Flow and Sequential Injection Manifolds for the Spectrophotometric Determination of Captopril Based on its Oxidation by Fe(III)

 Two new simple and rapid methods are reported for the accurate and precise spectrophotometric determination of captopril (CPL) using flow (FI) and sequential injection (SI) analysis. The methods are based on the fast oxidation of CPL by Fe(III). The produced Fe(II) reacts with 2,2′-dipyridyl-2-pyridylhydrazone (DPPH) in acidic medium to form a colored complex which is monitored spectrophotometrically at 535 nm. Both methods allow the determination of the analyte up to 1000 mg L⁻¹ at a sampling rate of 120 and 60 injections per hour for FI and SI, respectively. The methods are very precise [s _r=0.8 and 1.2% at 500 mg L⁻¹ CPL (n=12) for FI and SI, respectively] and the 3σ detection limits (c _L=4.0 and 7.0 mg L¹, respectively) are quite satisfactory. Their application to a variety of anti-hypertensive commercial pharmaceutical formulations showed excellent results (relative errors, e _r, < ± 1.6% in all cases compared to an official HPLC method), while common pharmaceutical excipients were found not to interfere. Recovery experiments further verified the accuracy of the developed methods, as the percent recoveries were in the range of 98.1–102.5%. Author for correspondence. E-mail: themelis@chem.auth.gr Received May 9, 2002; accepted January 8, 2003 Published online May 5, 2003     

Luminescence and thermal behavior of copper (I) complexes with heterocyclic thiones. Part II: cationic complexes

Copper (I) halide complexes formulated as [(L)₂Cu(μ₂-L)₂Cu(L)₂]²⁺ 2Χ⁻, (X = Cl, Br and L = pyridine-2-thione (py2SH) or 4,6-dimethylpyrimidine-2-thione (dmpymtH)) were prepared, and their photoluminescence and thermal properties were investigated. The complexes are strongly emissive in the solid state, with the emissions being dominated by large Stokes shifts (>200 nm), which are depending on both the heterocyclic thione and the nature of the halogen. These emissions can be assigned to MLCT with some mixing of the halide-to-ligand (XL) CT characters. Simultaneous TG/DTG–DTA technique was used for two complexes with the dmpymtH ligand to determine their thermal degradation, which was found to be very complicated. In inert atmosphere the residues at 1,000 °C (verified with PXRD) were mainly Cu₂S, while at 1,300 °C a mixture of Cu₂S and Cu. In oxygen atmosphere the residues were CuO.     

Kinetic Modeling of α‐Hydrogen Abstractions from Unsaturated and Saturated Oxygenate Compounds by Carbon‐Centered Radicals

Hydrogen abstractions are important elementary reactions in a variety of reacting media at high temperatures in which oxygenates and hydrocarbon radicals are present. Accurate kinetic data are obtained from CBS‐QB3 ab initio (AI) calculations by using conventional transition‐state theory within the high‐pressure limit, including corrections for hindered rotation and tunneling. From the obtained results, a group‐additive (GA) model is developed that allows the Arrhenius parameters and rate coefficients for abstraction of the α‐hydrogen from a wide range of oxygenate compounds to be predicted at temperatures ranging from 300 to 1500 K. From a training set of 60 hydrogen abstractions from oxygenates by carbon‐centered radicals, 15 GA values (ΔGAV^os) are obtained for both the forward and reverse reactions. Among them, four ΔGAV^os refer to primary contributions, and the remaining 11 ΔGAV^os refer to secondary ones. The accuracy of the model is further improved by introducing seven corrections for cross‐resonance stabilization of the transition state from an additional set of 43 reactions. The determined ΔGAV^os are validated upon a test set of AI data for 17 reactions. The mean absolute deviation of the pre‐exponential factors (log A) and activation energies (E_a) for the forward reaction at 300 K are 0.238 log(m³ mol^?1 s^?1) and 1.5 kJ mol^?1, respectively, whereas the mean factor of deviation <ρ> between the GA‐predicted and the AI‐calculated rate coefficients is 1.6. In comparison with a compilation of 33 experimental rate coefficients, the <ρ> between the GA‐predicted values and these experimental values is only 2.2. Hence, the constructed GA model can be reliably used in the prediction of the kinetics of α‐hydrogen‐abstraction reactions between a broad range of oxygenates and oxygenate radicals.     

Copper(I) halide complexes with 2,2′-bis(diphenylphosphano)-1,1′-binaphthyl (rac-binap) and heterocyclic thiones. Racemic compounds in chiral and achiral crystal space groups

Reactions of copper(I) halides with racemic 2,2′-bis(diphenylphosphano)-1,1′-binaphthyl (rac-binap) in 1:1 molar ratio afforded mononuclear complexes of the type [CuX(rac-binap)] (X = Cl, Br, I) which, on further treatment with 1 equiv. of pyridine-2-thione (py2SH), pyrimidine-2-thione (pymtH) or 4,6-dimethyl-pyrimidine-2-thione (dmpymtH) gave rise to the formation of mixed-ligand complexes of the formula [CuX(rac-binap)(thione)]. The molecular structures of [CuBr(rac-binap)(py2SH)] · 2CH₂Cl₂, [CuBr(rac-binap)(py2SH)] · CH₂Cl₂ and [CuBr(rac-binap)(dmpymtH)] · CH₂Cl₂ have been established by single-crystal X-ray diffraction. Each of the complexes features a distorted tetrahedral copper(I) center with the phosphane acting in a chelating fashion. The complexes are strongly luminescent in the solid state at ambient temperature. Unusually, the [CuBr(rac-binap)(py2SH)] · 2CH₂Cl₂ molecules crystallise in a chiral space group with independent S- and R-enantiomers in the asymmetric unit.     

Mechanism of a chemical classic: quantum chemical investigation of the autocatalyzed reaction of the serendipitous wöhler synthesis of urea

The detailed reaction pathways for the ammonium cyanate transformation into urea (W?hler's reaction) in the gas phase, in solution, and in the solid state have exhaustively been explored by means of first-principles quantum chemical calculations at the B3LYP level of theory using the 6-31G(d,p) basis set. This serendipitous synthesis of urea is predicted to proceed in two steps; the first step involves the decomposition of the ammonium cyanate to ammonia and isocyanic or cyanic acid, and the second one, which is the main reaction step (and probably the rate-determining step), involves the interaction of NH(3) with either isocyanic or cyanic acid. Several alternative pathways were envisaged for the main reaction step of W?hler's reaction in a vacuum involving the formation of "four-center" transition states. Modeling W?hler's reaction in aqueous solution and in the solid state, it was found that the addition of NH(3) to both acids is assisted (autocatalyzed) by the active participation of extra H(2)O and/or NH(3) molecules, through a preassociative, cooperative, and hydrogen-transfer relay mechanism involving the formation of "six-center" or even "eight-center" transition states. The most energetically economic path of the rate-determining step of W?hler's reaction is that of the addition of NH(3) to the C=N double bond of isocyanic acid, directly affording urea. An alternative pathway corresponding to the anti-addition of ammonia to the Ctbd1;N triple bond of cyanic acid, yielding urea's tautomer HN=C(OH)NH(2), seems to be another possibility. In the last case, urea is formed through a prototropic tautomerization of its enolic form. The energies of the reactants, products, and all intermediates along with the barrier heights for each reaction path have been calculated at the B3LYP/6-31G(d,p) level of theory. The geometry optimization and characterization of all of the stationary points found on the potential energy hypersurfaces was performed at the same level of theory.     

Luminescence and thermal behavior by simultaneous TG/DTG–DTA coupled with MS of neutral copper (I) complexes with heterocyclic thiones

Copper(I) halide complexes formulated as [(L)CuX(μ₂-L)₂CuX(L)] (X = Cl, Br and L = pyridine-2-thione (py2SH), or 4,6-dimethylpyrimidine-2-thione (dmpymtH)) were prepared, and their photoluminescence and thermal properties were investigated. The complexes are strongly emissive in the solid state, with the emissions being dominated by large Stokes shifts (>200 nm), which are depending on both the heterocyclic thione and the nature of the halogen. These emissions can be assigned to MLCT with some mixing of the halide-to-ligand (XL) CT characters. Simultaneous TG/DTG–DTA technique, coupled with MS for the analysis of the gaseous decomposition products, was used for two complexes with the dmpymtH ligand to determine their thermal degradation, which was found to be very complicated. In inert atmosphere the residues at 1,000 °C (verified with PXRD) were a mixture of Cu₂S and CuX, while at 1,300 °C a mixture of Cu₂S and Cu. In oxygen atmosphere the residues were CuO.     

Structural and electronic properties of luminescent copper(I) halide complexes of bis[2-(diphenylphosphano)phenyl] ether (DPEphos). Crystal structure of [CuCl(DPEphos)(dmpymtH

Heteroleptic copper(I) halide complexes containing the bis[2-(diphenylphosphano)phenyl]ether (DPEphos) ligand and the heterocyclic thioamides pyridine-2(1H)-thione (py2SH), pyrimidine-2(1H)-thione (pymtH) or 4,6-dimethylpyrimidine-2(1H)-thione (dmpymtH) have been synthesized and characterized by (1)H-NMR, IR spectroscopy, elemental analyses and melting point determinations. The complexes can be readily obtained by the addition of the thione ligand to a CuX-diphosphane adduct in dichloromethane-ethanol solution. The molecular structure of [CuCl(DPEphos)(dmpymtH)] complex has been established by single-crystal X-ray diffraction. The structure features a tetrahedral copper(I) center with two phosphorus atoms from the chelating diphos ligand, one halogen atom and the exocyclic sulfur atom of the heterocyclic thioamide unit. The complexes are strongly emissive in the solid state at ambient temperature. DFT and TD-DFT calculations were employed to study the structural, electronic and photophysical properties of the novel complexes. Electronic absorption spectra show two broad bands in the regions 275-290 and 380-398 nm of mixed MLCT/IL character. Intense blue-green emission is observed in the region 500-558 nm for complexes having py2SH or dmpymtH thione ligands. The emitting first triplet excited state, T(1) is mainly localized on the thione ligand.     

Cooperative oligopoly games with boundedly rational firms

We analyze cooperative Cournot games with boundedly rational firms. Due to cognitive constraints, the members of a coalition cannot accurately predict the coalitional structure of the non-members. Thus, they compute their value using simple heuristics. In particular, they assign various non-equilibrium probability distributions over the outsiders’ set of partitions. We construct the characteristic function of a coalition in such an environment and we analyze the core of the corresponding games. We show that the core is non-empty provided the number of firms in the market is sufficiently large. Moreover, we show that if two distributions over the set of partitions are related via first-order dominance, then the core of the game under the dominated distribution is a subset of the core under the dominant distribution.