Protonation Equilibria Research Articles

Effect of dielectric constant of medium on protonation equilibria of ethylenediamine

The effect of dielectric constant of medium on protonation equilibria has been studied by determining protonation constants of ethylenediamine pH metrically in various concentrations (0–60%v/v) of acetoni-trile– and ethylene glycol–water mixtures, at an ionic strength of 0.16mol L−1 and at 303.0 K. MINIQUAD75 computer program has been used for the calculation of protonation constants. Linear and non-linear variations of step-wise protonation constants with reciprocal of dielectric constant of the solvent mixtures have been attributed to the dominance of the electrostatic and non-electrostatic forces, respectively. The trend is explained on the basis of solute–solute and solute–solvent interactions, solvation, proton transfer processes and dielectric constants of the media.

Sensitivity study for the rate-based simulation of the reactive absorption of CO2

Abstract The most promising process for Post Combustion Carbon Capture (PCC) is reactive absorption. For the design and scale-up of this process, reliable process models are needed. To achieve this, the models need to not only reliably describe the physical and chemical equilibria in the reacting systems but also to account for both mass transfer and reaction kinetics. Consequently, the models are complex and contain a large number of input parameters for describing fluid dynamics and physico-chemical properties. In order to assess the required quality of the input, the sensitivity of the simulation results to the parameters has to be known. This paper presents results from such a sensitivity study for a typical operating point of a PCC pilot plant with 0.3 g/g monoethanolamine (MEA) in water as the solvent. A rate-based model and its input parameters are introduced and describe the pilot plant results well. The most relevant input parameters of the model are systematically varied and the influence of that variation on the simulation results is monitored. Absorber and desorber are considered separately. The results are sensitive to most of the studied parameters. High sensitivities are found for the interfacial area, the Henry’s law constant and the enthalpy of absorption as well as for the parameters describing the equilibrium and kinetics of the carbamate formation and the amine protonation equilibrium.

Protonation of 5,10,15,20-Tetrakis(4-sulfonatophenyl)porphine in Water

Protonation equilibriums of the porphyrin core H 2 P in H 2 P(PhSO 3 - ) 4 with perchloric acid in water are investigated by methods of spectropotentiometry and computer chemistry (DFT/B3LYP and РМ3). It is shown that leveling of step constants is caused by formation of aquacomplex [H 4 P 2+ (PhSO 3 - ) 4 ](H 2 O) 2 of diprotonated porphyrin platform showing properties of anion-molecular receptor. Values of lgK 1 (4.85 ± 0.03), lg(K 2 ·K 3 )(1.22 ± 0.03) are defined, and rough value of lgK 2 (-0.23) for reactions H 2 P + H + H 3 P + (1); H 3 P + + H + H 4 P 2+ (2); H 4 P 2+ + 2H 2 O [H 4 P 2+ ](H 2 O) 2 (3) is calculated. Molecular parameters [H 4 P 2+ (PhSO 3 - ) 4 ](H 2 O) 2 are calculated and it is established that receptor H 4 P 2+ possesses very high complementarity concerning two water molecules.

Effect of anionic micelles on speciation of L-glutamine in presence/absence of metals

Protonation equilibria of L-glutamine and speciation of its complexes with Co(II), Ni(II) and Cu(II) have been studied in 0–2.5% w/v SLS–water media using pH-metric method. The protonation constants and binary stability constants have been calculated with the computer program MINIQUAD75. Selection of the best fit chemical models is based on standard deviation in stability constants and residual analysis using crystallographic R-factor and sum of squares of residuals in all mass-balance equations. The trend in the variation of stability constants of the complexes with mole fraction of the medium is attributed to the compartmentalisation of complexation equilibria. Distribution of species and effect of influential parameters on chemical speciation have also been presented.

Influence of Heterogeneity of Confined Water on Photophysical Behavior of Acridine with Amines: A Time-Resolved Fluorescence and Laser Flash Photolysis Study

The photophysical behavior of acridine (Acr) shows facilitated water-assisted protonation equilibrium between its deprotonted (Acr* ∼ 10 ns) and protonated forms (AcrH(+*) ∼ 28 ns) within confined region of ordered water molecules inside AOT/H(2)O/n-heptane reverse micelles (RMs). The time-resolved-area-normalized-emission spectra confirm both Acr* and AcrH(+*), while time-resolved-emission spectra depict time evolution between them. Quenching of AcrH(+*) with N,N-dimethylaniline (DMA) is a purely diffusion-controlled bimolecular quenching with linear Stern-Volmer (S-V) plot, while nonlinearity arises with triethylamine (TEA) that forms ground state complex with AcrH(+) (AcrH(+)··H(2)O··TEA) indicating both static and dynamic quenching. Transient intermediates, DMA(•+) and AcrH(•) infer photoinduced electron transfer from DMA to Acr, while those from AcrH(+)··H(2)O··TEA complex suggest water mediated excited-state proton transfer (ESPT) between AcrH(+) and TEA. The ESPT becomes faster in larger RMs due to enhanced mobility of hydronium ions in AcrH(+)··H(2)O··TEA, which reduces in smaller RMs as water becomes much more constrained owing to stronger complexation by excess confinement.

Synthesis, characterization, and crystal structure of a tricadmium complex of 3,4-diamino-5-methyl-1,2,4-triazole

The ligand 3,4-diamino-5-methyl-4H-1,2,4-triazole (DAMT) and its trinuclear cadmium complex [Cd3(DAMT)2(DAMTH)2Cl8] have been prepared and identified by elemental analyses, infrared, Raman, 1H and 13C NMR spectroscopy, and single crystal X-ray diffraction. The protonation equilibria of the ligand have been studied by pH-potentiometry, with pK 1 13.29 and pK 2 5.40 determined. The free ligand consists of triazole cations (DAMTH) and chloride counter ions. The neutral cadmium compound is a complex of the cationic ligand in which three Cd's are coordinated by two unidentate cation forms of the ligands (DAMTH), two ambidentate neutral forms of the ligand (DAMT), and eight chlorides. Cadmium ions have slightly distorted octahedral geometry.

The organophosphate-degrading enzyme from Agrobacterium radiobacter displays mechanistic flexibility for catalysis

The OP (organophosphate)-degrading enzyme from Agrobacterium radiobacter (OpdA) is a binuclear metallohydrolase able to degrade highly toxic OP pesticides and nerve agents into less or non-toxic compounds. In the present study, the effect of metal ion substitutions and site-directed mutations on the catalytic properties of OpdA are investigated. The study shows the importance of both the metal ion composition and a hydrogen-bond network that connects the metal ion centre with the substrate-binding pocket using residues Arg254 and Tyr257 in the mechanism and substrate specificity of this enzyme. For the Co(II) derivative of OpdA two protonation equilibria (pKa1 ~5; pKa2 ~10) have been identified as relevant for catalysis, and a terminal hydroxide acts as the likely hydrolysis-initiating nucleophile. In contrast, the Zn(II) and Cd(II) derivatives only have one relevant protonation equilibrium (pKa ~4-5), and the μOH is the proposed nucleophile. The observed mechanistic flexibility may reconcile contrasting reaction models that have been published previously and may be beneficial for the rapid adaptation of OP-degrading enzymes to changing environmental pressures.

THE COORDINATION CHEMISTRY OF HNO: FROM WARREN ROPER TO HEMOGLOBIN

The coordination chemistry of the one-electron reduced form of nitric oxide, termed a nitroxyl or nitrosyl hydride (NO- or HNO), is described anecdotally through the author's discovery of a stable HNO adduct of myoglobin, HNO-Mb, as an illustration of how different areas of chemistry, such as organometallic and bioinorganic, overlap and influence one another in unexpected ways. HNO is short-lived in solution, and many fundamental characteristics of HNO, such as the widely cited values of its oxidation potential and pKa, have been significantly revised through collaborations of theoreticians, biologists, and physical chemists. The very first HNO metal complex was discovered in Warren Ropers lab in 1970, with some 27 years before the next fully characterized example was reported by Hillhouse. Hillhouse in turn helped guide our characterization of HNO-Mb. As with oxymyoglobin, there are several possible descriptions of bonding for HNO-adducts, we present an analogy to the π-bonding interactions of a Fischer carbene. Roper's initial work demonstrated the importance of redox and protonation equilibria in these species, a current focus of ongoing work.

Solid State NMR Spectroscopy as a Precise Tool for Assigning the Tautomeric Form and Proton Position in the Intramolecular Bridges of o-Hydroxy Schiff Bases

Two analogous Schiff bases, (S,E)-2-((1-hydroxy-3-methyl-1,1-diphenylbutan-2-ylimino)methyl)phenol (1) and (S,Z)-2-hydroxy-6-((1-hydroxy-3-methyl-1,1-diphenylbutan-2-ylamino)methylene)cyclohexa-2,4-dienone (2), exist in the solid state as phenol-imine and keto-amine tautomers, respectively. Their crystal structures were solved using the X-ray diffraction method. Sample 1 forms orthorhombic crystals of space group P2(1)2(1)2(1), while 2 forms monoclinic crystals of space group P2(1). In each sample, one molecule is in the asymmetric unit of the crystal structure. One-dimensional and two-dimensional solid state NMR techniques were used for structure assignment and for inspection of the (13)C and (15)N δ(ii) of the chemical shift tensor (CST) values. NMR study indicates that the span (Ω = δ(11)-δ(33)) and the skew (κ = 3(δ(22)-δ(iso)/Ω) are extremely sensitive to change in the tautomeric form of the Schiff bases. Theoretical calculations of NMR shielding parameters for 1 and 2 and a model compound with reduced aliphatic residue were performed using the GIAO method with B3LYP functional and 6-311++g(d,p) basis sets. From comparative analysis of the experimental and theoretical parameters, it was concluded that the position of hydrogen in the intramolecular bridge has tremendous influence on (13)C and (15)N CST parameters. Inspection of Ω and κ parameters allowed for the establishment of the nature of the hydrogen bonding and the assignment of the equilibrium proton position in the intramolecular bridges in the solid state.

Mechanistic Studies of the Oxygen Evolution Reaction by a Cobalt-Phosphate Catalyst at Neutral pH

The mechanism of the oxygen evolution reaction (OER) by catalysts prepared by electrodepositions from Co(2+) solutions in phosphate electrolytes (Co-Pi) was studied at neutral pH by electrokinetic and (18)O isotope experiments. Low-potential electrodepositions enabled the controlled preparation of ultrathin Co-Pi catalyst films (<100 nm) that could be studied kinetically in the absence of mass transport and charge transport limitations to the OER. The Co-Pi catalysts exhibit a Tafel slope approximately equal to 2.3 × RT/F for the production of oxygen from water in neutral solutions. The electrochemical rate law exhibits an inverse first order dependence on proton activity and a zeroth order dependence on phosphate for [Pi] ≥ 0.03 M. In the absence of phosphate buffer, the Tafel slope is increased ∼3-fold and the overall activity is greatly diminished. Together, these electrokinetic studies suggest a mechanism involving a rapid, one electron, one proton equilibrium between Co(III)-OH and Co(IV)-O in which a phosphate species is the proton acceptor, followed by a chemical turnover-limiting process involving oxygen-oxygen bond coupling.

Synthesis of a New Zwitterionic Surfactant Containing an Imidazolium Ring. Evaluating the Chameleon-like Behavior of Zwitterionic Micelles

Synthesis of a new zwitterionic surfactant containing the imidazolium ring 3-(1-tetradecyl-3-imidazolio)propanesulfonate (ImS3-14) is described. The solubility of ImS3-14 is very low but increases on addition of a salt which helps to stabilize the micellized surfactant. Fluorescence quenching and electrophoretic evidence for ImS3-14 shows that the micellar aggregation number is only slightly sensitive to added salts, as is the critical micelle concentration, but NaClO(4) markedly increases zeta potentials of ImS3-14 in a similar way as in N-tetradecyl-N,N-dimethylammonio-1-propanesulfonate (SB3-14) micelles. The rate of specific hydrogen ion catalyzed hydrolysis of 2-(p-heptoxyphenyl)-1,3-dioxolane and equilibrium protonation of 1-hydroxy-2-naphthoate ion in zwitterionic micelles of ImS3-14 and SB3-14 are increased markedly by NaClO(4) which induces anionoid character and uptake of H(3)O(+), but NaCl is much less effective in this respect. Comparison of ImS3-14 with SB3-14 is based on experimental evidence, and computational calculations indicate similarities and differences in structures of both compounds.

Fluorescence Quenching by Photoinduced Electron Transfer in the Zn2+ Sensor Zinpyr-1: A Computational Investigation

We report a detailed study of luminescence switching in the fluorescent zinc sensor Zinpyr-1 by density functional methods. A two-pronged approach employing both time-dependent density functional theory (TDDFT) and constrained density functional theory (CDFT) is used to characterize low-lying electronically excited states of the sensor. The calculations indicate that fluorescence activation in the sensor is governed by a photoinduced electron transfer mechanism in which the energy level ordering of the excited states is altered by binding Zn(2+). While the sensor is capable of binding two Zn(2+) cations, a single Zn(2+) ion appears to be sufficient to activate moderate fluorescence in aqueous solution at physiological pH. We show that it is reasonable to consider the tertiary amine as the effective electron donor in this system, although the pyridyl nitrogens each contribute some density to the xanthone ring. The calculations illustrate an important design principle: because protonation equilibria at receptor sites can play a determining role in the sensor's fluorescence response, receptor sites with a pK(a) near the pH of the sample are to be disfavored if a sensor governed by a simple PET fluorescence quenching model is desired.

Analysis of the Bromate−Sulfite−Ferrocyanide pH Oscillator Using the Particle Filter: Toward the Automated Modeling of Complex Chemical Systems

This study was aimed at identifying a quantitatively accurate reaction model of the bromate-sulfilte-ferrocyanide (BSF) pH oscillator by using the simulation-based model estimation algorithm known as the particle filter. The Rbai-Kaminaga-Hanazaki (RKH) model proposed for the BSF system was extended by adding the protonation equilibrium of SO42-, for which the particle filter analysis was carried out to optimize the rate constants involved with reference to the measured pH oscillation data. The extended RKH model with the optimized rate constants almost completely reproduced the measured pH oscillations and the state diagram, showing the validity of the present analysis. Chemical oscillators such as the BSF system show drastic switching of the dominant reaction path, which strongly disturbs the convergence of the rate constants if the objective function is defined in a conventional manner to reflect only a single time step datum. In this study, the objective function was defined as the residual sum of squares with respect to pH taken over an interval longer than one oscillatory period, so that all of the relevant reaction steps can contribute to the objective function. This is the first report which exemplifies the effectiveness of the particle filter in the analysis of real complex chemical systems.

ChemInform Abstract: Solvent Extraction Investigation of Molybdenum(VI) Equilibria.

The extraction of molybdenum(VI) by tri-n-butyl phosphate (TBP) or a 50% TBP-cyclohexane mixture from 1.0 M NaClO4 medium has been investigated as a function of molybdenum concentration at various approximately constant pH values in the range 1.0–2.0. A satisfactory description of the extraction system could be obtained by computer treatment of the data when all the relevant protonation and condensation equilibria in both phases were taken into account. In the pH range investigated the existence of two large polyanions in the aqueous phase has been confirmed, i.e. [Mo36O112(OH2)16]8− and most probably [Mo18O56]4−. Formation constants have been calculated for these ions and also for some other species which agree well with values previously determined by other methods.

ChemInform Abstract: Synthesis and Crystal Structure of Propane-1,2,3-triamine Trihydrochloride Monohydrate. Reactions of Propane-1,2,3-triamine with Hydrogen Ion. Thermodynamic Functions and Molecular Mechanics Calculations

The crystal and molecular structure of propane-1,2,3-triamine trihydrochloride monohydrate (1·3HCl·H2O) is reported. 1·3HCl·H2O crystallizes in the triclinic system, space group P(no. 2), a= 11.078(1), b= 11.935(2), c= 7.862(1)A, α= 102.88(2), β= 100.79(1), γ= 81.07(1)°, Z= 4. The structure refinement converged to R= 0.036 and Rw= 0.038. Two 1·H33+ cations, six Cl– anions and two water molecules are contained in the asymmetric unit. The two triammonium ions have almost the same conformation. The four C–C bond lengths average 1.521 (4)A while the six C–N bond lengths average 1.485(4)A. A network of strong hydrogen-bond interactions involve all of the chloride ions, the triprotonated nitrogen atoms as well as the water molecules. These forces are responsible for the non-equivalence in the two C–C–N(terminal) bond angles for the two 1·H33+ cations.The protonation equilibria of propane-1,2,3-triamine (1) in aqueous solution (0.15 mol dm–3 NaCl, 298 K) have been studied with potentiometric and calorimetric techniques. The logarithms of the protonation constants are 9.642 ± 0.002, 7.981 ± 0.001 and 3.715 ± 0.002 for 1, 1·H+ and 1·H22+, respectively. A comparison with the log K values previously determined for similar triamines containing three equivalent –CH2NH2 groups, such as 2-methyl-2-aminomethylpropane-1,3-diamine (2) and 2-ethyl-2-aminomethylpropane-1,3-diamine (3), shows that 1 is the least basic ligand. The decrease in the basicity is particularly evident for the third step. The enthalpy changes for 1 are – 10.97 ± 0.04, –11.07 ± 0.07 and –8.52 ± 0.08 kcal mol–1‡ in the first, second and third protonation step, respectively.A molecular-mechanics analysis was carried out on the 1·H33+ cation to obtain a proper force field for this kind of triamine. Atomic charges for all the atoms were calculated by means of the semiempirical MNDO method. The energy-minimized conformation for the folded 1·H33 + cation is in good agreement with the molecular structure found in the solid state for 1·3HCl·H2O. However, the two calculated C–C–N (terminal) bond angles are equal to each other showing that intermolecular interactions are responsible for the distortion found in the solid-state structure. In the most stable conformation of the 1·H22+ species, the protonated amino groups are the terminal ones. The cation shows an elongated chain similar to that previously found for the protonated linear polyamine spermine when co-crystallized with nucleic acid molecules.

Electronic and Steric Effects: How Do They Work in Ionic Liquids? The Case of Benzoic Acid Dissociation

The need to have a measure of the strength of some substituted benzoic acids in ionic liquid solution led us to use the protonation equilibrium of sodium p-nitrophenolate as a probe reaction, which was studied by means of spectrophotometric titration at 298 K. In order to evaluate the importance of electronic effect of the substituents present on the aromatic ring, both electron-withdrawing and -donor substituents were taken into account. Furthermore, to have a measure of the importance of the steric effect of the substituents both para- and ortho-substituted benzoic acids were analyzed. The probe reaction was studied in two ionic liquids differing for the ability of the cation to give hydrogen bond and pi-pi interactions, namely [bm(2)im][NTf(2)] and [bmpyrr][NTf(2)]. Data collected show that benzoic acids are less dissociated in ionic liquid than in water solution. Furthermore, the equilibrium constant values seem to be significantly affected by both the nature of ionic liquid cation and the structure of the acid. In particular, the ortho-steric effect seems to operate differently in water and in the aromatic ionic liquid, determining in this solvent medium a particular behavior for ortho-substituted benzoic acids.

Protonation Equilibria and Lipophilicity of Sarafloxacin

The main objective of this work was to quantify the lipophilicity of sarafloxacin in the form of the apparent and true partition coefficient in an n-octanol/buffer system. For that purpose the ionization constants were determined by a spectrophotometric method, and the distribution profiles of microspecies in water as a function of pH for this fluoroquinolone were established. The maximum concentration of the neutral species occurs at the sarafloxacin isoelectric point, pH = 5.5. The apparent partition coefficient in the n-octanol/buffer system versus the pH profile of sarafloxacin displayed a parabolic behavior that reached a maximum near the pH of the isoelectric point. The true partition coefficient was calculated from the log Papp and microconstant values and has the value of 0.846 ± 0.021.

Self-organization of grafted polyelectrolyte layers via the coupling of chemical equilibrium and physical interactions.

The competition between chemical equilibrium, for example protonation, and physical interactions determines the molecular organization and functionality of biological and synthetic systems. Charge regulation by displacement of acid-base equilibrium induced by changes in the local environment provides a feedback mechanism that controls the balance between electrostatic, van der Waals, steric interactions and molecular organization. Which strategies do responsive systems follow to globally optimize chemical equilibrium and physical interactions? We address this question by theoretically studying model layers of end-grafted polyacids. These layers spontaneously form self-assembled aggregates, presenting domains of controlled local pH and whose morphologies can be manipulated by the composition of the solution in contact with the film. Charge regulation stabilizes micellar domains over a wide range of pH by reducing the local charge in the aggregate at the cost of chemical free energy and gaining in hydrophobic interactions. This balance determines the boundaries between different aggregate morphologies. We show that a qualitatively new form of organization arises from the coupling between physical interactions and protonation equilibrium. This optimization strategy presents itself with polyelectrolytes coexisting in two different and well-defined protonation states. Our results underline the need of considering the coupling between chemical equilibrium and physical interactions due to their highly nonadditive behavior. The predictions provide guidelines for the creation of responsive polymer layers presenting self-organized patterns with functional properties and they give insights for the understanding of competing interactions in highly inhomogeneous and constrained environments such as those relevant in nanotechnology and those responsible for biological cells function.

Toward understanding the thermodynamics of TALSPEAK process. Medium effects on actinide complexation

The ingenious combination of lactate and diethylenetriamine-N,N,N',N",N"-pentaacetic acid (DTPA) as an aqueous actinide-complexing medium forms the basis of the successful separation of americium and curium from lanthanides known as the TALSPEAK process. While numerous reports in the prior literature have focused on the optimization of this solvent extraction system, considerably less attention has been devoted to the understanding of the basic thermodynamic features of the complex fluids responsible for the separation. The available thermochemical information for both lactate and DTPA protonation and metal complexation reactions are representative of the behavior of these ions under idealized conditions. Significant departures from the speciation predicted based on reported thermodynamic values may be present in the TALSPEAK aqueous environment. Thus, thermodynamic parameters describing the separation chemistry of this process require further examination at conditions significantly removed from conventional ideal systems commonly employed in fundamental solution chemistry. In this report, calorimetric studies of lactate protonation equilibrium illustrate a distinct influence of nitrate, perchlorate, methylsulfonate and triflate on the thermodynamic parameters associated with TALSPEAK aqueous chemistry. Triflate is identified a closer non-complexing representative of lactate solution chemistry. The reported initial two stability constants for the complexation of americium and neodymium by lactate in triflate medium suggest that the identified medium effects do not significantly affect the speciation.

Electrochemical Proton-Coupled Electron Transfer of an Osmium Aquo Complex: Theoretical Analysis of Asymmetric Tafel Plots and Transfer Coefficients

Electrochemical proton-coupled electron transfer of an osmium aquo complex attached to a self-assembled monolayer on a gold electrode is studied with a recently developed theoretical formulation. The calculated hydrogen/deuterium kinetic isotope effect for the standard rate constant, the cathodic transfer coefficient at zero overpotential, and the Tafel plot are in excellent agreement with experimental data. The input quantities to the heterogeneous rate constant expressions were calculated with density functional theory in conjunction with dielectric continuum models, and no parameters were fit to experimental data. The theoretical calculations indicate that the asymmetry of the Tafel plot and the deviation of the transfer coefficient at zero overpotential from the standard value of one-half arise from the change in the equilibrium proton donor-acceptor distance upon electron transfer. The direction of the asymmetry and deviation from one-half is determined by the sign of this distance change, and the magnitude of these effects is determined by the magnitude of this distance change, as well as the reorganization energy and the distance dependence of the overlap between the initial and final proton vibrational wave functions. This theory provides experimentally testable predictions for the impact of specific system properties on the qualitative behavior of the Tafel plots.