissolving 400 ppm carrot pomace extract. According to the adsorption isotherm computations, the standard adsorption equilibrium constant (ΔG°ads)varied between –33.774 and −38.213 kJ.mol−1. The thermodynamic calculations evidenced that the activation energy (E*) was increased to 44.25 kJ mol−1, whereas both activation enthalpy (H*) and activation entropy (S*) were diminished by raising the inhibitor concentration, 41.67 kJ·mol−1 and −141.83 J·K−1·mol−1, respectively.

Frustrated Lewis Pair Stabilized Phosphoryl Nitride (NPO), a Monophosphorus Analogue of Nitrous Oxide (N2O).

Phosphoryl nitride (NPO) is a highly reactive intermediate, and its chemistry has only been explored under matrix isolation conditions so far. Here we report the synthesis of an anthracene (A) and phosphoryl azide based molecule (N3P(O)A) that acts as a molecular synthon of NPO. Experimentally, N3P(O)A dissociates thermally with a first-order kinetic half-life that is associated with an activation enthalpy of ΔH⧧ = 27.5 ± 0.3 kcal mol-1 and an activation entropy of ΔS⧧ = 10.6 ± 0.3 cal mol-1 K-1 that are in good agreement with calculated DLPNO-CCSD(T)/cc-pVTZ//PBE0-D3(BJ)/cc-pVTZ energies. In solution N3P(O)A undergoes Staudinger reactivity with tricyclohexylphosphine (PCy3) and subsequent complexation with tris(pentafluorophenyl)borane (B(C6F5)3, BCF) to form Cy3P-NP(A)O-B(C6F5)3. Anthracene is cleaved off photochemically to form the frustrated Lewis pair (FLP) stabilized NPO complex Cy3P⊕-N═P-O-B⊖(C6F5)3. An intrinsic bond orbital (IBO) analysis suggests that the adduct is zwitterionic, with a positive and negative charge localized on the complexing Cy3P and BCF, respectively.

Berberine isolated from Mahonia nepalensis as an eco-friendly and thermally stable corrosion inhibitor for mild steel in acid medium

The environmental and economic benefits have been the driving force in search of efficient corrosion inhibitors for iron/steel used in industrial acidic medium. This study reports on berberine isolated from methanol extract of high-altitude (1347 m) shrub Mahonia nepalensis as a highly efficient and thermally stable corrosion inhibitor for mild steel (MS) in 1.0 M H 2 SO 4 simulating acid pickling condition. The weight-loss and electrochemical methods revealed the fast adsorption of berberine. Significant Findings: It achieved above 91% inhibition efficiency ( IE ) in 0.25 h and reached 94% in 6 h for 1000 ppm berberine. The IE increased with concentration and temperature, giving an IE of 97.2% at 328 K, which makes it a promising candidate for industrial application. It behaved as a mixed type of inhibitor as revealed by open circuit potential and polarization curves. The results indicated suppression of the corrosion by effectively forming an adsorbed berberine layer on the MS surface. Adsorption of the berberine followed a Langmuir adsorption isotherm. The thermodynamic parameters such as activation energy (43.19 kJ/mol), free energy (−35.05 kJ/mol), enthalpy (40.55 kJ/mol), and entropy (−97.83 J/molK) of adsorption supported both physical and chemical interactions of berberine with MS surface. The obtained results also revealed that the adsorption process was endothermic and spontaneous in nature.

Impact of the Local Concentration of Hydronium Ions at Tungstate Surfaces for Acid-Catalyzed Alcohol Dehydration.

Tungstate domains supported on ZrO2, Al2O3, TiO2, and activated carbon drastically influence the hydronium-ion-catalyzed aqueous-phase dehydration of alcohols. For all catalysts, the rate of cyclohexanol dehydration normalized to the concentration of Brønsted acid sites (turnover frequencies, TOFs) was lower for monotungstates than for polytungstates and larger crystallites of WO3. TOFs were constant when reaching or exceeding the monolayer coverage of tungstate, irrespective of the specific nature of surface structures that continuously evolve with the surface W loading. However, the TOFs with polytungstates and large WO3 crystallites depend strongly on the underlying support (e.g., WOx/C catalysts are 10-50-fold more active than WOx/Al2O3 catalysts). The electrical double layer (EDL) surrounding the negatively charged WOx domains contains hydrated hydronium ions, whose local concentrations change with the support. This varying concentration of interfacial hydronium ions ("local ionic strength") impacts the excess chemical potential of the reacting alcohols and induces the marked differences in the TOFs. Primary H/D kinetic isotope effects (∼3), together with the substantially positive entropy of activation (111-195 J mol-1 K-1), indicate that C-H(D) bond cleavage is involved in the kinetically relevant step of an E1-type mechanistic sequence, regardless of the support identity. The remarkable support dependence of the catalytic activity observed here for the aqueous-phase dehydration of cycloalkanols likely applies to a broad set of hydronium-ion-catalyzed organic reactions sensitive to ionic strength.

Adsorption Isotherm and Activation Energy of Inhibition of Alkaloids on Mild Steel Surface in Acidic Medium

Alkaloids as green inhibitors were extracted from three different plants Rhynchostylis retusa, Artimesia vulgaris,and Solanum tuberosum. Weight loss measurement in mild steel has been carried out in the presence and absence of green inhibitors individually in an acidic medium. Weight loss measurements at different temperatures are used to calculate thermodynamic parameters. The weight loss measurements at different concentrations are used to find adsorption isotherm and found that it obeys Langmuir adsorption isotherm with R2 values 1, 1, 0.996 for three inhibitors. Activation energy, enthalpy, and entropy of the three inhibitors have been calculated. It is found that the value of all these parameters increased in the addition of inhibitors. The free energy of the system is calculated and found (-17.46 kJ mol-1) indicating that the adsorption process is spontaneous and there is physical adsorption at the MS-Inhibitor interface.

Advancement of Microwave-Assisted Biosynthesis for Preparing Au Nanoparticles Using Ganoderma lucidum Extract and Evaluation of Their Catalytic Reduction of 4-Nitrophenol.

This study describes the biosynthesis of gold nanoparticles (AuNPs) using the extract of Ganoderma lucidum in the buffer zone of Bach Ma National Park, Vietnam, as a reducing and protecting agent using microwave-assisted synthesis. The as-synthesized AuNPs were characterized using transmission electron microscopy, scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, and Fourier transform infrared spectroscopy. Compared to the conventional method, the proposed microwave-assisted method produced AuNPs having a small size of 22.07 ± 8.11 nm in a short synthesis time period. In excess NaBH4, the as-prepared AuNPs demonstrated good catalytic activity for reducing 4-nitrophenol to 4-aminophenol. Furthermore, AuNPs demonstrated improved reusability after four cycles. The pseudo-first-order apparent rate constant was estimated to be 0.086 min–1 at 303 K. Both the catalytic mechanism and reaction path of reduction were proposed. Moreover, activation energy and thermodynamic parameters, including activation enthalpy and entropy, were examined.

Circular versus linear RNA topology: different modes of RNA–RNA interactions in vitro and in human cells

ABSTRACT Circular RNA is progressively reported to occur in various species including mammals where it is thought to be involved in the post-transcriptional regulation of gene expression, partly via interactions with microRNA. Here, we asked whether the circular topology causes functional differences to linear forms when interacting with short RNA strands in vitro and in human cells. Kinetic studies with human bladder cancer-derived synthetic circular RNA versus linear transcripts, respectively, with short oligoribonucleotides showed similar association rates for both topologies. Conversely, a substantial topology-related difference was measured for the activation entropy and the activation enthalpy of RNA–RNA annealing. This finding strongly indicates a significant difference of the mechanism of RNA–RNA interactions. To investigate whether these characteristics of circular RNA are biologically meaningful we performed transient transfection experiments with a microRNA-regulated expression system for luciferase in bladder cancer-derived cells. We co-transfected linear or circular RNA containing one microRNA binding site for the target-suppressing microRNA mlet7a. Here, the circular isoform showed a strongly increased competition with microRNA function versus linear versions. In summary, this study suggests novel topology-related characteristics of RNA–RNA interactions involving circRNA in vitro and in living cells.

Reactions of a Bis(pentalene)dititanium complex with alkenes; the molecular structure of the butadiene complex [Ti2(µ: η5,η5-Pn††)2(μ: η2,η2-s-trans-C4H6)] (Pn†† = 1,4-(SiiPr3)2-C8H4)

Reaction of the syn-bimetallic complex [Ti2(µ:η5,η5-Pn††)2] (1) (Pn†† = 1,4-(SiiPr3)2-C8H4) with 1,3-trans-butadiene in toluene results in the clean formation of the 1:1 adduct [Ti2(µ:η5,η5-Pn††)2(μ: η2,η2-s-trans-C4H6)] (2) featuring an essentially planar butadiene ligand. Complex (2) represents the first example of a bimetallic early transition metal complex where a coordinated butadiene adopts such a conformation. When (1) is reacted with propene an unexpected “tuck-over” alkene π-complex (3) is formed with co-current loss of propane. Complex (3) features a coordinated η2,η1 vinylic (H2C = CMe)-SiiPr2-Pn† moiety as a result of CH activation of one of the isopropyl substituents of the SiiPr3 groups on the Pn†† supporting ligand. One of the hydrogens of this secondary vinylic moiety is significantly shifted upfield in the 1H NMR spectrum of (3) and a single crystal XRD study shows an interaction between this hydrogen and one of the Ti centres. Preliminary kinetic studies of the formation of (3) show a slightly negative entropy of activation and 1st order consumption of (1) both of which suggest the involvement of a Ti-H(iPr) agostic interaction during the cyclometallation reaction.

Activation free energy gradient controls interfacial mobility gradient in thin polymer films.

We examine the mobility gradient in the interfacial region of substrate-supported polymer films using molecular dynamics simulations and interpret these gradients within the string model of glass-formation. No large gradients in the extent of collective motion exist in these simulated films, and an analysis of the mobility gradient on a layer-by-layer basis indicates that the string model provides a quantitative description of the relaxation time gradient. Consequently, the string model indicates that the interfacial mobility gradient derives mainly from a gradient in the high-temperature activation enthalpy ΔH0 and entropy ΔS0 as a function of depth z, an effect that exists even in the high-temperature Arrhenius relaxation regime far above the glass transition temperature. To gain insight into the interfacial mobility gradient, we examined various material properties suggested previously to influence ΔH0 in condensed materials, including density, potential and cohesive energy density, and a local measure of stiffness or u2(z)-3/2, where u2(z) is the average mean squared particle displacement at a caging time (on the order of a ps). We find that changes in local stiffness best correlate with changes in ΔH0(z) and that ΔS0(z) also contributes significantly to the interfacial mobility gradient, so it must not be neglected.

Enthalpy-Entropy Compensation Effect in Oxidation Reactions by Manganese(IV)-Oxo Porphyrins and Nonheme Iron(IV)-Oxo Models.

"Enthalpy-Entropy Compensation Effect" (EECE) is ubiquitous in chemical reactions; however, such an EECE has been rarely explored in biomimetic oxidation reactions. In this study, six manganese(IV)-oxo complexes bearing electron-rich and -deficient porphyrins are synthesized and investigated in various oxidation reactions, such as hydrogen atom transfer (HAT), oxygen atom transfer (OAT), and electron-transfer (ET) reactions. First, all of the six Mn(IV)-oxo porphyrins are highly reactive in the HAT, OAT, and ET reactions. Interestingly, we have observed a reversed reactivity in the HAT and OAT reactions by the electron-rich and -deficient Mn(IV)-oxo porphyrins, depending on reaction temperatures, but not in the ET reactions; the electron-rich Mn(IV)-oxo porphyrins are more reactive than the electron-deficient Mn(IV)-oxo porphyrins at high temperature (e.g., 0 °C), whereas at low temperature (e.g., -60 °C), the electron-deficient Mn(IV)-oxo porphyrins are more reactive than the electron-rich Mn(IV)-oxo porphyrins. Such a reversed reactivity between the electron-rich and -deficient Mn(IV)-oxo porphyrins depending on reaction temperatures is rationalized with EECE; that is, the lower is the activation enthalpy, the more negative is the activation entropy, and vice versa. Interestingly, a unified linear correlation between the activation enthalpies and the activation entropies is observed in the HAT and OAT reactions of the Mn(IV)-oxo porphyrins. Moreover, from the previously reported HAT reactions of nonheme Fe(IV)-oxo complexes, a linear correlation between the activation enthalpies and the activation entropies is also observed. To the best of our knowledge, we report the first detailed mechanistic study of EECE in the oxidation reactions by synthetic high-valent metal-oxo complexes.

A cleaner route of biodiesel production from waste frying oil using novel potassium tin oxide catalyst: A smart liquid-waste management.

The valorization of waste frying oil (WFO) to biodiesel has been carried out via solid base catalyzed transesterification reaction. A novel potassium tin oxide (KSO) catalyst was synthesized via polymer precursor auto combustion method. The catalyst showed the best physicochemical properties when it was calcined at 800°C. Using KSO 800 catalyst, the highest FAME conversion (99.5%) of WFO found at moderated reaction condition within very short time (35min); moreover, no leaching of K-species was observed in reusability test upto 5th cycle. Kinetics proved that the above catalytic reaction followed pseudo-first-order kinetics and the rate of the reaction was doubled with increasing 10°C reaction temperature. The reaction activation energy, enthalpy of activation, entropy of activation, and Gibb's free energy of activation of the reaction were found to be 66.52kJ/mol, 62.95kJ/mol, -74.07J/mol/K and 88kJ/mol respectively. Evaluation of the green parameters revealed that KSO 800 catalyzed transesterification process approached a cleaner route with excellent efficacy in terms of turnover frequency and yield. KSO 800 helped to produce high quality biodiesel from WFO adopting faster and greener reaction pathway. Thus, KSO 800 was considered as a potential and green catalyst for transforming waste oil into biofuel.

Amphoteric surfactant of octadecyl dimethyl betaine as an efficient corrosion inhibitor for cold rolled steel in phosphoric acid solution

The inhibition effect of amphoteric surfactant of octadecyl dimethyl betaine (BS-18) on cold rolled steel (CRS) in H3PO4 solution was studied by weight loss, electrochemical technique, surface analysis and quantum chemical calculations. The results show that BS-18 acts as an efficient inhibitor on CRS in 1.0 M H3PO4 medium at 20–50 °C, and the maximum inhibition efficiency is 87.1% with 50 mg L−1 at 20 °C. Its adsorption on steel surface obeys Langmuir adsorption isotherm, which is endothermic process accompanying with the increase of entropy. Compared with uninhibited solution, apparent activation energy, pre-exponential factor, activation enthalpy and activation entropy become higher in inhibited solution, while both reaction constant and rate constant decrease after adding BS-18. Inhibition performance remains more stable from 3 to 48 h, and fluctuates slightly with the acid concentration from 0.5 to 5.0 M. BS-18 is a mixed-type inhibitor through “geometric blocking effect”. EIS is a time constant with one depressed capacitive loop in Nyquist plot and one phase angle in Bode phase plot, and the charge transfer resistance turns to become more larger in the presence of BS-18. SEM and AFM micrographs of inhibited CRS surface appear little corrosion, and its contact angle is increased. There are functional groups of C–N and C–N+ in the adsorptive film of BS-18 on CRS surface using XPS analysis. Critical micelle concentration (CMC) of BS-18 is about 20 mg L−1 from surface tension measurements. Quantum chemical calculations confirm that the active adsorption site of BS-18 is the hydrophilic group of N+ and –CH2COO−.

Determination of ignition temperature and kinetics and thermodynamics analysis of high-volatile coal based on differential derivative thermogravimetry

In order to accurately determine the ignition temperature (Ti) of coal, Coal spontaneous combustion (CSC) process was tested by a simultaneous thermal analyzer. Next, the abrupt change point of the differential derivative thermogravimetric curve (DDTG) of coal after the high adsorption temperature was taken as the Ti of CSC. Besides, variations of kinetics and thermodynamics during the CSC before and after the Ti were calculated. It was found that the mass loss, heat releases and gaseous products of coal changed slowly before the Ti and surged sharply after the Ti. The coal at ignition temperature was in thermodynamic equilibrium with low activity. However, when the temperature was greater than Ti, the aromatic hydrocarbons in coal begin to decompose, resulting in the increased of active sites and the release of volatiles, and the coal enters an irreversible combustion stage. At this time, upward trends were obtained for the apparent activation energy, pre-exponential factor, enthalpy change, and entropy change, while a gradual decrease was observed for the Gibbs free energy change. Moreover, reaction intensity between oxygen and coal would be increased due to increased of active sites in the coal and enhanced of volatiles release under a slower heating rate.

Kinetics of the Release of Nicotinamide Absorbed on Partially Neutralized Poly(acrylic-co-methacrylic acid) Xerogel under the Conditions of Simultaneous Microwave Heating and Cooling.

The kinetics of release of nicotinamide (NIAM) that was absorbed on partially neutralized poly(acrylic-co-methacrylic) (PAM) xerogel/hydrogel, under the conditions of simultaneous microwave heating and cooling (SMHC) were examined. The kinetics curves of NIAM release into an aqueous solution at temperatures of 308–323 K were recorded. By applying the model-fitting method (MFM), it was found that the kinetics of NIAM release can be modeled by a kinetic model of a first-order chemical reaction. The values of the release rate constants (kM) at different temperatures were calculated, and their values were found to be within the range 8.4 10−3 s −1−15.7 10−3 s−1. It has been established that the Arrhenius equation was valid even in the conditions of SMHC. The values of the kinetic parameters (activation energy (Ea) and pre-exponential factor (A) of the NIAM release process adsorbed on PAM xerogel/hydrogel were calculated as follows: Ea = 25.6 kJ/mol and ln (A/s−1) = 5.21. It has been proven that the higher value of the rate constant at SMHC in relation to CH is not a consequence of the overheating of the reaction system or the appearance of “hot-points”. The values of change of the enthalpy of activation (ΔH*) and the change of entropy of activation (ΔS*) were calculated as follows: ΔH* = +23.82 kJ/mol and ΔS* = −201.4 J/mol K. The calculated higher values of the kinetic parameters and thermodynamic parameters of activation are explained by the formation of a specific activated complex under SMHC, whose structure and degree of order are different than in the one formed under CH.

Gas-phase thermolysis of azines. Part 3. Kinetics and mechanism of pyrolysis of substituted arylidenepyrazin-2-yl- and pyrimidin-2-ylhydrazines

Five N-arylidene-N’-pyrazin-2-ylhydrazines, series (1a-e) and five N-arylidene-N’-pyrimidin-2-ylhydrazines, series (2a-e) were synthesized and characterized: four of the former series and two of the latter are novel compounds. The ten substrates were then subjected to: (i) flow flash vacuum pyrolysis (FVP) at 700 °C and short (ms) contact time; (ii) static sealed-tube pyrolysis (STP) at temperatures in the range of 300–380 °C, and relatively longer substrate residence time of 15–60 min. The products of thermolysis were analyzed, identified, characterized and compared. Further, the kinetics of pyrolysis was investigated over the temperature range ca 550–620 K and 500–615 K, respectively, for substrates (1a,c,e) and (2a,c,e). Rates, Arrhenius log A and Ea, and entropy of activation (∆S≠) were calculated. The nature and composition of the products of pyrolysis were ascertained, and substrate molecular reactivities were adequately explained based on plausible reaction mechanisms, and further rationalized using known effects of common substituent groups and specific nitrogen replacement (N = ) substituents.

Residue-Specific Kinetic Insights into the Transition State in Slow Polypeptide Topological Isomerization by NMR Exchange Spectroscopy.

The characterization of the transition state is a central issue in biophysical studies of protein folding. NMR is a multiprobe measurement technique that provides residue-specific information. Here, we used exchange spectroscopy to characterize the transition state of the two-state slow topological isomerization of a 27-residue lantibiotic peptide. The exchange kinetic rates varied on a per-residue basis, indicating the reduced kinetic cooperativity of the two-state exchange, as well as the previously observed reduced thermodynamic cooperativity. Furthermore, temperature-dependent measurements revealed large variations in the activation enthalpy and entropy terms among residues. Interestingly, we found a linear relationship between the logarithm of the equilibrium constants and that of the exchange rates. Because the data points are derived from amino acid residues in one polypeptide chain, we refer to the linear relationship as the residue-based linear free energy relationship (rbLFER). The rbLFER offers information about the transition state of the two-state exchange.

Evidence of Rate Limiting Proton Transfer in an SNAr Aminolysis in Acetonitrile under Synthetically Relevant Conditions.

An early synthetic step in the synthesis of adavosertib, AZD1775, is the SNAr reaction between 4-fluoronitrobenzene and 1-methylpiperazine in acetonitrile. A simple kinetics-based design of four reaction profiling experiments was used to investigate the kinetics of the reaction for the purpose of building a kinetic model. Fitting of the reaction profile data from two experiments conducted at 70 °C with a different excess of 1-methylpiperazine showed the reaction to follow a third-order rate law with a second-order dependence upon 1-methylpiperazine. This was rationalized in terms of the reaction following a rate-limiting proton transfer mechanism (base catalyzed) in which the progress to product is driven by a proton transfer involving a second molecule of 1-methylpiperazine. The experimentally determined entropy of activation of -180 J K-1 is consistent with this mechanism. The formation of a low level impurity was found to be due to the presence of traces of piperazine in the 1-methylpiperazine, which was shown to react approximately 15 times faster than 1-methylpiperazine at 70 °C. The rate constants for the 1-methylpiperazine catalyzed reaction of piperazine, 1-methylpiperazine, and the piperazine derived impurity were found to correlate in a Brønsted type analysis with the pKa's (acetonitrile) of the amine nucleophile.

The Influence of Impurities on the Dehydration and Conversion Process of Calcium Sulfate Dihydrate to α-Calcium Sulfate Hemihydrate in the Two-Step Wet-Process Phosphoric Acid Production

The dehydration process and kinetics of calcium sulfate dihydrate to α-calcium sulfate hemihydrate were studied under simulated dihydrate–hemihydrate two-step wet-process phosphoric acid, and Fe3+, Al3+, Mg2+, SiF62–, and mixed impurities were introduced to explore their influence on the values of kinetic parameters. The properties of the dehydration conversion product α-calcium sulfate hemihydrate were characterized and analyzed by using scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and thermogravimetric and differential scanning calorimetry to reveal the interaction mechanism between impurities and α-calcium sulfate hemihydrate crystals. Results showed that the dehydration process of calcium sulfate dihydrate to α-calcium sulfate hemihydrate follows the dispersive kinetic model in the wet-process phosphoric acid solution. After Fe3+, Al3+, Mg2+, SiF62–, and mixed impurities were introduced into the system, the activation enthalpy increased, and the activation entropy decreased due to the reduction of the kinetic parameters α and β, causing the nucleation barrier of α-calcium sulfate hemihydrate crystals to increase, thereby inhibiting the dehydration and transformation of calcium sulfate dihydrate to α-calcium sulfate hemihydrate. The order of inhibition is mixed impurities > Al3+ > SiF62– > Fe3+ > Mg2+. Fe3+, Al3+, and Mg2+ impurities were combined with phosphate ions to form phosphate precipitates, which were selectively covered on different crystal surfaces of α-calcium sulfate hemihydrate, thereby hindering the growth of α-calcium sulfate hemihydrate crystals. Several Ca2+ ions existed on the (111) crystal plane in the c-axial direction, and SiF62– ions were combined with Ca2+ ions, thereby hindering the one-dimensional growth in the c-axial direction. Consequently, the aspect ratio of α-calcium sulfate hemihydrate crystal decreased, and the morphology gradually evolved to a short columnar structure.

Slow 3MLCT Formation Prior to Isomerization in Ruthenium Carbene Sulfoxide Complexes.

A series of photochromic complexes with general formulas of [Ru(bpy)2(NHC-SR)]2+ and [Ru(bpy)2(NHC-S(O)R)]2+ were prepared and investigated by X-ray crystallography, electrochemistry, and ultrafast transient absorption spectroscopy {where bpy is 2,2'-bipyridine and NHC-SR and NHC-S(O)R are chelating thioether (-SR) and chelating sulfoxide [-S(O)R] N-heterocyclic carbene (NHC) ligands}. The only differences between these complexes are the nature of the R group on the sulfur (Me vs Ph), the identity of the carbene (imidazole vs benzimidazole), and the number of linker atoms in the chelate (CH2 vs C2H4). A total of 13 structures are presented {four [Ru(bpy)2(NHC-SR)]2+ complexes, four [Ru(bpy)2(NHC-S(O)R)]2+ complexes, and five uncomplexed ligands}, and these reveal the expected coordination geometry as predicted from other spectroscopy data. The data do not provide insight into the photochemical reactivity of these compounds. These carbene ligands do impart stability with respect to ground state and excited state ligand substitution reactions. Bulk photolysis reveals that these complexes undergo efficient S → O isomerization, with quantum yields ranging from 0.24 to 0.87. The excited state reaction occurs with a time constant ranging from 570 ps to 1.9 ns. Electrochemical studies reveal an electron transfer-triggered isomerization, and voltammograms are consistent with an ECEC (electrochemical-chemical electrochemical-chemical) reaction mechanism. The carbene facilitates an unusually slow S → O isomerization and an unusally fast O → S isomerization. Temperature studies reveal a small and negative entropy of activation for the O → S isomerization, suggesting an associative transition state in which the sulfoxide simply slides along the S-O bond during isomerization. Ultrafast studies provide evidence of an active role of the carbene in the excited state dynamics of these complexes.

Kinetic analysis of nitrophenol reduction and colourimetric detection of hydrogen peroxide based on gold nanoparticles catalyst biosynthesised from Cynomorium songaricum

The pseudo-first-order reaction kinetics of metallic nanocatalyst for the model reaction of nitrophenols/NaBH4 were early reported on low reaction temperatures. Here, we investigated reduction of nitrophenols in a wide range of reaction temperature with gold nanoparticles (AuNPs) catalyst that was fabricated via a low-cost and effective method using aqueous extract of Cynomorium songaricum (CS). The biosynthesised CS-AuNPs were well characterised by the analytic techniques such as X-ray diffraction (XRD), transmission electron microscope (TEM), scanning transmission electron microscopy (STEM). The TEM images showed multiple shapes and size in a range of 3–30 nm with an average size of 16 nm. The colloidal solutions were stored at the room temperature for 30 days with a stability of 63%. The kinetic study of the NiPs reduction showed that the reaction occurred via Langmuir–Hinshelwood mechanism with a half-order kinetic model for all tested reaction temperatures (30–70 °C). The thermodynamic parameters including activation energy, enthalpy, entropy and Gibbs free energy were calculated. The activation energies in the reduction of 2-nitrophenol and 3-nitrophenol were found to be as low as 4.88 kJ mol−1 and 8.66 kJ mol−1, respectively. Moreover, CS-AuNPs were used as a nanozyme for the detection of hydrogen peroxide (H2O2) via peroxidase-like reaction with 3,3′,5,5′-tetramethylbenzidine (red-TMB) that found the lowest limit of detection (LOD) of 2.25 µM.