Reactive Monte Carlo and grand-canonical Monte Carlo simulations of the propene metathesis reaction system

The influence of silicalite-1 pores on the reaction equilibria and the selectivity of the propene metathesis reaction system in the temperature range between 300 and 600 K and the pressure range from 0.5 to 7 bars has been investigated with molecular simulations. The reactive Monte Carlo (RxMC) technique was applied for bulk-phase simulations in the isobaric-isothermal ensemble and for two phase systems in the Gibbs ensemble. Additionally, Monte Carlo simulations in the grand-canonical ensemble (GCMC) have been carried out with and without using the RxMC technique. The various simulation procedures were combined with the configurational-bias Monte Carlo approach. It was found that the GCMC simulations are superior to the Gibbs ensemble simulations for reactions where the bulk-phase equilibrium can be calculated in advance and does not have to be simulated simultaneously with the molecules inside the pore. The confined environment can increase the conversion significantly. A large change in selectivity between the bulk phase and the pore phase is observed. Pressure and temperature have strong influences on both conversion and selectivity. At low pressure and temperature both conversion and selectivity have the highest values. The effect of confinement decreases as the temperature increases.     

Crosslinking of PVA and glutaraldehyde in water monitored by viscosity and pulse field gradient NMR: a comparative study

The crosslinking of poly(vinyl alcohol) (PVA) with glutaraldehyde at 80 °C was characterized by viscosity and pulse field gradient (PFG) nuclear magnetic resonance (NMR) techniques. NMR signified an initial dormant period of approximately 6 hr, in which the self‐diffusion coefficient of PVA was found to be constant and independent of time. During the next 7 hr (the “primary” gel period), this induction period was succeeded by a fast decay of the self‐diffusion coefficient of rate (9.13 ± 0.45) × 10⁻⁵ sec⁻¹ followed by a slower decay rate of (3.22 ± 0.30) × 10⁻⁵ sec⁻¹ (the “secondary” gel period). The viscosity of the solution showed the same time behavior, i.e. an initial dormant period (∼ 6 hrs), followed by a fast increase of the viscosity for the next 7 –8 hr. During the “secondary” gel regime, the viscosity became too large to be reliably determined. However, within the time regime where both techniques produced reliable data, they gave identical information regarding the kinetics of the gel process, suggesting that PFG NMR enables in situ monitoring of gelation within porous materials (for instance sandstone). Copyright © 2000 John Wiley & Sons, Ltd.     

Thermal Reaction of Azulene-1-carbaldehydes with Dimethyl Acetylenedicarboxylate

Azulene-1-carbaldehydes which have Me substituents at C(3) and C(8) and no substituent at C(6) react with excess dimethyl acetylenedicarboxylate (ADM) in decalin at 200° to yield exclusively the Diels-Alder adduct at the seven-membered ring (cf. Scheme 3). The corresponding 1-carboxylates behave similarly (Scheme 4). Azulene-1-carbaldehydes which possess no Me substituent at C(8) (e.g. 11 , 12 in Scheme 2) gave no defined products when heated with ADM in decalin. On the other hand, Me substitutents at C(2) may also assist the thermal addition of ADM at the seven-membered ring of azulene-1-carbaldehydes (Scheme 6). However, in these cases the primary tricyclic adducts react with a second molecule of ADM to yield corresponding tetracyclic compounds. The new tricyclic aldehydes 16 and 17 which were obtained in up to 50% yield (Scheme 3) could quantitatively be decarbonylated with [RhCl(PPh₃)₃] in toluene at 140° to yield a thermally equilibrated mixture of four tricycles (Scheme 8). It was found that the thermal isomerization of these tricycles occur at temperatures as low as 0° and that at temperatures > 40° the thermal equilibrium between the four tricycles is rapidly established via [1,5]-C shifts. The establishment of the equilibrium makes the existence of two further tricycles necessary (cf. Scheme 8). However, in the temperature range of up to 85° these two further tricycles could not be detected by ¹H-NMR. When heated in the presence of excess ADM in decalin at 180°, the ‘missing’ tricyclic forms could be evidenced by their tetracyclic trapping products ‘anti’- 45 and ‘anti’- 48 , respectively (Scheme 9).     

WYD-like Skew Information Measures

Following recent advances in the theory of operator monotone functions we introduce new classes of WYD-like skew information measures.     

Measurements of turbulent mixing due to Kelvin–Helmholtz instability in high-energy-density plasmas

Kelvin–Helmholtz (KH) turbulent mixing measurements were performed in experiments on the OMEGA Laser Facility [T.R. Boehly et al., Opt. Commun. 133 (1997) 495]. In these experiments, laser-driven shock waves propagated through low-density plastic foam placed on top of a higher-density plastic foil. Behind the shock front, lower-density foam plasma flowed over the higher-density plastic plasma. The interface between the foam and plastic was KH unstable. The experiments were performed with pre-imposed, sinusoidal 2D perturbations, and broadband 3D perturbations due to surface roughness at the interface between the plastic and foam. KH instability growth was measured using X-ray, point-projection radiography. The mixing layer caused by the KH instability with layer width up to ～100 μm was observed at a location ～1 mm behind the shock front. The measured mixing layer width was in good agreement with simulations using a K–L turbulent mixing model in the two-dimensional ARES hydrodynamics code. In the definition of the K–L model K stands for the specific turbulent kinetic (K) energy, and L for the scale length (L) of the turbulence.     

Comparison of ethanol-soluble proteins from different rye (Secale cereale) varieties by two-dimensional electrophoresis

The major storage proteins from six rye varieties, grown under the same conditions in 1997 and 1998 in Rønhave, Denmark, were analyzed by two‐dimensional (2‐D) polyacrylamide gel electrophoresis. The proteins were extracted from ground rye kernels with 70% ethanol and separated by 2‐D electrophoresis. The gels were scanned, compared using ImageMaster® software and the data sets were analyzed by principal component analysis (PCA) using THE UNSCRAMBLER software. Afterwards MATLAB was used to make a cluster analysis of the varieties based on PCA. The analysis of the gels showed, that the protein patterns (number of different proteins and their isoelectric points and molecular weights) from the six rye varieties were different. Based on the presence of unique cultivar‐specific spots it was possible to differentiate between all six varieties if the two harvest years were investigated separately. When the results were combined from the two years five varieties could be differentiated. The results from the PCA confirmed the finding of the unique spots and cluster analysis was made in order to illustrate the results. The combination of the results from 2‐D electrophoresis and other grain characteristics showed that one protein spot was located close to the parameters bread volume and bread height.     

High-temperature 2D Fermi surface of SrTiO3 studied by energy-filtered PEEM

Functional oxides displaying phenomena such as 2D electron gas (2DEG) at oxide interfaces represent potential technological breakthroughs for post-CMOS (Complementary Metal Oxide Semiconductor) electronics. Noninvasive techniques are required to study the surface chemistry and electronic structure underlying their often unique electrical properties. The sensitivity of photoemission electron microscopy (PEEM) to local potential, chemistry, and electronic structure makes it an invaluable tool for probing the near surface region of microscopic regions and domains of functional materials. In particular, PEEM allows single shot acquisition of the 2D Fermi surface and full angular probing of the symmetry-induced intensity modulations. We present results demonstrating a 2DEG at the surface of SrTiO₃(001) at 140 K. The 2DEG is created by soft X-ray irradiation and can be reversibly controlled by a combination of soft X-rays and oxygen partial pressure.     

A survey and new results on computer enumeration of polyhex and fusene hydrocarbons

After a short historic review, we briefly describe a new algorithm for constructive enumeration of polyhex and fusene hydrocarbons. In this process our algorithm also enumerates isomers and symmetry groups of molecules (which implies enumeration of enantiomers). Contrary to previous methods often based on the boundary code or its variants (which records orientation of edges along the boundary) or on the DAST code, which uses a rigid dualist graph (whose vertices are associated with faces and edges with adjacency between them), the proposed algorithm proceeds in two phases. First inner dual graphs are enumerated; then molecules obtained from each of them by specifying angles between adjacent edges are obtained. Favorable computational results are reported. The new algorithm is so fast that output of the structures is by far the most time-consuming part of the process. It thus contributes to enumeration in chemistry, a topic studied for over a century, and is useful in library making, QSAR/QSPR, and synthesis studies.     

Graphs with maximum connectivity index

Let G be a graph and d_v the degree (=number of first neighbors) of its vertex v. The connectivity index of G is χ=∑ (d_ud_v)^−1/2, with the summation ranging over all pairs of adjacent vertices of G. In a previous paper (Comput. Chem. 23 (1999) 469), by applying a heuristic combinatorial optimization algorithm, the structure of chemical trees possessing extremal (maximum and minimum) values of χ was determined. It was demonstrated that the path P_n is the n-vertex tree with maximum χ-value. We now offer an alternative approach to finding (molecular) graphs with maximum χ, from which the extremality of P_n follows as a special case. By eliminating a flaw in the earlier proof, we demonstrate that there exist cases when χ does not provide a correct measure of branching: we find pairs of trees T, T′, such that T is more branched than T′, but χ(T)>χ(T′). The smallest such examples are trees with 36 vertices in which one vertex has degree 31.