Formation and stabilization of radical pairs in the quinone-pyrocatechol-polymer system

The formation of radical pairs (RP) in solid mixtures of 3,6-di-tert-butyl-o-quinones (Q) and 3,6-di-tert-butylpyrocatechol (QH₂) under the action of shock-wave (SW) pulses was studied by ESR and IR spectroscopy. The radical pairs are formed only when a threshold value of SW power is achieved. The generation of RP is preceded by the formation of hydrogen-bonded associates QH₂…Q upon intimate mixing and fine dispergation of the reagents in a SW field. The incorporation of the reagents into a polymeric matrix favors the formation of associates and stabilization of the pairs, which leads to a decrease in the SW power threshold. Deceased. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1464–1467, August, 1997.     

Synthesis of cyclopropyl silyl ethers and their facile ring opening by photoinduced electron transfer as key step in radical/radical cationic cascade reactions

Various ring-fused cyclopropyl silyl ethers with an benzylic, olefinic or acetylenic side chain have been synthesized. Upon oxidative photoinduced electron transfer (PET) the cyclopropane ring opens and forms a reactive β-keto radical, which undergoes intramolecular cyclization. In some cases we observed only formation of ring opened non-cyclized products. With olefinic side chain 5-exo-trig mode of cyclization rather than 6-endo-trig mode of cyclization takes place whereas in case of acetylenic side chain we observed 6-endo cyclization.     

A DFT study of H-isomerisation in alkoxy-, alkylperoxy- and alkyl radicals: Some implications for radical chain reactions in polymer systems

Intramolecular 1-n H-shift (n = 2, 3… 7) reactions in alkoxy, alkyl and peroxy radicals were studied by density functional theory (DFT) at the B3LYP/6-311+G∗∗ level and compared with respective intermolecular H-transfers. It was found that starting from 1 to 3 H-shift the barrier heights stepwise decrease with increasing n reaching minimum for 1-5 and 1-6 H-shifts. This dependence can be ascribed to the decrease of the strain with increasing transition state (TS) ring size, which is minimal in six- and seven-member rings. The barrier heights of H-shifts in alkyl radicals are systematically larger than those in alkoxy radicals: the respective activation energies (E_a) of 1-5 and 1-6 H-shifts are about 59-67 kJ/mol for alkyl radical and 21-34 kJ/mol for alkoxy radicals. Further increase of the TS ring size in 1-7 H-shifts leads to the increase of the barrier to 44 kJ/mol in the hexyloxy radical and 84 kJ/mol for n-heptyl radical. We have also found that intermolecular H-transfer reactions in all three types of free radicals have smaller barriers than respective intramolecular 1-5 or 1-6 H-shifts by 4-25 kJ/mol. The mentioned difference can be explained in terms of enhanced nonbonding repulsion interaction in the cyclic TS structures compared to respective intermolecular TS. B3LYP/6-311+G∗∗ geometric parameters and imaginary frequencies for 1-n H-shifts TS are consistent with respective calculated barrier heights. Reactivity of some other radicals compared to alkoxy, peroxy and alkyl radicals as well as other factors influencing their reactivity (π-conjugation, steric effect and ring strain in cyclic TS, etc.) are also briefly discussed in relation to free radical reactions in polymer systems.     

Ion-pairing and reversed phase liquid chromatography for the determination of three different quinolones: Enrofloxacin,lomefloxacin and ofloxacin

Two simple and sensitive high performance liquid chromatographic (HPLC) methods have been developed for the simultaneous determination of three different quinolones: enrofloxacin, lomefloxacin and ofloxacin in their pure and dosage forms, one with reversed phase HPLC and the other with ion-pair HPLC. In reversed phase HPLC, method (A), the mobile phase consists of 2.18% aqueous solution of KH₂PO₄ with pH adjusted to 2.4 ± 0.2 with acetonitrile (80:20; v/v), the mobile phase pumped at flow rate of 1.2 ml min^?1. A Neucleosil C₁₈ column (10 μm, 100 Å), 250 mm length × 4.6 mm diameter was utilized as stationary phase. Detection was affected spectrophotometrically at 294 nm. While in ion-pair HPLC, method (B), the mobile phase was aqueous solution of 0.65% sodium perchlorate and 0.31% ammonium acetate adjusted to pH 2.2 ± 0.2 with orthophosphoric acid: acetonitrile (81:19; v/v), the mobile phase pumped at flow rate of 1.5 ml min^?1. A μ bondapack C₁₈ column (10 μm, 100 Å), 250 mm length × 4.6 mm diameter was utilized as stationary phase. Detection was affected spectrophotometrically at 294 nm. Linearity ranges for enrofloxacin, lomefloxacin and ofloxacin were 4.0–108, 7.0–112 and 8.0–113 μg ml^?1, respectively using method A and 8.0–112, 7.0–112 and 5.0–105 μg ml^?1, respectively applying method B. Minimum detection limits obtained were 0.013, 0.023 and 0.035 μg ml^?1 for enrofloxacin, lomefloxacin and ofloxacin, respectively using method A, and 0.028, 0.023 and 0.011 μg ml^?1 using method B. The proposed methods were further applied to the analysis of enrofloxacin in injection and tablets containing the ofloxacin and lomefloxacin drugs, and the results were satisfied.     

Additivity model calculations of UHF spin densities and charge densities in methyl-substituted radical cations

By use of a heteroatom model for the methyl group and an additivity model for spin densities, the unrestricted Hartree-Fock after annihilation (UHFAA) results for the radical cations of naphthalene, 1-methylnaphthalene and 2-methylnaphthalene are used to predict the spin densities in the -electron approximation in the corresponding cations of di-, tri- and tetramethylnaphthalenes. The additivity model approach is shown to be equally successful for charge densities.     

Quantitative determination of iron and copper in cotton material by TLC

Summary A TLC method for the separation of quantitative determination of copper and iron in cotton material is described. The optimal solvent system is 9:2:1 (v/v), ethanol-nitric acid-hydrochloric acid and the locating reagent is Na-diethyldithiocarbamate. Regression analysis shows that the most precise results can be obtained for the concentration range of 0.0400–0.0800 mg/ml of iron and copper.     

New azo-peroxidic initiators in the radical polymerization of styrene and methyl methacrylate

Two new azo-perester compounds, di-tert-butyl-6,6′-azobis-(6-cyanoperoxyheptanoate) (6,6-di-tBu) and di-tert-amyl-6,6′-azobis-(6-cyanoperoxyheptanoate) (6,6-di-tAm), synthesized on the basis of 6,6′-azobis-(6-cyanoheptanoic acid) (ACHpA), were investigated for their use in the radical polymerization of styrene (S) and methyl methacrylate (MMA). Their characteristics are given, including chemical (IR spectra), thermal (DSC) and kinetic, i.e., thermal decomposition studied by volumetric and gas chromatographic methods. The rate constants and activation energies of the decomposition of both the azo and perester bonds were determined. The new azo-peresters were utilized to initiate the radical solution polymerizations of S and MMA at 60 °C. The kinetic parameters of the processes, i.e., polymerization rate and overall rate constant, were determined. Subsequently, the polymerization products were characterized by IR and DSC. It was found that the perester groups were present in the obtained polymers, and hence, the polymers are “active” for further polymerization.     

The use of microemulsion for determination of sodium and potassium in biodiesel by flame atomic absorption spectrometry

A new method for F AAS determination of sodium and potassium in biodiesel using water-in-oil microemulsion as sample preparation is proposed. The method was investigated for biodiesel produced from different sources, as soybean, castor and sunflower oil and animal fat and was also applied for vegetable oils. The optimized condition for microemulsion formation was 57.6% (w/w) of n-pentanol, 20% (w/w) of biodiesel or vegetable oil, 14.4% (w/w) of Triton X-100 and 8% (w/w) of water (aqueous standard of KCl or NaCl in/or diluted HNO₃). The optimized instrumental parameters were: aspiration rate of 2 mL min⁻¹ and the flame composition of 0.131 of C₂H₂/air ratio. For comparison purpose, the determination of sodium and potassium were also carried out according to European norms (EN 14108 and EN 14109, respectively). These norms are applied for determination of sodium and potassium in fatty acid methylic ester samples and consist in the sample dilution using organic solvent and determination by F AAS. The stability of microemulsified aqueous standards and samples was investigated and it was found to be stable for at least 3 days while the organic standard diluted with xylene showed a decrease around of 15% in the analytical signal in 1 h. The limits of detection were 0.1 μg g⁻¹ and 0.06 μg g⁻¹ and the obtained characteristic concentrations were 25 μg L⁻¹ and 28 μg L⁻¹ for sodium and potassium, respectively. The proposed method presented two times better limits of detection and better precision (0.4–1.0%) when compared with the dilution technique (1.5–4.5%). The accuracy of the method was evaluated through recovery tests and comparison with the results obtained by dilution technique. The recoveries ranged from 95% to 115% for biodiesel and 90% to 115% for vegetable oil samples. Comparison between the results obtained for biodiesel by both methods showed no significant differences at the 95% confidence level according to a Student's t-test. This study shows that the proposed method based on microemulsion as sample preparation can be applied as an efficient alternative for sodium and potassium determination in biodiesel samples.     

Simultaneous determination of selenium and lead in whole blood samples by differential pulse polarography

The polarographic reduction of lead in the presence of selenite gives rise to an additional peak corresponding to the reduction of lead (Pb) on adsorbed selenium (Se) on mercury at −0.33 V. The selenium and lead content can be determined using this peak by the addition of a known amount of one of these ions first and then the second ion. The linear domain range of lead is 5.0×10⁻⁷–2.0×10⁻⁵ M and for selenium 5.0×10⁻⁷–1.0×10⁻⁵ M. Using this method 4.90×10⁻⁷ M Se(IV) and 1.47×10⁻⁶ M Pb(II) in a synthetic sample could be determined with a relative error of +2.0% and 1.8%, respectively (n=4). A recovery test after acid digestion for a synthetic sample was 97% for selenium and 96.5% for lead. The method was applied to 1 ml of digested blood, and 328±23 μg l⁻¹ Se(IV) and 850±62 μg l⁻¹ Pb(II) could be determined with a 90% (n=5) confidence interval.     

Triplet excited states and radical intermediates formed in electron pulse radiolysis of amino-substituted fluorenones

Electron pulse radiolysis of four differently substituted amino derivatives of fluorenone, namely, 1-amino-, 2-amino- 3-amino-, and 4-aminofluorenone, has been carried out to study the effect of structure on the spectroscopic and kinetic characteristics of the triplet excited states as well as the transient free radical intermediates formed under reducing and oxidizing conditions. The triplet states of these compounds have been generated in benzene by pulse radiolysis and in other solvents by flash photolysis technique and their spectral and kinetic properties have been investigated. Hydrated electron (e_aq⁻) has been found to react with these fluorenone derivatives to form the anion radical species with a diffusion-controlled rate constant. The spectral and kinetic properties of the transient ketyl and anion radicals have been studied by generating them in aqueous solutions of suitable pH. The pK_a values of ketylanion radical equilibria are in the range of 6.8–7.7 for these derivatives. The oxidized species have been generated by reaction with the azide radical. Hydrogen atom adducts as well as the cation radicals of these derivatives have also been generated by pulse radiolysis and characterized.     

Analytical applications of a differential technique in laser-induced fluorimetry: accurate and precise determination of uranium in concentrates and for designing microchemielectronic devices for on-line determination in processing industries

A novel instrumental technique for the direct, fast, accurate, and precise determination of uranium in concentrates and other U-rich materials (as well as to mineralized rocks) is presented. The proposed technique is an absolute methodology, based on the comparison of the fluorescence of the accurately known standard with a sample of similar but unknown concentration in the low operational range of the instrument (on same sample-dilution basis), by the use of H₃PO₄-NH₄H₂PO₄ as a fluorescence-enhancing reagent. The relative standard deviation of the proposed technique was 0.5-0.9% (n=9) at 18.1, 36.2, 61.2, and 99.6% U₃O₈. The proposed technique is suitable for the determination of uranium in samples arising from exploration projects, ores from mining operations, mill process samples, uranium ore concentrates leading to fuel fabrication as well as samples from environmental monitoring containing up to 100% uranium. The results are in good agreement with those obtained by titrimetric, gravimetric, and TBP extraction-H₂O₂ spectrophotometric methods. The precision of the technique is within the acceptable ‘pure geochemistry’ type of analysis (R.S.D. ∼ 1.0%) and is comparable even those obtained with titrimetric and gravimetric assay. The proposed differential technique coupled with flow injection may open up new advancement in instrumentation leading to design and development of microchemielectronic devices for direct on-line determination, more compatible with the tools of computer age as also to help in handling of radioactive solutions in chemical laboratories in uranium processing industries.     

The 5-exo-trig radical cyclization reaction under reductive and oxidative conditions in the synthesis of optically pure GABA derivatives

Free radical precursors 4a and 4b were synthesized and treated under reductive or oxidative conditions to obtain the corresponding optically pure pyrrolidinones 5-8, which were subsequently transformed into corresponding optically pure GABA derivatives 9-11. When reductive radical conditions (4a→7 and 8) were used, a Ph_1-5 migration product 14 was obtained; presumably depending upon the specific conformation of the rotamer precursor and also 14 was found to be a kinetic product.     

Optimization of sample preparation using statistical methods: spectrophotometric determination of Fe and Co in pharmaceutical samples

This work presents alternatives for Fe and Co determination in pharmaceutical samples using flow analysis. The first procedure describes Fe extraction in mineral/vitamin complexes. The best conditions were reached when HNO₃ concentration and volume, sample mass and shaking time were 1.0 mol l⁻¹, 5 ml, 25 mg and 10 min. Three mineral/vitamin complexes of known concentrations (ranging from 12 to 32 g kg⁻¹) were analyzed (10 authentic replicates for each) and recoveries of around 100% were obtained when compared with a well-established mineralization procedure employing concentrated HNO₃ and H₂O₂ (30% w/v). The second work part shows the employment of Tiron and H₂O₂ reaction for Co determination in a drug for inappetence. The results (352±18.7 mg kg⁻¹) were compared with those using Electrothermal Atomic Absorption Spectrometry—ETAAS (346±15.7 mg kg⁻¹). The proposed method showed detection and quantification limits of 0.20 and 0.70 μg l⁻¹, respectively. Both procedures for Fe and Co determination presented time, reagent and effort reduction.     

The first observation in an organic medium and DFT calculation of the TMM radical anion generated via a single electron reduction of a methylenecyclopropane

γ-Irradiation of 2,2-diphenyl-1-methylenecyclopropane (3) in a degassed 2-methyltetrahydrofuran glassy matrix at 77 K gave an intense UV/vis absorption band with λ_ab at 496 nm. This result and calculations based on density functional theory for its radical anion 3⁻ and the corresponding trimethylenemethane radical anion (2⁻) strongly suggest that single electron reduction of 3 followed by ready ring opening affords 2⁻, whose molecular geometry is largely twisted (θ = 45.5°), and the negative charge and spin are localized mainly in the diphenyl methyl and allyl moieties, respectively.     

Direct and simultaneous determination of Cr and Fe in crude oil using high-resolution continuum source graphite furnace atomic absorption spectrometry

A simple, fast and sensitive direct method for the simultaneous determination of Cr and Fe in crude oil samples is proposed using high-resolution continuum source graphite furnace atomic absorption spectrometry. No sample preparation is used except for a 10-minute homogenization in an ultrasonic bath. Aliquots of 0.1–4 mg of the samples are weighed onto solid sampling platforms and analyzed directly using aqueous standards for calibration. The simultaneous determination was possible because there is a secondary Fe line at 358.120 nm in the vicinity of the most sensitive Cr line at 357.868 nm, and both absorption lines were within the wavelength interval covered by the linear charge-coupled device array detector. It has also been of advantage that the sensitivity ratio between the two analytical lines corresponded roughly to the concentration ratio of the two elements found in crude oil, and that both analytes have very similar volatility, so that no compromises had to be made regarding pyrolysis and atomization temperatures. Two oil reference materials have been analyzed and the results were in agreement with the certified or reported values. Characteristic masses of 3.6 pg and 0.5 ng were obtained for Cr and Fe, respectively. The limits of detection (3σ, n = 10) were 1 µg kg^− 1 for Cr and 0.6 mg kg^− 1 for Fe, and the precision, expressed as the relative standard deviation, ranged from 4 to 20%, which is often acceptable for a rapid direct analytical procedure. Five crude oils samples were analyzed.     

Use of fractional factorial design for optimization of digestion procedures followed by multi-element determination of essential and non-essential elements in nuts using ICP-OES technique

Two digestion procedures have been tested on nut samples for application in the determination of essential (Cr, Cu, Fe, Mg, Mn, Zn) and non-essential (Al, Ba, Cd, Pb) elements by inductively coupled plasma-optical emission spectrometry (ICP-OES). These included wet digestions with HNO₃/H₂SO₄ and HNO₃/H₂SO₄/H₂O₂. The later one is recommended for better analytes recoveries (relative error < 11%). Two calibrations (aqueous standard and standard addition) procedures were studied and proved that standard addition was preferable for all analytes. Experimental designs for seven factors (HNO₃, H₂SO₄ and H₂O₂ volumes, digestion time, pre-digestion time, temperature of the hot plate and sample weight) were used for optimization of sample digestion procedures. For this purpose Plackett-Burman fractional factorial design, which involve eight experiments was adopted. The factors HNO₃ and H₂O₂ volume, and the digestion time were found to be the most important parameters. The instrumental conditions were also optimized (using peanut matrix rather than aqueous standard solutions) considering radio-frequency (rf) incident power, nebulizer argon gas flow rate and sample uptake flow rate. The analytical performance, such as limits of detection (LOD < 0.74 μg g⁻¹), precision of the overall procedures (relative standard deviation between 2.0 and 8.2%) and accuracy (relative errors between 0.4 and 11%) were assessed statistically to evaluate the developed analytical procedures. The good agreement between measured and certified values for all analytes (relative error <11%) with respect to IAEA-331 (spinach leaves) and IAEA-359 (cabbage) indicates that the developed analytical method is well suited for further studies on the fate of major elements in nuts and possibly similar matrices.     

Utility of the ion-pair formation for spectrophotometric determination of terfenadine in pure form and in some pharmaceutical formulations

A spectrophotometric procedure for the determination of terfenadine and a number of its pharmaceutical preparations has been developed that offers advantages of simplicity, rapidity, sensitivity and stability indication over the official USP (1995) method. The proposed method is based on the formation of ion-pairs by the reaction of terfenadine with some chromotropic acid mono- and bis-azo dyes. Different variables affecting the ion-pair formation were studied and optimized. At the maximum absorption of 557, 521, 592 and 543 nm, Beer's law is obeyed in the range 0.2–18.6, 0.2–16.4, 0.2–25.0 and 0.2–22.2 g ml^–1 on using reagents I, II, III and IV, respectively. The stoichiometric ratio and stability of each ion-pair were estimated and the mechanism of the reaction is discussed. The molar absorptivity and Sandell sensitivity of the produced ion-pairs were calculated in addition to Ringbom optimum concentration ranges. Statistical treatment of the experimental results indicates that the procedures are precise and accurate. Excipients used as additives in pharmaceutical formulations did not interfere in the proposed procedures. The reliability of the methods was established by parallel determination against the official USP method. The procedures described were successfully applied to the determination of the bulk drug and its pharmaceutical formulations by applying the standard addition technique.     

超高效液相色谱-串联质谱法快速测定镇咳祛痰药中阿片类物质的含量

建立了镇咳祛痰药中吗啡、可待因、海洛因、蒂巴因、罂粟碱、那可汀6种阿片类物质含量的LC-MS/MS快速测定方法.药品经超声浸取,甲醇稀释过滤后经Waters C18柱分离,以乙腈和10 mmol/L乙酸铵(含0.1％甲酸)溶液进行梯度洗脱,采用电喷雾正离子化(ESI+)、多反应监测模式进行测定.6种阿片类物质在相应的线...