Solubility, dissociation and complexation with Nd(III) and Th(IV) of oxine, thenoyltrifluoroacetone and 1,10-phenanthroline in 5.0 m NaCl

Equilibria in the system of Nd(III) and Th(IV) with 8-hydroxyquinoline (oxine), thenoyltrifluoroacetone (HTTA) and 1,10-phenanthroline (phen) in 5.0 m NaCl solution have been investigated by spectroscopy and potentiometry. The solubility and deprotonation constants of the three organics were measured to be: pK_s = 3.09 ± 0.01, pK_a1 = 5.82 ±0.02, pK_a2= 10.00 ±0.01 for oxine; pK_s = 2.49 ± 0.01, pK_a1 = 6.47 ±0.03 for HTTA; pK_s = 2.86 ± 0.02, pK_a2 = 5.82 ± 0.05 for phen. The stabilities of the corresponding metal complexes are in the order M(oxine) > M(TTA) > M(phen), where M = Nd(III), Th(IV). For all three organic ligands, the Th(IV) complexation is stronger than that of Nd(III).     

基于毛细管电泳和离子迁移率经验公式测定洛伐他汀的绝对淌度和解离常数

作为一种可以预防动脉粥样硬化和冠心病的潜在药物洛伐他汀,其绝对淌度m₀和解离常数pK_a值的测定有助于其性质与应用的研究。在前期相关研究基础上,该文提出了一种基于毛细管区带电泳(CZE)和离子淌度经验公式测定洛伐他汀m₀和pK_a的新方法。首先,根据经验公式由实际淌度(m_act)、有效淌度(m_eff)和m₀之间的关系推导出m₀的计算公式。对于一元酸HA,根据之前m₀的计算公式,以氢离子的浓度为自变量,m_eff的倒数为因变量可得到一条直线。根据这条直线的斜率计算得到pK_a。为了验证该方法的可行性和可靠性,应用该方法测定了巴比妥酸、苯甲酸、苄胺、苯酚、间甲酚等有机酸碱的m₀和pK_a值。同时,对于pH值低于6的缓冲体系,采用反向毛细管电泳技术,测定其pK_a,并将测得的实验结果与理论参考值进行对比,发现两者具有较高的一致性,m₀的标准偏差小于6.0%, pK_a的标准偏差小于6.2%,且由线性回归方程的相关系数(R)可以看出测定pK_a时的线性回归直线拟合度较好,说明该文建立的新方法具有较高的可靠性。最后基于这种CZE与经验公式结合的新方法,采用二甲基亚砜(DMSO)作为电渗流标记物测定了洛伐他汀的m₀和pK_a,得到的值分别为-1.70×10^-8 m²/(V·s)和9.00。该方法适用于酸性和碱性分析物m₀和pK_a等理化参数的测定,在药物分析尤其是新药理化特性研究中具有重要意义。     

黄豆黄素与黄豆黄苷吸收光谱和荧光光谱的比较研究

研究了黄豆黄素和黄豆黄苷在不同pH条件下的吸收光谱和荧光光谱, 从分子结构的角度解释了二者呈现不同光谱特征的原因. 黄豆黄素分子基本无荧光. 在弱碱性时, 黄豆黄素分子发生7-OH质子的电离, 导致吸收光谱中320 nm的吸收峰红移至348 nm. 采用pH-光度法测得7-OH质子的电离常数pK_a1=7.08±0.04. 黄豆黄素一价阴离子呈现较强荧光, 最大激发和发射波长λ_ex/λ_em分别为334 nm/464 nm, 荧光量子产率为0.049. 在碱性溶液中, 黄豆黄素4'-OH质子电离, 导致吸收光谱中254 nm的吸收峰红移至260 nm, 电离常数pK_a2=9.96±0.01. 黄豆黄苷分子基本无荧光. 在碱性条件下, 黄豆黄苷分子的4'-OH质子发生电离, 导致吸收光谱中256 nm的吸收峰红移至 280 nm, 电离常数pK_a=9.81±0.03. 黄豆黄苷阴离子基本无荧光, 但在热碱性条件下发生γ-吡喃酮环裂解反应而产生较强荧光, λ_ex/λ_em为288 nm/388 nm, 裂解产物的荧光量子产率为0.056. 虽然, 黄豆黄苷与黄豆黄素是苷与苷元的关系, 但黄豆黄苷不能在热碱性条件下通过糖苷水解转变为黄豆黄素, 二者的荧光增强机理存在本质不同.     

Critical validation of a new simpler approach to estimate aqueous pKa of drugs sparingly soluble in water

A simple linear approach to estimate the aqueous pK_a of compounds sparingly soluble in water, mainly drugs, from solely one pK_a value determined in any methanol/water mixture is evaluated. The parameters (slope and intercept) of the linear relationships are related to the solvent composition and can be easily calculated according to the acidic or basic functional group of the compound. The method has been tested using the available literature data for phenols, aliphatic carboxylic acids, benzoic acid derivatives, both ortho and non-ortho substituted, amines and imidazole derivatives. The study involves the whole range of solvent composition and about one hundred compounds which show a wide variety of aqueous pK_a, from 1.3 to 12.4. The differences between calculated and previously published aqueous pK_a values are less of 0.2 pK units. Consistent values are obtained whatever the composition of methanol/water mixture employed in the experimental measurements. The results support the usefulness of the tested method as a very simple approach to get reliable aqueous pK_a values for sparingly soluble drugs.     

Fundamental properties of isoelectric buffers for capillary zone electrophoresis

Different isoelectric buffers are analysed theoretically, taking into account a fundamental parameter, i.e., the ratio between intrinsic buffering power and conductivity (R=β/λ). For a model ampholyte, the above parameter is analysed both as a function of the pI and the pK_b–pK_a values. For natural pH gradients, the variation of R, connected with approaching the isoelectric point, is evaluated. A case of oligo-protic ampholytes is also considered.     

Intramolecular hydrogen-bonding in beta adrenergic blocking agents

The acid dissociation constants for nine beta-adrenergic blocking agents have been determined. Their unusually low pK_a, values are explained as being due to the formation of an intramolecular hydrogen bond, between the terminal amino group and the β-hydroxyl group on the alkanolamine side-chain which is common to this class of drugs.     

Formation of hydrogen-bonded chains between a strong base with guanidine-like character and phenols with various pKa values - an FT-IR study

The complexes formed by phenols with 1,3,4,6,7,8-hexahydro-1-methyl-2H-pyrimido[1,2-a]pyrimidine (mTBD), an N-base with guanidine-like character, were studied as a function of the pK_a of the phenols by FT-IR spectroscopy. The following phenols were used: 4-cyanophenol (4-CNPh), pentachlorophenol (PCP) and 2,6-dichloro-4-nitrophenol (DNPh). In the case of chloroform solutions of 1:1 mixtures of the phenols with MTBD the corresponding complexes are formed completely. With increasing acidity of the phenols the hydrogen bonds become increasingly asymmetrical. The OH … N ⁻O … H⁺N hydrogen bond in the 4-CNPh-MTBD complex shows large proton polarizability. In the other cases only the polar structure is realized. With increasing phenol MTBD ratio, the formation of chains with two phenol molecules is observed. With decreasing pK_a of the phenols the fluctuation is limited to the phenol-phenolate bond and finally, the phenol-protonated MTBD bond begins to dissociate. In acetonitrile solutions, N⁺H … O⁻ hydrogen bonds are observed in the case of the 1:1 mixture of 4-CNPh with MTBD. A weak continuum indicates the presence of homoconjugated phenol-phenolate bonds with large proton polarizability. In the case of 2:1 mixtures only protonated MTBD and homoconjugated phenol-phenolate bonds are observed, independent of the pK_a of the phenols. The results are discussed with regard to the proton pathway in bacteriorhodopsin.     

Correlation between hydration and deprotonation in hexaaqua metal complexes

The hydration energy of metallic cations determined with density functional calculations using a double-numerical plus p-polarization basis set, related to the acidity constants of hexaaqua metal complexes, was investigated in the present study. From the results calculated by Vosko-Wilk-Nusair (VWN), Becke-Perdew (BP) and Becke-Lee-Yang-Parr (BLYP) density functionals, a global linear correlation with the observed acidity constants in both main group [Mg(II), Ca(II) and Al(III)] and (post-)transition group [Mn(II), Zn(II), Cd(II), Sc(III), Cr(III), Fe(III), Ga(III) and In(III)] hexaaqua metal complexes has been established:

VWN density functional: pK_a = 16.5760 + 0.0173E_hydr kcal mol⁻¹

BP density functional: pK_a = 15.7329 + 0.0182E_hydr kcal mol⁻¹

BLYP density functional: pK_a = 15.9448 + 0.0185E_hydr kcal mol⁻¹     

Surface chemistry of nanoporous carbon and the effect of pH on adsorption from aqueous phenol and 2,3,4-trichlorophenol solutions

Granular nanoporous activated carbon prepared from polyacrylonitrile (PAN) was investigated as a means of removing weak aromatic acids from aqueous solutions. This carbon is highly nanoporous, the BET surface area being 544 m²/g with V_tot=0.278 cm³/g, and V_micro=0.266 cm³/g. Aqueous treatment reduced the surface area to 364 m²/g.

Granular nanoporous carbon prepared from PAN contains O and N related surface functional groups. The surface concentration of both oxygen and nitrogen atoms was found by XPS to be 5.3%. Surface groups containing these hetero atoms are responsible for the acid–base character of this carbon in aqueous solutions. The pH_PZC of the carbon is 8.4. The microporous pore network produces a wide hysteresis loop, observed when the granular carbon was studied by continuous titration. This loop, which was found to diminish, but not to disappear completely when the particle size was reduced, is attributed to irreversible hydrolysis of surface esters and/or lactones. The surface concentration of the functional groups titrated by the equilibrium Boehm method is 449.2 μeq./g, of which 112.9 μeq./g have acidic and 336.3 μeq./g have basic character. Within the acidic species three subgroups, namely the regions pK_a<6.37, 6.37K_a<10.25 and 10.25K_a<15.74.

The acid–base behavior influences the adsorption performance of the granular carbon in aqueous solutions of weak aromatic acids, in this case phenol and 2,3,4-trichlorophenol. Both the adsorption capacity and the overall interaction parameter, K (both derived from a fit to the Langmuir equation) depend on the adsorbed species and on the pH. The former is a consequence of the different water solubilities of the solute molecules, while the latter stems from the pH sensitivity of both the surface functional groups and these weak acids. The K values show a sequence pH=3K values indicate different adsorption mechanisms. The effect of pH is more marked for 2,3,4-trichlorophenol.     

Cell surface acid-base properties of Escherichia coli and Bacillus brevis and variation as a function of growth phase, nitrogen source and C:N ratio

Potentiometric titration has been conducted to systematically examine the acid–base properties of the cell surfaces of Escherichia coli K-12 and Bacillus brevis as a function of growth phase, nitrogen source (ammonium or nitrate), and carbon to nitrogen (C:N) ratio of the growth substrate. The two bacterial species revealed four distinct proton binding sites, with pK_a values in the range of 3.08–4.05 (pK₁), 4.62–5.57 (pK₂), 6.47–7.30 (pK₃), and 9.68–10.89 (pK₄) corresponding to phosphoric/carboxylic, carboxylic, phosphoric, and hydroxyl/amine groups, respectively. Two general observations in the data are that for B. brevis the first site concentration (N₁), corresponding to phosphoric/carboxylic groups (pK₁), varied as a function of nitrogen source, while for E. coli the fourth site concentration (N₄), corresponding to hydroxyl/amine groups (pK₄), varied as a function of C:N ratio. Correspondingly, it was found that N₁ was the highest of the four site concentrations for B. brevis and N₄ was the highest for E. coli. The concentrations of the remaining sites showed little variation. Finally, comparison between the titration data and a number of cell surface compositional studies in the literature indicates one distinct difference between the two bacteria is that pK₄ of the Gram-negative E. coli can be attributed to hydroxyl groups while that of the Gram-positive B. brevis can be attributed to amine groups.     

Use of factorial experimental design for the rapid evaluation of main and interactive factors affecting linearity in calibration curves for sulfate analysis by ion chromatography

Analytical results of anion determination by suppressed ion chromatography are significantly affected by calibration curve calculation. In this paper, as expected, eluent pK_a is shown to influence calibration linearity in the range 1–20 mg/l sulfate, with A carbonate-hydrogencarbonate mixture producing a larger non-linearity than NaOH. Evidence is given for very large errors (about 30–40%) in estimating sample sulfate concentration when linear regression is used instead of a quadratic calibration curve. This study was performed following a 2⁴ run full factorial experimental design, including eluent pK_a, counterion type, solution composition and current level for background suppression as main variables.     

Acid-base dissociation constants of 2,2'-bipyridyl in mixed protic solvents

The acid dissociation constants (K_a), base dissociation constants (K_b) and the autoprotolysis constants (K_s) for 2,2′-bipyridyl in water and in water+alcohol(methanol, ethanol, iso-propanol) mixed solvents have been determined at 25°C and an ionic strength of 0.1 mol l⁻¹, from a direct potentiometric method based on the treatment of the data of a single pH titration. It has been shown that K_a increases, whereas K_b and K_s decrease, with increasing proportion of the alcohol in the mixed solvents. Linear relations between pK_a, pK_b, pK_s and the mole fraction of the alcohol were observed in the composition range investigated. These results are discussed in terms of the properties of solvent and the interactions of the different species existing in dissociation equilibrium with solvents. It is concluded that the higher stabilization of both 2,2′-bipyridyl and its protonated form by dispersion forces and of the proton by its interaction with solvent molecules in the mixed solvents compared with that in water are largely responsible for the observed changes of pK_a with composition. On the other hand, the low stabilization of OH⁻ in the mixed solvents relative to that in water and the electrostatic effect are the main factors in determining the solvent effect on pK_b.     

Criegee中间体CH3CHOO与H2O反应机理及酸催化效应

采用CCSD（T）//B3LYP/6-311+G（d,p）方法研究了H₂O及甲酸等6种有机酸对CH₃CHOO与H₂O加成反应的催化作用。结果表明,非催化反应存在双质子迁移和加成反应2条通道,其中加成反应为优势通道。其加成机理为H₂O中OH加到CH₃CHOO的α-C上,同时H₂O中另一个H迁移到CH₃CHOO的端O上。催化剂H₂O及有机酸以氢键复合物的形式参与反应促进了H质子转移,可降低基元反应能垒和表观活化能,且催化效应与有机酸的强度成正比。例如,当分别用H₂O（pK_a=15.7）、甲酸（pK_a=3.75）和草酸（pK_a=1.23）催化时,生成syn-HAHP的基元反应能垒由非催化的69.12 kJ·mol^-1分别降至40.78、18.88和10.61 kJ·mol^-1。非催化反应具有正的表观活化能,而所有催化反应则均具有负的表观活化能。     

Primary radicals in the photo-oxidation of aromatics — reactions of xylenols with OH, N3 and H radicals and formation and characterization of dimethylphenoxyl, dihydroxydimethylcyclohexadienyl and hydroxydimethylcyclohexadienyl radicals by pulse radiolysis

Photo-oxidations of environmental organics in illuminated TiO₂ dispersions have implicated surface-bound ^•OH radicals and/or valence band holes. To explore the implications of the former oxidizing entity, six isomeric xylenols (dimethylphenols) were examined by pulsed (nanoseconds to milliseconds) radiolysis methods. The spectral and kinetic characteristics of formation and decay of the transients formed by the reaction of N₃^•, ^•OH and H^• radicals with these xylenols were assessed in buffered (pH 4, 10⁻³ M phosphate) aqueous media, where the xylenols exist in their protonated form (pK ≈ 10.19–10.65). The products from the reaction of N₃^• with 2,6- and 3,4-xylenol were exclusively the corresponding dimethylphenoxyl radicals, formed via electron transfer followed by deprotonation. In contrast, except with 3,4-xylenol, the principal radical intermediates formed initially upon reaction with ^•OH were the corresponding ^•OH adducts, the dihydroxydimethylcyclohexadienyl radicals. 3,4-Xylenol was examined in the pH range 4–10. At pH 8 the initial ^•OH adduct (dihydroxy-3,4-dimethylcyclohexadienyl radical) was subsequently transformed (about 20%–40%) via water elimination into the dimethylphenoxyl radical. In contrast, at pH 9 and 10 the ^•OH adduct and the dimethylphenoxyl radical were formed concurrently (about 60% ^•OH adduct and about 40% dimethylphenoxyl species), the latter through an inner-sphere electron transfer pathway. The switch in behaviour from pH 8 to pH 9 suggests that the pK_a of the dihydroxy-3,4-dimethylcyclohexadienyl radical is about 8–9, about 2 pK units below the pK_a of the parent substrate (10.4). A mechanism for the conversion of the ^•OH adduct to the dimethylphenoxyl radical is proposed. Reaction of 2,6-xylenol with H^• radicals gave exclusively the H^• adduct (hydroxycyclohexadienyl radical), whose spectral characteristics are similar to those of the related ^•OH adduct.     

Photosolvolysis of 2-aminobenzl alcohol in aqueous solution

The photosolvolytic behavior of 2-aminobenzyl alcohol (4) was studied in aqueous solution over a range of pH and in moderately concentrated aqueous H₂SO₄ solution. Although reactive at all pH values studied, clean solvolytic reaction was observed only when pHK_a (for ground state dissociation of aryl ammonium ion). A mechanism of reaction in acidic medium is proposed in which the protonated substrate (ArNH₃⁺) is excited to S₁ and then undergoes adiabatic deprotonation to give the electronically excited free base. Subsequent dehydroxylation, which can be assisted by a proton, gives o-quinone methide imine (o-QMI, 5) as the reactive intermediate. Nucleophilic attack by added ROH gives the photosolvolytic product. If pH>pK_a, a significant proportion of 5 reacts with free amine 4, which is sufficiently nucleophilic to compete with H₂O or ROH, eventually giving rise to oligomeric products.     

High-order torsional couplings in the infrared spectrum of 3,3,3-trifluoropropene

A 2 MHz resolution electric-resonance optothermal spectrometer and a microwave-sideband CO₂ laser have been used with microwave-infrared double resonance to investigate high-order torsional couplings in the 10 μm infrared spectrum of 3,3,3-trifluoropropene. Three normal mode vibrations are studied with band origins at 963.4, 980.2 and 1025.2 cm⁻¹. The 963.4 cm⁻¹ band is well characterized by an asymmetric-top Hamiltonian, except for the presence of a weak perturbation for J′ = 7, K_a′ = 2 affecting only the A-symmetry internal-rotor state. Microwave-infrared double resonance is used to study the microwave spectrum of the perturbing or ‘dark’ state. The observed dark-state K-doublet asymmetry splittings and rotational-state selection rules indicate that the perturbing state has five quanta of excitation in the torsional mode (ν₂₁) built upon the A″ ν₁₉ fundamental. The precise frequency determined for 5 β₂₁ of 421(2) cm⁻¹ leads to the first accurate determination of the barrier to CF₃ internal rotation as 641(5) cm⁻¹. In contrast to the 963.4 cm⁻¹ vibration, the 980.2 and 1025.2 cm⁻¹ modes show a large number of J′ and K_a′ perturbations which differentially affect the A and E symmetry internal-rotor states. The magnitude of the perturbation-induced A/E splittings indicate that the perturbing states must have at least four quanta of torsional excitation. The present results suggest that high-order vibrational interactions are important in the vibrational dynamics of molecules at low levels of overall vibrational excitation.     

Acidity constants of benzidine in aqueous solutions

The acidity constants of benzidine (Bz) in aqueous solutions determined potentiometrically at 25° were K_a1 = (1.11 ± 0.08) × 10⁻⁵, K_a2 = (1.45 ± 0.12) × 10⁻⁴. The apparent mixed constants in 0.1M sodium nitrate are K_a1 = (5.37 ± 0.28) × 10⁻⁶ and K_a2 = (1.14 ± 0.09) × 10⁻⁴. The ultraviolet spectra were recorded as a function of pH and analysed with these constants to obtain the absorption spectra of H₂Bz²⁺, HBz⁺ and Bz; the corresponding wavelengths of maximal absorption are 247, 273 and 278 nm, and molar absorptivities 1.63 × 10⁴, 1.76 × 10⁴ and 2.26 × 10⁴ 1.mole⁻¹.cm⁻¹.     

Organic photochemistry V: photochemistry of substituted Schiff''s bases—search for luminescent dyes for solar collectors

The pH dependence of the absorption and fluorescence spectra of Schiff's bases derived from 2-amino-4-phenylthiazole and aniline with substitued 2-hydroxybenzaldehydes was investigated. The pK_a values of the Schiff's bases associated with the ground state equilibria were determined spectrophotometrically. The excited state pK_a values were also estimated. Their Stokes shifts were also calculated.     

Ab initio calculations of pKa values of some organic acids in aqueous solution

The acidity of different classes of organic compounds in aqueous solution has been calculated. The calculations are carried out at the SCF level with inclusion of entropic and thermochemical correction to yield free energies of dissociations.

The polarized continuum model is used to describe the solvent. The model furnishes pK_a values in relatively good agreement with experimental data. Scaling different parts of solvation energies provides a significant improvement in results and signifies the importance of balance of individual contributions from electrostatic, cavity, dispersion and repulsion interactions.     

The electrolyte NRTL model and speciation approach as applied to multicomponent aqueous solutions of H2SO4, Fe2(SO4)3, MgSO4 and Al2(SO4)3 at 230–270 °C

This work presents chemical modeling of solubilities of metal sulfates in aqueous solutions of sulfuric acid at high temperatures. Calculations were compared with experimental solubility measurements of hematite (Fe₂O₃) in aqueous ternary and quaternary systems of H₂SO₄, MgSO₄ and Al₂(SO₄)₃ at high temperatures. A hybrid model of ion-association and electrolyte non-random two liquid (ENRTL) theory was employed to fit solubility data in three ternary systems H₂SO₄–MgSO₄–H₂O, H₂SO₄–Al₂(SO₄)₃–H₂O at 235–270 °C and H₂SO₄–Fe₂(SO₄)₃–H₂O at 150–270 °C. Employing the Aspen Plus™ property program, the electrolyte NRTL local composition model was used for calculating activity coefficients of the ions Al³⁺, Mg²⁺ Fe³⁺ and SO₄²⁻, HSO₄⁻, OH⁻, H₃O⁺, respectively, as well as molecular species. The solid phases were hydronium alunite (H₃O)Al₃(SO₄)₂(OH)₆, hematite Fe₂O₃ and magnesium sulfate monohydrate (MgSO₄)·H₂O which were employed as constraint precipitation solids in calculating the metal sulfate solubilities. A correlation for the equilibrium constants of the association reactions of complex species versus temperature was implemented. Based on the maximum-likelihood principle, the binary interaction energy parameters for the ionic species as well as the coefficients for equilibrium constants of the reactions were obtained simultaneously using the solubility data of the ternary systems. Following that, the solubilities of metal sulfates in the quaternary systems H₂SO₄–Fe₂(SO₄)₃–MgSO₄–H₂O, H₂SO₄–Fe₂(SO₄)₃–Al₂(SO₄)₃–H₂O at 250 °C and H₂SO₄–Al₂(SO₄)₃–MgSO₄–H₂O at 230–270 °C were predicted. The calculated results were in excellent agreement with the experimental data.