Spectrophotometric determination of acidity constants of 4-(2-pyridylazo)resorcinol in binary methanol-water mixtures

The acidity constants of 4-(2-pyridylazo)resorcinol (PAR, Scheme 1) in binary mixtures of methanol-water at 25 °C and an ionic strength of 0.1 M have been determined spectrophotometrically. DATa ANalysis (DATAN) program applied for determination of acidity constants. As percent of methanol increases in solvent mixtures the pK_a constants also increased. There is linear relationship between acidity constants and the mole fraction of methanol in the solvent mixtures. Effect of solvent composition on acidity constants and pure spectrum of each component are also discussed.     

Protonation Constants of Uridine 5′-Monophosphate in Different Aqueous Solutions of Methanol

The protonation equilibria of uridine 5′-monophosphate disodium salt (UMP) was determined in binary solvent mixtures of water–methanol containing 0, 10, 15, 20, 25, 30, 35, 40, 45, and 50 % (v/v) methanol, using a combination of spectrophotometric and potentiometric methods at 25 °C and constant ionic strength (0.1 mol·dm^?3 NaClO₄). The protonation constants were analyzed using Kamlet, Abboud, and Taft parameters. A good linear correlation of the protonation constants (on the logarithmic scale) was obtained. Dual-parameter correlation of log₁₀ K versus π* (dipolarity/polarizability) and α (hydrogen-bond donor acidity), as well as π* and β (hydrogen-bond acceptor basicity), gave good results in various aqueous solutions of methanol. Finally, the results are compared with CMP, a homolog of UMP, and are discussed in terms of the effect of the solvent on the protonation constants.     

A Correlation Between Solvatochromic Solvent Polarity Parameters and the Ionization Constants of Various Phenols in 1,4-Dioxane–Water Mixtures

Precise thermodynamic ionization constants K for 3-nitrophenol, 3,4-dichlorophenol, and 4-cyanophenol have been obtained in 1,4-dioxane-water mixtures (0–70% volume fraction in dioxane) at 25°C using a potentiometric method. The same information for another twelve cationic, neutral, and anionic phenols were taken from the literature. Three different methods were used to study the effects of the solvents on the ionization constants: one involves a single polarity parameter, E _T(30); the next involves the Kamlet–Taft multiparametric method; and the last involves the preferential solvation model. The pK values follow the preferential solvation model, but the parameters obtained are highly correlated. Using the data for the phenol molecule as reference, a linear correlation between ΔpK and E _T(30) has been used to develop a new method of obtaining pK values for any binary solvent composition, with only the pK in water known. The pK(s) values correlate with the molecular parameters for the dipolarity/polarizability of the solvent π* and its hydrogen-bond donor ability α. The preferential solvation parameter, f _12/1, correlates with the parameter for the hydrogen-bond donor ability of the solvent. All the phenols follow Hammett's equation and the reaction constants have been calculated for the different water–dioxane mixtures.     

Influence of Solute–Solvent Interaction on Acid Strength of Some Substituted Phenols in Ethanol–Water Media

The solute–solvent interactions of some phenol derivatives were investigated potentiometrically in 0–60 % (v/v) ethanol–water mixtures. The acidity constants values were correlated with either macroscopic parameters such as molar fraction, permittivity and the solvating ability or microscopic parameters, such as the Kamlet–Taft solvatochromic parameters. Moreover, it is demonstrated that the pK _a values in any ethanol–water mixtures are linearly related to the pK _a values of the phenols in pure water. The slope and intercept parameters of the linear correlations are related with the mole fraction of ethanol. These equations permit accurate calculation of the pK _a values of the studied phenols at any ethanol–water composition.     

Multiwavelength spectrophotometric determination of acidity constants of 1-（2-thiazolylazo）-2-naphthol in methanol-water mixtures

The acid-base properties of 1-(2-thiazolylazo)-2-naphthol (TAN) in mixtures of methanol-water at 25℃and an ionic strength of 0.1 mol/L are studied by a multi-wavelength spectrophotometric method.The acidity constants of all related equilibria are estimated using the whole spectral fitting of the collected data to an established factor analysis model DATAN program was used for determination of acidity constants.The corresponding pK_a values in methanol-water mixtures were determined.There is a linear relationship between acidity constants and the mole fraction of methanol in the solvent mixtures.     

Potentiometric and ab initio studies of acid-base interactions of substituted 4-halo(Cl, Br)pyridine N-oxide systems

By using the potentiometric method, acidity constants have been determined in systems of tri- and tetra-substituted pyridine N-oxides. The potentiometric measurements in systems of four 4-chloropyridine N-oxide derivatives containing the chlorine atom at position 4 to the NO₂ group and four bromine counterparts were carried out in polar non-aqueous solvents, viz. amphiprotic methanol (MeOH) and aprotic protophilic dimethyl sulfoxide (DMSO). It was found that in all the systems studied the pK_a values were readily determinable (as indicated by small standard deviations) in MeOH, whereas in DMSO large standard deviations were obtained making the pK_a values either hardly determinable or indeterminable from potentiometric measurements. Furthermore, it was demonstrated that the acidity constants of protonated N-oxides studied in MeOH changed according to the sequence of their acidity constants in water. It was also found that in the polar solvents studied, i.e. in the amphiprotic methanol and the highly basic aprotic dimethyl sulfoxide, the cationic homo-conjugation equlibrium constants could not be determined using potentiometric method. Also, by using ab initio methods at the RHF and MP2 levels and the PCM model, utilizing the Gaussian 6-31++G^∗∗ basis set, energies and Gibbs free energies of the protonation reactions of the N-oxides have been determined. The energy parameters have been compared with acidity constants of the protonated N-oxides determined by potentiometric titration in methanol to establish a correlation between these approaches.     

Novel, biologically imperative, highly versatile and planar systems: Synthesis, characterization, electrochemical behavior, DNA binding and cleavage properties of substituted β-diketimine copper(II) and zinc(II) complexes with dipyrido(3,2-a:2′, 3′ -c)phenazine ligand

Novel copper(II) and zinc(II) complexes of the type [ML(dppz)]Cl₂, [L = Schiff base derived from the condensation of 3-(3- phenyl-allylidene)-pentane-2,4-dione and para-substituted aniline; X = -NO₂ (L¹), -H (L²), -OH (L³) and -OCH₃ (L⁴); dppz = dipyrido (3,2-a:2′, 3′ -c)phenazine] were synthesized and characterized by various analytical and spectral techniques. The physicochemical studies and spectral data indicated that all the complexes were monomeric and cationic with square-planar geometry. Spectroscopic data and viscosity measurements showed that the complexes intercalated to DNA with large binding constants. The substituted groups such as -NO₂, -H, -OH and -OCH₃ in aniline moiety influenced the observed trend in the redox potentials of the complexes. The peak potential separation and formal potential of complexes were independent of sweep rate or scan rate (ν) indicating a quasireversible one-electron redox process. In all the cases, i _p was linear function of ν^1/2, as expected for diffusion controlled process, and i _pa/i _pc ≈ 1 at all sweep rates. It was found that the decrease in i _pc was due to the higher binding of copper complexes and slowly diffusing DNA. In the presence of a reducing agent like 3-mercaptopropionic acid (MPA), the chemical nuclease activity order of the copper complexes under dark reaction condition was -NO₂ > -H > -OH > -OCH₃. The hydrolytic cleavage of DNA by the zinc complexes was supported by the evidence from free radical quenching and T4 ligase ligation.     

The relationship between the association constants of phenol with ethers and those of tricholoroacetic acid with ethers : Estimation of association constants of trichloroacetic acid with oxirane

The association constants, K_p, of phenol with ethers (tetrahydrofuran, tetrahydropyran dibutyl ether, dipropyl ether, 1,4-dioxane) and those , K_T, of trichloroacetic acid with ethers were measured in CCl₄ over a temparature range 20°–40° using near IR spectra. A linear relationship between K_p and K_T was found. On the basis of this realtionship the association constants of trichloroacetic acid with propylene oxide were estimated from those of phenol with propylene oxide.     

Acidity of osmium tetroxide (OsO4) towards coordination with pyridine and its derivatives

This article reports on the Lewis acidity of osmium tetroxide (OsO₄), the most reliable reagent available for the cis-hydroxylation of alkenes. Three simple and straightforward scales, donor ability of N-donors, the Hammett reaction constant and the Electrostatic-Covalent model (ECW) were used for this purpose. Equilibrium constants (K_eq) and enthalpies for the complexation of OsO₄ with pyridine and its derivatives have been evaluated by spectrophotometeric measurements. The excellent linear relationships were observed between the equilibrium constants and either the pK_a values of the N-donors or the Hammett σ constants of the substituents on the ligands. The slope of the plot of log K_eq versus ligand’s pK_a and the Hammett ρ value derived suggest the use of these parameters as an index of the Lewis acidity strength for OsO₄. The enthalpies of complex formation of N-donors with OsO₄ also fit the ECW model to predict the values of E_A and C_A parameters for OsO₄ as 2.01 and 0.25 (kcal/mol)^1/2, respectively. The parameters can be used to make an acidity scale for OsO₄. These parameters also permit the chemists and biochemists to predict and correlate quantitatively the enthalpies of OsO₄ · Lewis base interactions.     

pK a Constant Determination of Two Triazole Pesticides: Tebuconazole and Penconazole

We determined the acidity constants of tebuconazole and penconazole, two fungicides from the group of 1,2,4-triazoles. Potentiometric titrations were performed in a 20 % (v/v) acetonitrile/water mixture at 25 °C and at a fixed ionic strength (KNO₃, 0.1 mol·dm^?3). The pK _a values (representing thermodynamic constants) were determined to be 5.0 ± 0.1 and 5.2 ± 0.1 for tebuconazole and penconazole, respectively. These values could be used in pure water solutions to consider the protonated or deprotonated forms when studying the field behavior of these fungicides. Molecular modeling calculations allowed identifying the N4 atom as the protonation site.     

The acidity functions of trifluoroethanol and hexafluoroisopropanol,and their mixtures with water

Acidity measurements in trifluoroethanol and hexafluoroisopropanol as solvents, and in their mixtures with water, are reported. The hydrogen electrode and the glass electrode were used for pH measurements. The “experimental” autoprotolysis constants were determined from measurements in strongly acidic (trifluoromethanesulfonic) media and in strongly basic (alcoholate) media: for trifluoroethanol, pK = -log K/mol² l^-2 = 15 and for hexafluoroisopropanol, pK = 14.8. Evaluation of the pH-indicator potential systems with reference to the ferrocene/ferricinium couple gives the acidity function R₀(H). The values obtained are compared to the H₀ values evaluated in the same conditions.     

Determination of configuration-dependent spin-orbit interaction constants by atomic spectroscopy

A new approach to the evaluation of spin-orbit interaction (SOI) constants is presented. The SOI constants ξ_p and ξ_d of 4p and 3d electrons have been evaluated by employing the available atomic spectral data for the first transition row atoms and ions possessing the electron configuration K(2)L(8)3s²3p⁶3d^α4s^β4p^γ.     

Protolytic properties of 2-acylthioacetamides

Concentration acidity of 2-acylthioacetamides dissolved in DMSO-H₂O was measured by the method of pH-metric titration. The data obtained were extrapolated for the determination of pK _a values of 2-acylthioacetamides in water and DMSO. We found that in aqueous solutions the acidity of thioacetarylamide (pK _a = 5.75?C6.01) was almost independent of the nature of substituents in their N-phenyl rings, whereas in DMSO solution the nature of substituents significantly affects their ionization constants (pK _a = 7.88?C11.70). The graphs were displayed and the acidity of N-aryl-3-oxobutanethioamides dependence on the Hammett constants of meta- and para-substituents in the phenyl rings derived. Water is shown to be a leveling, and DMSO a differentiating solvent for 2-acylthioacetamides.     

UV and MO study on the deprotonation of some 2-aryl-Δ2-1,3,4-oxadiazoline-5-thiones

Substituent effects on the deprotonation processes of a series of 2-aryl-Δ²-1,3,4-oxadiazoline-5-thione (1) derivatives have been studied experimentally as well as theoretically. The acid dissociation constants pK_a have been determined spectrophotometrically in ethanol-water solutions (7.5-92.5%) and vary between 3.76 and 5.80. Semiempirical molecular orbital (MO) calculations (AM1 and PM3) were used for the investigation of the existence of possible tautomeric thione and thiol forms of the studied compounds. Strong correlation between the pK_a values and the deprotonation enthalpies were evaluated.     

Synthesis of a new bluish pigment from the reaction of a methylpyranoanthocyanin with sinapaldehyde

The synthesis of a new pigment with a bluish color was obtained from the reaction of methylpyranomalvidin-3-glucoside with sinapaldehyde and its formation mechanism seems to involve a charge-transfer reaction pathway. The structure of this compound was fully characterized by LC/DAD-MS and NMR. Its equilibrium forms present in water at different pH values and the respective ionization constants were determined by UV-visible spectroscopy. The results revealed the presence of three equilibria involving only deprotonation reactions at the 7-OH and 4′-OH positions of the pyranoanthocyanin moiety and at the 4′″-OH of the syringol moiety (pK_a1 = 3.64 ± 0.01; pK_a2 = 8.02 ± 0.01; pK_a3 = 11.19 ± 0.01).     

Simultaneous determination of partition coefficients and acidity constants of chlorinated phenols and gualacols by gas chromatography

The partition coefficients, P, in the n-octanol/water system and the acidity constants, K_a, of 15 chlorinated phenols and guaiacols at 20° were simultaneously determined by investigating the pH dependence of the apparent partition coefficients, P_a. For determining P_a, the relative responses of individual phenols and guaiacols in the aqueous phase after equilibration were measured by glass-capillary gas chromatography with electron-capture detection and compared with those in the octanol phase before partition. Curvefitting, linear regression, and non-linear regression were used for treating the partition data. The values of pK_a and log P were compared with the values which could be found in the literature, and are discussed in relation to their affinity to methanol/water and acetonitrile/water mixtures.     

The SAMPL5 challenge for embedded-cluster integral equation theory: solvation free energies,aqueous p<Emphasis Type="Italic">K</Emphasis><Subscript>a</Subscript>, and cyclohexane–water log <Emphasis Type="Italic">D</Emphasis>

We predict cyclohexane–water distribution coefficients (log D _7.4) for drug-like molecules taken from the SAMPL5 blind prediction challenge by the “embedded cluster reference interaction site model” (EC-RISM) integral equation theory. This task involves the coupled problem of predicting both partition coefficients (log P) of neutral species between the solvents and aqueous acidity constants (pK _a) in order to account for a change of protonation states. The first issue is addressed by calibrating an EC-RISM-based model for solvation free energies derived from the “Minnesota Solvation Database” (MNSOL) for both water and cyclohexane utilizing a correction based on the partial molar volume, yielding a root mean square error (RMSE) of 2.4 kcal mol^?1 for water and 0.8–0.9 kcal mol^?1 for cyclohexane depending on the parametrization. The second one is treated by employing on one hand an empirical pK _a model (MoKa) and, on the other hand, an EC-RISM-derived regression of published acidity constants (RMSE of 1.5 for a single model covering acids and bases). In total, at most 8 adjustable parameters are necessary (2–3 for each solvent and two for the pK _a) for training solvation and acidity models. Applying the final models to the log D _7.4 dataset corresponds to evaluating an independent test set comprising other, composite observables, yielding, for different cyclohexane parametrizations, 2.0–2.1 for the RMSE with the first and 2.2–2.8 with the combined first and second SAMPL5 data set batches. Notably, a pure log P model (assuming neutral species only) performs statistically similarly for these particular compounds. The nature of the approximations and possible perspectives for future developments are discussed.     

Amidines-XVIII: Tautomeric equilibria and pka values of N,N'-disubstituted amidines. Substituent effects

The influence of substitution of amidine group on tautomeric equilibria constants and basicities is discussed. Equations based on correlation analysis methods are derived enabling predictions of both, microscopic pK_a, values of individual tautomers, measured macroscopic pK_a values of the tautomeric mixture, as well as the tautomeric equilibrium constant (as pK_T). It is shown that pK_arn values of unsymmetrically N,N'-disubstituted amidines should obey a non-linear relation with σ° constants, and only for symmetrically N,N'-disubstituted amidines obey the linear Hammett equation. Tautomeric equilibrium constants of N,N'-disubstituted amidines correlate withσ° substituent constants. The methods of prediction of pK_a value of both tautomers and pK_T value are proposed.Derived relations are applied to the series of N,N'-diphenylacetamidines and benzamidines.     

Protolytic properties and complexation with copper(II) ions of some azo derivatives of β-arylaminopropionic acids

The azo coupling reaction of N-(2-carboxyethyl)anthranilic acid and N,N,N′,N′-tetrabis(2-carboxyethyl)-1,3-phenylenediamine with diazosulfanilic acid yielded the complexones sodium 4-N-(2-carboxyethyl)amino-5-carboxyazobenzene-4′-sulfonate (I) and 2,4-N,N,N′,N′-tetrabis(2-carboxyethyl)diaminoazobenzene-4′-sulfonic acid (II), respectively. The acidity constants of I and II (20°C, μ = 0.1M KCl) were determined to be as follows: for I, pK ₀₀ = 1.29 ± 0.13, pK ₀ = 2.92 ± 0.07, pK ₁ = 3.92 ± 0.05, pK ₂ = 5.16 ± 0.03; for II, pK ₀₀ = 2.35 ± 0.06, pK ₀ = 2.81 ± 0.09, pK ₁ = 3.21 ± 0.11, pK ₂ = 3.81 ± 0.09, pK ₃ = 4.34 ± 0.04, pK ₄ = 5.03 ± 0.06, pK ₅ = 6.67 ± 0.07. The electronic absorption spectra of I and II were measured, and acid-base equilibrium scheme for I and II in aqueous solutions were suggested. The complexation constants of I and II with copper(II) ions were determined to be logK _CuQ^I= 5.47 ± 0.06 and logK _CuQ^II= 5.72 ± 0.13 (20°C, μ = 0.1 M KCl).