Spectroscopic properties of various quinolone antibiotics in aqueous-organic solvent mixtures

The spectroscopic properties of enoxacin (ENO), oxolinic acid (OXO) and nalidixic acid (NAL) were studied in various H2O-CH3OH and H2O-CH3CN mixed solvents because these solvents were thought to behave as a biological mimetic system. ENO has piperazinyl group, but OXO and NAL do not have this substituent. The fluorescence emission spectra of ENO were very sensitive to the composition of the solvents. In the Lippert-Mataga analysis of the steady-state fluorescence data, clear reverse solvatochromism was exhibited for ENO in both mixed solvents. This observation can be explained using the excited state twisted intramolecular charge transfer (TICT) from the nitrogen of the piperazinyl group to the keto oxygen. Theoretical calculations further support this observation. The nonradiative and radiative rate constants of these molecules were analyzed as a function of dipolarity-polarizability (pi*) and hydrogen bond donor acidity (alpha) of the mixed solvents. These results for ENO were consistent with the suggested mechanism of the TICT very well. The influence of bulk dielectric effect was more significant relative to the specific hydrogen bonding interactions. The emission spectra of OXO and NAL do not exhibit any characteristic responses to the properties of the solvent.     

The acidity constants of some pyrimidine bases in various water-organic solvent media

The acid ionization constants of some pyrimidine bases of nucleic acids were determined pH-metrically at 25 degrees C and at the constant ionic strength I = 0.10 mol l(-1) (KNO3) in pure water as well as in aqueous media containing variable mole percentages (5-30%) of organic solvents. The organic solvents used were methanol, ethanol, N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), acetonitrile, acetone and dioxane. The results obtained indicated that the acidity constants are generally decreased as the content of an organic solvent in the medium is increased. It was deduced that the hydrogen bonding interactions and the solvent basicity in addition to the electrostatic effect are the major effects influencing significantly the acid ionization process of pyrimidine bases in the different water-organic solvent media. Some thermodynamic parameters (deltaH, deltaG degrees, deltaS degrees) of the ionization process over the temperature range 5-45 degrees C in pure water were also determined and discussed.     

The acid-base equilibrium constants of 1,2,3,4-tetrahydro-8-oxyquinoline in aqueous solution

The dissociation constants of 1,2,3,4-tetrahydro-8-oxyquinoline in aqueous solution, pK ₁ = 5.51 ± 0.07 and pK ₂ = 9.7 (298 K), were determined by pH-metric titration and computer simulation. The influence of heteroring hydrogenation on the acid-base properties of 8-oxyquinoline is discussed.     

Autoprotolysis constants and acid-base properties of propylene glycol mixtures

The autoprotolysis constants of propylene glycol and its mixtures with water, acetone, propan-2-ol and chloroform have been determined potentiometrically. In the same solvent mixtures the protolysis constants of the phthalic acid-hydrogen phthalate system have been evaluated and indicate that the solvent is more acidic than water, but less acidic than ethylene glycol.     

The Morita-Baylis-Hillman reaction in aqueous-organic solvent system

We have found that water and organic solvents mixed at different proportions can give good to excellent yields and short reaction times for the Morita-Baylis-Hillman reaction. The present Letter details our findings in the Morita-Baylis-Hillman reaction between several aromatic aldehydes and methyl acrylate or acrylonitrile. The selection of the catalyst was also evaluated and DABCO affored the best results when compared to DBU, DMAP, HMT, Imidazole and Triethylamine.     

Further calculations on solubility of dipyrone in some binary solvent mixtures at various temperatures

The recently reported solubility data of dipyrone in binary solvent mixtures of {ethanol + water}, {methanol + ethanol} and {methanol + 1-propanol} at various temperatures have been used to report further numerical results based on the Jouyban–Acree model.     

Picosecond dynamics of proton transfer of a 7-hydroxyflavylium salt in aqueous-organic solvent mixtures

The intermediacy of the geminate base-proton pair (A*···H(+)) in excited-state proton-transfer (ESPT) reactions (two-step mechanism) has been investigated employing the synthetic flavylium salt 7-hydroxy-4-methyl-flavylium chloride (HMF). In aqueous solution, the ESPT mechanism involves solely the excited acid AH(+)* and base A* forms of HMF as indicated by the fluorescence spectra and double-exponential fluorescence decays (two species, two decay times). However, upon addition of either 1,4-dioxane or 1,2-propylene glycol, the decays become triple-exponential with a term consistent with the presence of the geminate base-proton pair A*···H(+). The geminate pair becomes detectable because of the increase in the recombination rate constant, k(rec), of (A*···H(+)) with increasing the mole fraction of added organic cosolvent. Because the two-step ESPT mechanism splits the intrinsic prototropic reaction rates (deprotonation of AH(+)*, k(d), and recombination, k(rec), of A*···H(+)) from the diffusion controlled rates (dissociation, k(diss), and formation, k(diff)[H(+)], of A*···H(+)), the experimental detection of the geminate pair provides a wealth of information on the proton-transfer reaction (k(d) and k(rec)) as well as on proton diffusion/migration (k(diss) and k(diff)).     

Polarography of some anions in aqueous organic solvent mixtures

The present investigation deals with the polarographic behaviour of iodate ions in various aqueous mixtures of methanol, ethanol, n-propanol, iso-propanol, 1,2-propanediol, 1,3-propanediol, and glycerol at 30°C. 0.1M-NaOH was used as a base electrolyte. The half-wave potential ranges from 1.24 to 1.46 and the diffusion current decreases with increasing percentage of the solvent. The plots ofi _d vs. andi _d vs.c were linear and pass through origin for all the solvent mixtures, showing that the limiting current is diffusion controlled. The plots of logi/(i_d–i) vs.E _d.e. were linear for all the aqueous organic solvent mixtures, but the value of slope suggested that the reductions are irreversible. The electrode kinetics was studied byKoutecky's method for irreversible electrode process.     

The role of solvent in some oxygen-transfer reactions of peroxy compounds

Solvent–solute interactions in the peroxyacid oxidations are believed to be specific rather than electrostatic in nature. The kinetic solvent effects reported for the oxidations of organic sulfides, olefins, acetylenes, nitrosobenzenes, thioketones, and aryl sulfines reveal that in each case the rates are fast in nonbasic solvents (e.g., benzene, nitrobenzene, and halogenated hydrocarbons) relative to those in basic solvents such as DMF, dioxane, and alcohols. The rates in CF₃CH₂OH and aqueous or partially aqueous media are again higher than those in the basic solvents. This remakably similar pattern of sensitivity of rates to changes in the solvent nature appears to be characteristic of these oxidations as demonstrated by the existence of linear free-energy relationship. The behavior is best understood in terms of cyclic transition states for these oxidations in which charge separation is avoided by intra- or intermolecular hydrogen bonding depending on the nature of the solvent. Solvent effects on sulfoxide oxidation and on oxidations by hydrogen peroxide and t-butylhydroperoxide are also briefly discussed.     

13C NMR measurements of acid-base equilibrium constants in alkaline aqueous solutions of D-glucose anomers

The concentration dissociation constants pK _c of the α and β D-glucose anomers were measured at pH 12–14 by the ¹³C NMR method. The pK _c values for L ? H_?1L + H equilibria at 25°C and an ionic strength of 1.0 mol/l (NaCl) were pK _c ^α = 12.31 ± 0.03 and pK _c ^β = 12.03 ± 0.01. Dissociation was found to shift all the spectrum lines downfield. The most probable hydroxyl group responsible for the dissociation of β-D-glucose was determined.     

Effect of solvent on spin-spin coupling constants in pmr spectra of some organophosphorus compounds containing p=o groups

Theoretical and Experimental Chemistry -     

TLC separations of some metal ions with aqueous and aqueous-organic solvent systems containing formic acid

Summary The chromatographic behaviour of some metal ions on silica gel thin layers has been studied in aqueous formic acid and mixed aqueous-organic solvent systems containing formic acid. Many analytically important separations of cations have been achieved in varying concentrations of formic acid solution in DMSO, acetone, and methyl ethyl ketone. Semiquantitative determination of nine cations has also been attempted.The most suitable concentration range of acid for reliable and reproducible separations was found to be 0.1M to 1.0M.     

Development of quinhydrone electrode in chloroform. Application to the determination of equilibrium constants of acid-base reactions and potentiometric titration of some amine drugs

The optimum conditions for the functioning of quinhydrone electrode as acid indicator in chloroform was determined by voltammetric and potentiometric studies. This electrode was used for the determination of equilibrium constants of acid-base reactions and the potentiometric determination of the end point of acidimetric titrations of some amine drugs such as phenothiazinic derivatives in pharmaceutical preparations, after extraction into chloroform. The relative standard deviations of 20 mg of aliphatic amines, piperidine and piperazine substituted phenothiazines were 0.5, 0.7 and 1 % respectively.     

Spectrophotometric study of acidity constants of Alizarine Red S in various water-organic solvent

The acidity constants of Alizarine Red S were determined spectrophotometrically at 25 degrees C and at constant ionic strength 0.1 M (KNO3) in pure water as well as in aqueous media containing variable mole percentages (5-70%) of organic solvents. The organic solvents used were methanol, ethanol, N,N-dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), acetonitrile and dioxan. 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 applied for determining of acidity constants and pure spectra of different form of Alizarine Red S. The obtained results indicated that acidity constants decrease as the content of an organic solvent in the medium increases. There are linear relationship between acidity constants and the mole fraction of various organic solvents in the solvent mixtures. Effect of various solvents on acidity constants and pure spectrum of each component are also discussed.