Hard-soft Acid-base Principle Research Articles

Halide Replacement with Complete Preservation of Crystal Lattice in Mixed-Anion Lanthanide Oxyhalides.

A challenge in anion control in periodic solids is to preserve the crystal lattice while substituting for different anions of widely varying size and hardness. Post-synthetic modification routes that place cations or anions in non-equilibrium configurations are promising; however, such methods remain relatively unexplored for anion placement. Here, we report the synthesis of LaOI nanocrystals by a non-hydrolytic sol-gel condensation reaction and their transformation into LaOBr, LaOCl, and LaOF nanocrystals along hard-soft acid-base principles using post-synthetic metathesis reactions with ammonium halides. Anion displacement proceeds along halide planes, preserving the tetragonal matlockite structure. Energy-variant X-ray excited optical luminesce signatures of alloyed Tb3+ -ions is a sensitive quantum reporter of the preservation of the cation sublattice and hardening of the crystal structure upon anion replacement.

Influence of polyethylene glycol plasticizer on the structural and electronic properties of PEO-NaI complex: a density functional study.

Ab initio study has been carried out to investigate the influence of low molecular weight polyethylene glycol (PEG) plasticizer on structural and electronic properties of the polyethylene oxide-sodium iodide (PEO-NaI) polymer-metal complex. DOS and PDOS analysis provided a quantitative explanation of the electronic bandgap of the PEO-NaI and PEO-PEG-NaI system. Hirshfeld population charge analysis (HPA) explains better dissociation of NaI in presence of polyethylene glycol, based on the Hard Soft Acid Base Principle. Also, an increase in amorphic content of polymer system is observed with the addition of PEG, evident from the increment in the strength of anti-bonding orbitals in COOP plot. Bond strength of the polymeric system is also found to be affected with the addition of plasticizer. The findings provide an avenue that the present polymer system [PEO-PEG-NaI] is a potential candidate to be used as an electrolyte for next-generation energy storage technology.

Nitrogen-Enhanced Charge Transfer Efficacy on the Carbon Sheet: A Theoretical Insight Into the Adsorption of Anionic Dyes

Dye removal over the surface of activated carbon is a well-established approach for the treatment of wastewater effluents from pharmaceutical, food, and textile industries. Tuning the surface chemistry of the adsorbent plays a vital role in improving the dye adsorption performance. Herein, we report the mechanistic insights from first-principles calculations on the removal of common anionic dyes using nitrogen-enriched carbon sheets. The adsorption characteristics and chemical reactivity patterns of methyl blue, methyl orange, and methyl red were thoroughly accounted for based on their computed reactivity descriptors, adsorption energies, and structural and electronic properties. Nitrogen enrichment of the carbon sheet resulted in the significant decrease in the energy gap and global hardness values, suggesting an improvement in charge transfer efficacy and dye adsorption capacity. Among the three dyes investigated, methyl blue exhibited the lowest energy gap of 0.960 and 1.064 eV and the global hardness of 0.480 and 0.532 eV at the B3LYP/6-31G(d) and the B3LYP/6-311G(d,p) levels of theory, respectively, displaying a higher electrostatic attraction toward nitrogen-enriched carbon sheets in accordance with the hard-soft acid–base principle. Predicted adsorption energies of 28.69, 13.18, and 16.90 kcal/mol at 6-31G(d); and 28.91, 12.08, and 16.88 kcal/mol at 6-311G(d,p) basis sets were obtained for methyl blue, methyl orange, and methyl red, respectively, suggesting the selectivity of the nitrogen-enriched carbon sheet to the methyl blue dye. Further, the protonation of the nitrogen-enriched carbon sheets would tune the selectivity of the adsorbent resulting in a significant enhancement of the electrostatic attraction, in line with the experimental findings.

Introduction of a new nitrite-selective optical sensor for food analysis using a copper(II) Schiff base complex

A copper(II) Schiff base complex, [Cu(cd4OMeSalMeen)], methyl-2-{[1-methyl-2-(4-methoxy-phenolate)mehylidynenitrilo]ethyl}amino-1-cyclopentene dithiocarboxylate copper(II), was incorporated into triacetylcellulose membrane and applied as ionophore in order to develop an anion-selective optical sensor for the analysis of nitrite by absorption spectrophotometry. At optimum pH 3.0, a linear calibration curve was observed for nitrite in the range of 0.50 to 7.00 mgL-1 with a detection limit of 0.04 mgL-1. The response time of the optode (t95%) was found to be 8–10 min, depending on the nitrite ion concentration. The proposed sensor was fully recovered in nitric acid solution (0.1 M) and had acceptable reproducibility (RSD=1.9%). The optode exhibited good selectivity toward nitrite ion according to hard-soft acid base (HSAB) principle and the formation constant was calculated. The application of the sensor for determination of nitrite content in meat products was quiet successful.

Synthesis of silica coated on iron sand magnetic materials modified with 2- mercaptobenzimidazole through sol-gel

Synthesis of silica coated on iron sand magnetic materials modified with mercapto was carried out through a sol-gel method. This study aims to obtain adsorbent material that has magnetic properties. The purpose of this research are gift magnetic proprties of adsorbent. It was easy access to the separation of the adsorbent after the adsorption process in aqueous system. Modified silica surface with mercapto group provides spesific and selective for soft metals according to hard soft acid base (HSAB) principle. Solution of Na 2 SiO 3 as a silica precursor were coated on the surface of iron sand magnetic materials (MM) that has been activated by acid. Silica which have been coated on MM then modified with compound 2- mercaptobenzimidazole as a functional group of mercapto (-SH). The compound 2- mercaptobenzimidazole does not have a functional group of silanol (≡Si-OH) and siloxane (Si- O-Si) to bind to the functional group on silica surface. Therefore, chloropropyltrimethoxysilane (CPTMS) was added as a link between silica and 2-mercaptobenzimidazole. Chloropropyltrimethoxysilane (CPTMS) was reacted with 2-mercaptobenzimidazole as mixture I then mixed into silica which has coated MM as mixture II by sol-gel process. Material characterization was carried out by Fourier Transform Infrared (FTIR) spectrophotometer, X- ray diffractometer (XRD) and Vibrating Sample Magnetometer (VSM), and the stability of coated magnetite against acid. The characterization results showed that synthesized adsorbent of MM/SiO 2 /SH was successful. Coating of iron sand magnetic material with silica modified mercapto increases the stability of the acid and decreases the magnetic properties.

Facile and efficient nitrogen modified porous carbon derived from sugarcane bagasse for CO2 capture: Experimental and DFT investigation of nitrogen atoms on carbon frameworks

The use of sustainable feedstock and low-cost precursor is an important prerequisite in developing sorbents for CO2 capture. In this research, nitrogen-modified porous carbon materials with varying ratio of carbon-melamine (1:1, 1:2 and 1:3) were synthesized from a sustainable and green-initiated feedstock, sugarcane bagasse, and solid-state impregnation of melamine and investigated as alternative sorbent in CO2 capture. This gives an insight into the effect of increasing the nitrogen content of porous carbon material on CO2 capture. The CO2 uptake of the nitrogen-modified carbon was at least 35% more than that of the pristine porous carbon. Modified carbon with carbon-melamine ratio (1:2) demonstrates the highest CO2 uptake at ambient pressure and temperature (3.34 mmol/g), making it comparable to similar materials that have been reported. The superiority demonstrated by this adsorbent was attributed to a balance between its textural properties and chemical functionalization using nitrogen. The synthesized material also showed high selectivity and regeneration capacities. Adsorption of CO2 and N2 on pristine and nitrogen modified carbon material was also examined using density functional theory (DFT) calculation at B3LYP/6-31G (d) level. The result obtained revealed that nitrogen-doped carbon material exhibited higher tendency to CO2 adsorption than pristine carbon material in accordance with the hard-soft acid-base (HSAB) principle. This work provides a new and efficient route for the synthesis of nitrogen modified carbon and application for similar adsorption processes.

Sorption of different metal ions on magnetic nanoparticles and their effect on nanoparticles settlement

The present work describes the sorption of Cs+, Sr2+, La3+ and Th4+ on Fe3O4 nanoparticles (NPs). The selection of metal ions was done on the basis of different charges to see its effect on sorption. Experiments were carried out at two different pH, 6 and 9 and in presence of NaCl salt to see the effect of these factors on metal ion removal rate. Interestingly no sorption of metal ions were observed at pH ∼ 6 while at pH ∼ 9 only Sr2+ and La3+ ions can be selectively sorbed on NPs surface. Experiments in presence of high concentration of salt suggest that the presence of salt does not have an appreciable effect on sorption suggesting the formation of an inner sphere complex of these metal ions on NPs surface. Sr2+ and La3+ sorption over NPs surface facilitates the enhancement of settlement of NPs. NPs settlement rate was monitored in presence of different metal ions and interestingly was found to vary with metal ions. The results were explained on the basis of DLVO theory, HSAB (hard soft acid base) principle, hydrogen bonding and different charges on metal ions.

Nanostructured Semiconducting Metal Oxides for Ammonia Sensors. A Novel HSAB Sensing Paradigm

The paper demonstrates how the Hard-Soft Acid Base (HSAB) theory can be used as a valuable criterion in the selection process of semiconducting metal oxides (MOX) suitable as sensing layers for ammonia detection. Six different cases of ammonia detection performed by chemiresistive sensors employing MOX and related nanocomposites as sensing layers are identified and discussed. The role of HSAB as an efficient selection tool for appropriate sensing layer (any type of gas), is further reinforced by analyzing and discussing literature results on MOX-based trimethylamine sensing layers. By analyzing the operation of a fiber-optic ammonia sensor, we demonstrate that the HSAB principle can be also successfully applied to the selection of sensing layers for detectors employing other sensing principles, different than the chemiresistive one. Changing the sensing paradigm (i.e., the amino groups-based compounds are part of the sensing layer, rather than part of the analyte), the paper shows that these types of molecules (polymers, carbon nanotubes, ionic liquids) are appropriate constituents of a CO2 sensing layer, in full accordance to the HSAB criteria.

Enhanced visible-light photocatalytic activity to volatile organic compounds degradation and deactivation resistance mechanism of titania confined inside a metal-organic framework

Poor visible-light-driven activity and deactivation property as well as wide band gap of the most common TiO2 photocatalyst significantly limits its practical application in volatile organic compounds (VOCs) purification. In this study, tiny TiO2 nanoparticles incorporated into a typical metal-organic framework (MOF), NH2-UiO-66, with controllable TiO2 content and size, were synthesized based on the hard-soft acid-base (HSAB) principle and applied to VOCs purification. Compared to bare TiO2, the TiO2@NH2-UiO-66 composites could extend the optical absorption to the visible light range and accelerate the photogenerated electrons-holes separation, due to the excellent interface contact between TiO2 and NH2-UiO-66. Moreover, the abundant interconnected 3D cavities of the outer MOF allowed for VOCs to easily diffuse into the pores, producing a concentration microenvironment around the encapsulated TiO2. The TiO2@NH2-UiO-66 composites exhibited a markedly improved photocatalytic efficiency and a good resistance to deactivation during the photocatalytic degradation of gaseous styrene under visible light illumination, which were associated with the synergetic effects between the TiO2 and MOF. The TiO2@NH2-UiO-66 with 5 wt% TiO2 could efficiently mineralize styrene to CO2 to some extent companying with the removal ratio >99% within 600 min, whereas the removal efficiency over the bare TiO2 only 32.5%.

Non‐classical Design of High‐Efficiency Sensitizers for Dye‐Sensitized Solar Cells

The dye regeneration step in a dye‐sensitized solar cell (DSSC) affects significantly the device efficiency. To be able to predict the dye regeneration efficiency by the electrolyte this paper provides a facile way to design high‐efficiency sensitizers for DSSC. This paper proposes, for the first time, a simple and ingenious way to identify the dye regeneration sites and their relative efficiencies when a specific electrolyte is used. Two steps are proposed to identify the dye regeneration sites and their relative regeneration efficiencies: (1) drawing all the resonance structures of the oxidized dye to determine the regeneration sites, and (2) choosing the most favored site for dye regeneration as the chemically softest (when the redox couple used is soft I−/I3− pair) and the least spatially hindered site. The regeneration sites identified by the resonance structures are consistent with the β‐LUSO (β lowest unoccupied spin orbital) distribution, which is generally used for identifying the dye regeneration sites, calculated with DT‐DFT theory. The relative dye regeneration efficiency and photovoltaic performance of both ruthenium and metal‐free organic dyes predicted by the method reported here are supported by experimental data and the proposed dye regeneration mechanism. Several types of dye molecules are used to demonstrate the correctness of this new tool. This non‐classical tool, which uses the well‐known chemical knowledge of the resonance structure and hard–soft acid–base principle, without any computer calculation or physicochemical measurement, provides a very simple and powerful tool to quickly conceive high‐efficiency sensitizers for DSSCs.

Rationalization of Benzazole-2-carboxylate versus Benzazine-3-one/Benzazine-2,3-dione Selectivity Switch during Cyclocondensation of 2-Aminothiophenols/Phenols/Anilines with 1,2-Biselectrophiles in Aqueous Medium.

The cyclocondensation reaction of 2-aminothiophenols with 1,2-biselectrophiles such as ethyl glyoxalate and diethyl oxalate in aqueous medium leads to the formation of benzothiazole-2-carboxylates via the 5-endo-trig process contrary to Baldwin's rule. On the other hand, the reaction of 2-aminophenols/anilines produced the corresponding benzazine-3-ones or benzazine-2,3-diones via the 6-exo-trig process in compliance with Baldwin's rule. The mechanistic insights of these cyclocondensation reactions using the hard-soft acid-base principle, quantum chemical calculations (density functional theory), and orbital interaction studies rationalize the selectivity switch of benzothiazole-2-carboxylates versus benzazine-3-ones/benzazine-2,3-diones. The presence of water facilitates these cyclocondensation reactions by lowering of the energy barrier.

Highly Selective Dehydrochlorination of 1,1,1,2‐Tetrafluoro‐2‐chloropropane to 2,3,3,3‐Tetrafluoropropene over Alkali Metal Fluoride Modified MgO Catalysts

AbstractGas‐phase selective dehydrochlorination of 1,1,1,2‐tetrafluoro‐2‐chloropropane (HCFC‐244bb) to 2,3,3,3‐tetrafluoropropene was investigated over various K and Cs halides modified MgO catalysts. The reaction proceeded in the absence of additives such as air and HF. High dehydrochlorination selectivity (>97 %) with moderate activity (>14 μmol h−1 m−2) was achieved on KF and CsF modified MgO catalysts, in which the dehydrofluorination process was inhibited. Compared with alkali metal modified MgO, the dehydrofluorination product, 2‐chloro‐3,3,3‐trifluoropropene increased over Zr and Al modified MgO and fluorinated MgO catalysts (sel. of 20–76 %). XRD and energy‐dispersive‐spectrometry results revealed that surface chlorination and fluorination occurred on the MgO‐based catalysts during the reaction. Introducing alkali metal fluorides into the catalyst can reduce the fluorination of MgO and the deposition of chlorine species, thus improving the dehydrochlorination activity and stability of MgO. The NH3 temperature‐programmed desorption (TPD) results show that decreasing the surface acidity of catalyst benefits the dehydrochlorination of HCFC‐244bb, whereas the relationship between the basicity of the catalyst and its dehydrohalogenation behavior is complicated as shown by the CO2 TPD results. According to the hard–soft acid–base principle, the dehydrohalogenation selectivity can be correlated with the surface chemical hardness of the catalyst.

Influence of metal impregnants on adsorption of dimethylsulfide vapors by activated carbons

In this paper, effect of metal impregnation was studied on the textural and sorptive properties of activated carbon cloth towards adsorption of dimethyl sulfide (DMS) vapors. The activated carbon was impregnated by three different transition metal ions (Ag+, Cr6+, Cu2+). Isotherms of dimethyl sulfide were measured gravimetrically at different temperatures. The results show that the adsorption capacity of activated carbon for DMS decreased after impregnation with metals. The variation in adsorbed amount of DMS vapors on the activated carbon impregnated with different metals was discussed on the basis of the hard-soft acid base (HSAB) principle. Kinetics of adsorption has been studied by using the Linear driving force model (LDF) and Empirical diffusion model. The kinetic parameters such as rate constant (k), diffusional exponent (n), adsorbate-adsorbent interaction constant (K) and activation energy (Ea) have also been determined. The trends in isosteric heats of adsorption indicated the effect of surface heterogeneity and lateral interactions. On the basis of equilibrium, kinetics and thermodynamic studies, the suitable mechanisms for the adsorption processes occurring on impregnated carbons have been suggested.

Sunlight and ultrasound-assisted synthesis of photoluminescent silver nanoclusters: A unique ‘Knock out’ sensor for thiophilic metal ions

We demonstrate the fine tuning of sunlight and ultrasound in green synthesis of photoluminescent silver nanoclusters (AgNCs) having quantum dot (QD) character for the first time. The use of hazardous additives (e.g. reducing agents, organic solvents, etc.) is minimized. The role of sunlight (which ruptures the S–S linkage) and ultrasound (which produces free radicals) in controlling various synthetic parameter as well as properties are carefully monitored and scientifically explained. The synthesized materials have the capabilities in sensing thiophilic metals ions (Hg2+, Pb2+, Cd2+, Zn2+) up to nanomolar (nM) and milimolar (mM) ranges by photoluminescence (PL) spectroscopy and PL microscopy respectively. The sensing of metal ions is based on fluorescence quenching. Hard soft acid base principle (HSAB) plays a major role to recognise the synthesized material as a unique ‘knock out’ sensor. The HR-MS and HR-TEM show that the silver nanocluster is made of 2–6 silver atoms having diameter in the range of 2–3nm respectively. The synthesized materials may be a good candidate for bio-labelling, bio-sensing and cell imaging.

Inhibitory kinetic spectrophotometric method for the quantitative estimation of d-penicillamine at micro levels

Biologically active thiol, d-penicillamine (d-PA), based on the hard soft acid base (HSAB) principle, binds strongly with Hg(II) ions leading to the inhibition of the rate of Hg(II) catalyzed substitution of cyanide in hexacyanoruthenate(II) i.e. [Ru(CN)6]4− by nitroso-R-salt (NRS). This formed the basis for the development of a novel kinetic spectrophotometric method for the quantitative determination of d-PA at micro levels. The reaction was followed spectrophotometrically at 525nm (λmax of [Ru(CN)5NRS]3− complex) under optimized reaction conditions at 8.75×10−5M [Ru(CN)64−], 3.50×10−4M [NRS], pH7.00±0.02, ionic strength (μ) 0.1M (KCl) and temperature 45.0±0.1°C. The modified mechanistic scheme is proposed to understand the inhibition caused by d-PA on Hg(II) catalyzed substitution of cyanide by NRS in [Ru(CN)6]4−. The detection limit of the proposed method was found to be 2.5×10−7M. The method has successfully been applied for the determination of d-PA in real samples with quantitative results.

The fundamental flaw of the HSAB principle is revealed by a complete speciation analysis of the [PtCl(6-n)Br(n)](2-) (n = 0-6) system.

Bjerrum's model of step-wise ligand exchange is extended to compute a complete speciation diagram for the [PtCl6-nBrn](2-) (n = 0-6) system including all 17 equilibrium constants concerning the Pt(IV) chlorido-bromido exchange reaction network (HERN). In contrast to what the hard soft acid base (HSAB) principle "predicts", the thermodynamic driving force for the replacement of chloride by bromide in an aqueous matrix, for each individual ligand exchange reaction present in the Pt(IV) HERN, is due to the difference in halide hydration energy and not bonding interactions present in the acid-base complex. A generalized thermodynamic test calculation was developed to illustrate that the HSAB classified class (b) metal cations Ag(+), Au(+), Au(3+), Rh(3+), Cd(2+), Pt(2+), Pt(4+), Fe(3+), Cd(2+), Sn(2+) and Zn(2+) all form thermodynamically stable halido complexes in the order F(-) ≫ Cl(-) > Br(-) > I(-) irrespective of the sample matrix. The bonding interactions in the acid-base complex, e.g. ionic-covalent σ-bonding, Π-bonding and electron correlation effects, play no actual role in the classification of these metal cations using the HSAB principle. Instead, it turns out that the hydration/solvation energy of halides is the reason why metal cations are categorized into two classes using the HSAB principle which highlights the fundamental flaw of the HSAB principle.

EXAFS study into the speciation of metal salts dissolved in ionic liquids and deep eutectic solvents.

The speciation of metals in solution controls their reactivity, and this is extremely pertinent in the area of metal salts dissolved in ionic liquids. In the current study, the speciation of 25 metal salts is investigated in four deep eutectic solvents (DESs) and five imidazolium-based ionic liquids using extended X-ray absorption fine structure. It is shown that in diol-based DESs M(I) ions form [MCl2](-) and [MCl3](2-) complexes, while all M(II) ions form [MCl4](2-) complexes, with the exception of Ni(II), which exhibits a very unusual coordination by glycol molecules. This was also found in the X-ray crystal structure of the compound [Ni(phen)2(eg)]Cl2·2eg (eg = ethylene glycol). In a urea-based DES, either pure chloro or chloro-oxo coordination is observed. In [C6mim][Cl] pure chloro complexation is also observed, but coordination numbers are smaller (typically 3), which can be explained by the long alkyl chain of the cation. In [C2mim][SCN] metal ions are entirely coordinated by thiocyanate, either through the N or the S atom, depending on the hardness of the metal ion according to the hard-soft acid-base principle. With weaker coordinating anions, mixed coordination between solvent and solute anions is observed. The effect of hydrate or added water on speciation is insignificant for the diol-based DESs and small in other liquids with intermediate or strong ligands. One of the main findings of this study is that, with respect to metal speciation, there is no fundamental difference between deep eutectic solvents and classic ionic liquids.

Critical Study of the Charge Transfer Parameter for the Calculation of Interaction Energy Using the Local Hard–Soft Acid–Base Principle

Local hard-soft acid-base (HSAB) principle is semiquantitative in nature due to the presence of an ad hoc charge transfer parameter. The accuracy of HSAB principle significantly depends on the definition of this ad hoc parameter. In this paper, for the first time we have introduced the second-order approximation of ΔN (ΔNsecond) as an ad hoc parameter for charge transfer to calculate interaction energies of multiple site based interactions using local hard soft acid base principle. The second-order approximation of ΔN has been derived from Sanderson's electronegativity equalization principle. To validate our approach, we have studied interaction energies of some prototype molecules. The interaction energies obtained from our approach have been further compared with the interaction energies of those obtained using other charge transfer parameters (ΔNfirst and λ) and the conventional methods. We have also discussed the advantages and limitations of the approach.

Aluminum Pitting Corrosion in Halide Media: A Quantum Model and Empirical Evidence

The phenomenon of localized damage of aluminum oxide surface in the presence of halide anions was scrutinized at an atomistic level, through the cluster approach and density functional theory. The phenomenon was also investigated empirically through Tafel polarization plots and scanning electron microscopy. A distinct behavior witnessed in the fluoride medium was justified through the hard-soft acid-base principle. The atomistic investigations revealed the greatest potency for chloride entrance into the metal oxide lattice and rationalized to the severity of damage. The interaction of halide anions with the oxide surface causing some displacements on the position of Al atoms provides a mechanistic insight of the phenomenon.

How reliable is the hard–soft acid–base principle? An assessment from numerical simulations of electron transfer energies

By computing the electron-transfer energies for two million simulated double acid-base exchange reactions, we assess the reliability of the global hard-soft acid-base (HSAB) principle. We find that the HSAB principle is often thwarted by the tendency of strong acids to prefer strong bases. We define the strong-weak and hard-soft driving forces to characterize the strength of these two competing effects, and assess the reliability of the HSAB principle for different strengths and directions of the hard-soft and strong-weak driving forces. We provide a series of probability tables for making informed predictions about the preferred products of double acid-base exchange reactions.