Vitrification and physical ageing on isothermal curing of an epoxy resin

Vitrification phenomena and further structural relaxation processes or physical ageing occurring in the isothermal curing reaction of an epoxy resin are studied by Differential Scanning Calorimetry (DSC). The vitrification time,t _v , the limiting conversion degree and the limiting glass transition temperature (T _g) are evaluated at curing temperatures (T _c) between 30 and 100C. The dependence of limitingT _g withT _c permits the determination of the maximumT _g of the resin (109C). The physical ageing, which appears as the the last step of curing reaction for curing times above to vitrification, is analyzed through the endothermic peak superposed to the glass transition temperature. The results obtained in partially reticulated resin show the kinetics of the physical ageing to slow down asT _c increases, as a consequence that the segmental mobility is reduced.

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Zusammenfassung VitrifikationphÄnomeme und strukturelle Relaxations-respektive AlterungsvorgÄnge von Epoxiden wurden mitells isothermer DSC Messungen untersucht. Die Vitrifikationszeit, der Grenzwert des Unwandlungsgrades und die Grenztemperatur der Glasumwandlung wurden in Bereich von 30 bis 100C ermittelt. Werden Epoxide für Temperaturen unterhalb der Glasumwandlungstemperatur einer thermischen Behandlung ausgesetzt, so bilden sich SpannungszustÄnde aus, die bei der ErwÄrmung unmittelbar über der Glasumwandlungstemperatur mit einem endothermen Vorgang relaxieren.

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Financial support of this work has been provided by CICYT (project No. PBO395/85). Special thanks are addressed to CIBA-GEIGY for supplying the materials.     

Hydroformylation of oct-1-ene catalyzed by dinuclear gem-dithiolato-bridged rhodium(I) complexes and phosphorus donor ligands

The dinuclear gem-dithiolato bridged compounds [Rh₂(μ-S₂Cptn)(cod)₂] (1) (CptnS₂²⁻ = 1,1-cyclopentanedithiolato), [Rh₂(μ-S₂Chxn)(cod)₂] (2) (ChxnS₂²⁻ = 1,1-cyclohexanedithiolato), [Rh₂(μ-S₂CBn₂)(cod)₂] (3) (Bn₂CS₂²⁻ = 1,3-diphenyl-2,2-dithiolatopropane) and [Rh₂(μ-S₂CⁱPr₂)(cod)₂] (4) (ⁱPr₂CS₂²⁻ = 2,4-dimethyl-2,2-dithiolatopentane) dissolved in toluene in the presence of monodentate phosphine or phosphite P-donor ligands under carbon monoxide/hydrogen (1:1) atmosphere are efficient catalysts for the hydroformylation of oct-1-ene under mild conditions (6.8 atm of CO/H₂ and 80 °C). The influence of the gem-dithiolato ligand, the P-donor co-catalyst and the P/Rh ratio on the catalytic activity and selectivity has been explored. Aldehyde selectivities higher than 95% and turnover frequencies up to 245 h⁻¹ have been obtained using P(OMe)₃ as modifying ligand. Similar activity figures have been obtained using P(OPh)₃ although the selectivities are lower. Regioselectivities toward linear aldehyde are in the range 75–85%. The performance of the catalytic systems [Rh₂(μ-S₂CR₂)(CO)₂(PPh₃)₂]/PPh₃ has been found to be comparable to the systems [Rh₂(μ-S₂CR₂)(cod)₂] at the same P/Rh ratio. The system [Rh₂(μ-S₂CBn₂)(cod)₂] (3)/P(OPh)₃ has been tested in the hydroformylation-isomerization of trans-oct-2-ene. Under optimized conditions up to 54% nonanal was obtained. Spectroscopic studies under pressure (HPNMR and HPIR) evidenced the formation of hydrido mononuclear species under catalytic conditions that are most probably responsible for the observed catalytic activity.     

The influence of the node criticality relation on some measures of component importance

For different reliability importance measures we prove that the criticality relation between nodes can completely determine the most important component in a system. In particular, we prove that in k-out-of-n systems, the ranking of component reliabilities determines the ranking of component importance for, at least, three different reliability importance measures.     

Divisorial components of the Petri locus for pencils

The locus in the moduli space of curves where the Petri map fails to be injective is called the Petri locus. In this paper we provide a new proof on the existence of Divisorial components in the Petri locus for the case of pencils. For this proof we produce some special reducible curves (chains of elliptic components) in the Petri locus and we show that such curves have only a finite number of pencils for which the Petri map is not injective.     

Quantitative determination of clenbuterol, salbutamol and tulobuterol enantiomers by capillary electrophoresis

Enantiomers of clenbuterol, salbutamol and tulobuterol were directly separated and quantitated from a spiked sample by capillary electrophoresis (CE) using sulfated β-cyclodextrin (SCD) as chiral selector and phosphate as running buffer. The SCD and buffer concentration, pH and field strength were the parameters studied to optimize the separation. Optimal separation was obtained using 50 mM of phosphate monobasic at pH = 2.24, 0.25% (w/w) of sulfated cyclodextrin and a field strength of 10 kV, with 20 min total time analysis. Comparison between two different injection modes (hydrodynamic and electrokinetic) was made. In the hydrodynamic mode, repeatability (expressed as relative standard deviation, RSD) was less than 1.2% for migration times for all the analyte peaks and less than 2% for peak area percentages. With respect to reproducibility, RSD was less than 3.8% for migration time and less than 3% for peak area percentages. Calibration curves were set up for two different sample concentration ranges (1 to 10 μg mL^–1 and 160– 800 ng mL^–1, of each of the racemates studied). Although the electrokinetic injection mode for an aqueous sample appeared to suffer from some enantiodiscrimination, calibration curves were linear in the range between 1 and 10 ng mL^–1 with regression coefficients ranging from 0.9996 to 0.9952. As in the case of hydrodynamic injection, the method was tested with a spiked sample.     

Hydrogen bonding in the inner-salt zwitterion and in two different charged forms of 5,6-bis(2-pyridyl)pyrazine-2,3-dicarboxylic acid

5,6-Bis(2-pyridyl)­pyrazine-2,3-di­carboxyl­ic acid exists as an inner-salt zwitterion, 3-carboxy-5-(2-pyridinio)-6-(2-pyridyl)­pyrazine-2-carboxyl­ate, (Ia), C₁₆H₁₀N₄O₄. The adjacent pyridine and pyridinium rings are almost coplanar due to the presence of an intramolecular hydrogen bond involving the pyridine N atom and the NH H atom of the pyridinium group. In the crystal of (Ia), symmetry-related mol­ecules are hydrogen bonded via the carboxyl­ic acid OH group and one of the carboxyl­ate O atoms to form a polymer, which exhibits a channel-type structure. In the HCl, HClO₄ and HPF₆ salts, 6-­carboxy-5-carboxyl­ato­pyrazine-2,3-diyldi-2-pyridinium chloride 2.25-hydrate, (II), C₁₆H₁₁N₄O₄⁺·Cl⁻·2.25H₂O, 6-­carboxy-5-carboxyl­ato­pyrazine-2,3-diyldi-2-pyridinium perchlor­ate trihydrate, (IIIa), C₁₆H₁₁N₄O₄⁺·ClO₄⁻·3H₂O, and 6-car­boxy-5-carboxyl­ato­pyrazine-2,3-diyldi-2-pyridinium hexa­fluoro­phosphate trihydrate, (IIIb), C₁₆H₁₁N₄O₄⁺·PF₆⁻·3H₂O, both pyridine rings are protonated. In the perchlorate form, and in the isomorphous hexa­fluoro­phosphate form, the mol­ecule possesses C₂ symmetry, with has a symmetrical intramolecular hydrogen bond involving the adjacent carboxyl­ate and carboxyl­ic acid substituents. In the crystals of the chloride and perchlorate (or hexa­fluoro­phosphate) salts, hydrogen-bonded polymers are formed which are three-dimensional and one-dimensional, respectively.     

Copper(II) hexaaza macrocyclic binuclear complexes obtained from the reaction of their copper(I) derivates and molecular dioxygen

Density functional theory (DFT) calculations have been carried out for a series of Cu(I) complexes bearing N-hexadentate macrocyclic dinucleating ligands and for their corresponding peroxo species (1c-8c) generated by their interaction with molecular O2. For complexes 1c-7c, it has been found that the side-on peroxodicopper(II) is the favored structure with regard to the bis(mu-oxo)dicopper(III). For those complexes, the singlet state has also been shown to be more stable than the triplet state. In the case of 8c, the most favored structure is the trans-1,2-peroxodicopper(II) because of the para substitution and the steric encumbrance produced by the methylation of the N atoms. Cu(II) complexes 4e, 5e, and 8e have been obtained by O2 oxidation of their corresponding Cu(I) complexes and structurally and magnetically characterized. X-ray single-crystal structures for those complexes have been solved, and they show three completely different types of Cu(II)2 structures: (a) For 4e, the Cu(II) centers are bridged by a phenolate group and an external hydroxide ligand. The phenolate group is generated from the evolution of 4c via intramolecular arene hydroxylation. (b) For 5e, the two Cu(II) centers are bridged by two hydroxide ligands. (c) For the 8e case, the Cu(II) centers are ligated to terminally bound hydroxide ligands, rare because of its tendency to bridge. The evolution of complexes 1c-8c toward their oxidized species has also been rationalized by DFT calculations based mainly on their structure and electrophilicity. The structural diversity of the oxidized species is also responsible for a variety of magnetic behavior: (a) strong antiferromagnetic (AF) coupling with J = -482.0 cm(-1) (g = 2.30; rho = 0.032; R = 5.6 x 10(-3)) for 4e; (b) AF coupling with J = -286.3 cm(-1) (g = 2.07; rho = 0.064; R = 2.6 x 10(-3)) for 5e; (c) an uncoupled Cu(II)2 complex for 8e.     

Combined effects of diamines and carboxylate bridges on structural and magnetic properties of a series of polynuclear copper(II) complexes with 1,1-cyclobutanedicarboxylic acid

Six new copper(II) complexes of formula [Cu(mu-cbdca)(H2O)]n (1) (cbdca = cyclobutanedicarboxylate), [Cu2(mu-cbdca)2(mu-bipy)2]n (2) (bipy = 4,4'-bipyridine), [Cu(mu-cbdca)(mu-bpe)]n (3) (bpe = 1,2-bis(4-pyridyl)ethane), [Cu(mu-cbdca)(bpy)]2 (4) (bpy = 2,2'-bipyridine), [Cu(terpy)(ClO4)]2(mu-cbdca).H2O (5) (terpy = 2,2':6',2' '-terpyridine), and [Cu(cbdca)(phen) (H2O)].2H2O (6) (phen = 1,10-phenanthroline) were obtained and structurally characterized by X-ray crystallography. Complex 1 is a two-dimensional network with a carboxylate bridging ligand in syn-anti (equatorial-equatorial) coordination mode. Complexes 2 and 3 are formed by chains through syn-anti (equatorial-apical) carboxylate bridges, linked to one another by the corresponding amine giving two-dimensional nets. Complexes 4 and 5 are dinuclear, with the copper ions linked by two oxo (from two different carboxylate) bridging ligands in 4 and with only one carboxylate showing the unusual bis-unidentate mode in complex 5. Complex 6 is mononuclear, with the carboxylate linked to copper(II) in a chelated form. Intermolecular hydrogen bonds and pi-pi stacking interactions build an extended two-dimensional network. Magnetic susceptibility measurements of complexes 1-5 in the temperature range 2-300 K show the occurrence of weak ferromagnetic coupling for 1 and 4 (J = 4.76 and 4.44 cm(-1), respectively) and very weak antiferromagnetic coupling for 2, 3, and 5 (J = -0.94, -0.67, and -1.61 cm(-1), respectively). Structural features and magnetic values are compared with those reported for the similar copper(II) malonate and phenylmalonate complexes.     

First successful application of diphosphite ligands in the asymmetric hydroformylation of dihydrofurans

Good enantioselectivities and excellent regioselectivities are achieved in the Rh-catalyzed asymmetric hydroformylation of 2,5- and 2,3-dihydrofuran using diphosphite ligands; whereby the backbone of the ligand is crucial to suppressing isomerization and obtaining high ee's.     

Versatility of 2,6-diacetylpyridine (dap) hydrazones in stabilizing uncommon coordination geometries of Mn(II): synthesis, spectroscopic, magnetic and structural characterization

Five seven- or eight-coordinate manganese complexes of hydrazone ligands have been prepared. Three seven-coordinate neutral Mn(II) complexes: [Mn(dapA2)]n (1), [Mn(dapB2)(H2O)2] (2), [Mn(dapS2)(H2O)2] (3) have been synthesized from the bis-Schiff bases of 2,6-diacetylpyridine: dap(AH)2, dap(BH)2 and dap(SH)2 (AH = anthraniloyl hydrazide, BH = benzoyl hydrazide, SH = salicyloyl hydrazide), respectively. Two eight-coordinate Mn(II) complexes: [Mn(dapS)2] (4) and [Mn(dapB)2].3H2O (5) have been synthesized from the mono-Schiff bases dapBH and dapSH, respectively. The complexes have been characterized by elemental analyses and by IR, UV-Vis., FAB mass, EI mass and EPR spectroscopy. The molecular structures of 1, 3.DMF and 4.DMF have been determined by single-crystal X-ray diffraction. The mono-Schiff bases are monoanionic and the bis-Schiff bases are dianionic. The octa-coordinated mono-Schiff base complex 4 adopts a dodecahedral geometry, while the hepta-coordinated bis-Schiff base complex 1 forms a one-dimensional linear polymeric chain. A weak antiferromagnetic exchange interaction (J=-0.15 cm(-1)) between the Mn(II) ions in is attributed to weak Mn...Mn interaction through the PhNH(2) moiety of the ligand, as indicated by extended-Hückel molecular orbital calculations. A good simulation of the EPR spectrum of a frozen solution (DMSO at 4 K) of compound 1 was obtained with g=2.0, D=0.1 cm(-1), E=0.01 cm(-1). The EPR spectrum of a powdered sample of compound 1 shows a large broadening of the signal, due in part, to the important zero-field splitting of the hepta-coordinated Mn(II) ion.