Combination of <Superscript>13</Superscript>C/<Superscript>113</Superscript>Cd NMR,potentiometry, and voltammetry in characterizing the interactions between Cd and two models of the main components of soil organic matter

This work allowed the characterization of the Cd-binding sites of two compounds taken as models for exudates, the main components of soil organic matter (SOM). The studied compounds were exopolysaccharides (EPS), specifically exudates of roots (polygalacturonic acid) and of soil bacteria (Phytagel). Potentiometric acid–base titrations were performed and fitting of the obtained results indicated the presence of two main classes of acidic sites, defined by their pK _a values, for both EPS but of a different nature when comparing the two compounds. The two studied exopolysaccharides presented different acidic/basic site ratios: 0.15 for Phytagel and 0.76 for polygalacturonic acid. Spectroscopic techniques (¹³C/¹¹³Cd NMR, FTIR) distinguished different Cd surroundings for each of the studied EPS, which is in agreement with the titration results. Furthermore, these analyses indicated the presence of –COOH and –OH groups in various proportions for each exopolysaccharide, which should be linked to their reactivity towards cadmium. Cadmium titrations (voltammetric measurements) also differentiated different binding sites for each compound and allowed the determination of the strength of the Cd-binding site of the EPS. Fitting of the results of such voltammetric measurements was performed using PROSECE (Programme d’Optimisation et de Speciation Chimique dans l’Environnement), a software coupling chemical speciation calculation and binding parameter optimization. The fitting, taking into account the Cd²⁺/H⁺ competition towards exopolysaccharides, confirmed the acid-base titrations and spectroscopic analyses by revealing two classes of binding sites: (i) one defined as a strong complexant regarding its Cd²⁺–EPS association (logK = 9–10.4) and with basic functionality regarding H⁺–EPS association (pK _a = 11.3–11.7), and (ii) one defined as a weak complexant (logK = 7.1–8.2) and with acidic functionality (pK _a = 3.7–4.0). Therefore the combination of spectroscopic analyses, voltammetry, and fitting allowed the precise characterization of the binding sites of the studied exopolysaccharides, mimicking the main SOM components. Furthermore, the binding parameters obtained by fitting can be used in biogeochemical models to better define the role of key SOM compounds like exudates of roots and of soil bacteria on trace metal transport or assimilation.     

Decomposition mechanism of dinitramide onium salts

Thermal decomposition of dinitramide onium salts proceedsvia the dissociative mechanism when pK _a of the base is lower than 5.0 andvia the monomolecular decay of the anion at pK _a>7.0. On going from the melt to the solid state, the reaction mechanism does not change, and the rate decreases by 1–2 orders of magnitude. No anomalous effects inherent in dinitramide metal salts in the solid phase are observed during decomposition of onium salts. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1951–1953, November, 1997     

Nucleophilicity of micellar systems based on amphiphilic derivatives oF 2-(oximinomethyl)imidazole in the decomposition of 4-nitrophenyl diethyl phosphate

The differences in the acid–base properties and nucleophilic reactivity within a series of 1-methyl-3-alkyl-2-(oximinomethyl)imidazolium halides in the dissociation of 4-nitrophenyl diethyl phosphate were analyzed. The positive deviation of points for comicellar systems derived from compounds of this series from the Br?nsted plot was shown to be related to a decrease in the values of p K_\texta^\textapp K_{\text{a}}^{\text{app}} of the oximate fragment of the functional surfactants due to the action of the added cationic codetergent (CTAB).     

Interaction of aromatics with Br?nsted sites in zeolites: Demarcation line between regions of stable existence of H-complexes and ion pairs for various types of bases

Interaction of naphthalene, α-methylnaphthalene, and hexamethylbenzene with OH groups of SiO₂, HNaY and H-ZSM-5 zeolites was studied by IR spectroscopy. The results are compared with data obtained for other types of organic bases. Specificity in the generation of H-complexes of olefins, arenes, naphthalenes, N- and O-bases with Br?nsted surface sites was elucidated. Each type of base appears to have its particular correlation of (PA^a-PA^b) and the shift of Δυ_OH band in H-complexes. It has been found that if (PA^a-PA^b) is less than 85±5 kcal/mol, the formation of ion pairs is thermodynamically more favorable than that of H-complexes for all types of organic compounds.     

Theoretical pKa calculations of proteins; the tyrosine and lysine residues of b-elicitin

Elicitins are small proteins that are secreted by plant pathogenic fungi. In this work we have used a computer program that utilizes the boundary element method for heterogeneous dielectrics with ionic strength to calculate the pK _a of all titrating groups in the 98-residue protein β-cryptogein. Our results are in reasonable agreement with the experimentally determined pK _a values for the Tyr residues in the protein. We find that the functionally important Lys13 residue has a normal pK _a of 10.3. Our work also shows that there is no direct correlation between the exposure of an amino acid sidechain and its pK _a. Received: 24 April 1998 / Accepted: 4 August 1998 / Published online: 11 November 1998     

Enantioselective Addition Reaction of Azlactones with Styrene Derivatives Catalyzed by Strong Chiral Brønsted Acids

An enantioselective intermolecular addition reaction of azlactones, as carbon nucleophiles, with styrene derivatives, as simple olefins, was demonstrated using a newly developed chiral Brønsted acid catalyst, namely, F₁₀BINOL‐derived N‐triflyl phosphoramide. Addition products having vicinal tetrasubstituted carbon centers, one of which is an all‐carbon quaternary stereogenic center, were formed in good yields with moderate to high stereoselectivities. Extremely high acidity of the new chiral Brønsted acid was confirmed by its calculated pK_a value based on DFT studies and is the key to accomplishing not only high catalytic activity but also efficient stereocontrol in the intermolecular addition.     

Protonation acidity constants for benzotoluidides in sulfuric acid solutions

The protonation of o-, m-and p-benzotoluidide in sulfuric acid solutions is studied by UV spectroscopy in the 190–350 nm region. Principal component analysis is applied to estimate the contributions of the effect of protonation and the medium effect. For the substances studied in this work, the first principal component (PC) captures about 98 % of the variance and the second PC ∼100 % of the cumulative percentage variance in the 210–350 nm region. The same spectral region is used for calculation of the ionization ratio from the coefficients of the first PC and mole fractions of the base and its conjugate acid. Using these data and Hammett's equation (pK _{BH +} = H _X + log I), the pK _{BH +} values for the protonation reaction are obtained. The dissociation constants as well as the solvent parameters m* (∼0.43) and ϕ (∼0.60) are calculated using the Excess Acidity Method (-pK _{BH +}= 2.28–2.30) and the Bunnett-Olsen Method (-pK _{BH +}= 2.24–2.28). The probable sites of protonation are discussed.     

Improving the study of proton transfers between amino acid side chains in solution: choosing appropriate DFT functionals and avoiding hidden pitfalls

We have studied the influence of implicit solvent models, inclusion of explicit water molecules, inclusion of vibrational effects, and density functionals on the quality of the predicted pK _a of small amino acid side chain models. We found that the inclusion of vibrational effects and explicit water molecules is crucial to improve the correlation between the computed and the experimental values. In these micro-solvated systems, the best agreement between DFT-computed electronic energies and benchmark values is afforded by BHHLYP and B97-2. However, approaching experimental results requires the addition of more than three explicit water molecules, which generates new problems related to the presence of multiple minima in the potential energy surface. It thus appears that a satisfactory ab initio prediction of amino acid side chain pK _a will require methods that sample the configurational space in the presence of large solvation shells, while at the same time computing vibrational contributions to the enthalpy and entropy of the system under study in all points of that surface. Pending development of efficient algorithms for those computations, we strongly suggest that whenever counterintuitive protonation states are found in a computational study (e.g., the presence of a neutral aspartate/neutral histidine dyad instead of a deprotonated aspartate/protonated histidine pair), the reaction profile should be computed under each of the different protonation micro-states by constraining the relevant N–H or O–H bonds, in order to avoid artifacts inherent to the complex nature of the factors contributing to the pK _a.     

Laws of complex formation in a series of azo-substituted pyrocatechol derivatives and their tin(II) complexes

Physicochemical properties of new reagents, azo-substituted pyrocatechol derivatives and their tin(II) complexes, are studied. The acid-base properties of the hydroxy groups (pK′_i, pK″_i), parameters of complex formation reactions (pH, temperature, time), and instability constants of the complexes formed (pK _i) are determined. Quantitative correlations between the dissociation constants (pK′_a) of the functional analytical group, and the electronic Hammett constant σ for a substituent (pK′_a-pH₅₀ of the complex formation reaction), as well as between pK′_a and instability constants of the complexes (pK _a), are established. The quantitative correlations established allow the prediction of the physicochemical properties of the reagents and tin(II) complexes with new reagents of this class with the same functional analytical group (FAG) but other substituents.     

Resonance Raman spectroscopy and DFT calculations of the protonation of 4-(2-pyridylazo)-N,N-dimethylaniline in solution and adsorbed on oxide surfaces

The protonation of 4-(2-pyridylazo)-N,N-dimethylaniline (PYAD) in aqueous solution and its adsorption on oxide surfaces has been studied by resonance Raman (RR) spectroscopy. The gas phase structures of neutral, protonated and diprotonated forms of PYAD were modelled by SCF-DFT calculations at the B3-LYP/DZ level, enabling determination of the simulated vibrational spectra of these species, together with vibrational assignments, and providing confirmation that protonation occurs initially at the pyridyl nitrogen atom. Electronic absorption spectra were interpreted using time-dependent DFT calculations. Adsorption of PYAD on SiO₂ or Al₂O₃ surfaces is mainly via the neutral species, hydrogen bonded to surface OH groups, although a small proportion of adsorbed molecules are protonated. By contrast, adsorption on SiO₂–Al₂O₃ results in complete protonation, indicating the presence of Brønsted acidic sites with pK_a values ? 4.5, whereas adsorption on H-mordenite results in diprotonation, indicating the presence of Brønsted acidic sites with pK_a values ? 2.     

A Spiro Phosphamide Catalyzed Enantioselective Proton Transfer of Ylides in a Free Carbene Insertion into N−H Bonds

Free carbene readily causes multiple side reactions due to its high energy, thus its asymmetric transformation is very difficult. We present here our findings of high-pK_a Brønsted acid catalysts that enable free carbene insertion into N−H bonds of amines to prepare chiral α-amino acid derivatives with high enantioselectivity. Under irradiation with visible light, diazo compounds produce high-energy free carbenes that are captured by amines to form free ylide intermediates, and then the newly designed high-pK_a Brønsted acids, chiral spiro phosphamides, promote the proton transfer of ylides to afford the products. Computational and kinetic studies uncover the principle for the rational design of proton-transfer catalysts and explain how the catalysts accelerate this transformation and provide stereocontrol.     

Acid-base properties of N-methyl-substituted 1,2-dihydroquinolines in the ground and excited states

Changes in the absorption and fluorescence spectra of 1,2,2,3-tetramethyl-(1), 1,2,2,4-tetramethyl-(2), 6-ethoxy-1,2,2,4-tetramethyl-(3), and 1,2,6-trimethyl-1,2-dihydroquinolines (4) were studied in aqueous solution over a wide pH range from 1.0 to 12.0. The quantum yields of fluorescence and the values of pK _a of dihydroquinolines (DHQs) under study in the ground and excited states were determined, pK _a = 4.5, 3.8, 4.5, and 4.2 for the ground state of compounds 1–4, respectively, and pK _a ∼ 1.7 for the S ₁^* state for all DHQs.     

(E)-2-Benzylidenebenzocyclanones, part VIII: spectrophotometric determination of pKa values of some natural and synthetic chalcones and their cyclic analogues

Abstract  

UV–Vis spectrophotometry was used to determine acid dissociation constant (pK _a) values of the natural flavonoids phloretin, phlorizin, naringenin, and naringin, as well as 4′-hydroxychalcone, 4′-(dimethylamino)chalcone, and their cyclic analogues. Comparison of the results with those previously reported for the natural flavonoids showed the applied method is a relatively straightforward and easy-to-perform technique for the determination of pK _a values of compounds with relatively low solubility. Comparative analysis of the pK _a values of the synthetic chalcones showed a strong correlation between the degree of conjugation and the acid strength of the respective compounds with different geometry. Our results provide further evidence that modification of the three-dimensional structure of open-chain bioactive compounds is the method of choice to modify not only their stereochemistry but also their physicochemical properties.     

The influence of surfactants on the acid—base and redox properties of reagents of the diphenylamine series

The influence of aqueous solutions of surfactants on the redox and acid—base properties of diphenylamine redox reagents was studied by potentiometric titration and UV spectroscopy. The formal redox potentials, the pK _a values of the reagents in aqueous solutions and in the presence of surfactants, and the constants of binding of the reagents by micellar solutions of surfactants were estimated. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1386–1388, August. 2000.     

Lewis Superacidic Tellurenyl Cation-Induced Electrophilic Activation of an Inert Carborane

The aryltellurenyl cation [2-(tBuNCH)C₆H₄Te]⁺, a Lewis super acid, and the weakly coordinating carborane anion [CB₁₁H₁₂]⁻, an extremely weak Brønsted acid (pK_a=131.0 in MeCN), form an isolable ion pair complex [2-(tBuNCH)C₆H₄Te][CB₁₁H₁₂], in which the Brønsted acidity (pK_a 7.4 in MeCN) of the formally hydridic B−H bonds is dramatically increased by more than 120 orders of magnitude. The electrophilic activation of B−H bonds in the carborane moiety gives rise to a proton transfer from boron to nitrogen at slightly elevated temperatures, as rationalized by the isolation of a mixture of the zwitterionic isomers 12- and 7-[2-(tBuN{H}CH)C₆H₄Te(CB₁₁H₁₁)] in ratios ranging from 62 : 38 to 80 : 20.     

Quantum-chemical study on the relation between thermodynamic parameters in the iodination of benzene with N-I and O-I reagents and p<Emphasis Type="Italic">K</Emphasis><Subscript>a</Subscript> values of the corresponding NH and oh acids

Thermodynamic parameters of direct iodination of benzene with several iodinating agents were calculated using semiempirical (PM3), ab initio (3–21G**), and DFT (B3LYP/LanL2DZ) methods, as well as in terms of the polarization continuum model (PCM or Tomasi model). A close to linear correlation was found between the calculated thermodynamic parameters (ΔH ^≠, ΔG ^≠) and pK _T and experimental pK _a values of acids whose anions are incorporated into iodine-containing intermediates.     

A New Approach to Analysis of the Nonlinearity of the Brønsted Relationship: I. Role of Solvation Factors in Variations of the Kinetic and Thermodynamic Acidity of CH Acids

A procedure for eliminating the electrostatic component from pK _a of CH acids in DMSO wassuggested, substantiated in detail, used to construct the most suitable pK _a scale from the viewpoint of thechoice of standard, solvent-independent states of species participating in acid-base equilibria. Deviationsfrom the linearity of the Brönsted relationship are explained using a corrected scale of pK _a cor in combination with the concept of hard and soft acids and bases. The reactions of hydrogen exchange of CH acids of various structures in media of various protolytic activities were chosen as critical objects when checking in what cases and why derivations from the Brönsted relationship should be expected. Hard electrostatiuc interaction of carbanions with an aprotic polar solvent, nonequivalent to a less hard interaction between the transition state of the reaction and a protic solvent, is one of the factors responsible for deivations from the linearity.     

Identification of adsorption forms by ir spectroscopy for formaldehyde and formic acid on K3PMo12O40

Interaction of HCOOH and H₂CO with K₃PMo₁₂O₄₀ has been studied by IR spectroscopy. HCOOH adsorbed mainly in molecular form due to hydrogen bonds with surface oxygen ions. Two forms of adsorbed H₂CO were observed depending on the pressure. At low pressures it adsorbed on Br?nsted acid sites with the formation of hydrogen-bonded complexes. Dioxymethylene groups and a small amount of polyoxymethylene groups were formed at higher pressures. In contrast to oxide catalysts, formates were formed only in small amounts in the absence of oxygen. This can be explained by the formation of new surface adsorption sites Mo ions-due to a partial reduction of K₃PMo₁₂O₄₀ during its interaction with HCOOH and H₂CO.