ansa-Zirconocenium catalysis of syndiospecific polymerization of propylene: Theory and experiment

The syndiospecific propylene polymerizations catalyzed by isopropylidene(cyclopentadienyl)(fluorenyl)- and (2,2-dimethylpropylidene)(cyclopentadienyl)(fluorenyl)-zirconocenium ( 1 ⁺ and 2 ⁺) have been investigated theoretically and compared with experimental observations. With the ab initio calculated structures for the transition state (TS) of 1 ⁺(M)P and 2 ⁺(M)P (M = propylene, P = 2-methylpentyl), their steric energies (E°) have been computed using MM2 force-field. The difference between steric energies E°(m) and E°(r) for the meso and racemic enchainment of propylene, respectively, is defined as the stereocontrol energy [δE°(m ? r)] for syndiotactic propagation. The δE°(m ? r) for the TS of 1 ⁺ (M)P is about 2.1 kcal/mol, the value is 1 kcal/mol greater for 2 ⁺(M)P. The observed steric pentad distributions of the syndiotactic poly(propylene) obtained by these catalysts are consistent with smaller effective stereocontrol energy, which is about two-third as large as δE°(m ? r) values calculated for the MM2 optimized structure. Syndiotactic enchainment is favored over isotactic enchainment for all combinations of site configurations in the catalyst. α-Agostic interaction seems to enhance syndioselectivity, whereas γ-agostic interaction changes the stereoselectivity to meso enchainment. The mirror plane symmetry of the syndiotactic propagating species renders the stereoselectivity of the polymerization insensitive to reaction conditions. These catalysts are also highly regiospecific. © 1995 John Wiley & Sons, Inc.     

手性氮磷配体与钌的配位过程的研究—原位变温 31P核磁及分子模拟计算

用原位变温 ³¹P NMR和分子模拟研究了手性氮磷配体与金属钌的配位过程。首先确定了配体及其与金属的配合物的化学位移信号分别在δ=-13.0ppm和δ=48.2ppm。配位过程在293～343K温度范围内,相继生成四配位和五配位中间体。四配位体在 ³¹P NMR上对应于30.8ppm和-15.0ppm的两个信号;五配位中间体对应于35.1ppm和-16.5ppm的两个信号;最后在343K,产物中只有六配位的配合物存在,其信号出现在48.2ppm。为了得到各种中间体的结构,用分子模拟方法进行了结构优化计算,并比较了它们的构象能。计算结果表明,四配位中间体有两种构象,能量相差9kcal·mol^-1,它们可能同时存在于平衡状态。而五配位中间体只有一种绝对优势构象,尚有一个磷原子未参与配位。六配位的钌金属配合物的结构优化结果显示,两个氯原子分别位于PNNP原子所构成平面的两侧。其构象能为162.0kcal·mol^-1,其中键角能的贡献是112.5kcal·mol^-1,而非键静电作用是-41.4kcal·mol^-1,这表明分子内的静电吸引力对于形成完全配位产物是十分有利的,但是收敛的配体分子却承受了较大的键角张力。     

Particle dynamics and particle heat and mass transfer in thermal plasmas. Part III. Thermal plasma jet reactors and multiparticle injection

Thermal plasma processing involves complex interactions of particulates with plasmas. In previous studies (see Parts I and II of this series), an assessment of different effects has been made considering the dynamics and heat and mass transfer of a single particle immersed into a thermal plasma. The last paper of this sequence is concerned with the simulation of thermal plasma jet reactors and the effects caused by multiparticle injection.A mathematical model is proposed for the simulation of thermal plasma jet reactors, including the mixing phenomena between the jet and the surrounding gases by generalizing the governing equations for simple mixing flows. Also included is the density fluctuation effect by extending the K- model to a four-equation turbulence model combined with a probability density function. This model is internally consistent covering additional physical phenomena which are not covered by existing models. Unfortunately, its expected higher accuracy cannot be proven because of the present uncertainties associated with the input.For multiparticle injection, the simulation repeats calculations for single-particle injection, but with different initial conditions correcting the solutions by considering the coupling effects between particles and the plasma.The results indicate that (i) thermal plasmas show different mixing behavior in different gases; (ii) the density fluctuation effect is important since it causes large differences between the mass-weighted and unweighted time-averaged temperatures of thermal plasma jets; (iii) coupling effects become important when the particle loading rate exceeds half of the plasma mass flow rate; (iv) there are 16 constraints imposed on the modeling work which have to be considered for establishing a base for comparison with future experimental studies.     

A comparative study of various computational approaches in calculating the structure of pyridoxal 5'-phosphate (PLP)-dependent beta-lyase protein. The importance of protein environment

Various computational approaches, using molecular mechanics (Amber), semiempirical (AM1), density functional (B3LYP), and various ONIOM methods, have been comparatively investigated for the structure of Escherichia coli NifS CsdB protein. The structure of the entire monomer containing 407 amino acid residues and 579 surrounding water molecules has been optimized. The full geometry optimization in the "active site-only" approach (including only active site atoms) has been found to give the largest root-mean-square (RMS) deviation from the X-ray structure; a much better agreement has been achieved by keeping the atoms leading to the backbones of some amino acids frozen in their positions in the X-ray structure. The best agreement has been attained by including the surrounding protein in the calculations using the two-layer ONIOM (B3LYP:Amber) approach. The results presented in this study conclusively demonstrate the importance of the protein/active-site interaction on the active-site structure of the enzyme. The present theoretical study represents the largest system studied at the ONIOM level to date, containing 7992 atoms, including 84 atoms in the QM region and rest in the MM region.     

Vapor pressure of R227ea + ethanol at 343.13 K by molecular simulation

A new molecular model for 1,1,1,2,3,3,3-heptafluoropropane (R227ea) was developed on the basis of quantum chemical calculations and optimized using experimental vapor pressure and bubble density data. In combination with an existing model for ethanol, a molecular model for the binary mixture R227ea + ethanol was defined, using the Lorentz–Berthelot combining rule. It was validated at 283.17 K, where, considering the statistical uncertainties, it agrees to the experimental vapor pressure. The vapor–liquid equilibrium, comprising both bubble line and dew line data, was predicted at 343.13 K by molecular simulation. The Peng–Robinson equation of state fails for this system.     

Human topoisomerase I poisoning: docking protoberberines into a structure-based binding site model

Summary Using the X-ray crystal structure of the human topoisomerase I (top1) – DNA cleavable complex and the Sybyl software package, we have developed a general model for the ternary cleavable complex formed with four protoberberine alkaloids differing in the substitution on the terminal phenyl rings and covering a broad range of the top1-poisoning activities. This model has the drug intercalated with its planar chromophore between the −1 and +1 base pairs flanking the cleavage site, with the nonplanar portion pointing into the minor groove. The ternary complexes were geometry-optimized and relative interaction energies, computed by using the Tripos force field, were found to rank in correct order the biological potency of the compounds; in addition, the model is also consistent with the top1-poisoning inactivity of berberine, a major prototype of the protoberberine alkaloids. The model might serve as a rational basis for elaboration of the most active compound as a lead structure, in order to develop more potent top1 poisons as next generation anti-cancer drugs.     

Tricyclo[2.2.0.0]hexane: a new hypothetical molecule which should have only one inverted carbon atom

HF and MP2 calculations with the 6-31G** and 6-311G** basis sets and those at MP2/cc-pVTZ level were carried out for the hypothetical tricyclo[2.2.0.0^1,3]hexane. The results indicate that the molecule under study should have one carbon atom with highly unusual inverted configuration. The analysis of the vibrational frequencies of this molecule as well as the analysis of its plausible decomposition routes performed at the DFT level indicate that this unique molecule could be a plausible synthetic target.     

Structural study of pyrimidine nucleoside analogues. I: Molecular mechanics and semiempirical calculations of 2′-deoxy-2′-fluoroarabinofuranosyluracils

Molecular mechanics (MM) calculations have been performed on the title compounds. For the MM minimum energy conformation obtained by conformational analysis, molecular orbital (MO) calculations (MNDO and AM1) have also been performed. The geometries obtained have been compared with the experimental ones extracted from the Cambridge Structural Database (CSD). A qualitative structure-activity relationship has been pointed out based on the electrostatic potentials calculated at different positions on the electronic surface.     

Host–Guest Interactions of Risperidone with Natural and Modified Cyclodextrins: Phase Solubility, Thermodynamics and Molecular Modeling Studies

The solubility of risperidone (Risp) in aqueous buffered cyclodextrin (CD) solution was investigated for α-, β-, γ- and HP-β-CD. The effects of pH, ionic strength and temperature on complex stability were also explored. Neutral Risp tends to form higher order complexes (1:2) with both β- and HP-β-CD, but only 1:1 type complexes with α-, and γ-CD. The tendency of Risp to complex with cyclodextrins is in the order β-CD > HP-β-CD > γ-CD > α-CD. The 1:1 complex formation constant of Risp/HP-β-CD increases with increasing ionic strength in an opposite trend to the inherent solubility (S ₀) of Risp, thus indicating significant hydrophobic effect. The hydrophobic effect contributes to the extent of 72% towards neutral Risp/HP-β-CD complex stability, while specific interactions contribute only 4.7 kJ/mol. Thermodynamic studies showed that 1:1 Risp/HP-β-CD complex formation is driven by a favorable enthalpy change (ΔH ⁰=−31.2 kJ/mol, ΔS ⁰=−7 J/mol.K) while the 1:2 complex is largely driven by entropy changes (ΔH ⁰=−5.0 kJ/mol, ΔS ⁰=42 J/mol.K). Complex stability was found to vary with pH, with a higher formation constant for neutral Risp. Molecular mechanical computations using MM (atomic charges and bond dipole algorithms) and Amber force fields, which were carried out to explore possible sites of interactions between Risp and CDs and to rationalize complex stoichiometry, produced similar results concerning optimal inclusion complex geometries and stoichiometries.     

Modeling of H2 and H2/CH4 Moderate-Pressure Microwave Plasma Used for Diamond Deposition

One-dimensional transport models of moderate-pressure H ₂ and H ₂ /CH ₄ plasmas obtained in a diamond deposition microwave reactor are presented. These models describe the plasma as a thermochemically nonequilibrium flow with three different energy modes. The solution of the one-dimensional plasma transport equations enabled the estimation of plasma species concentrations and temperatures on the axis of the reactor. As far as pure H ₂ plasmas are concerned, results showed that the model predictions of gas and vibration temperatures are in good agreement with experimental measurements. The model also yields a relatively good qualitative prediction of the variations of H-atom mole fraction with the power density absorbed by the plasma. The results obtained for H ₂ /CH ₄ discharges showed that the model prediction on the variations of H-atom mole fraction with methane percentage in the discharge is in good qualitative agreement with experimental results. They also showed that methane is rapidly converted to acetylene before reaching the discharge zone. The concentrations of neutral hydrocarbon species in the reactor are mainly governed by thermal chemistry. The addition of methane strongly affects the ionization kinetics of the plasma. Three major ions are generally obtained in H ₂ /CH ₄ plasmas: C ₂ H ₂ ⁺ , C ₂ H ₃ ⁺ , and C ₂ H ₅ ⁺ . The relative predominance of these ions depends on the considered plasma region and on the discharge conditions. The ionic species concentrations are also mainly governed by chemistry, except very near the substrate surface. Finally the use of this transport model along with the surface chemistry model of Goodwin ⁽¹⁾ enabled us to estimate the diamond growth rate for several discharge conditions.