Hard And Soft Acids And Bases Principle Research Articles

Modelling of chemical species of Al, Mn, Zn, and Pb in river body waters of industrial areas of West Rhodope Mountain, Bulgaria.

The assessment of the ecological status of natural surface water, in terms of dominant trace metals, within an area subject to various sources of pollution including a non-ferrous metal ore mining, such as the West Rhodope Mountain, Bulgaria, is significant. The present study estimates the ecological status of river body waters at industrial areas of the West Rhodope Mountain, Bulgaria, simultaneously evaluating the possibility of state forecasting, together with assessing the potential risks, through the study of scenarios focusing on (i) possible variations of physicochemical parameters such as pH, concentration levels of trace metals, sulphates, and dissolved organic carbon (DOC) of surface water and (ii) consideration of potential spontaneous precipitation reactions in the studied waters. The ecological status of river body waters was assessed through a combination of experimental field, laboratory, and computational techniques. Al, Mn, Zn, and Pb were found to be the dominant pollutants with a variety of chemical species and distribution. The most significant difference characterizing the chemical species distribution in light of total spontaneous crystallization in the systems was found for Pb, followed by Zn and Mn, with the differences being more significant at lower trace metal levels. The calculated species were discussed on the basis of HSAB (hard and soft acids and bases) principle.

Ambident Nucleophilic Substitution: Understanding Non-HSAB Behavior through Activation Strain and Conceptual DFT Analyses.

The ability to understand and predict ambident reactivity is key to the rational design of organic syntheses. An approach to understand trends in ambident reactivity is the hard and soft acids and bases (HSAB) principle. The recent controversy over the general validity of this principle prompted us to investigate the competing gas‐phase SN2 reaction channels of archetypal ambident nucleophiles CN−, OCN−, and SCN− with CH3Cl (SN2@C) and SiH3Cl (SN2@Si), using DFT calculations. Our combined analyses highlight the inability of the HSAB principle to correctly predict the reactivity trends of these simple, model reactions. Instead, we have successfully traced reactivity trends to the canonical orbital‐interaction mechanism and the resulting nucleophile–substrate interaction energy. The HOMO–LUMO orbital interactions set the trend in both SN2@C and SN2@Si reactions. We provide simple rules for predicting the ambident reactivity of nucleophiles based on our Kohn–Sham molecular orbital analysis.

Synthesis of the perovskite-type oxyfluoride AgTiO2F: an approach adopting the HSAB principle.

A synthetic approach for oxyfluorides involving the hard and soft acids and bases (HSAB) principle was examined. Using AgF composed of the Ag+ ion as a soft acid and F- as a hard base, we attempted to synthesize an oxyfluoride, AgTiO2F, that included the O2- ion as a softer base than the F- ion and the Ti4+ ion as a harder acid than the Ag+ ion. Consequently, a perovskite (Pv)-type oxyfluoride AgTiO2F was synthesized by a solid-state reaction under ambient pressure and under high pressure, and a heat treatment at 1000 °C for 30 min under a pressure of 4 GPa produced the Pv-type phase with minimal impurity phases. The lack of an optical second harmonic generation (SHG) response and the result of Rietveld refinement indicated that the compound is centro-symmetric and shows the anti-phase tilt of Ti(O, F)6 octahedra along the c-axis on the basis of the space group I4/mcm. O/F ordering on the anion site was not clarified. The phase stability of a Pv-type AgTiO2F is discussed in terms of the HSAB principle and the thermodynamics in the formation of the Pv-type phase by comparing KTiO2F and NaTiO2F. AgTiO2F has a light yellow color and a band gap energy of 2.8 eV in the visible region, which originates from the covalent bonding between the soft cation Ag+ and O2- as a softer anion than F-. We propose that the HSAB principle is useful for selecting a cation for the synthesis of oxyfluorides.

The Reactivity of Ambident Nucleophiles: Marcus Theory or Hard and Soft Acids and Bases Principle?

The model reactions CH3 X + (NH-CH=O)M ➔ CH3 -NH-NH═O or NH═CH-O-CH3 + MX (M = none, Li, Na, K, Ag, Cu; X = F, Cl, Br) are investigated to demonstrate the feasibility of Marcus theory and the hard and soft acids and bases (HSAB) principle in predicting the reactivity of ambident nucleophiles. The delocalization indices (DI) are defined in the framework of the quantum theory of atoms in molecules (QT-AIM), and are used as the scale of softness in the HSAB principle. To react with the ambident nucleophile NH═CH-O- , the carbocation H3 C+ from CH3 X (F, Cl, Br) is actually a borderline acid according to the DI values of the forming C…N and C…O bonds in the transition states (between 0.25 and 0.49), while the counter ions are divided into three groups according to the DI values of weak interactions involving M (M…X, M…N, and M…O): group I (M = none, and Me4 N) basically show zero DI values; group II species (M = Li, Na, and K) have noticeable DI values but the magnitudes are usually less than 0.15; and group III species (M = Ag and Cu(I)) have significant DI values (0.30-0.61). On a relative basis, H3 C+ is a soft acid with respect to group I and group II counter ions, and a hard acid with respect to group III counter ions. Therefore, N-regioselectivity is found in the presence of group I and group II counter ions (M = Me4 N, Li, Na, K), while O-regioselectivity is observed in the presence of the group III counter ions (M = Ag, and Cu(I)). The hardness of atoms, groups, and molecules is also calculated with new functions that depend on ionization potential (I) and electron affinity (A) and use the atomic charges obtained from localization indices (LI), so that the regioselectivity is explained by the atomic hardness of reactive nitrogen atoms in the transition states according to the maximum hardness principle (MHP). The exact Marcus equation is derived from the simple harmonic potential energy parabola, so that the concepts of activation free energy, intrinsic activation barrier, and reaction energy are completely connected. The required intrinsic activation barriers can be either estimated from ab initio calculations on reactant, transition state, and product of the model reactions, or calculated from identity reactions. The counter ions stabilize the reactant through bridging N- and O-site of reactant of identity reactions, so that the intrinsic barriers for the salts are higher than those for free ambident anions, which is explained by the increased reorganization parameter Δr. The proper application of Marcus theory should quantitatively consider all three terms of Marcus equation, and reliably represent the results with potential energy parabolas for reactants and all products. For the model reactions, both Marcus theory and HSAB principle/MHP principle predict the N-regioselectivity when M = none, Me4 N, Li, Na, K, and the O-regioselectivity when M = Ag and Cu(I). © 2019 Wiley Periodicals, Inc.

Characterization of chalcogen bonding interactions via an in-depth conceptual quantum chemical analysis.

The chalcogen bond has been acknowledged as an influential noncovalent interaction (NCI) between an electron-deficient chalcogen (donor) and a Lewis base (acceptor). This work explores the main features of chalcogen bonding through a large-scale computational study on a series of donors and acceptors spanning a wide range in strength and character of this type of bond: (benzo)chalcogenadiazoles (with Ch = Te/Se/S) versus halides and neutral Lewis bases with O, N, and C as donor atoms. We start from Pearson's hard and soft acids and bases (HSAB) principle, where the hard nature of the chalcogen bond is quantified through the molecular electrostatic potential and the soft nature through the Fukui function. The σ-holes are more pronounced when going down in the periodic table and their directionality matches the structural orientation of donors and acceptors in the complexes. The Fukui functions point toward an n→σ*-type interaction. The initial conjectures are further scrutinized using quantum mechanical methods, mostly relating to the systems' electron density. A Ziegler-Rauk energy decomposition analysis shows that electrostatics plays a distinctly larger role for the soft halides than for the hard, uncharged acceptors, associated with the softness matching within the HSAB principle. The natural orbital for chemical valence analysis confirms the n→σ* electron donation mechanism. Finally, the electron density and local density energy at the bond critical point in the quantum theory of atoms in molecules study and the position of the spikes in the reduced density gradient versus density plot in the NCI theory situate the chalcogen bond in the same range as strong hydrogen bonds. © 2017 Wiley Periodicals, Inc.

Diacetyl and related flavorant α-Diketones: Biotransformation, cellular interactions, and respiratory-tract toxicity.

Exposure to diacetyl and related α-diketones causes respiratory-tract damage in humans and experimental animals. Chemical toxicity is often associated with covalent modification of cellular nucleophiles by electrophilic chemicals. Electrophilic α-diketones may covalently modify nucleophilic arginine residues in critical proteins and, thereby, produce the observed respiratory-tract pathology. The major pathway for the biotransformation of α-diketones is reduction to α-hydroxyketones (acyloins), which is catalyzed by NAD(P)H-dependent enzymes of the short-chain dehydrogenase/reductase (SDR) and the aldo-keto reductase (AKR) superfamilies. Reduction of α-diketones to the less electrophilic acyloins is a detoxication pathway for α-diketones. The pyruvate dehydrogenase complex may play a significant role in the biotransformation of diacetyl to CO2. The interaction of toxic electrophilic chemicals with cellular nucleophiles can be predicted by the hard and soft, acids and bases (HSAB) principle. Application of the HSAB principle to the interactions of electrophilic α-diketones with cellular nucleophiles shows that α-diketones react preferentially with arginine residues. Furthermore, the respiratory-tract toxicity and the quantum-chemical reactivity parameters of diacetyl and replacement flavorant α-diketones are similar. Hence, the identified replacement flavorant α-diketones may pose a risk of flavorant-induced respiratory-tract toxicity. The calculated indices for the reaction of α-diketones with arginine support the hypothesis that modification of protein-bound arginine residues is a critical event in α-diketone-induced respiratory-tract toxicity.

Pulmonary effects by diacetyl exposure

Exposure to diacetyl and related α-diketones causes respiratory-tract damage in humans and experimental animals. Chemical toxicity is often associated with covalent modification of cellular nucleophiles by electrophilic chemicals. Electrophilic α-diketones may covalently modify nucleophilic arginine residues in critical proteins and, thereby, produce the observed respiratory-tract pathology. The major pathway for the biotransformation of α-diketones is reduction to α-hydroxyketones (acyloins), which is catalyzed by NAD(P)H-dependent enzymes of the short-chain dehydrogenase/reductase (SDR) and the aldo-keto reductase (AKR) superfamilies. Reduction of α-diketones to the less electrophilic acyloins is a detoxication pathway for α-diketones. The pyruvate dehydrogenase complex may play a significant role in the biotransformation of diacetyl to CO2. The interaction of toxic electrophilic chemicals with cellular nucleophiles can be predicted by the hard and soft, acids and bases (HSAB) principle. Application of the HSAB principle to the interactions of electrophilic α-diketones with cellular nucleophiles shows that α-diketones react preferentially with arginine residues. Furthermore, the respiratory-tract toxicity and the quantum-chemical reactivity parameters of diacetyl and replacement flavorant α-diketones are similar. Hence, the identified replacement flavorant α-diketones may pose a risk of flavorant-induced respiratory-tract toxicity. The calculated indices for the reaction of α-diketones with arginine support the hypothesis that modification of protein-bound arginine residues is a critical event in α-diketone-induced respiratory-tract toxicity.

The Pearson’s HSAB principle in the quantum-chemical model of formation of the MN2O2 or MN2S2 coordination node in the bischelates of Be(II) and Hg(II) with ambidentate ligands based on azomethines and their cyclic analogs

The density functional theory method (DFT) has been applied to investigate the competitive coordination upon formation of the bivariant (with respect to the coordination node composition; MN2O2 or MN2S2) isomers of Be(II) and Hg(II) bischelates based on thioxoindanone derivatives of azomethines, azines, and azoles. The simulation-revealed effects of the properties of the central metal atom and the composition of the ligating donor atoms on the relative stability of the complexes isomers (the MN2O2 or MN2S2 node) has been in agreement with the prediction based on the empirical Pearson’s hard and soft acids and bases (HSAB) principle.

1,3-Dipolar cycloaddition of nitrones to transition metal-bound isocyanides: DFT and HSAB principle theoretical model together with analysis of vibrational spectra

The hard and soft acids and bases (HSAB) principle together with the density functional theory (DFT) calculations were used to analyze the driving forces of the hypothetical 1,3-dipolar cycloaddition (CA) of the nitrone CH2N(Me)O to the uncomplexed CNMe and to isocyanide ligands in various transition metal complexes. The calculated activation (ΔGs≠) and reaction (ΔGs) energies depend from the electronic chemical potential (μ), the global electrophilicity (ω) and nucleophilicity (N) indexes of the dipolarophiles, the static charge transfer “nucleophile → electrophile” (ΔN0), and the maximum amount of electronic charge that the electrophile system may accept (ΔNmax). Thus, the HSAB principle may be successfully apply for the study of reactivity of such compounds. The ΔGs≠ and ΔGs and the calculated stretching vibration frequencies ν(CN) of isocyanides well correlated with each other, i.e. higher values of ν(CN) correspond to lower values of ΔGs≠ and ΔGs. The application of the reactivity descriptors discussed in this work is a possible alternative and a complement addition to the estimates based on the classical frontier molecular orbital (FMO) theory.

Intermolecular interactions in organic crystals: gaining insight from electronic structure analysis by density functional theory

Conceptual density functional theory is exploited to extend the HSAB (hard and soft acids and bases) principle for investigating the locality and regioselectivity of intermolecular interactions in organic crystals.

HSAB principle: Applications of its global and local forms in organic chemistry

The hard and soft acids and bases (HSAB) principle, both in its local and global versions, is applied, to a series of chemical reactions, including eleminations and substitutions, Diels–Alder cycloadditions, enolate–ion alkylation, and 1,3-dipolar additions. It will be shown that this principle, in the different resolutions, provides a powerful tool in the study of regioselectivity problems in organic chemistry. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 80: 227–235, 2000

Extractive separation of cadmium(II) from zinc(II) with capriquat-diethyldithiocarbamate.

Based on the Hard and Soft Acids and Bases (HSAB) principle, the possibility of extractive separation of cadmium(II) from zinc(II) with some chelating agents was discussed. The separation factor (S.F. = logDCd-logDZn) for extraction of cadmium(II) from zinc(II) is derived from the HSAB principle as follows: S.F.=(SACd-SAZn)SB +(σACd-σAZn)σB where SA, σA, SB and σBrepresent strength factor of a metal ion, soft factor of a metal ion, strength factor of chelating agent and soft factor of chelating agent, respectively. If it is assumed that S.F.≥4 is required for the quantitative separation of cadmium(II) from zinc(II), the above equation leads to the following relationship (where SAZn =32.2, SACd=28.1, σAZn=-26.6, σACd =-12.6): σB≥0.286+0.293SB. This equation suggests that the possibility of the quantitative separation of cadmium(II) from zinc (II) with a suitable chelating agent can be estimated from its SB and σB values. In this paper, some chelating agents such as thenoyltrifluoroacetone (TTA, SB = 0.312, σB=0.101), monothiothenoyltrifluoroacetone (STTA, SB = 0.26, σB-=0.31), diethyldithiocarbamic acid (DDTC, SB=0.569, σB=0.433), dithizone (SB=0.42, σB=0.20) and 8-quinolinol (SB=0.623, σB=0.133) were applied. As a result, it was concluded that DDTC is the most effective chelating agent for the separation of cadmium(II) from zinc(II) although quantitative separation cannot be achieved. However, the use of an ionpair extraction system using a mixture of DDTC and capriquat enables the quantitative separation of cadmium(II) from zinc(II).

The inhibition of pit growth on an iron surface in a borate buffer solution containing chloride ion by inhibitors classified as soft bases in the HSAB principle

The pitting potential E pit and the repassivation potential E rep were measured for a previously passivated iron electrode in an aerated Cl −-containing borate buffer solution (pH 8.49) with and without various inhibitors using potentiodynamic polarization measurements. The values of E pit and E rep were discussed based on the hard and soft acids and bases (HSAB) principle. Iodide and bromide ions and 1,3-di- n-butyl-2-thiourea, which are classified as soft bases, shifted E rep in the positive direction, indicating inhibition of the pit growth process. Electron-probe micro-analyses showed that these soft base inhibitors were adsorbed on the surface of iron substrate acting as a soft acid within a pit. However, inhibition effects of these inhibitors on E pit were not observed. Organic anion inhibitors of hard bases, which have been reported to suppress pit formation by repair of defects or pores in the passive film with precipitates of complexes or salts formed by the hard acid-hard base reactions, were ineffective for E rep because the hard base inhibitors prefer not to bind to the iron substrate surface within the pit.

Studies of the local reactivity of surfaces using chemical based principles

We apply the local version of the hard and soft acids and bases (HSAB) principle to infer the chemical reactivity of surface-like systems. Based on properties of the isolated reactants, the HSAB principle allows us to predict the most reactive sites of a polyatomic system. For the sake of simplicity the surface was simulated by a planar cluster (Ga 2As 2). We perform an ab initio all-electron self-consistent-field calculation to determine the electronic and structural properties of the cluster and the probe atoms (Na, Al, As and Cl) that are supposed to react with it. The predictions based on the HSAB principle are compared against total energy calculations of the system (atom + cluster) in configurations where we expect the reaction to initiate. The results encourages one to apply this method to a large variety of problems that involve chemical reactions of complex systems.

Regioselective addition of 2-nitropropane anion to NAD+ analogues

Various NAD+ analogues have been reduced regioselectively by the tetramethylammonium salt of 2-nitropropane anion in acetonitrile at 298 K to yield the corresponding 4-alkylated NADH analogues. The one-electron oxidation potential of the tetramethylammonium salt of 2-nitropropane anion has been determined as 0.10 V (vs. SCE) by using second harmonic ac voltammetry as well as by analysing the cyclic voltammograms at various sweep rates. The rate constants for the reduction of NAD+ analogues by 2-nitropropane anion (>1 × 106 dm3 mol–1 s–1) are much larger than those estimated for outer-sphere electron transfer from 2-nitropropane anion to NAD+ analogues based on the one-electron oxidation potential of 2-nitropropane anion and the one-electron reduction potentials of NAD+ analogues. The origin of the regioselectivity is discussed in terms of the HSAB (hard and soft acids and bases) principle.

Hardness and softness in the ab initio study of polyatomic systems.

A combination of the maximum hardness (MH) principle and the local version of the hard and soft acids and bases (HSAB) principle is tested as a tool to describe the stability and reactivity of polyatomic systems. The local HSAB principle describes regional differences in reactivity whereas the MH principle selects the most stable configuration. This approach is tested using ab initio density functional theory techniques. Total-energy calculations fully confirm the validity of both principles

Adsorption of Model Compounds of Lubricant on Nascent Surfaces of Mild and Stainless Steels under Dynamic Conditions

Mild steel and stainless steel specimens were activated mechanically by cutting under high vacuum at room temperature, and sub-monolayer levels were adsorbed. It was found that the compounds could be divided into two groups according to their adsorption activity on the cut surface. Benzene, 1-hexene, methyl proprionate and diethyl disulfide exhibited a high adsorption activity, while the polar compounds propionic acid, propylamine, trimethyl phosphate and trimethyl phosphite exhibited comparatively low activity. The functional groups of the former compounds have a higher polarizability than that of the latter compounds. The results are discussed on the basis of the Pearson's HSAB (Hard and Soft Acids and Bases) principle. It was found that the nascent surface was so active that benzene decomposition and the reaction of diethyl disulfide with the surface occurred even at room temperature.

ChemInform Abstract: Adsorption and Corrosion Inhibition Effect of Anions Plus an Organic Cation on Iron in 1M HClO4 and the HSAB (hard and soft acids and bases) Principle.

AbstractThe effect of nine anions (Cl‐, Br‐, I‐, SO42‐, S2O32‐, SH‐, SCN‐, NO3‐, N3‐) and/or nBu4N+ on the corrosion rate of an Fe electrode in 1M HClO4 and the adsorption behavior of these ions have been studied by impedance measurements.

Adsorption of Lubricating Oil Components on the Cut Steel Surface

The adsorption of model compounds of lubricating oil components on the cut steel surfaces was carried out under high vacuum. The estimated amount of adsorption was less than a monolayer level. The adsorption of nitrogen and n-hexane was not observed. The other adsorbates used were divided into two groups according to their adsorption activity on the cut surface. Benzene, cyclohexene and diethyl disulfide exhibited a high adsorption activity, while, propionic acid, propyl amine and trimethyl phosphate exhibited comparatively low activity. It is noteworthy that the functional group of the latter compounds was characterized by a lower polarizability than that of the former. The results were discussed on the basis of HSAB (Hard and Soft Acids and Bases) principle.

Displacement of Activated Amino Groups. C,N‐ vs. N,S‐Bond Cleavage in the Reaction of N‐Alkyl‐disulfonamides with Nucleophiles

AbstractSodiumthiophenoxide and sodiumphenylselenide react with N‐benzyl‐ and N‐hexyl‐di‐p‐toluenesulfonamides (1 and 2) via displacement at the C‐atom to afford the corresponding organosulfides and selenides in yields of 68–96%. In contrast, sodium cyanide converts disulfonamides to monosulfonamides by attack on the S‐atom. The different selectivities of phenylsulfide and selenide as compared to cyanide anions with respect to attack on the C‐ and S‐atoms are rationalized on the grounds of the HSAB (hard and soft acids and bases) principle of Pearson.