Hard And Soft Acids And Bases Research Articles

UHV friction of tribofilms derived from metal dithiophosphates

The friction‐reduction mechanisms of Modtp and Zndtp were highlighted by submitting tribofilms to friction in ultra‐high vacuum (UHV). The use of an UHV tribometer to understand these phenomena is justified by the fact that the friction coefficient recorded in UHV is close to the friction coefficient obtained in traditional tests in oil. After UHV friction, the transfer films on the pin were analyzed by in situ AES, XPS and AES mapping. Low friction is associated with the transfer to the pin of a sulfur‐rich film. In the case of Modtp, we observe a very thin MoS2 film. The UHV friction coefficient approaches 0.04. In the case of Zndtp, the transfer film contains ZnS together with some phosphates. Because of the poor capacity of ZnS to reduce friction, the UHV friction coefficient recorded is near 0.15. A global model of the action of dithiophosphates in reducing friction is described on the basis of the hard and soft acids and bases (HSAB) principle.

In situ MoS2 formation and selective transfer from MoDPT films

UHV friction tests and complementary analyses using AES , XPS, TEM and EELS give a global view of the friction reduction mechanisms of MoDTP antiwear and friction-reducing additive. The study was performed with MoDTP tribofilms composed mainly of an amorphous phosphate glass containing flexible and highly dispersed MoS 2 single sheets. The transfer films created under UHV friction were studied by AES. The hard and soft acids and bases (HSAB) principle was used to exploit the data. We observed that relatively high friction ( μ ∼0.3) was associated with the transfer of the whole tribofilm from the flat to the pin by a chemical reaction between the phosphates present in the tribofilm and the native oxide layer on the pin. When the oxide layer was removed, the MoS 2 sheets reacted with the nascent metal surface of the pin. The transfer film observed on the pin is very thin (4 nm) and it can clearly be seen that only a few single sheets of MoS 2 are needed to achieve friction at the centirange.

Hard and Soft Acid-Base Model Applied to Bivalent Cation Selectivity on a 2:1 Clay Mineral

AbstractWe applied the hard and soft acid-base (HSAB) model (Xu and Harsh 1990a, 1990b) to bivalent cation exchange on a purified Ca-montmorillonite. As a result, a satisfactory model is proposed to describe the gradual selectivity of exchange with Ca for 4 of the 6 metals studied (Cd, Cu, Pb, Zn). The selectivity is predicted as a function of the differences of electronegativity and softness of the metals. The deviation of Ni and Co data from the predicting model is interpreted in terms of hydration (Ni and Co being the most strongly hydrated ions). The fitting parameters of the model, α and β, are related to the electronegativity and softness characteristics of the surface, respectively. Their ratio gives information on the nature of bonding. Results suggest that covalent bonding modifies electrostatic interactions, which in turn affect selectivity, with an increasing influence of covalent bondings in the order: Pb < Cd < Zn < Cu.To balance the lack of representativity of the model for the small molar fraction (NM), we propose to associate to the HSAB model an equation describing the variation of the Vanselow selectivity coefficient as a function of the molar fraction of metal on clay.

Bond Energies of Copper Ion−Ligand L Complexes CuL2+ Determined in the Gas Phase by Ion−Ligand Exchange Equilibria Measurements

The free energy changes, ΔG2, for the reaction CuL2+ = Cu+ + 2L were obtained in the gas phase for some 23 different ligands L. These results were based on the determination of ligand exchange equilibria of the type CuA2+ 2B = CuB2+ 2A. The equilibria were observed in a gas-phase ion−molecule reaction chamber sampled with a mass spectrometer. The entropy changes were determined by evaluation of the entropies S°, of the reactants from vibrational frequencies and moments of inertia obtained with HF/3-21G* basis sets. Combining and one obtained also the values. The range of values extended from 72 kcal/mol for the most weakly bonded ligand ethyl chloride to 135 kcal/mol for the most strongly bonding one, 1-methylimidazole. The results provide a partial confirmation of the Hard and Soft Acids and Bases (HSAB) principle. Comparison with available data for M+ = Li+ and Ag+ show that soft bases such as Me2S bond relatively more strongly to soft acids such as Cu+ and Ag+. However the actual bond energies are affe...

Binding Energies of Silver Ion−Ligand, L, Complexes AgL2+ Determined from Ligand-Exchange Equilibria in the Gas Phase

The free energy ΔG°, entropy ΔS°, and enthalpy ΔH° for the reaction: AgL2+ = Ag+ + 2L in the gas phase were determined for 16 different ligands L which included oxygen bases such as H2O, MeOH, EtOH, Me2CO, and Me2SO, nitrogen bases NH3 and MeCN, and sulfur bases Me2S. The determinations were based on measurements of the exchange equilibria AgA2+ + B = AgAB+ + A and AgAB+ + B = AgB2 + A where A and B are different ligands L. The exchange equilibria were determined in a “high”-pressure ion source at 10 Torr bath gas containing A and B in the 10−100 mTorr range and using Ag(MeOH)2+ ions produced by electrospray. A scale of ΔG° values for the exchange reaction AgA2+ + 2B = AgB2+ + 2A was established and calibrated to the ΔG° for Ag(H2O)2+ = Ag+ + 2H2O, due to Holland and Castleman, obtaining thus absolute ΔG° values for all ligands involved. Theoretically calculated ΔS° values led to ΔH°. Comparison of the ΔG° and ΔH° results with binding energies for the same ligands in complexes with the alkali ions Li+ and K+ showed that while a good correlation is observed for the alkali ions as a group, only a very poor correlation is observed between the alkali ions and Ag+. In particular, the “soft” base Me2S showed, relative to the “hard” oxygen bases, a much stronger bonding to Ag+. The comparison was extended to CuL2+ on the basis of additional exchange equilibria determinations. The CuL2+ and AgL2+ results combined with earlier determinations of MnL2+, CoL2+ and CuL2+ by Jones and Staley provide a partial confirmation of the hard and soft acid−base (HSAB) concept for these systems. However, a more detailed comparison on the basis of the absolute hardness, η, scale developed by Pearson and co-workers indicates only a very limited agreement when a larger variety of bases is included.

An ab initio quantum chemical study on the structure, stability and polymerization of C 28 and its derivatives

STO-3G calculated reaction energies of the radical reactions producing C 28H n ( n = 1, 2, 3, 4) from the unstable C 28 are used to discuss the stabilization of these derivatives. The influence of the nature of various groups X (X is F, OH, NH 2, CI, CH 3, iPr, CN, CF 3 and tBu) on the stabilization of C 28 is investigated using the isodesmic reactions in which one or more hydrogens of C 28H 4 are substituted by X. In general, all our calculations predict that C 28 is stabilized by the addition of hard monovalent atoms (or groups) such as H and F on the four dangling bonds. When applying the Hard and Soft Acids and Bases (HSAB) principle to these isodesmic reactions, C 28H 3 is predicted to be harder than CH 3. The stability of C 28H 3 and the localization of its unpaired electron suggest that this molecule may be used as a monomer in a radical polymerization. The stability analysis of the dimer, trimer, tetramer and pentamer (in linear or branched form) shows that these molecules are stable with strong C 28C 28 bonds.

Chemistry of [(perfluoroalkyl)methyl] oxiranes. Regioselectivity of ring opening with O-nucleophiles and the preparation of amphiphilic monomers

The reactions of oxiranes R FCH 2CH(–O–)CH 2 (R F≡C 4F 9, C 6F 13, C 8F 17; 4a– 4c) with a series of alkanols in the presence of a Lewis acid took place at the terminal carbon atom with complete regioselectivity. 2-Hydroxyethyl methacrylate and acrylate reacted similarly. The reaction with alkane diols was controlled to proceed with one or two molecules of the oxiranes chemoselectively. Non-regioselective, base-catalysed ring opening by methacrylic acid (83% terminal attack) was discussed on the basis of the hard and soft acids and bases (HSAB) concept. A convenient transformation of the oxiranes to the corresponding diols 13a– 13c via dioxolane intermediates, and their conversion to bis-methacrylates, was accomplished with overall yields of 75%–79%. Thiourea converted the oxiranes into the corresponding thiiranes ( 15a– 15c). The reactions afforded products generally in yields of 82%–98%.

Hard and Soft Acid-Base Behavior in Aqueous Solution: Steric Effects Make Some Metal Ions Hard: A Quantitative Scale of Hardness-Softness for Acids and Bases

The idea of hard and Soft Acids and Bases (HSAB), developed by R. G. Pearson some 30 years ago, has been used as a unifying principle in several texts. In spite of this, HSAB has remained qualitative and largely intuitive. In this article, the behaviour of HSAB in aqueous solution is addressed. A quantitative scale of hardness-softness for acids and bases is presented also.

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.

Adsorption of monoethanolamine on clean, oxidized and hydroxylated aluminium surfaces: a model for amine-cured epoxy/aluminium interfaces

The interaction of 1-amino-2-hydroxyethane (monoethanolamine) with clean Al(100), in-situ oxidized Al(100) and in-situ hydroxylated Al(100) has been investigated using Auger electron spectroscopy (AES) and high-resolution electron energy-loss spectroscopy (HREELS). It has been shown that, on all these surfaces, adsorption of monoethanolamine can be interpreted in the framework of the Hard and Soft Acid Base (HSAB) principle. However, the detailed mechanism differs for each surface: (i) monoethanolamine interacts with clean Al(100) through its amine termination via a Lewis-like acid-base reaction; (ii) monoethanolamine interacts with oxidized Al(100) through its alcohol termination via a Lewis-like acid-base reaction; (iii) monoethanolamine interacts with hydroxylated Al(100) through its alcohol termination via a Brönsted-like acid-base reaction.

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.

The inhibition of passive film breakdown on iron in a borate buffer solution containing chloride ions by organic anion inhibitors

The hard and soft acids and bases (HSAB) principle closely relates to the adsorption of an anion on the oxidized metal surface and the formation of its complex with a highly oxidized metallic ion. Inhibition effects of several organic anions on passive film breakdown of iron in a borate buffer solution at pH 8.49 in the presence of chloride ions were investigated on the basis of the principle. Organic anions 3-octylthiopropionate, N-dodecanoyl sarcosinate, oleate, benzoate and 8-quinolinolate ions were examined as inhibitors. Except for the 8-quinolinate ion, they shifted the pitting potential in the positive direction, indicating inhibition of passive film breakdown. From the results of electron probe micro-analysis, these inhibitors suppressed the breakdown by repairing defects or pores in the passive layer. 8-Ouinolinolate ions stimulated the breakdown because a non-adherent deposit of its chelate compound with Fe 3+ flaked off the passive film surface.

1.3-Dimethyl-1,3-diaza-2-R-5,6-benzo-2λ3-phosphorinan-4-one (R = F, Me2N, 2-Methylpiperidino, MeC(: 0 )N H -) als Liganden in Übergangsmetallkomplexen; Synthese und Struktur von Dichloro-Platin(II)- und Tetracarbonyl-Metall(O)- Koordinationsverbindungen (Metall = Cr, Mo und W) / 1.3-Dimethyl-1,3-diaza-2-R-5,6-benzo-2λ3-phosphorinan-4-ones (R = F, Me2N, 2-Methylpiperidino, MeC(: 0 )NH-) as Ligands in Transition-Metal Complexes; Synthesis and Structure of Dichloro-Platinum(II)- and Tetracarbonyl-Metal(0) Coordination Compounds (Metal = Cr, Mo and W)

Abstract 1,3-Dimethyl-1,3-diaza-2-fluoro-5,6-benzo-1,3,2-phosphorinane-4-one (1) reacted with (COD)PtCl2 (COD = 1,5-cyclooctadiene) or K2PtCl4 to give the cis-dichloro-platinum(II) complex (2). The reactions of 1 with the tetracarbonyl norbornadiene derivatives of Cr, Mo and W furnished the complexes 3 - 6 . In the case of the chromium and tungsten complexes the trans-isomers 3 and 6 were formed, due to a cis-trans-isomerization. Room temperature and low temperature 31P NMR spectra of a mixture of 5 and 6 show no significant temperature dependence of the cis-trans equilibrium. The reaction of 1 with the pentacarbonyl tetrahydrofuran derivative of W led to the product 7. The spirophosphoranes 8 and 9 were obtained in low yields. The analogous reactions of 10, 15 and 20 with (COD)PtCl2 and the tetracarbonyl norbornadiene derivatives of Cr and Mo furnished the coordination compounds 11-14, 16-18, 21 and 23. In the case of all chromium complexes only the trans-isomers 11,16 and 21 were formed, whereas in the case of molybdenum cis-trans-isomer mixtures 12 and 13, as well as 17 and 18 were obtained. An attempt to obtain the cis-isomer of the molybdenum complex 22 failed. In the case of platinum the expected cis-complexes 14 and 23 were formed, while the attempted synthesis of 19 was unsuccessful. All these results are interpreted in terms of both steric and electronic factors. The ligands 1, 10, 15 and 20 coordinate via phosphorus. The validity of the concept of hard and soft acids and bases (HSAB) was confirmed. The characterization of 2 - 7 , 11-14, 16-18, 21 and 23 rests, especially, on their 1H, 13C and 31P NMR spectra, and their mass and infrared spectra. 31P - 31P coupling constants 2J(PP) for the cis-coordinated complexes 4, 5 and 14 involving two equivalent phosphorus ligands are also reported.

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.

Characterization of conducting polymers by inverse gas chromatography Part II. Effect of dopant on the dispersive and specific properties of polypyrrole

A series of polypyrroles, synthesized using iron(III) oxidizing agent, was characterized by inverse gas chromatography (IGC). The dispersive component to the surface tension of these materials falls in the range 40–60 mJ/m 2 for Cl −- and NO 3 −-doped polypyrroles, whereas that of Fe(CN) 6 − is extremely high (106 mJ/m 2) when compared to a conventional polymer. These values rank conducting polypyrroles between conventional insulating polymers and high surface energy materials such as metals, metal oxides and graphite. In addition, all polymers exhibit strong specific adsorption of acidic and basic molecular probes, confirming the Lewis amphoteric behaviour of conducting polypyrroles. The hard and soft acids and bases (HSAB) concept is also emphasized in an original manner based on acidic and basic Lewis probes and in connection with the nature of the dopant. This IGC study shows that the dopant plays an important role in the surface thermodynamics of conducting polypyroles, as evidenced by the magnitude of the dispersive and acid-base properties.

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

Local softness, scanning tunneling microscopy, and surface reactivity

Within the finite temperature extension of density functional theory, it is shown that local softness measures the ability of a chemical system to gain or donate charge as the external potential changes. This feature allows us to establish a formal connection between local softness and scanning tunneling microscopy (STM) images. We show that, under appropriate conditions, STM images can be used to measure local softness for surfaces. Finally, a potential application of those images as reactivity criteria within the context of the hard and soft acids and bases (HSAB) principle is discussed

Alkali Cation Selectivity and Surface Charge of 2:1 Clay Minerals

AbstractA critical demand in environmental modeling and a desirable but elusive goal of research on the ion exchange properties of the charged solid surface has been to determine the selectivity coefficient from fundamental properties of the ions and surface. We developed a Hard and Soft Acid and Base (HSAB) Model to describe exchangeable cation selectivity on solid surfaces. Our previous work has shown that the model quantitatively describes alkali cation exchange on clay minerals in terms of the absolute electronegativity and softness of the exchangeable cations and two fitting parameters: α and β. This study was conducted to determine the relationship between α and β and surface charge characteristics of 2:1 clays. The layer charge and cation selectivity of seven smectites and one vermiculite were used. The regression of log Kvo against four combinations of charge properties was performed and the appropriate relationship between α, β, and surface charge was selected based on both statistical criteria (R2) and their consistency with the assumptions of the HSAB model. The selected model was then cross-validated using separate cation exchange data from the literature. It was found that α and β are linearly related to the amount of charge arising from mineral tetrahedral and octahedral sites, respectively. These results make it possible to predict the alkali cation selectivity of 2:1 clay minerals from their chemical composition data and the alkali cation properties.

Thermogravimetric Studies on Bimetallic Dithiocarbamate Complexes

Thermogravimetric (TG) studies have been carried out for some bimetallic dithiocarbamate complexes of the type ZnML4 and NiM′L′4 [M = Co(II), Cu(II); M′ = Zn(II), Hg(II); L = N-methylcyclohexyldithiocarbamate; L′ = N-ethylcyclohexyldithiocarbamate]. From TG curves, the order, apparent activation energy and apparent activation entropy for the thermal decomposition reaction have been calculated. The thermal stabilities have been correlated with the structures of the complexes on the basis of hard and soft acid base (HSAB) theory.