Conformal solutions: which model for which application?

Shukla, K.P., Luckas, M., Marquardt, H. and Lucas, K., 1986. Conformal solutions: which model for which application? Fluid phase Equilibria, 26: 129–147.Various forms of conformal solution theories are discussed and compared to computer simulations for Lennard-Jones mixtures under rather different conditions. It is found that the simple VDW1-theory is applicable when the ratio of the energy parameters is 1.5, and the ratio of the size parameters is 1.1. The same region of applicability is found for the Mean Density Approximation (MDA). Two models using explicitly known properties of the hard-sphere mixture, i.e., the Hard-Sphere Expansion theory (HSE) and a particular version of the Hard-Sphere Perturbation theory (WCA-LL-GH), have also been tested. The HSE is not complete unless a consistent method is found to calculate the hard-sphere diameter. The overall best results are found for WCA-LL-GH. While WCA-LL-GH turns out to be limited in applicability to size ratios smaller than 1.3 and appears to be inaccurate for particular cases, it is definitely the best model available today.     

Understanding d(0)-olefin metathesis catalysts: which metal, which ligands?

Density functional theory (DFT, B3PW91) calculations have been carried out on the reactivity of ethene with model systems M(NR)(=CHCH3)(X)(Y) for M = Mo or W, R = methyl or phenyl, X = CH2CH3, OCH3, or OSiH3, and Y = CH2CH3, OCH3, or OSiH3, which are representative of experimental olefin metathesis catalysts, and the results are compared to those previously obtained for Re(CCH3)(=CHCH3)(X)(Y). The general pathway comprises four steps: olefin coordination, [2+2] cycloaddition, cycloreversion, and olefin de-coordination. Two key factors have been found to control the detailed shape of the energy profiles: the energy of distortion of the tetrahedral catalyst and the stability of the metallacycle intermediate, which is controlled by the M-C bond strength. The efficiency has been evaluated by calculating the turnover frequency (TOF) based on the steady-state approximation, and the most striking feature is that the unsymmetrical catalysts (X not equal to Y) are systematically more efficient for all systems (Mo, W, and Re). Overall, the Re complexes have been found to be less efficient than the Mo and W catalysts, except when Re is unsymmetrically substituted: it is then calculated to be as efficient as the best Mo and W catalysts.     

Which genetic marker for which conservation genetics issue?

Conservation genetics focuses on the effects of contemporary genetic structuring on long-term survival of a species. It helps wildlife managers protect biodiversity by identifying a series of conservation units, which include species, evolutionarily significant units (ESUs), management units (MUs), action units (AUs), and family nets (FNs). Although mitochondrial DNA (mtDNA) evolves 5-10 times faster than single-copy nuclear DNA (scnDNA), it records few traces of contemporary events. Thus, mtDNA can be used to resolve taxonomic uncertainties and ESUs. Variable number of tandem repeats (VNTRs) evolve 100-1000 times faster than scnDNA and provide a powerful tool for analyzing recent and contemporary events. VNTR analysis techniques include polymerase chain reaction (PCR)-based microsatellite assays and oligonucleotide probing. Size homoplasy problems in PCR-based microsatellite assays can strongly affect the inference of recent population history. The high homozygosity in endangered species is reflected in a relatively low number and level of variability in microsatellite loci. This combined with "allelic dropout" and "misprinting" errors contributes to the generation of highly biased genetic data following analyses of natural populations. Thus, in conservation genetics, microsatellites are of limited use for identifying ESUs, MUs, and AUs. In contrast to PCR-based microsatellite analysis, oligonucleotide probing avoids errors resulting from PCR amplification. It is particularly suitable for inferring recent population history and contemporary gene flow between fragmented subpopulations. Oligonucleotide fingerprinting generates individual-specific DNA banding patterns and thus provides a highly precise tool for monitoring demography of natural populations. Hence, DNA fingerprinting is powerful for distinguishing ESUs, MUs, AUs, and FNs. The use of oligonucleotide fingerprinting and fecal DNA is opening new areas for conservation genetics.     

Chaotrope vs. kosmotrope: which one has lower friction?

We examine the frictional properties of zwitterionic surfaces and explore whether chaotropic or kosmotropic charged groups are preferred to achieve lower friction. Self-assembled monolayers of carboxybetaine (CB-SAMs) and sulfurbetaine (SB-SAMs) are used as model surfaces as they contain the same positively charged group, but different negatively charged ones. The negatively charged groups are kosmotropic carboxylates in the CB-SAM surfaces and chaotropic sulfonate groups in the SB-SAM surfaces, respectively. The results show that the friction of the SB-SAM surfaces is even lower than that of the CB-SAM surfaces although both surfaces have low friction. This suggests that chaotropic charged groups are better in reducing friction than kosmotropic groups. The lower friction of the SB-SAM surfaces over the CB-SAM can be explained by the higher mobility of water near the SB-SAM surfaces, as shown in the survival autocorrelation function and the dipole autocorrelation function of hydration water molecules.     

Ring-enlargement reactions of donor-acceptor-substituted cyclopropanes: which combinations are most efficient?

A detailed theoretical study of ring-enlargement reactions of 72 differently substituted donor-acceptor-substituted cyclopropanes is presented. Transition states, activation barriers, and, for representative examples, the behavior in solution were additionally determined using the B3LYP/6-311G(d) level of theory.     

Sulfonyl vs. carbonyl group: which is the more electron-withdrawing?

Unexpectedly high reactivity of nitrogenated aromatics protected as amides or carbamates, when compared to sulfonamides, can be explained by a decrease of the aromaticity due to a greater ability of the carbon-centered groups to achieve delocalisation of the nitrogen lone pair, resulting in stronger global withdrawing effects.     

Chemistry or biology: which comes first after the genome is sequenced?

In drug discovery, chemical screening is performed after a protein's function has been determined. By screening for ligands that alter the function of a cell or an organism, new proteins that participate in poorly understood biological processes can be identified. Genomic and expression-cloning technologies can rapidly identify the protein targets of these ligands, enhancing the power of chemical screening as a tool for the initial stages of biological discovery.     

Detaching thiolates from copper and gold clusters: which bonds to break?

The interaction of alkanethiolates with small coinage metal clusters of copper and gold was studied based on density functional theory with a focus on the metal-thiolate junction. Calculation of fragmentation energies indicate that for Cu(n)-thiolate (n = 1,3,5,7, and 9) there is a progressive lowering in energy for the fragmentation of the S-C bond in the thiolate from a value of 2.9 eV for n = 1 to 1.4 eV for n = 9. The detailed electronic origins of this specific weakening are attributed to a polarization of electron density in the S-C bond as induced by bonding with the Cu(n) cluster. For the gold analogues, this effect is not observed and fragmentation at the S-C bond experiences only a slight 10% destabilization as n increases from 3 to 9. The relativistic origin of this difference between Cu and Au is discussed, and an analysis of bonding considerations is presented.     

Pushing the limits on possibilities for large scale gas separation: which strategies?

Opportunities abound to extend membrane markets for gas and vapor separations; however, the existing membrane materials, membrane structures and formation processes are inadequate to fully exploit these opportunities. The requirements for viability of membranes vary somewhat with each application. Nevertheless, the key requirements of durability, productivity and separation efficiency must be balanced against cost in all cases. The various ‘contender’ technologies for large scale gas separation membrane applications and the gas transport mechanisms are considered. The current spectrum of applications of gas separation membranes include; nitrogen enrichment, oxygen enrichment, hydrogen recovery, acid gas CO₂, H₂S removal from natural gas and dehydration of air and natural gas. The current status and the limitations faced by the available membrane materials for each of these applications are discussed. Two key technical challenges exist. Achieving higher permselectivity for the relevant application with at least equivalent productivity is the first of these challenges. Maintaining these properties in the presence of complex and aggressive feeds is the second challenge. Attractive avenues to overcome these challenges for each application will be presented. Finally, several new membrane applications with immense potential (e.g. fuel cells and olefin-paraffin separations) are discussed.     

Two secondary modes in decahydroisoquinoline: which one is the true Johari Goldstein process?

Broadband dielectric measurements were carried out at isobaric and isothermal conditions up to 1.75 GPa for reconsidering the relaxation dynamics of decahydroisoquinoline, previously investigated by Richert et al. [R. Richert, K. Duvvuri, and L.-T. Duong, J. Chem. Phys. 118, 1828 (2003)] at atmospheric pressure. The relaxation time of the intense secondary relaxation tau(beta) seems to be insensitive to applied pressure, contrary to the alpha-relaxation times tau(alpha). Moreover, the separation of the alpha- and beta-relaxation times lacks correlation between shapes of the alpha-process and beta-relaxation times, predicted by the coupling model [see for example, K. L. Ngai, J. Phys.: Condens. Matter 15, S1107 (2003)], suggesting that the beta process is not a true Johari-Goldstein (JG) relaxation. From the other side, by performing measurements under favorable conditions, we are able to reveal a new secondary relaxation process, otherwise suppressed by the intense beta process, and to determine the temperature dependence of its relaxation times, which is in agreement with that of the JG relaxation.     

Oxidic or metallic palladium: which is the active phase in pd-catalyzed aerobic alcohol oxidation?

In situ X-ray absorption spectroscopy combined with on-line catalytic measurements using FT-IR spectroscopy unequivocally identified that metallic palladium is the more active phase in the aerobic oxidation of benzyl alcohol than palladium oxide. The aerobic oxidation of benzyl alcohol in cyclohexane at 50 degrees C was low over oxidized 0.5%Pd/Al2O3 and 5%Pd/Al2O3 catalysts. XANES and EXAFS showed that the catalysts in the as-received state were almost fully oxidized and no reduction of the palladium constituent was observed during time-on-stream. After in situ reduction by hydrogen-saturated cyclohexane, the catalysts were much more active (over 50 times) than before reduction. Both XANES and EXAFS uncovered that the palladium constituent was mainly in a reduced state under these conditions of high catalytic activity. This demonstrates that metallic palladium is the active phase for alcohol dehydrogenation.     

Microparticles based on chitosan–hyaluronic acid interpolyelectrolyte complex,which provide stability of aqueous dispersions

Turbidimetric titration and laser scattering methods were used to assess the phase behavior of mixtures of chitosan and hyaluronic acid in aqueous solutions in relation to interaction conditions. Conditions were found in which stable water dispersions with particle size of up to 573 nm can be obtained. It was found that the molar ratios and the overall concentration mode of the starting-polyelectrolyte solutions affect the particle size.     

Pore size or geometry: which determines the shape and inverse-shape selective adsorption of alkane isomers?

The adsorption of pure pentane (C(5)) isomers and their ternary mixture is simulated in a series of carbon nanoslits. With decreasing nanoslit pore size, shape selective adsorption first occurs in the order of nC(5) > or = iC(5) > neoC(5) due to the configurational entropy effect, then inverse-shape selective adsorption occurs in the order of nC(5) < iC(5) < or = neoC(5) due to the area entropy effect, and finally no adsorption occurs. The entropy effects lead to a large adsorptive separation among the C(5) isomers from their mixture. Similar behavior has been observed from the simulation of C(5) adsorption in carbon nanotubes with variation in pore size. These results reveal that pore size rather than geometry determines the shape and inverse-shape selective adsorption of alkane isomers in nanopores.     

Number of terminal groups versus generation of the dendrimer, which criteria influence the catalytic properties?

We report the synthesis of a dense dendrimer ended by the PTA ligand (1,3,5-triaza-7-phosphaadamantane), and the use of the corresponding Rh and Ru complexes for catalysis. The catalytic properties of these dendrimers are compared with those of two other dendrimers: a dendrimer of the same generation but having half the number of ligands, and a dendrimer of the next generation, but having the same number of ligands. The positive influence of the density of catalytic sites on the surface of these dendrimers for alcohol isomerization in water has been evidenced.     

Dissociation and ionization competing processes for H2+ in intense laser field: which one is larger?

Competition between dissociation and ionization of H(2)(+) in intense laser field has been investigated by using an accurate three-dimensional time-dependent wavepacket approach. The disagreement between the experiment and the former one-dimensional theory has been resolved. In a comparison of the calculated results with the available experimental data, a good agreement is reached, not only for the relative probabilities between dissociation and ionization but also for the two-peak structures and the peak energy locations for these two processes.     

Photochemical electrocyclization of α,β-unsaturated anilides to give zwitterionic intermediates which eliminate carboxylate and phenolate leaving groups

α,β-Unsaturated anilides bearing allylic leaving groups undergo photochemical electrocyclic ring closure to produce zwitterionic intermediates which eliminate carboxylate and phenolate leaving groups.