Hard Soft Acid Base Research Articles

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.

Modeling continuous changes in substituent electronegativity and chemical hardness using fictitious nuclear potentials

Electronegativity and chemical hardness are important conceptual tools in organic and organometallic chemistry. Computational chemistry can aid understanding of these concepts by simulating fictitious substituents with smoothly varying electronegativity and chemical hardness. This work shows that Gaussian smearing of an atom’s nuclear charge distribution enables variation of its electronegativity and chemical hardness, without changing the total molecule charge. Tunable covalent, Lewis acid, and Lewis base substituents give chemically reasonable results in several canonical chemical reactions. These include hard–soft acid–base effects on thiocyanate and enolate isomerization, ligand trans influences on the stability of organometallic complexes and the thermodynamics of reductive elimination, and substituent effects on the proton affinities of benzene derivatives. The results illustrate the utility of these fictitious substituents and motivate their use as interpretive tools.

Estimation of stability constants for metal–ligand complexes containing neutral nitrogen donor atoms with applications to natural organic matter

Linear free energy relationships (LFERs) were developed for estimating 1:1 metal–ligand stability constants (logKML) for small organic molecules containing neutral nitrogen donor atoms. A data set of 44 monodentate and 112 bidentate ligands for six metal ions: Mn2+, Co2+, Ni2+, Cu2+, Zn2+ and Cd2+ was employed to parameterize the LFER equations. Monodentate and bidentate logKML values were adequately described using Irving–Rossotti LFERs previously developed for ligands containing negatively-charged oxygen functional groups. Modifications to the LFER equations were necessary to account for steric hindrances to metal complexation by primary, secondary, and tertiary amines. The resulting LFER equations can be used to estimate logKML values for monodentate and bidentate ligands with neutral nitrogen donor groups where such values do not currently exist in the literature. Comparison of these results to our previous work with negatively-charged oxygen donor atoms reveals that neutral nitrogen functional groups are weaker than their oxygen counterparts for metal ions classified as “hard” on the basis of Hard–Soft Acid–Base (HSAB) theory. For “soft” metals, the opposite is true. These LFERs can be used to incorporate nitrogen functional groups in models for metal ion binding to natural organic matter (NOM).

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.

Oxorhenium Complexes Bearing the Water-Soluble Tris(pyrazol-1-yl)methanesulfonate, 1,3,5-Triaza-7-phosphaadamantane, or Related Ligands, as Catalysts for Baeyer–Villiger Oxidation of Ketones

New rhenium(VII or III) complexes [ReO3(PTA)2][ReO4] (1) (PTA = 1,3,5-triaza-7-phosphaadamantane), [ReO3(mPTA)][ReO4]I (2) (mPTA = N-methyl-1,3,5-triaza-7-phosphaadamantane cation), [ReO3(HMT)2][ReO4] (3) (HMT = hexamethylenetetramine), [ReO3(η(2)-Tpm)(PTA)][ReO4] (4) [Tpm = hydrotris(pyrazol-1-yl)methane, HC(pz)3, pz = pyrazolyl], [ReO3(Hpz)(HMT)][ReO4] (5) (Hpz = pyrazole), [ReO(Tpms)(HMT)] (6) [Tpms = tris(pyrazol-1-yl)methanesulfonate, O3SC(pz)3(-)] and [ReCl2{N2C(O)Ph}(PTA)3] (7) have been prepared from the Re(VII) oxide Re2O7 (1-6) or, in the case of 7, by ligand exchange from the benzoyldiazenido complex [ReCl2{N2C(O)Ph}(Hpz)(PPh3)2], and characterized by IR and NMR spectroscopies, elemental analysis and electrochemical properties. Theoretical calculations at the density functional theory (DFT) level of theory indicated that the coordination of PTA to both Re(III) and Re(VII) centers by the P atom is preferable compared to the coordination by the N atom. This is interpreted in terms of the Re-PTA bond energy and hard-soft acid-base theory. The oxo-rhenium complexes 1-6 act as selective catalysts for the Baeyer-Villiger oxidation of cyclic and linear ketones (e.g., 2-methylcyclohexanone, 2-methylcyclopentanone, cyclohexanone, cyclopentanone, cyclobutanone, and 3,3-dimethyl-2-butanone or pinacolone) to the corresponding lactones or esters, in the presence of aqueous H2O2. The effects of a variety of factors are studied toward the optimization of the process.

Probing the Acidity of Carboxylic Acids in Protic Ionic Liquids, Water, and Their Binary Mixtures: Activation Energy of Proton Transfer

Acidity functions were used to express the ability of a solvent/solution to donate/accept a proton to a solute. The present work accounts for the acidity determination of HCOOH, CH3COOH, and CH3CH2COOH in the alkylimidazolium-based protic ionic liquids (PILs), incorporated with carboxylate anion, water, and in a binary mixture of PIL and water using the Hammett acidity function, H0. A reversal in the acidity trend was observed, when organic acids were transferred from water to PIL. It was emphasized that an increased stabilization offered by PIL cation toward the more basic conjugate anion of organic acid was responsible for this anomalous change in acidity order in PILs, which was absent in water. The greater stabilization of a basic organic anion by PIL cation is discussed in terms of the stable hard–soft acid base (HSAB) pairing. A change in the H0 values of these acids was observed with a change in temperature, and a linear correlation between the ln H0 and 1/T was noted. This relationship points toward the activation energy of proton transfer (E(a,H+)), a barrier provided by the medium during the proton transfer from Brønsted acid to indicator. The H0 function in binary mixtures points to the involvement of pseudosolvent, the behavior of which changes with the nature and concentration of acid. The presence of the maxima/minima in the H0 function is discussed in terms of the synergetic behavior of the pseudosolvent composed of the mixtures of aqueous PILs.

Industrial Application of Reactivity Indices within Density Functional Theory

Reactivity index is that theory which was developed at the right time to rational chemical bonding. Density Functional theory (DFT) has given precision to chemical concepts such as electro negativity, hardness, and softness and has embedded them in a perturbational approach to chemical reactivity. Since the majority of the reactions can be analyzed through the electrophilicity/nucleophilicity of various species involved, a proper understanding of these properties becomes essential. The hard soft acid-base (HSAB) principles classify the interaction between acids and bases in terms of global softness. In last few years the reactivity index methodology is well established and had found its application in a wide variety of systems. This study is to revisit the definition of reactivity index using density functional theory, within the domain of hard soft acid base (HSAB) principle and then to discuss its application in industrial applications; in combination with intra and intermolecular reactivity in materials.

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.

Chemoaffinity-mediated crystallization of Cu2O: a reaction effect on crystal growth and anode property

We present a chemoaffinity-mediated crystallization strategy to fabricate Cu2O with different morphologies for studying their electrochemical performances as lithium-ion battery anode materials. The chemical reaction routes were finely-designed by including important anions such as OH−, SO42−, NO3−, Ac−, and Cl− in the redox system of N2H4 and Cu2+. With different affinity abilities of the anions in terms of hard–soft acid–base theory, two growth mechanisms were evidenced, which can lead to the crystallization of spherical aggregations and octahedra, respectively. The crystallization of Cu2O octahedra occurred in the presence of OH−, which is independent of anions, due to the higher chemoaffinity of OH− with Cu2+. Without adding additional OH−, anions such as SO42−, NO3−, Ac−, and Cl− can induce diverse chemical reactions to effectively control the growth of Cu2O spherical aggregations. Electrochemical measurements indicated that Cu2O octahedra can show better cyclability than that of spherical aggregations. The present work validates that the chemoaffinity-mediated method is a useful bottom-up strategy to crystallize high performance anode materials.

Kinetics of thiocyanate orientation conversion on Pt surface studied by in situ step-scan time-resolved microscope FTIR spectroscopy

Fast surface dynamic processes of thiocyanate adsorbed on a nanostructured Pt microelectrode were studied by in situ step-scan time-resolved microscope FTIR spectroscopy (in situ SSTR-MFTIRS) at a time resolution of 100 μs. It was found that SCN− adsorbs preferentially on Pt surface through N atom at low potential, while through S atom at high potential. The potential-induced orientation conversion between N- and S-bounded forms is very reversible, and occurs only within 2 ms after potential alternation. However, the rate constant (4×103 s−1) of conversion from N- to S-bounded adsorption is nearly double to that of the reverse process (2×103 s−1). The difference in kinetics of two processes may be explained through the hard-soft acid-base principle.

Synthesis of Five‐Membered Cyclic Ethers by Reaction of 1,4‐Diols with Dimethyl Carbonate

The reaction of 1,4-diols with dimethyl carbonate in the presence of a base led to selective and high-yielding syntheses of related five-membered cyclic ethers. This synthetic pathway has the potential for a wide range of applications. Distinctive cyclic ethers and industrially relevant compounds were synthesized in quantitative yield. The reaction mechanism for the cyclization was investigated. Notably, the chirality of the starting material was maintained. DFT calculations indicated that the formation of five-membered cyclic ethers was energetically the most favorable pathway. Typically, the selectivity exhibited by these systems could be rationalized on the basis of hard-soft acid-base theory. Such principles were applicable as far as computed energy barriers were concerned, but in practice cyclization reactions were shown to be entropically driven.

Electrochemical Properties of Solid Polymer Electrolytes Prepared by the Hard/Soft Acid Base Principle

In order to improve the electrochemical properties including ionic conductivity of SPEs (solid polymer electrolytes), understanding of the interaction between the polymer and salt in the SPE is important. In this study, four types of polymer matrices and four types of salts were used according to the hard/soft acid base (HSAB) principle. The results of impedance measurement reveal that the ionic conductivities are affected by the HSAB principle at low salt concentration. With increasing salt content, however, the SPEs are influenced by the ion hopping property of salt rather than by the solubility of the polymer with salt. In contrast, the PPS-based SPE shows different characteristics because it is prepared as a slurry phase at high salt content.

Chemical reactivity parameters (HSAB) applied to magma evolution and ore formation

Magmas are commonly described through the usual content of 10 major oxides. This requires a complex dimensional plot. Concepts of hard–soft acid–base (HSAB) interactions allow estimating chemical reactivity of elements, such as electronegativity, i.e. the chemical potential changed of sign, hardness and electrophilicity. For complex system, those values result from equalization methods, i.e. the equalization of the respective chemical potentials, or from ab-initio computations through density functional theory (DFT). They help to characterize silicate magmas by a single value describing their reactivity. Principles of minimum electrophilicity (mEP), maximum hardness (MHP) and minimum polarizability (mPP) indicate trends towards regions of higher stability. Those parameters are plotted within a fitness landscape diagram, highlighting toward which principle reactions trend. Major oxides, main minerals and magmas determine the respective fields in which evolve natural rocks. Three poles are identified, represented by silica and alkalis, whereas oxidation forms the third trend. Mantle-derived rocks show a large variation in electrophilicity compared to hardness. They present all characters of a closed chemical system, being simply described by the free Gibbs energy. Conversely, rocks contaminated within the continental crust show a large variation in hardness between a silica pole and an alkaline, defining two separate trends. The trends show the character of an open chemical system, requiring a Grand Potential description (i.e. taking into account the difference in chemical potential). The terms open and closed systems refer to thermodynamical description, implying contamination for the crust and recycling for the mantle. The specific role of alkalis contrasts with other cations, pointing to their behavior in modifying silicate polymer structures. A second application deals with the reactivity of the melt and its fluid phase. It leads to a better understanding on the mechanisms that control sequestration and transport of metals within the different phases during igneous activity. Based on high gas/melt partitioning for metals and similar reactivity, the gaseous phase is more attractive for metals than silicate melts. The presence of halogens in the fluid phase tends to reinforce hardness, making the fluid phase attractive for hard metals such as Sn or W. Conversely, the presence of S decreases hardness of the fluid phase that becomes attractive for soft metals such as Au, Ag and Cu.

Effect of digestive ripening agent on nanoparticle size in the digestive ripening process

Digestive ripening is an efficient method that leads to monodispersed nanoparticles from polydispersed ones. A systematic investigation of digestive ripening with different agents viz. dodecanethiol and dodecylamine on different metallic systems is presented. It is shown that both the metallic system involved and the digestive ripening agent have influences on the final size distribution. Further, it is demonstrated that the interaction strength between the different metals and the digestive ripening agents based on Hard Soft Acid Base principles could be invoked to explain the variation in nanoparticle size.

Complexation behavior of trivalent actinides and lanthanides with 1,10-phenanthroline-2,9-dicarboxylic acid based ligands: insight from density functional theory

We have investigated the complexation behavior of preorganized 1,10-phenanthroline-2,9-dicarboxylic acid (PDA) based ligands with trivalent lanthanides and actinides using density functional theory with various GGA type exchange-correlation functionals and different basis sets. New ligands have been designed from PDA through functionalization with soft donor atoms such as sulfur, resulting in mono-thio-dicarboxylic acids (TCA/TCA1) and di-thio-dicarboxylic acid (THIO). It has been found that selectivity in terms of complexation energy of actinides over lanthanides is the maximum with TCA1 where the metal-ligand binding is through the O atoms. This unusual feature where a softer actinide metal ion is bonded strongly with hard donor oxygen atoms has been explained using the popular chemical concepts, viz., Pearson's Hard-Soft-Acid-Base (HSAB) principle and the frontier orbital theory of chemical reactivity as proposed by Fukui. Detailed analysis within the framework of the HSAB principle indicates that the presence of softer nitrogen atoms in the phenanthroline moiety (which also act as donors to the metal ion) has a profound influence in changing the soft nature of the actinide ion, which in turn binds with the hard oxygen atoms in a stronger way as compared to the valence isoelectronic lanthanide ion. Also, the trends in the variation of calculated values of the metal-ligand bond distances and the corresponding complex formation energies have been rationalized using the Fukui reactivity indices corresponding to the metal ions and the donor sites. All the calculations have also been done in the presence of solvent. The "intra-ligand synergistic effect" demonstrated here for PDA or TCA1 with soft and hard donor centers might be very important in designing new ligands for selective extraction of various metal ions in a competitive environment. However, for TCA and THIO ligands with only soft donor centers, "intra-ligand synergism" may not be very efficient although reports are available demonstrating soft-soft inter-ligand synergism. Nevertheless, in the case of TCA and THIO complexes, a shorter Am-S bond distance in conjunction with lower metal ion charge and a higher percentage of orbital interaction energy corroborate the presence of a higher degree of covalency in Am-S bonds, which in turn may be responsible for selectivity towards Am(3+).

Cation radii induced structural variation in fluorescent alkaline earth networks constructed from tautomers of a nucleobase analogue

Nucleobase tautomers and their metal complexes have attracted considerable attention due to their fascinating architectures along with wide applications. In this paper, 4,6-dihydroxypyrimidine (H(2)DHP), an analogue of uracil and thymine, was employed to react with the vital elements of alkaline earth metals in an aqueous solution and lead to the formation of four novel complexes, [Mg(HDHP)(2) (H(2)O)(4)] (1), [Ca(HDHP)(2)(H(2)O)(3)](n)·nH(2)O (2), [Sr(HDHP)(2)(H(2)O)(3)](n)·nH(2)O (3), and [Ba(HDHP)(2)(H(2)O)(2)](n)·nH(2)O (4), which have been characterized by elemental analysis, IR, TG, UV-Vis, PL, powder and single-crystal X-ray diffraction and progressively evolve from zero-dimensional (0D) mononuclear, one-dimensional (1D) zig-zag double chain, two-dimensional (2D) double layer, to a three-dimensional (3D) porous network along with the increase of cation radii. This tendency in dimensionality follows salient crystal engineering principles and can be explained by considering factors such as hard-soft acid-base principles and cation radii. The deprotonated H(2)DHP ligand exhibits four new coordination modes, namely, O-monodentate (complex 1), N,O-chelating (complexes 2 and 3), O,O-bridging (complexes 2 and 3), and κ(1)O:κ(2)O-bridging mode (complex 4). Interestingly, the structural investigation indicates that the HDHP(-) monoanion shows three unusual types of tautomers, which are essential for the diagnosis of disease and investigation of medicine. Furthermore, the four complexes exhibit strong blue emission compared to free H(2)DHP ligand at room temperature and may be potential candidates for blue fluorescent biological materials used in organisms.

Investigation on Protectional Ability on Steel Bar of Compound Corrosion Inhibitor Applied in Seawater-and-Sea Sand Concrete

Seawater and sea sand mixed in concrete will cause rebar of structural concrete corrosion, referring to the principles of composition and design of MCI and Pearson’s Hard-Soft Acid-Base (HSAB) theory, as well as to synergy effect of corrosion inhibitor applied in chemical industry, compound corrosion inhibitor has been designed consisting of diisopropanolamine, surface active agent, inorganic salts and so on. This paper research the relation between polarization potential and time of rebar in seawater-and-sea sand concrete through a series of experiments by adding corrosion inhibitor into seawater-and-sea sand concrete to improve its corrosive resistance characteristics. The results indicate that Z5 has the best inhibition effect. Corrosion inhibitor Z5 comprises 50% of triethanolamine, 20% of dimethylethanolamine (DMEA), 25% of triethoxysilane and 5%lithium nitrate, and its proper content applied in seawater-and-sea sand concrete is 1.5%. Rebar have been embedded in seawater-and-sea sand concrete for the long-term test. The results show that corrosion resistance performance of seawater-and-sea sand concrete with corrosion inhibitor Z5 was significantly improved, and the embedded bar was not corroded at all within 420 days.

Describing the chemical character of a magma

We introduce the concepts of hard–soft acid–base (HSAB) interactions and derive parameters to characterise a magma that consists either of a solid rock, a melt or its exsolved gaseous phase. Those parameters are the electronegativity, i.e. the chemical potential changed of sign, hardness, electrophilicity and polarisability. They determine the chemical reactivity of each individual element, or its equivalence in the case of a complex system of elements or compounds. The parameter values result from equalisation methods for complex systems, i.e. the equalisation of the respective chemical potentials, or from direct computation through density functional theory (DFT). They help in characterising magma by a single value. They also provide insights into the reactivity of the melt or its fluid phase when in contact with another magma, or when considering the affinity of elements for each phase. The description leads to a better understanding on the mechanisms that control sequestration and transport of metals within the different phases during igneous activity. The presence of halogens tends to reinforce hardness, making the fluid phase attractive for hard metals such as Sn or W. Conversely, the presence of S decreases hardness of the fluid phase that becomes attractive for soft metals such as Au, Ag and Cu. Theoretical principles of maximum hardness and minimum electrophilicity can be used to identify the poles toward which the reactions should naturally evolve. The trends observed during magma evolution are also observed using these parameters, indicating two different trends toward a pole of greater stability. One pole consists in increasing the hardness at constant electrophilicity as observed for felsic magmas toward a silica-rich pole. Conversely mantle-derived magmas evolve at a constant hardness and decreasing electrophilicity in between the two poles of iron-rich and alkali-rich minerals. They respectively correspond to the Bowen and Fenner trends, though the experimental and field evidences have now some chemical explanation.

Phase Selection and Site-Selective Distribution by Tin and Sulfur in Supertetrahedral Zinc Gallium Selenides

Doping is among the most important methods to tune the properties of semiconductors. For dense phase semiconductors, the distribution of dopant atoms in crystal lattices is often random. However, when the size of semiconductors becomes increasingly smaller and reaches the extreme situation as is the case in chalcogenide supertetrahedral clusters, different chemically distinct sites (e.g., corner, edge, face, and core) occur, which can dramatically affect the doping chemistry at different sites and also spatial assembly of such clusters into covalent superlattices. In this work, we use the Zn-Ga-Se supertetrahedral clusters and their frameworks as the model system to examine the doping chemistry of Sn(4+) and S(2-) in the Zn-Ga-Se clusters. A series of selenide clusters (undoped supertetrahedral T4-ZnGaSe, S-doped T4-ZnGaSeS, Sn-doped T4-ZnGaSnSe, and dual S- and Sn-doped T4-ZnGaSnSeS) have been prepared with various levels of Sn- and S-doping and with different superlattice structures (OCF-1, -5, -40, and -42). The complex compositional and structural features of these materials are dictated by the convoluted interplay of three key factors: (1) the overall charge density and size/shape matching between clusters/frameworks and protonated guest amines determine the framework topology and the doping levels of Sn(4+) and S(2-); (2) the site selectivity of Sn(4+) is dictated by the local charge balance surrounding anionic Se/S sites as required by the electrostatic valence sum rule; and (3) the site selectivity and doping levels of sulfur is dictated by the location and amount of Sn based on hard soft acid base (HSAB) principle. The cooperative effect of amine-templating and doping by Sn and/or S leads to a rich chemical system with tunable framework compositions, topologies, and electronic properties.

Nanostructure Modified Gas Sensor Detection Matrix for NO Transient Conversion of NO to NO2

Porous silicon (PS) conductometric gas sensors are used to create a sensitivity matrix for the room temperature detection of NOx (NO, NO2). “P-type” nanopore coated microporous silicon is treated with tin, nickel, copper, and gold, electrolessly deposited onto the PS surface to form SnOx, NiO, CuxO, and AuxO nanostructured centers as confirmed by XPS measurements. The relative sensitivities of these modified PS gas sensor surface sites have been measured under 1–5 ppm NO exposure. An improved sensitivity of up to 10 times that of untreated PS is observed for a 1 ppm exposure. The choice of deposits is based on the hard to soft acid character of the nanostructured metal oxide islands that are fractionally deposited on the semiconductor interface and their effect on the physisorption of NO, a weak base, dictated by an inverse pattern (IHSAB) to the hard-soft acid base concept. NO, a free radical, can interact with oxygen sites on the modified PS sensor interfaces, to effect a transient NO2 signal unique to PS-based NO sensors, which is not observed as other basic analytes including NH3, PH3, H2S, SO2, and CO interact with “p-type” PS.A comparison is made between the current PS sensor systems which operate at room temperature and electrochemical and traditional metal oxide sensors.