Dynamic properties of metallocenium ion pairs in solution by atomistic simulations

We report a molecular dynamics study of the dynamics and energetic of the [H(2)Si(Cp)(2)ZrMe(+)][MeB(C(6)F(5))(3)(-)], IP1, and [Me(2)Si(Cp)(2)ZrMe(+)][B(C(6)F(5))(4)(-)], IP2, ion pairs in benzene. The metrical parameters obtained for the IP1 ion pair are in excellent agreement with the NMR data reported for the strictly related [Me(2)Si(Cp)(2)ZrMe(+)][MeB(C(6)F(5))(3)(-)] ion pair (J. Am. Chem. Soc. 2004, 126, 1448). This validates the molecular modeling protocol we developed. Simulation of the IP2 ion pair suggests that the counterion oscillates between two geometries characterized by a different coordination pattern of the F atoms to the Zr cation. In one case the B(C(6)F(5))(4)(-) coordinates to the metal with two F atoms of the same aryl ring, whereas in the other case two F atoms of different aryl rings are involved in the coordination. Strong solvent reorganization occurs around IP1 and IP2, as well as around the two isolated cations. In the case of the two ion pairs solvent is never coordinated directly to the metal, whereas in the absence of the counterion one benzene molecule is coordinated to the metal through a cation-pi interaction. Free energy calculations result in ion pair free energies of separation of 36.8 and 23.3 kcal/mol for IP1 and IP2, respectively. Simulations with the Zr-B distance fixed at values > 7 A have been also performed. This mimics the situation occurring after counterion displacement by an inserting monomer molecule during olefin polymerization by the title catalysts.     

Towards Comprehensive Coal Combustion Modelling for LES

Large eddy simulations of pulverised coal combustion (PCC-LES) stabilised on a laboratory-scale piloted jet burner are carried out. The joint simulation effort of three research groups at Freiberg University (FG), Imperial College (IC) and Stuttgart University (ST) is presented, and the details of the comprehensive coal combustion models and their numerical implementation in three different computer programs are discussed. The (standard) coal sub-models and parameters used by the different groups are unified wherever possible. Differences amongst the groups are a different code basis and an Eulerian treatment of the coal particles by IC, while FG and ST use the Lagrangian framework for particle transport. The flow modelling is first validated for the corresponding non-reacting case and all LES calculations accurately capture the experimental trends. Velocity field statistics for the PCC case are in good accordance with the experimental evidence, but scalar statistics illustrate the complexity of coal combustion modelling. The results show notable differences amongst the groups that cannot only be attributed to the different treatment of the particle phase, and they highlight the difficulty to assess and interpret the quality of specific modelling approaches, and a need for further work by the research community. The present study is the first to compare three originally independent transient coal simulations and a step towards comprehensive PCC-LES.     

Anomalous atomic hydrogen shock pattern in a supersonic plasma Jet

A two-photon laser-induced fluorescence study on the transport of ground-state atomic hydrogen in a supersonic plasma jet, generated from an Ar-H (2) mixture, reveals an unexpected shock pattern. Whereas both the axial-velocity profile and the temperature profile of hydrogen atoms along the jet centerline can be interpreted in terms of a supersonic expansion of an Ar-H gas mixture, the H-atom density profiles do not satisfy the well established Rankine-Hugoniot relation leading to a nonconservation of the forward flux. The experimental results show that H atoms escape from the supersonic expansion by a diffusion process due to strong density gradients between the core of the jet and its vicinity.     

Radical Intermediates in the Jacobsen - Katsuki Epoxidation

Insight into the controversial mechanism of the Mn - salen-catalyzed epoxidation of olefins is provided in a theoretical study based on density functional theory. The calculations suggest that radical species A, but not manganaoxetanes B, are likely candidates for viable intermediates.     

Biodegradable Polyhydroxyalkanoates by Stereoselective Copolymerization of Racemic Diolides: Stereocontrol and Polyolefin-Like Properties

Bacterial polyhydroxyalkanoates (PHAs) are a unique class of biodegradable polymers because of their biodegradability in ambient environments and structural diversity enabled by side-chain groups. However, the biosynthesis of PHAs is slow and expensive, limiting their broader applications as commodity plastics. To overcome such limitation, the catalyzed chemical synthesis of bacterial PHAs has been developed, using the metal-catalyzed stereoselective ring-opening (co)polymerization of racemic cyclic diolides (rac-8DL^R, R=alkyl group). In this combined experimental and computational study, polymerization kinetics, stereocontrol, copolymerization characteristics, and the properties of the resulting PHAs have been examined. Most notably, stereoselective copolymerizations of rac-8DL^Me with rac-8DL^R (R=Et, Bu) have yielded high-molecular-weight, crystalline isotactic PHA copolymers that are hard, ductile, and tough plastics, and exhibit polyolefin-like thermal and mechanical properties.     

A combined mechanistic and computational study of the gold(I)-catalyzed formation of substituted indenes

Substituted indenes can be prepared after a sequence [1,3] O-acyl shift-hydroarylation-[1,3] O-acyl shift. Each step is catalyzed by a cationic NHC-Gold(I) species generated in situ after reaction between [(IPr)AuOH] and HBF(4)·OEt(2). This interesting silver-free way is fully supported by a computational study justifying the formation of each intermediate.     

Comparison of different ruthenium-alkylidene bonds in the activation step with N-heterocyclic carbene Ru-catalysts for olefins metathesis

Density functional theory calculations have been used to investigate the activation steps involving three of the most used alkylidene groups in Ru-catalysts for olefins metathesis. Specifically, we compared the benzylidene, the indenylidene and a phosphonium alkylidene groups. Calculations reveal that the benzylidene and the indenylidene groups behave rather similarly, despite their structural differences. The phosphonium alkylidene group seems to have the most favourable activation pathway.     

Exploring Electronic and Steric Effects on the Insertion and Polymerization Reactivity of Phosphinesulfonato PdII Catalysts

Thirteen different symmetric and asymmetric phosphinesulfonato palladium complexes ([{( ^X 1 ‐Cl)‐μ‐M}_n], M=Na, Li, 1 =^X(P^O)PdMe) were prepared (see Figure 1). The solid‐state structures of the corresponding pyridine or lutidine complexes were determined for ^(MeO)2 1‐py , ^(iPrO)2 1‐lut , ^(MeO,Me2) 1‐lut , ^(MeO)3 1‐lut , ^CF3 1‐lut , and ^Ph 1‐lut . The reactivities of the catalysts ^X 1 , obtained after chloride abstraction with AgBF₄, toward methyl acrylate (MA) were quantified through determination of the rate constants for the first and the consecutive MA insertion and the analysis of β‐H and other decomposition products through NMR spectroscopy. Differences in the homo‐ and copolymerization of ethylene and MA regarding catalyst activity and stability over time, polymer molecular weight, and polar co‐monomer incorporation were investigated. DFT calculations were performed on the main insertion steps for both monomers to rationalize the effect of the ligand substitution patterns on the polymerization behaviors of the complexes. Full analysis of the data revealed that: 1) electron‐deficient catalysts polymerize with higher activity, but fast deactivation is also observed; 2) the double ortho‐substituted catalysts ^(MeO)2 1 and ^(MeO)3 1 allow very high degrees of MA incorporation at low MA concentrations in the copolymerization; and 3) steric shielding leads to a pronounced increase in polymer molecular weight in the copolymerization. The catalyst properties induced by a given P‐aryl (alkyl) moiety were combined effectively in catalysts with two different non‐chelating aryl moieties, such as ^{c HexO/(MeO)2} 1 , which led to copolymers with significantly increased molecular weights compared to the prototypical ^MeO 1 .     

Multinuclear Solid‐State Magnetic Resonance as a Sensitive Probe of Structural Changes upon the Occurrence of Halogen Bonding in Co‐crystals

Although the understanding of intermolecular interactions, such as hydrogen bonding, is relatively well‐developed, many additional weak interactions work both in tandem and competitively to stabilize a given crystal structure. Due to a wide array of potential applications, a substantial effort has been invested in understanding the halogen bond. Here, we explore the utility of multinuclear (¹³C, ^14/15N, ¹⁹F, and ¹²⁷I) solid‐state magnetic resonance experiments in characterizing the electronic and structural changes which take place upon the formation of five halogen‐bonded co‐crystalline product materials. Single‐crystal X‐ray diffraction (XRD) structures of three novel co‐crystals which exhibit a 1:1 stoichiometry between decamethonium diiodide (i.e., [(CH₃)₃N⁺(CH₂)₁₀N⁺(CH₃)₃][2 I^?]) and different para‐dihalogen‐substituted benzene moieties (i.e., p‐C₆X₂Y₄, X=Br, I; Y=H, F) are presented. ¹³C and ¹⁵N NMR experiments carried out on these and related systems validate sample purity, but also serve as indirect probes of the formation of a halogen bond in the co‐crystal complexes in the solid state. Long‐range changes in the electronic environment, which manifest through changes in the electric field gradient (EFG) tensor, are quantitatively measured using ¹⁴N NMR spectroscopy, with a systematic decrease in the ¹⁴N quadrupolar coupling constant (C_Q) observed upon halogen bond formation. Attempts at ¹²⁷I solid‐state NMR spectroscopy experiments are presented and variable‐temperature ¹⁹F NMR experiments are used to distinguish between dynamic and static disorder in selected product materials, which could not be conclusively established using solely XRD. Quantum chemical calculations using the gauge‐including projector augmented‐wave (GIPAW) or relativistic zeroth‐order regular approximation (ZORA) density functional theory (DFT) approaches complement the experimental NMR measurements and provide theoretical corroboration for the changes in NMR parameters observed upon the formation of a halogen bond.     

Effect of cooling rate on the crystal/mesophase polymorphism of polyamide 6

The solidification of the quiescent polyamide 6 (PA 6) melt has been analyzed as a function of the cooling rate in a wide range between 1.67 × 10⁻² and close to 2 × 10² K s⁻¹, by means of differential scanning calorimetry at a low cooling rate of up to about 1 K s⁻¹, and by the recording of continuous cooling curves and time-resolved X-ray diffraction on cooling at a higher rate. The performed experiments allowed for the first time to establish the relationship between the cooling rate, the crystallization temperature, and the X-ray structure of PA 6. The exclusive formation of monoclinic α-crystals is only detected if the crystallization temperature is higher than about 430 K or if the cooling rate is slower than about 5 K s⁻¹, respectively. The formation of α-crystals is increasingly replaced by the development of mesophase with increasing cooling rate, accompanied with a decrease of the temperature of crystallization/ordering. Finally, completely amorphous samples were obtained on cooling faster than about 10² K s⁻¹. The continuous decrease of the temperature of crystallization with increasing cooling rate, regardless of the specific structure formed, precludes a primary effect of the nucleation mechanism on the α-crystal/mesophase polymorphism of PA 6. A preliminary discussion of the effect of molar mass of PA 6 on the cooling rate-dependent polymorphism is also included.