From the lab bench to the plant: how to commercialize a metallocene catalyst?

In order to apply metallocene catalysts for the polymerization of ethylene in the commercial “slurry loop process” it is necessary to heterogenize the homogeneous catalysts and to produce resins with attractive properties. A solution to these problems is the self‐immobilization of ω‐alkenyl functionalized catalysts that can produce polyethylenes with evenly distributed ethyl branches in the polymer chain without a comonomer. The new resin is produced at Chevron Phillips Company, USA, and has the trade name mPact.     

How to revise the GUM?

The announcement of a revision of the Guide to the expression of uncertainty in measurement has renewed the debate about the topic of measurement uncertainty. In this paper the author, chairman of Working Group 1 of the Joint Committee for Guides in Metrology, replies to the theses given in two recent papers by Semion Rabinovich. His opinions are personal, and are not necessarily shared by the JCGM/WG1. They are to be intended as a further contribution to the present discussion. Papers published in this section do not necessarily reflect the opinion of the Editors, the Editorial Board and the Publisher.     

Where is the road to bio-water-saving for the globe?

The First International Conference on the Theory and Practices in Bio-water-saving (ICTPB) was held from May 21 to 25, 2006 in Beijing, China. This indicated that the work related to this hot topic on the globe has been paid more attention to. Most progress in this field has been presented from near 300 participating people worldwide, who were meeting together to discuss about the theory and practices of water-saving biology and how to serve global agricultural and ecological sustainable development. The work related to bio-water-saving has been involved in different scales and soil–plant root biointerfaces. On the basis of this background and in combination with the work from our laboratory and the center, we provided some ideas for global bio-water-saving in this paper, sharing the achievement in this field and advocating true bio-water-saving for the world and promoting the pace of global bio-water-saving.     

Transfer matrix approach to the hydrogen-bonding in cellulose Iα fibrils describes the recalcitrance to thermal deconstruction

Cellulosic biomass has the potential to serve as a major renewable energy source. However, its strong recalcitrance to degradation hampers its large-scale use in biofuel production. To overcome this problem, a detailed understanding of the origins of the recalcitrance is required. One main biophysical phenomenon leading to the recalcitrance is the high structural ordering of natural cellulose fibrils, that arises largely from an extensive hydrogen-bond network between and within cellulose polymers. Here, we present a lattice-based model of cellulose I(α), one of the two major natural forms, at the resolution of explicit hydrogen bonds. The partition function and thermodynamic properties are evaluated using the transfer matrix method. Two competing hydrogen-bond patterns are found. This plasticity of the hydrogen-bond network leads to an entropic contribution stabilizing the crystalline fibril at intermediate temperatures. At these temperatures, an enhanced probability of bonding between the individual cellulose chains gives rise to increased resistance of the entire cellulose fibril to degradation, before the final disassembly temperature is reached. The results are consistent with the available crystallographic and IR spectroscopic experiments on the thermostability of cellulose I(α).     

New Routes to the Synthesis of Mes

New　Routes　to　the　Synthesis　of　Mes...     

Can the ebselen derivatives catalyze the isomerization of peroxynitrite to nitrate?

The reaction of ebselen and its derivatives (1-7) with peroxynitrite anion (ONOO(-); PN) has been studied in gas phase and in aqueous, dichloromethane, benzene, and cyclohexane solutions using B3LYP/6-311+G(d,p)//B3LYP/6-311G(d,p) and PCM-B3LYP/6-311+G(d,p)//B3LYP/6-311G(d,p) approaches, respectively. It was shown that the reaction of 2 (R=H) with PN proceeds via 2 + PN --> 2-PN --> 2-TS1 (O-O activation) --> 2-O(NO(2)(-)()) --> 2-SeO + NO(2)(-) pathway with a rate-determining barrier of 25.3 (14.8) kcal/mol at the NO(2)(-) dissociation step (numbers presented without parentheses are enthalpies, and those in parentheses are Gibbs free energies). The NO(3)(-) formation process, starting from the complex 2-O(NO(2)(-)()), requires by (7.9) kcal/mol more energy than the NO(2)(-) dissociation process and is unlikely to compete with the latter. Thus, in the gas phase, the peroxynitrite --> nitrate isomerization catalyzed by complex 2 is unlikely to occur. It is shown that the NO(3)(-) formation process is slightly more favorably than the NO(2)(-) dissociation process for complex 4, with a strongest electron-withdrawing ligand R=CF(3). Therefore, complex 4 (as well as complex 6 with R=OH) is predicted to be a good catalyst for peroxynitrite <--> nitrite isomerization in the gas phase. Solvent effects (a) change the rate-determining step of the reaction 2 + PN from NO(2)(-) dissociation in the gas phase to O-O activation, which occurs with barriers of (13.9), (8.4), (8.4), and (8.2) kcal/mol in water, dichloromethane, benzene, and cyclohexane, respectively, and (b) significantly reduce the NO(2)(-) dissociation energy, while only slightly destabilizing the NO(3)(-) formation barrier, and make the peroxynitrite <--> nitrate isomerization process practically impossible, even for complex 4.     

Extending the applicability of the O-ring theory to protein–DNA complexes

Many biological processes depend on protein-based interactions, which are governed by central regions with higher binding affinities, the hot-spots. The O-ring theory or the “Water Exclusion” hypothesis states that the more deeply buried central regions are surrounded by areas, the null-spots, whose role would be to shelter the hot-spots from the bulk solvent. Although this theory is well-established for protein–protein interfaces, its applicability to other protein interfaces remains unclear. Our goal was to verify its applicability to protein–DNA interfaces. We performed Molecular Dynamics simulations in explicit solvent of several protein–DNA complexes and measured a variety of solvent accessible surface area (SASA) features, as well as, radial distribution functions of hot-spots and null-spots. Our aim was to test the influence of water in their coordination sphere. Our results show that hot-spots tend to have fewer water molecules in their neighborhood when compared to null-spots, and higher values of ΔSASA, which confirms their occlusion from solvent. This study provides evidence in support of the O-ring theory with its applicability to a new type of protein-based interface: protein–DNA.     

How to explain the difficulties in the coffinite synthesis from the study of uranothorite?

The preparation of Th(1-x)U(x)SiO(4) uranothorite solid solutions was successfully undertaken under hydrothermal conditions (T = 250 °C). From XRD and EDS characterization, the formation of a complete solid solution between x = 0 (thorite) and x = 0.8 was evidenced. Nevertheless, additional (Th,U)O(2) dioxide and amorphous silica were systematically observed for the highest uranium mole loadings. The influence of kinetics parameters was then studied to avoid the formation of such side products. The variation of the synthesis duration allowed us to point out the initial formation of oxide phases then their evolution to a silicate phase through a dissolution/precipitation process close to that already described as coffinitization. Also, the uranium mole loading initially considered was found to significantly influence the kinetics of reaction, as this latter strongly slows down for x > 0.3. Under these conditions, the difficulties frequently reported in the literature for the synthesis of pure USiO(4) coffinite were assigned to a kinetic hindering associated with the coffinitization reaction.     

A New Route to the Synthesis of Ninhydrin

邻苯二甲酸二丁酯同乙酰乙酸乙酯的钠盐反应得到１，３－二酮氢化茚，再经二氧化硒氧化合成出较高收率的茚三酮。     

Is the dynamical polarization a significant part of the contribution of the triples to the correlation energy?

One may call dynamical polarization of doubly excited configurations the energy lowering of these configurations under the response of the other electrons to the so-created fluctuation of the electric field. This contribution of triply excited configurations may be identified and calculated through a computation that only requires a computation time proportional to the sixth power of the number of molecular orbitals (MOs), instead of the seventh power for the total contribution of the triples. Its amplitude depends on the choice of the MOs and becomes important when localized MOs are used.     

Pointing the way to the products? Comparison of the stress tensor and the second-derivative tensor of the electron density

The eigenvectors of the electronic stress tensor can be used to identify where new bond paths form in a chemical reaction. In cases where the eigenvectors of the stress tensor are not available, the gradient-expansion-approximation suggests using the eigenvalues of the second derivative tensor of the electron density instead; this approximation can be made quantitatively accurate by scaling and shifting the second-derivative tensor, but it has a weaker physical basis and less predictive power for chemical reactivity than the stress tensor. These tools provide an extension of the quantum theory of atoms and molecules from the characterization of molecular electronic structure to the prediction of chemical reactivity.     

A new approach to δ-lactones,related to the Prelog-Djerassi lactone.

The reductive rearrangement of 2-ethoxy-5-(2-alkenyl)-2H-tetrahydropyrane systems, in the presence of AlBu¹₃, was used as the key-step in the synthesis of Prelog-Djerassi related lactones.     

Some remarks to the derivation of the “generalized Lippmann equation”

In the present communication, an attempt is made to demonstrate (once again) some of the problems with the derivation of the “generalized Lippmann equation” considered to be valid by many researchers for solid electrodes and to address the problems in the framework of the Gibbs model of the interface by using only the basic principles of thermodynamics. By surveying the relevant literature, it has been shown that during the derivation of the equation, it was completely ignored that the Gibbs-Duhem equation (i.e., the electrocapillary equation) is a mathematical consequence which follows directly from the homogeneous degree one property of the corresponding thermodynamic potential function; consequently, the resulting expression cannot be correct. Some alternative approaches have also been considered. The adequacy of the open system and the partly closed system approach has been critically discussed, together with the possibility of introducing new thermodynamic potential functions.

     

On the applicability of the Kasha—Vavilov rule to C60

Via full correction for the instrumental response function, the fluorescence excitation spectrum of C₆₀ in methylcyclohexane between 240 and 580 nm is shown to match the absorption spectrum. This proves that, in contrast to earlier reports, also after UV excitation into higher S_n states, deactivation proceeds virtually completely via the S₁ state, in accordance with the Kasha—Vavilov rule.     

An enantioselective access to the (−)-tubotaiwine skeleton

An efficient induction of chirality is obtained when using α-phenylethylamine as a chiral starting material to produce in few steps the skeleton of anilinoacrylic indole alkaloids. Ring enlargement is described to produce the complete five-ring system of tubotaiwine⁽²⁾.     

Some questions on the exchange contribution to the effective potential of the Kohn–Sham theory

The current success of Density Functional Theory applications hinges upon the availability of explicitly density-dependent functionals to self-consistently solve a set of one-electron equations, the Kohn–Sham (KS) equations, which determine the occupied orbitals and its associated electronic density. In KS theory, a local exchange potential is proposed as part of an effective potential. This potential is compared to the exchange operator of the Hartree–Fock theory, which is of a non-local nature. The present paper discusses the variational framework of the KS equations, and the equivalence between both exchange potentials within a correlation-free theory. The common difficulties of current local exchange functionals to correctly simulate the non-locality of the exchange energy density in chemical systems are also analyzed and explained through an exactly solvable model. We give then numerical arguments and conclude by analyzing the performance of various commonly used approximations to exchange functionals.     

An efficient Approach to Modify the Catalyst Activity for the Hydrogenation of Nitrobenzene

Transition metal complexes have been used extensively for the hydrogenation in homogeneous system probably due to their high catalytic selectivities under mild operating conditions. In order to improve the homogeneous catalyst system, some studies on the homogeneous or soluble polymer-supported bimetallic catalysts have been recently carried out and enhanced activity, better selectivity were observed in selective hydrogenation, hydrodehalogenation, carbonylation, hydroformylation and regioselec…     

Is the Kissinger Equation Applicable to the Processes that Occur on Cooling?

The activation energy, E, of nonisothermal crystallization of polymer melts has been frequently evaluated by the Kissinger equation with positive rates of temperature variation, β. It is demonstrated that dropping the negative sign for β is a mathematically invalid procedure that generally makes the Kissinger equation inapplicable to the processes that occur on cooling. Simulations and an experimental example demonstrate that the use of this invalid procedure may result in erroneous values of E. Alternative methods of estimating the correct E values are suggested.