Fragmentation of water clusters: Molecular-dynamics simulation study

The fragmentation of water clusters, [(H ₂ O)_n;n = 2-8], have been investigated by using molecular-dynamics simulation method. In the simulations a polarizable-dissociable potential energy function for water has been used. Particular attention has bee paid to investigate the effect of structural properties and cluster size on the fragmentation. Received 27 April 2000 and Received in final form 6 October 2000     

Neutron activation analysis of Turkish clays

Neutron activation analysis was used in the determination of thirteen trace element concentrations in clays from various regions of Turkey. Comparison of results in terms of enrichment factors with those of lignite samples from the same regions revelaed no significant correlation between clay and lignite concentrations. Structural effects as well as local soil composition seem to affect the enrichment.Supported by the Turkish Atomic Energy Authority, and by IEAE-Vienna.     

Sample geometry and efficiency determination of bremsstrahlung radiation of 90Sr on gamma detection systems

Summary The bremsstrahlung radiation energy spectra were produced by hard beta-emitters the ⁹⁰Sr-⁹⁰Y contaminated tea sample sources placed in a copper cylinder (cylinder counting geometry) and encapsulated in two Cu discs (sandwich counting geometry). These energy spectra were directly measured by using two separate gamma-ray spectrometers with a coaxial 110% efficient HpGe detector and a 110 cm³ HpGe well-type detector. The minimum detectable activity and the absolute efficiency of beta-activity for the sandwich and cylinder geometries were found to be 23 Bq ^. kg^-1 and (1.67±0.04)% and 55 Bq ^. kg^-1 and (2.61±0.05)%, respectively. These results indicate that the bremsstrahlung radiation counting method can be applied to some environmental studies when high efficient HpGe detectors, especially well-type HpGe are used.     

Modeling the cyclopolymerization of diallyl ether and methyl α‐[(allyloxy)methyl]acrylate

The free‐radical cyclopolymerization of diallyl ether (1) and methyl α‐(allyloxymethyl)acrylate (2) has been modeled with the B3LYP/6‐31G* methodology, by making use of model compounds for the growing radicals. The cyclization of both monomers is exo, with activation barriers of 5.33 and 9.82 kcal/mol, respectively. To account for the polymerizabilities of these monomers, competing reactions have also been modeled. Although both monomers have a lower barrier for homopolymerization than for cyclization, cyclization dominates due to entropy. This explains the high cyclopolymerization vs. homopolymerization of monomer 2, although its monofunctional counterpart has been reported to homopolymerize well. It has also been shown that the degradative chain transfer by H‐abstraction from the allylic carbon is not effective with this monomer. Poor cyclopolymerization of the monomer 1 has been demonstrated by modeling the degradative chain transfer by H‐abstraction from the allylic carbon, which has been shown to compete very efficiently with polymerization reactions. Additionally, intermolecular propagation reaction has been shown to be facile due to cyclization, since the attacking monomer adopts a cyclic structure. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Determination of the surface properties of untreated and chemically treated kaolinites by inverse gas chromatography

Inverse gas chromatography is used to study the surface properties of the untreated and chemically treated kaolinite samples. Changes in the enthalpy of adsorption for a variety of probes and in the surface energy of clays are measured and the effect of modification of the natural clay after chemical treatment with Na₂CO₃ is determined. The surface energy of the natural clay increased by the modification due to an increase in the surface area. It can be concluded that the dispersive component of surface free energy, γ _s ^d , decreases with temperature in the 200–275°C temperature range for both samples. Temperature coefficients of γ _s ^d for untreated and modified kaolinites are −0.1185 and −0.3966 mJ/(m² °C) with the correlation coefficients (R ²) of 0.8479 and 0.965, respectively. From the retention data for polar probes at infinite dilution, information on the accessibility of surface sites to the probes and on the acid-base character of the surface is obtained. The specific free energy of adsorption, the specific enthalpy of adsorption (ΔH ^sp), and the specific entropy of adsorption of polar probes on initial and modified kaolinites are determined. The ΔH ^sp values correlated with the donor numbers and modified acceptor numbers of the probes to quantify the acidity (K _A) and basicity (K _D) parameters of clay surfaces. The values of K _A and K _D for initial and modified kaolinites are determined to be 0.1202 and 0.2803; 0.0130 and 0.0408 with the correlation coefficients of −0.9805 and −0.9782, respectively. The unmodified clay sample indicated a more acidic character, while the modified clay sample conferred a largely basic character. Consequently, the predominant surface basicity of the modified kaolinite agrees with expectation, bearing in mind the treatment with Na₂CO₃, taking into consideration that such a modification contributes to a decrease in the hydrophilicity of the surface and also results in the surface showing only weak Lewis acidity. The text was submitted by the authors in English.     

Aza-Diels–Alder Reactions with Lanthanide Triflates: Syntheses of Quinoline and Phenanthridine Derivatives

The aza-Diels–Alder reactions of cyclopentadiene and cyclo-1,3-hexadiene with various substituted N-arylimines in the presence of Yb/Sc triflates as catalyst in MeCN at room temperature gave quinoline and phenanthridine derivatives in moderate to high yields. Some of the cycloaddition reactions were carried out in ionic liquid.