Thermoluminescence study of persistent luminescence materials: Eu2+- and R3+-doped calcium aluminates, CaAl2O4:Eu2+,R3+

Thermoluminescence properties of the Eu2+-, R3+-doped calcium aluminate materials, CaAl2O4:Eu2+,R3+, were studied above room temperature. The trap depths were estimated with the aid of the preheating and initial rise methods. The seemingly simple glow curve of CaAl2O4:Eu2+ peaking at ca. 80 degrees C was found to correspond to several traps. The Nd3+ and Tm3+ ions, which enhance most the intensity of the high-temperature TL peaks, form the most suitable traps for intense and long-lasting persistent luminescence, too. The location of the 4f and 5d ground levels of the R3+ and R2+ ions were deduced in relation to the band structure of CaAl2O4. No clear correlation was found between the trap depths and the R3+ or R2+ level locations. The traps may thus involve more complex mechanisms than the simple charge transfer to (or from) the R3+ ions. A new persistent luminescence mechanism presented is based on the photoionization of the electrons from Eu2+ to the conduction band followed by the electron trapping to an oxygen vacancy, which is aggregated with a calcium vacancy and a R3+ ion. The migration of the electron from one trap to another and also to the aggregated R3+ ion forming R2+ (or R3+-e-) is then occurring. The reverse process of a release of the electron from traps to Eu2+ will produce the persistent luminescence. The ability of the R3+ ions to trap electrons is probably based on the different reduction potentials and size of the R3+ ions. Hole trapping to a calcium vacancy and/or the R3+ ion may also occur. The mechanism presented can also explain why Na+, Sm3+, and Yb3+ suppress the persistent luminescence.     

Silver Halide and Silver Chalcogenide Systems. IV. Excess enthalpies of the liquid mixtures of AgCl + Ag2Se,Agl + Ag2Se,AgCl + Ag2Te,AgBr + Ag2Te,and Agl + Ag2Te

The integral enthalpies of mixing of the liquid mixtures AgCl + Ag₂Se, AgI + Ag₂Se, AgCl + Ag₂Te, AgBr + Ag₂Te and AgI + Ag₂Te were determined in an isoperibolic calorimeter. The systems have endothermic effects which can be explained by a misfit energy, due to the substitution of ions of different size in the polymeric network of these melts.     

Minor and Trace Sterols in Marine Invertebrates. XII. Occurrence of 24 (R + S)-isopropenylcholesterol, 24 (R + S)-methylcholesta-5, 25-dien-3β-ol,and 24 (R + S)-methylcholesta-7, 25-dien-3β-ol in the caribbean sponge,Verongia cauliformis

In addition to the two new sterols verongulasterol 11 and 25-dehydroaplysterol 13 of Verongia cauliformis³), which were reported earlier [2] [3], the minor and trace sterols of this sponge include five new sterols listed in the title (with the exception of the known 24 S-methyl-cholesta-5, 25-dien-3β-ol (codisterol, 1b ). The isolation of the 24(R)-epimer of codisterol is of interest, as this compound is a possibly biosynthetic precursor for aplysterol 12 , 25-dehydroaplysterol 13 , and verongulasterol 11 (all 24R) which occur in the same sponge [2]. A partial synthesis from fucosterol ( 4 ) of 24 (R + S)-isopropenylcholesterol ( 9 ), and of 24-isopropylcholesterol ( 10 ) is described.     

Thermodynamics of the early stages in homogeneous ziegler-natta polymerization: Density functional calculations on model systems TiCl2R+ and ZrCl2R+

The ethylene polymerization enthalpy, calculated through quantum-mechanical ab-initio methods on model systems of homogeneous Ziegler-Natta cationic catalysts, is reported to be from two to three times greater than the experimental value of 22,3 kcal/mol. In this paper we analyze the origin of this discrepancy and show that it is mainly due to the intrisnic instability of the cationic system in vacuum. We also demonstrate that the growing polymer chain can act as a quite efficient stabilizing agent. We examined, through density functional calculations, the model systems MCl₂R⁺, where M = Ti, Zr and R = CH₃, C₃H₇, C₅H₁₁, C₇H₁₅ and their analogues obtained by neutralizing the positive charge with a chloride anion. On the basis of our computational results, we found that: (i) for the ideal reaction of ethane with ethylene to give butane, considered as a thermodynamical model of the single insertion step, the calculated enthalpy value of 35,6 kcal/mol is in closer agreement with the experimental value and is taken as theoretical reference value; (ii) the same value is obtained also for the neutral systems MCl₃R independently of the nature of the metal and of the alkyl chain length; (iii) for cationic systems, when R = CH₃, high insertion enthalpies are obtained in agreement with the calculated values reported in literature, but, for R = C₃H₇, C₅H₁₁ and C₇H₁₅, the insertion enthalpy remarkably decreases converging towards the theoretical reference value. We conclude that the high enthalpy value obtained for the first monomer insertion is not only a mere consequence of the computational method, but is mainly due to the weak stabilizing effect of the methyl group. A longer alkyl chain produces a stabilization of the cationic system through the inductive effect as well as through formation of agostic bonds. This leads us to formulate the hypothesis that, in real polymerization conditions, the role of a stabilizing agent, which is mainly played by the counterion in the early stages of the propagation reaction, could be performed by the growing chain as the polymerization proceeds.     

Convergent synthesis of (2R,3R,8R,9R)-N-Boc-ADDA

The convergent synthesis of N-Boc-(2R,3R,8R,9R,4E,6E)-3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldecadenoic acid (enantio-N-Boc-ADDA) is reported. Our flexible approach takes advantage of highly efficient non-aldol aldol and cross-metathesis methodologies.     

Theoretical and Experimental Study of Pb2+ and Hg2+ Adsorption on Biopolymers, 2. Experimental Part

Chitosan and pectin were studied as adsorbents for Hg²⁺ and Pb²⁺ from acidic aqueous solutions. The conditions were selected to analyze the adsorption efficiency of pellets of chitosan–pectin. Kinetics data are reported in this work for conditions under which ppm of contaminants are adsorbed. The ability of these biopolymers to remove Pb and Hg from polluted water was investigated using the unmodified biopolymers and the pellets under identical conditions. Results from Molecular Mechanics calculations (MM2) and semiempirical calculations are discussed together to get more insight into the mechanism of metal retention and the effect of conformational factors on adsorption. Several adsorption sites were considered for the biopolymers and many adsorption mechanisms proposed, taking into account the theoretical and experimental results.     

Ketene-forming eliminations from aryl phenylacetates promoted by R2NH/R2NH2+ in aqueous MeCN. Mechanistic borderline between E2 and E1cb

Elimination reactions of 2-X-4-NO2C6H3CH2C(O)OC6H3-2-Y-4-NO2 [X = H (1), NO2 (2)] promoted by R2NH/R2NH2+ in 70 mol % MeCN(aq) have been studied kinetically. The base-promoted eliminations from 1 proceeded by the E2 mechanism when Y = Cl, CF3, and NO2. The mechanism changed to the competing E2 and E1cb mechanisms by the poorer leaving groups (Y = H, OMe) and to the E1cb extreme by the strongly electron-withdrawing beta-aryl group (2, X = NO2). The values of beta = 0.14 and beta(lg) = 0.10-0.21 calculated for elimination from 1 (Y = NO2) indicate a reactant-like transition state with small extents of proton transfer and C(alpha)-OAr bond cleavage. The extent of proton transfer increased with a poorer leaving group, and the degree of leaving group bond cleavage increased with a weaker base. Also, the changes in the k(1) and k(-1)/k(2) values with the reactant structure variation are consistent with the E1cb mechanism. From these results, a plausible pathway of the change of the mechanism from E2 to the E1cb extreme is proposed.     

Studies on the transformation of nitrosugars into iminosugars III: synthesis of (2R,3R,4R,5R,6R)-2-(hydroxymethyl)azepane-3,4,5,6-tetraol and (2R,3R,4R,5R,6S)-2-(hydroxymethyl)azepane-3,4,5,6-tetraol

A divergent synthesis of the two novel polyhydroxylated azepanes (2R,3R,4R,5R,6R)-2-(hydroxymethyl)azepane-3,4,5,6-tetraol and (2R,3R,4R,5R,6S)-2-(hydroxymethyl)azepane-3,4,5,6-tetraol from d-mannose is described. The method involves a Henry reaction between dimethyl-tert-butylsilyl 2,3-O-isopropylidene-α-d-lyxo-pentodialdo-1,4-furanoside and 2-nitroethanol followed by a reductive ring closure of the resulting epimeric nitro aldols. Glycosidase inhibition tests showed that (2R,3R,4R,5R,6S)-2-(hydroxymethyl)azepane-3,4,5,6-tetraol exhibits a weak but selective inhibition against α-l-fucosides.