Collective motion artifacts arising in long‐duration molecular dynamics simulations

We tested a variety of molecular dynamics simulation strategies in long‐duration (up to several nanoseconds) constant‐temperature simulations of liquid water under periodic boundary conditions. Such long durations are necessary to achieve adequate conformational sampling in simulations of membrane assemblies and other large biomolecular systems. Under a variety of circumstances, serious artifacts arise in the form of spurious collective behavior that becomes obvious only after the simulation has gone at least several hundred picoseconds. The potential energy of the system drops and the system changes from a liquid to an icy or glassy state. The underlying cause is accumulated center‐of‐mass motion of the system, coupled with velocity rescaling associated with constant‐temperature control. The velocity rescaling in the constant‐temperature algorithm reduces the thermal velocity as the net center‐of‐mass velocity grows, effectively causing the kinetic energy of the system to drain from thermal motions into coordinated motions. We found that the incidence and magnitude of the underlying artifactual motion leading to the spurious transition is mediated by: choice of method for computing electrostatic interactions; choice of ensemble; size of the simulation cell; SHAKE tolerance; frequency of nonbonded pairlist updating; and closeness of coupling to the temperature bath. The appearance of the spurious transition can be avoided by periodically subtracting net center‐of‐mass motion during the dynamics, or by improving the accuracy of the simulation by means of tightening SHAKE tolerance and updating nonbonded pairlists every timestep. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 121–131, 2000     

Multiresolution reproducing kernel particle method for computational fluid dynamics

Multiresolution analysis based on the reproducing kernel particle method (RKPM) is developed for computational fluid dynamics. An algorithm incorporating multiple-scale adaptive refinement is introduced. The concept of using a wavelet solution as an error indicator is also presented. A few representative numerical examples are solved to illustrate the performance of this new meshless method. Results show that the RKPM is a good candidate for tackling the widespread large-scale problems in fluid dynamics. © 1997 John Wiley & Sons, Ltd.     

Synthesis and lithographic applications of highly metallized cluster-based polyferrocenylsilanes

We report the use of a cobalt-clusterized polyferrocenylsilane (Co-PFS) as a precursor to patterned ferromagnetic ceramics. Co-PFS was synthesized. Functioning as a negative resist, Co-PFS lines with widths of 10-300 μm were patterned using UV-photolithography, while features as small as 500 nm were afforded by electron-beam lithography. Subsequent pyrolytic treatment of the lithographically patterned Co-PFS yielded ferromagnetic ceramics containing Fe/Co nanoparticles. Due to its high metal-loading, Co-PFS is a good etch resist for oxygen and hydrogen plasma reactive ion etching. Reactive ion etching of a thin film of Co-PFS in a secondary magnetic field allowed direct access to ferromagnetic ceramic films, providing a viable alternative to pyrolysis.     

Modification of Zeolite Morphology via NH4F Etching for Catalytic Bioalcohol Conversion

Various commercial zeolites, including FER, MOR, ZSM-5, BEA, and FAU frameworks, were treated with NH₄F aqueous solutions to study the effects of fluoride etching on different zeolite frameworks. NH₄F-treated small-medium pore FER, MOR, and ZSM-5 samples showed much higher mesoporosities than the untreated ones without alteration of the structural compositions and acidic properties. On the other hand, the 12-membered ring zeolites BEA and FAU showed severe dissolution of the framework aluminosilicate structure after NH₄F etching due to the high accessibility of fluoride species into the framework structures. The effect of NH₄F concentration on the fluoride treatment of H-ZSM-5 zeolite was specifically studied. From the results, we observed that structural etching with 20 wt % NH₄F was optimal for fabricating open-pore H-ZSM-5 zeolite and resulted in a high mesoporosity with comparable relative crystallinity and acidity with respect to the untreated H-ZSM-5. The catalytic activities of the open-pore H-ZSM-5 were evaluated with acid-catalyzed methanol and bioethanol conversions. Remarkably, the hierarchical open-pore H-ZSM-5 zeolite fabricated via fluoride etching exhibited an enhanced catalytic performance in bioethanol conversion with >85 % conversion over 34 h TOS and a higher catalytic stability in methanol conversion than the parent H-ZSM-5 (~50 % of bioethanol conversion at 34 h TOS).     

激光诱导荧光诊断中荧光频率极限的实验研究

在用激光诱导荧光法测量多偶极腔体中的氩等离子体的离子速度分布函数的实验中，研究了激光频率与信号强度的关系。在给定等离子体放电参数条件下，发现存在一个入射激光诱导荧光的极限频率，大于该频率时荧光强度不再增加。该诱导荧光的极限频率随荧光收集系统中透镜组中的小孔大小而改变。     

Generation of Helical States – Breaking of Symmetries,Curie's Principle,and Excited States**

Herein, we deeply detail for the very first time mathematical concepts behind the generation of helical molecular orbitals (MOs) for linear chains of atoms. We first give a definition of helical MOs and we provide an index measuring how far a given helical states is from a perfect helical distribution. Structural properties of helical distribution for twisted -cumulene and cumulene version of Möbius systems are given. We then give some simple structural assumptions as well as symmetry requirements ensuring the existence of helical MOs. Considering molecules which do not admit helical MOs, we provide a first way to induce helical states by the breaking of symmetries. We also explore an alternative way using excited conformations of given molecules as well as different electronic multiplicities.     

Synthesis of a diazacrown ether containing polymers and formation of complexes with alkaline earth metals

We report the synthesis of a polymer that contains the diaza‐18‐crown‐6 moiety on the polymer main chain. Metal containing polymers were obtained when the respective barium or strontium complexes were used as monomers. These polymers emitted greenish blue light in solution, and the metal had little influence on absorption and emission energies. In the solid state, the position of the emission peaks was different probably due to a different degree of aggregation.     

Polymerization of norbornene with Co(II) complexes

The norbornene polymerization was studied in the presence of 6 pyridine bis(imine) cobalt(II) complexes activated with methylaluminoxane (MAO). Norbornene was also polymerized with CoCl₂ associated to MAO. All these catalytic systems generate an addition polymerization of norbornene, yielding fully saturated polymers. It was shown that the polymerization yield and the molar masses are highly dependant on several reaction parameters (monomer concentration, [Al]/[Co] ratio, polymerization temperature and time) and the frame of the ligand.     

Stereospecific synthesis of 2‐phthalimido‐2a,3,4,5‐tetrahydro‐1H‐azeto[2,1‐d][1,5]benzothiazepin‐1‐ones

2a,4‐Disubstituted 2‐phthalimido‐2a,3,4,5‐tetrahydro‐1H‐azeto[2,1‐d][1,5]benzothiazepin‐1‐ones were synthesized by cycloaddition reactions of 2,4‐disubstituted 2,3‐dihydro‐1,5‐benzothiazepines and phthalimidoketene, generated from phthalimidoacetyl chloride, in the presence of triethylamine in anhydrous benzene. The stereochemistry was discussed for the cycloaddition reaction. © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:276–279, 2002; Published online in Wiley Interscience (www.interscience.wiley.com). DOI 10.1002/hc.10029