L Mol (l + mol)

Distribution by Scientific Domains

Selected Abstracts

Development and validation of fixed-time method for the determination of isoxsuprine hydrochloride in commercial dosages forms

Dr Nafisur Rahman
Abstract The main aim of this work was to develop a kinetic spectrophotometric method for the quantitative analysis of isoxsuprine hydrochloride in commercial tablets. The method is based on the reaction of isoxsuprine hydrochloride (ISx) with hydroxylamine hydrochloride and ammonium cerium (IV) nitrate in sulphuric acid medium at room temperature which resulted in the formation of yellow-coloured product peaking at 380 nm. The reaction is followed spectrophotometrically by measuring the absorbance as a function of time. Fixed time method (,A = A4,A2, where A2 and A4 refer to absorbance measurements taken at 2 and 4 min, respectively) was adopted for constructing the calibration curve which was found to be linear over the concentration range of 30,80 µgmL,1 with molar absorptivity of 5.95 × 103 L mol,1 cm,1. The method has been applied successfully to the determination of isoxsuprine hydrochloride in tablets. Statistical comparison (point and interval hypothesis tests) of the results showed that there is no significant difference between the proposed method and reference method. Copyright © 2010 John Wiley & Sons, Ltd. [source]

Anionic ring-opening polymerization of small phosphorus heterocycles and their borane adducts: An ab initio investigation

Michelle L. Coote
The kinetics and thermodynamics of anionic ring-opening reactions of phosphiranes, phosphetanes, and phospholanes and their borane adducts have been studied by high-level ab initio procedures. For the free heterocycles, model propagation reactions involving nucleophilic attack by Me2P, at the ring ,-carbon were found to be feasible for the three- and four-membered rings, but not for the five-membered ring. For the borane adducts, nucleophilic attack by Me2(BH3)P, was only facile for the three-membered ring, despite an increased thermodynamic tendency toward ring opening for the borane adducts of both the three- and four-membered rings. The formation constants of the borane adducts of methylphosphirane and methylphosphetane were K = 2.6 × 1013 L mol,1 and K = 1.2 × 1020 L mol,1, respectively. A Marcus analysis of the ring-opening reactions of methylphosphirane, methylphosphetane, and their borane adducts showed that the release of ring strain and an "additional factor" contribute to rate enhancement compared with their strain-free analogues. The additional factor was larger for the three-membered rings than for the four-membered rings and was larger in the free heterocycles than in their borane adducts. The additional factor is complex in origin and appears to reflect an increase in the separation between the bonding and antibonding orbitals of the breaking bond on going from the three-membered rings to the four-membered rings, and on going from the free heterocycles to the borane adducts. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:118,128, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20323 [source]

Kinetics and mechanism of myristic acid and isopropyl alcohol esterification reaction with homogeneous and heterogeneous catalysts

Tuncer Yalçinyuva
The reaction of myristic acid (MA) and isopropyl alcohol (IPA) was carried out by using both homogeneous and heterogeneous catalysts. For a homogeneously catalyzed system, the experimental data have been interpreted with a second order, using the power-law kinetic model, and a good agreement between the experimental data and the model has been obtained. In this approach, it was assumed that a protonated carboxylic acid is a possible reaction intermediate. After a mathematical model was proposed, reaction rate constants were computed by the Polymath* program. For a heterogeneously catalyzed system, interestingly, no pore diffusion limitation was detected. The influences of initial molar ratios, catalyst loading and type, temperature, and water amount in the feed have been examined, as well as the effects of catalyst size for heterogeneous catalyst systems. Among used catalysts, p -toluene sulfonic acid (p -TSA) gave highest reaction rates. Kinetic parameters such as activation energy and frequency factor were determined from model fitting. Experimental K values were found to be 0.54 and 1.49 at 60°C and 80°C, respectively. Furthermore, activation energy and frequency factor at forward were calculated as 54.2 kJ mol,1 and 1828 L mol,1 s,1, respectively. © 2008 Wiley Periodicals, Inc. 40: 136,144, 2008 [source]

The kinetics of complex formation between Ti(IV) and hydrogen peroxide

Daniel W. O'Sullivan
The kinetics of the formation of the titanium-peroxide [TiO2+2] complex from the reaction of Ti(IV)OSO4 with hydrogen peroxide and the hydrolysis of hydroxymethyl hydroperoxide (HMHP) were examined to determine whether Ti(IV)OSO4 could be used to distinguish between hydrogen peroxide and HMHP in mixed solutions. Stopped-flow analysis coupled to UV-vis spectroscopy was used to examine the reaction kinetics at various temperatures. The molar absorptivity (,) of the [TiO2+2] complex was found to be 679.5 ± 20.8 L mol,1 cm,1 at 405 nm. The reaction between hydrogen peroxide and Ti(IV)OSO4 was first order with respect to both Ti(IV)OSO4 and H2O2 with a rate constant of 5.70 ± 0.18 × 104 M,1 s,1 at 25°C, and an activation energy, Ea = 40.5 ± 1.9 kJ mol,1. The rate constant for the hydrolysis of HMHP was 4.3 × 10,3 s,1 at pH 8.5. Since the rate of complex formation between Ti(IV)OSO4 and hydrogen peroxide is much faster than the rate of hydrolysis of HMHP, the Ti(IV)OSO4 reaction coupled to time-dependent UV-vis spectroscopic measurements can be used to distinguish between hydrogen peroxide and HMHP in solution. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 457,461, 2007 [source]

Radical polymerization of n -butyl methacrylate initiated by stibonium ylide

Sumita Srivastava
Abstract 1,2,3,4-Tetraphenylcyclopentadiene triphenyl stibonium ylide initiated radical polymerization of n -butyl methacrylate (n -BMA) in dioxane at (60 ± 0.2)°C for 90 min under nitrogen atmosphere has been carried out. The system follows nonideal kinetics, i.e., Rp , [ylide]0.2 [n -BMA]1.8. The value of k/kt and overall energy of activation have been computed as 0.133 × 10,2 L mol,1 s,1, 33 kJ/mol, respectively. The FTIR spectrum shows a band at 1745 cm,1 due to acrylate group of n -BMA. The 1H NMR spectrum shows a peak of two magnetically equivalent protons of methylene group at 2.1 , ppm. The DSC curve shows glass transition temperature (Tg) as 41°C. The presence of six hyperfine lines in ESR spectrum indicates that the system follows free radical polymerization and the initiation is brought about by phenyl radical. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2457,2463, 2007 [source]

Kinetics of reactions between chlorine or bromine and the herbicides diuron and isoproturon

Juan L Acero
Abstract The chemical oxidation of two herbicide derivatives of the phenylurea group,diuron and isoproturon,has been carried out by means of chlorine, in the absence and in the presence of bromide ion. Apparent second-order rate constants for the reactions between chlorine and the herbicides were determined to be below 0.45 L mol,1 s,1. Hypobromous acid reacts faster with the investigated herbicides, especially with isoproturon (kapp = 24.8 L mol,1 s,1 at pH 7). While pH exerts a negative effect on the bromination rate, the maximum chlorination rate was found to be at circumneutral pH. In a second stage, the oxidation of each compound was conducted in different natural waters, in order to simulate the processes which take place in water purification plants. Again, chlorine was used as an oxidant, and bromide ion was added in some experiments with the aim of producing the more reactive HOBr oxidant. The herbicide oxidation rate was inversely proportional to the organic matter content of the natural water. However, the formation of trihalomethanes (THMs) was directly proportional to the organic matter content and constitutes a limitation for the application of chlorine during drinking water treatment. Finally, the evolution of herbicide concentration was modeled and predicted by applying a kinetics approach based on the rate constants for the reactions between the herbicides and the active oxidants. Copyright © 2007 Society of Chemical Industry [source]

Extraction of water-soluble organic matter from mineral horizons of forest soils

Thilo Rennert
Abstract Dissolved organic matter (DOM) is involved in many important biogeochemical processes in soil. As its collection is laborious, very often water-soluble organic matter (WSOM) obtained by extracting organic or mineral soil horizons with a dilute salt solution has been used as a substitute of DOM. We extracted WSOM (measured as water-soluble organic C, WSOC) from seven mineral horizons of three forest soils from North-Rhine Westphalia, Germany, with demineralized H2O, 0.01 M CaCl2, and 0.5 M K2SO4. We investigated the quantitative and qualitative effects of the extractants on WSOM and compared it with DOM collected with ceramic suction cups from the same horizons. The amounts of WSOC extracted differed significantly between both the extractants and the horizons. With two exceptions, K2SO4 extracted the largest amounts of WSOC (up to 126 mg C,kg,1) followed by H2O followed by CaCl2. The H2O extracts revealed by far the highest molar UV absorptivities at 254 nm (up to 5834 L mol,1,cm,1) compared to the salt solutions which is attributed to solubilization of highly aromatic compounds. The amounts of WSOC extracted did not depend on the amounts of Fe and Al oxides as well as on soil organic C and pH. Water-soluble organic matter extracted by K2SO4 bore the largest similarity to DOM due to relatively analogue molar absorptivities. Therefore, we recommend to use this extractant when trying to obtain a substitute for DOM, but as WSOM extraction is a rate-limited process, the suitability of extraction procedures to obtain a surrogate of DOM remains ambiguous. [source]

Incubation period in the 2,2,4,4-tetramethyl-1-piperidinyloxy-mediated thermal autopolymerization of styrene: Kinetics and simulations

Enrique Saldívar-Guerra
Abstract Mechanisms and simulations of the induction period and the initial polymerization stages in the nitroxide-mediated autopolymerization of styrene are discussed. At 120,125 °C and moderate 2,2,4,4-tetramethyl-1-piperidinyloxy (TEMPO) concentrations (0.02,0.08 M), the main source of radicals is the hydrogen abstraction of the Mayo dimer by TEMPO [with the kinetic constant of hydrogen abstraction (kh)]. At higher TEMPO concentrations ([N,] > 0.1 M), this reaction is still dominant, but radical generation by the direct attack against styrene by TEMPO, with kinetic constant of addition kad, also becomes relevant. From previous experimental data and simulations, initial estimates of kh , 1 and kad , 6 × 10,7 L mol,1 s,1 are obtained at 125 °C. From the induction period to the polymerization regime, there is an abrupt change in the dominant mechanism generating radicals because of the sudden decrease in the nitroxide radicals. Under induction-period conditions, the simulations confirm the validity of the quasi-steady-state assumption (QSSA) for the Mayo dimer in this regime; however, after the induction period, the QSSA for the dimer is not valid, and this brings into question the scientific basis of the well-known expression kth[M]3 (where [M] is the monomer concentration and kth is the kinetic constant of autoinitiation) for the autoinitiation rate in styrene polymerization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6962-6979, 2006 [source]

Synthesis and properties of the polythiourethanes obtained by the cationic ring-opening polymerization of cyclic thiourethanes

Daisuke Nagai
Abstract The cationic ring-opening polymerization of a five-membered thiourethane [3-benzyl-1,3-oxazolidine-2-thione (BOT)] with boron trifluoride etherate afforded the corresponding polythiourethane with a narrow molecular weight distribution in an excellent yield. The molecular weight of the polymers could be controlled by the feed ratio of the monomer to the initiator. A kinetic study of the polymerization revealed that the polymerization rate of BOT (1.3 × 10,2 L mol,1 min,1) was two times larger than that of the six-membered thiourethane [3-benzyltetrahydro-1,3-oxazolidine-2-thione (BTOT); 6.8 × 10,3 L mol,1 min,1], and the monomer conversion obeyed the first-order kinetic equation. These observations, along with the successful results in the two-stage polymerization, supported the idea that this polymerization proceeded in a controlled manner. Block copolymerizations of BOT with BTOT were also carried out to afford the corresponding di- and triblock copolymers with narrow molecular weight distributions. The order of the 5% weight loss temperatures was as follows: poly(3-benzyltetrahydro-1,3-oxazolidine-2-thione) [poly(BTOT)] > poly(BTOT54 - b -BOT46) > poly(3-benzyl-1,3-oxazolidine-2-thione) [poly(BOT)]. This indicated that an increase in the BTOT unit content raised the decomposition temperature. The order of the refractive indices was poly(BOT) > poly(BTOT54 - b -BOT46) > poly(BTOT54 - b -BOT46 - b -BTOT50) > poly(BTOT); this was in accord with the order of the sulfur content in the polymer chain. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4795,4803, 2006 [source]

Scirpusin A, a hydroxystilbene dimer from Xinjiang wine grape, acts as an effective singlet oxygen quencher and DNA damage protector

Qingjun Kong
Abstract BACKGROUND: Grapes and red wines are rich sources of phenolic compounds such as anthocyanins, catechins, flavonols and stilbenes, most of which are potent antioxidants showing cardioprotective properties. We first isolated scirpusin A, a hydroxystilbene dimer, from a wine grape of Xinjiang, and studied its antioxidant activity. RESULTS: Reactive oxygen species scavenging effects and the protection against reactive singlet oxygen-induced DNA damage of scirpusin A have been investigated in our experiments. The concentration of scirpusin A required to inhibit 50% of 1O2 generation was 17 µmol L,1, while addition of scirpusin A at 140 µmol L,1 caused complete inhibition. Further kinetic study revealed that the reaction of Scirpusin A with singlet oxygen has an extremely high rate constant (ka = 4.68 × 109 L mol,1 s,1). Scirpusin A (140 µmol L,1) exhibited significant inhibition effects on pBR322 DNA breakage. However, scavenging effects of scirpusin A on superoxide anion O2,, and hydroxyl radical ·OH were not potent as the inhibitor rates at a concentration of 1400 µmol L,1 were 28.83% and 19.5%, respectively. CONCLUSION: The present study shows that scirpusin A is a selective quencher of singlet oxygen and a protector against reactive singlet oxygen-induced pBR322 DNA damage at very low concentrations. Copyright © 2010 Society of Chemical Industry [source]

Photosensitizing Properties of Triplet ,-Lapachones in Acetonitrile Solution

José Carlos Netto-Ferreira
The photochemical reactivity of ,-lapachone (1), nor -,-lapachone (2) and 1,2-naphthoquinone (3) towards amino acids and nucleobases or nucleosides has been examined employing the nanosecond laser flash photolysis technique. Excitation (, = 355 nm) of degassed solutions of 1,3, in acetonitrile, resulted in the formation of their corresponding triplet excited states. These transients were efficiently quenched by l -tryptophan, l -tryptophan methyl ester, l -tyrosine, l -tyrosine methyl ester and l -cysteine (kq , 109 L mol,1 s,1). For l -tryptophan, l -tyrosine and their methyl esters new transients were formed in the quenching process, which were assigned to the corresponding radical pair resulting from an initial electron transfer from the amino acids or their esters to the excited quinone, followed by a fast proton transfer. No measurable quenching rate constants could be observed in the presence of thymine and thymidine. On the other hand, efficient rate constants were obtained when 1,3 were quenched by 2,-deoxyguanosine (kq , 109 L mol,1 s,1). The quantum efficiency of singlet oxygen (1O2) formation from 1 to 3 was determined employing time-resolved near-IR emission studies upon laser excitation and showed considerably high values in all cases (,, = 0.6), which are fully in accord with the ,,* character of these triplets in acetonitrile. [source]

Controlling the photoluminescence of water-soluble conjugated poly[2-(3-thienyl)ethyloxy-4-butylsulfonate)] for biosensor applications

Enrique López-Cabarcos
Abstract The photoluminescence of poly[2-(3-thienyl)ethyloxy-4-butylsulfonate)] (PTE-BS) in aqueous solution increases threefold on addition of the surfactant tetrabutylammonium perchlorate (TBA). Furthermore, the luminescence of the PTE-BS/TBA system is reduced by more than five times by the addition of small amounts of the cationic electron acceptor methyl viologen (MV2+). The Stern,Volmer constant KSV = 1.4 × 104 L mol,1 for the quenching of the polymer,surfactant complex by MV2+ is approximately 60 times smaller than the KSV = 8.4 × 105 L mol,1 obtained in water polymer solutions without surfactant. Thus, the luminescence of PTE-BS in aqueous solution can be modulated by complexing the polymer either with a surfactant or with a quencher. In this contribution we show that the surfactant/quencher tuning effect found in polymers of the phenylenevinylene family, such as poly(2,5-methoxy-propyloxysulfonate phenylenevinylene), also appears in polymers of the thiophene family such as PTE-BS. Copyright © 2007 Society of Chemical Industry [source]

Electrochemical Quartz Crystal Impedance and Fluorescence Quenching Studies on the Binding of Carbon Nanotubes (CNTs)-Adsorbed and Solution Rutin with Hemoglobin

Yuhua Su
Electrochemical quartz crystal impedance (QCI) technique was utilized to monitor in situ the adsorption of rutin (RT) onto a carbon nanotubes (CNTs)-modified gold electrode and to study the binding process of solution hemoglobin (Hb) to RT immobilized on the electrode. Time courses of the QCI parameters including crystal resonant frequency were simultaneously obtained during the RT adsorption and Hb-RT binding. In contrast to the negligible RT adsorption at a bare gold electrode, the modification by CNTs notably enhanced the amount of adsorption, and almost all of the adsorbed RT molecules were found to be electroactive. On the basis of the frequency response from the binding of adsorbed RT to solution Hb and the diminished electroactivity of adsorbed RT after the formation of the electrochemically inactive RT-Hb adduct, the average binding molar ratio of adsorbed RT to Hb was estimated to be 23.9:1, and the association constant (Ka) for the binding was estimated to be 2.87 × 106 (frequency) and 3.92 × 106 (charge) L mol,1, respectively. Comparable results were obtained from fluorescence quenching measurements in mixed solutions containing RT of fixed concentration and Hb of varying concentrations, demonstrating that the interfacial RT here behaved equivalently in the RT-Hb binding activity compared to that in solution. This work may have presented a new and general protocol involving CNTs to study many other electroactive natural antioxidants or drugs that are at the interface or in solution, their binding with proteins or other biomolecules, and changes of their antioxidant activity after the binding. [source]

Photoinitiated alternating copolymerization of vinyl ethers with chlorotrifluoroethylene

Manuel Gaboyard
Abstract The photoinitiated copolymerization of chlorotrifluoroethylene (CTFE) with several vinyl ethers [ethyl vinyl ether (EVE), 2-chloroethyl vinyl ether (CEVE), cyclohexyl vinyl ether (CHVE), 4-hydroxybutyl vinyl ether (HBVE)] was studied. CTFE is an acceptor monomer (e,,,1.5) whereas vinyl ethers are donor monomers (e,,,,1.5), and therefore their copolymerization led to alternating copolymers, as indicated by elementary analysis. The equilibrium constant (KF) of the charge-transfer complex formation (CTC) was determined by 19F NMR spectroscopy. Under our experimental conditions, KF was low for CHVE/CTFE and HBVE/CTFE systems, 0.058 and 0.013,l mol,1 respectively. It can be assumed that the copolymerization involves the free monomers rather than propagation via the donor,acceptor complex. The alternating structure arises from the great difference in polarity between the two types of monomers. Several functional copolymers were prepared in good yield and with molecular weight close to 15,000,g,mol,1. © 2002 Society of Chemical Industry [source]

Photooxidation of Pterin in Aqueous Solutions: Biological and Biomedical Implications

Studies of the photochemical reactivity of pterin (=2-aminopteridin-4(3H)-one; PT) in acidic (pH,5.0,6.0) and alkaline (pH,10.2,10.8) aqueous solutions have been performed. The photochemical reactions were followed by UV/VIS spectrophotometry, thin layer chromatography (TLC), high-performance liquid chromatography (HPLC), and an enzymatic method for H2O2 determination. PT is not light-sensitive in the absence of molecular oxygen, but it undergoes photooxidation in the presence of O2, yielding several nonpteridinic products. The quantum yields for PT disappearance were found to be 8.2 (±0.6)×10,4 and 1.2 (±0.2)×10,3 in acidic and alkaline media, respectively. H2O2 was detected and quantified in irradiated solutions of PT; and its importance from a biomedical point of view is discussed. The rate constant of the chemical reaction between singlet oxygen (1O2) and PT was determined to be 2.5 (±0.2)×105,l mol,1 s,1 in alkaline medium, and the role of 1O2 in the photooxidation of pterin was evaluated. [source]