Spectrophotometric investigation of protolytic equilibria of rutin

The dissociation constants of rutin in aqueous-methanol medium (6040 v/v), the values of which are pK₁=–2.92±0.06, pK₂=6.72±0.11, pK₃=8.26±0.05, pK₄=12.57±0.09, were estimated by using the spectrophotometric method. They were ascribed to the dissociation of protonated oxygen atom at position 1 and then to hydroxyl groups at positions 7, 4', 5, respectively. Resonance structures of H₃L^– ion of rutin were suggested and, by using them, the greater dissociability of the hydroxyl group at position 7 in relation to the –OH group at position 4' was explained.     

Investigation of the protolytic equilibria of nitraminotetrazoles in aqueous media

The UV and Raman spectra of a number of nitraminotetrazoles were recorded in aqueous media. The dependence of the spectral characteristics on the pH and H_o was studied, and the dissociation constants of neutral and protonated molecules were determined. It was established that 5-nitramino- and 1-methyl-5-nitraminotetrazole exist in the nitrimine form in aqueous solutions.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1569–1571, November, 1984.     

Influence of isomerism on the chromatographic behaviour of porphyrins

Summary The chromatographic behaviour of some porphyrins and their complexes with zinc has been studied by HPLC on 150×3.9 mm and 300×3.9 mm columns packed with Nova-Pak C₁₈ and μ-Bondapak C₁₈, respectively, and on a microcolumn (64×2 mm) packed with Nucleosil C₁₈. The effect of the nature and the arrangement of side substituents in the porphyrin molecules on retention is considered. It is demonstrated that HPLC can be used for the separation ofcis-andtrans-isomers (atropisomers) of the zinc complex of 5,15-di(phenyl-2-CH₃O)-3,7,13,17-tetramethyl-2,8,12,18-tetrabutylporphyrin and other porphyrins with a similar structure. The efficiency of separation has been compared on different columns.     

Micellar effects on absorption spectra and protolytic equilibria of phenyl-pyridyl-ketones

The absorption spectra of the three isomeric phenyl-pyridyl-ketones were studied in anionic (SLS), cationic (CTAB) and non-ionic (Triton X-100) micellar solutions. The spectral changes of the n, π* transition in micelles compared to net water provided information on the location of the ketone molecules in the micelle and the partitioning of ketones between micelles and water. The apparent pK^a values of the three pyridyl ketones were determined in anionic, cationic and non-ionic micelles and compared to those in aqueous solution. The results indicate that the ketones reside within the interfacial head group region of the micelle. This location affects the protolytic equilibria: a decrease in nitrogen basicity in Triton X-100 (ΔK^a∼−0.6) and CTAB (ΔpK^a−0.5) and an increase in SLS (ΔpK^a+1.5) are attributed to a reduced polarity at the micelle surface and to an electrostatic surface potential.     

Thermochemical characteristics of nicotinamide protolytic equilibria in water-dimethylsulfoxide mixtures

The heat effects of nicotinamide protonation in water-dimethylsulfoxide (DMSO) solutions over the concentration range 0–0.75 DMSO mole fractions were determined calorimetrically at 25.00 ± 0.01°C and ionic strength 0.25 (NaClO₄). Changes in the enthalpy of protonation as the content of DMSO increased were found to be described by an S-shaped curve. This curve shape was caused by the dynamics of reagent solvation contributions as the concentration of DMSO grew with the predominance of the nicotinamide solvation contribution.     

Use of lithium perchlorate media in the study of protolytic equilibria

A study of the H⁺/F^– system by fluoride ion-selective electrode potentiometry over a range of ionic strengths in LiClO ₄ gives protolysis constants which differ markedly from those obtained in other media such as NaClO ₄ . This is shown to be consistent with the competitive complexation of the fluoride ion by the lithium ion. An examination of literature data indicates that lithium ion complexation is a general phenomenon and supports the conclusion that LiClO ₄ is not preferable to NaClO ₄ as a swamping electrolyte for the study of protolytic equilibria as has previously been thought.     

Thermodynamic characteristics of protolytic equilibria of L-serine in aqueous solutions

The heat effects of the reaction of aqueous solution of L-serine with aqueous solutions of HNO₃ and KOH were determined by calorimetry at temperatures of 288.15, 298.15, and 308.15 K, and ionic strength values of 0.2, 0.5, and 1.0 (background electrolyte, KNO₃). Standard thermodynamic characteristics (Δ_r H ^o, Δ_r G ^o, Δ_r S ^o, ΔC _p ^o) of the acid-base reactions in aqueous solutions of L-serine were calculated. The effect of the concentration of background electrolyte and temperature on the heats of dissociation of amino acid was considered. The combustion energy of L-serine by bomb calorimetry in the medium of oxygen was determined. The standard combustion and formation enthalpies of crystalline L-serine were calculated. The heats of dissolution of crystalline L-serine in water and solutions of potassium hydroxide at 298.15 K were measured by direct calorimetry. The standard enthalpies of formation of L-serine and products of its dissociation in aqueous solution were calculated.     

Thermodynamic characteristics of protolytic equilibria of L-valyl-L-valine in aqueous solution

Direct calorimetric method was used to determine the heat of reaction of L-valyl-L-valine stepwise dissociation. The standard thermodynamic characteristics of the studied equilibria were established. The results are compared with the corresponding data for related compounds.     

Microscopic acid-base equilibria of arginine

Three and two protonation macroconstants as well as twelve and four microconstants have been determined for arginine and citrulline, respectively. These data include microscopic basicity values of the arginine guanidino group, for which no reliable constants have been reported earlier. The determinations were carried out by a combination of potentiometric and deductive methods. The guanidino basicity proved to be extremely high, and can be characterized by microconstants between 14.7 and 15.0 log k units. The interactivity parameters could also be determined from the microscopic protonation constants. These values for the guanidino—amino and the guanidino—carboxylate interactions are 0.2 and 0.1 log k units, respectively. The pH-dependent distribution of all eight arginine microspecies is made visual by a microspeciation diagram. The microspeciation diagram is the most useful tool to predict bidentate binding isomerism in metal complex formation and site-specific bioligand—bioligand associations.     

Ion-pair association and acid-base equilibria in nonaqueous capillary electrophoresis of weakly basic compounds

CE in nonaqueous media was used to study the migrating behavior of two weakly basic s-triazine pesticides and one of their metabolites. The target pesticides were selected to be representative for each of the two main groups: propazine and deethylatrazine for the chloro-s-triazines group and ametryn for the methylthio-s-triazines group. To elucidate the phenomena involved, systematic studies were carried out in the different organic media studied. Absolute mobilities were determined in 50% v/v methanol (MeOH)/ACN by extrapolation of the effective mobilities to zero ionic strength in the presence of different concentrations of perchloric acid. Conductivity measurements performed in MeOH and 50 and 20% v/v methanol/ACN permitted the evaluation of the associations of the components of the BGE. The effects of ionic strength on the actual mobilities of the compounds were determined in the presence of perchloric acid and SDS in different organic media. Two different ion-pair equilibria were considered: one due to the presence of perchlorate anions present in the BGE and second that from the added dodecyl sulfate anions. Bearing in mind that these weakly basic compounds can exhibit ion-pair and acid-base equilibria, the acid-base and ion-pair parasite reaction coefficients were determined. Finally, the effects of ionic strength, ion-pair interactions and acid-base properties on the effective electrophoretic mobilities of the analytes are discussed.     

Influence of bond flexibility on the vapor-liquid phase equilibria of water

The authors performed Gibbs ensemble simulations on the vapor-liquid equilibrium of water to investigate the influence of incorporating intramolecular degrees of freedom in the simple point charge (SPC) water model. Results for vapor pressures, saturation densities, heats of vaporization, and the critical point for two different flexible models are compared with data for the corresponding rigid SPC and SPC/E models. They found that the introduction of internal vibrations, and also their parametrization, has an observable effect on the prediction of the vapor-liquid coexistence curve. The flexible SPC/Fw model, although optimized to describe bulk diffusion and dielectric constants at ambient conditions, gives the best prediction of saturation densities and the critical point of the examined models.     

Thermodynamic characteristics of protolytic equilibria of β-alanylglycine in aqueous solution

Previously unknown thermal effects of stepwise dissociation of β-alanylglycine in aqueous solution were determined by direct calorimetry. The standard thermodynamic characteristics of these equilibria were calculated. The results obtained were compared with the corresponding data for related compounds.     

Prediction of the chromatographic behaviour for a series of diuretic compounds

Summary The proportion of organic modifier and the pH of the acetonitrile-water mixtures used as mobile phases were optimized in order to separate a group of diuretic compounds covering a wide range of physyco-chemical properties. The Linear Solvation Energy Relationship (LSER) formalism based either on the multiparameter π^*, β and α scales or the single solvent polarity parameterE _T ^N , have been used to predict their chromatographic behaviour as a function of the percentage of acetonitrile in the eluent. Moreover, correlation established between retention and pH of the aqueous-organic mobile phases have been used to predict the chromatographic behaviour of the diuretic compounds studied as a function of the eluent pH. Linear correlation between a function of the eluent pH. Linear correlation between the chromatographic retention and theE _T ^N polarity parameter of mobile phases containing different percentages of organic modifier has been obtained Based on the knowledge of the acid-base dissociation constant the relation between retention and mobile phase pH has also been linearized. These relationship allowed an important reduction of the experimental retention data needed for developing a given separation and a great improvement in chromatographic optimization schemes.     

Influence of the solvent on the gas chromatographic behaviour of urea herbicides

Summary Degradation products of chlorsulfuron, chlortoluron, diuron, fluometuron, isoproturon, linuron, metabenzthiazuron, metobromuron, and monuron formed in the gas chromatographic injector have been used for identification of the respective herbicides. Mass spectra of the derived compounds were obtained with a quadrupole mass spectrometric detector working in scan mode (20–450 amu). The compounds generated often depended on the solvent used for phenylurea herbicide injection (ethanol, methanol, dichloromethane, and acetonitrile). When methanol and ethanol were used as solvents the major products formed from phenylureas were carbamic acid esters. When acetonitrile or dichloromethane were used the main derivatives were phenylisocyanates. Chlorsulfuron and metabenzthiazuron, however, generated a triazine plus a phenylsulfonamide and a benzothiazolamine, respectively, irrespective of the solvent used. Linuron and diuron behaved similarly and gave degradation products with the same mass spectra. The thermal reactions occurred instantaneously in the injector block and were promoted by the high temperature selected (300°C). Detemination of the compounds derived from urea herbicides, by use of a 30 m BP10 column and a selected ion registering (SIR) program based on two or three ions, can be used for sensitive detection of the presence of urea herbicides in environmental extracts. With standards in methanol instrument detection limits ranged from 0.1 pg for chlorsulfuron (detected as 2-chlorobenzensulfonamide) to 1 pg for monuron and metobromuron (both detected as their carbamic acid methyl esters).RSD were below 9% at the 5 ng L⁻¹ level. The response was linearly dependent on quantily (r>0.9986) in the 5 ng L⁻¹ to 25 μg L⁻¹ range. Unequivocal identification of some phenylurea herbicides was not always possible because some herbicides with similar structures, for example diuron and linuron, gave the same derivative.     

A potentiometric and spectrophotometric study on acid-base equilibria in ethanol-aqueous solution of acetazolamide and related compounds

Acid-base equilibria in ethanol-aqueous solution of 5-acetamido-1,3,4-thiadiazole-2-sulfonamide (acetazolamide, H(2)acm), 5-tertbutyloxycarbonylamido-1,3,4-thiadiazole-2-sulfonamide (B-H(2)ats), 5-amino-1,3,4-thiadiazole-2-sulfonamide (Hats) and 5-amino-1,3,4-thiadiazole-2-thiol (Hatm) at 25 degrees C, 0.15 mol dm(-3) ionic strength (NaNO(3)), have been investigated by potentiometry and UV spectrophotometry. The ionization constants were calculated with SUPERQUAD program from potentiometric measurements and by a method according to Edsall et al. using the mole fractions determined by complementary tri-stimulus colorimetry (CTS). The constants obtained by potentiometry were: B-H(2)ats, pk(a(1))=7.33(3) and pk(a(2))=9.27(1); Hats, pk(a(1))=2.51(3) and pk(a(2))=8.49(1); Hatm, pk(a(1))=1.92(1) and pk(a(2))=6.81(1); whereas the constants determined by spectrophotometry were: H(2)acm, pk(a(1))=7.78(1) and pk(a(2))=9.57(2); B-H(2)ats, pk(a(1))=7.71(2) and pk(a(2))=9.61(2); Hats, pk(a(1))=2.19(3) and pk(a(2))=8.61(2); Hatm, pk(a(2))=6.90(2). Theoretical calculations using MO semiempirical and ab-initio RHF/6-31G* computations for the compounds were also performed. It was possible to clarify the preferred deprotonation mechanism of acetazolamide and B-H(2)ats in which the first deprotonation takes place at the carbonamido group.