On semiperfect associative pairs

It is known [8] that a semiperfect ring is characterized by the existence of a frame, i.e, a complete set of local orthogonal idempotents. We prove in this paper that a similar behaviour occurs when dealing with an associative pair A, namelyAis semiperfect if and only if Acontains a frame and ā=A/Rad Ais unital. Moreover, we show that, when ā is unital, the existence of a frame for Ais equivalent to the condition that every irreducible right A-module is isomorphic to _e A/e(RadA) for some idempotent e of A.     

Early Main Group Metal Catalysts for Imine Hydrosilylation

The efficient catalytic reduction of imines with phenylsilane is achieved by using the potassium, calcium and strontium based catalysts [(DMAT)K (THF)]_∞, (DMAT)₂Ca⋅(THF)₂ and (DMAT)₂Sr⋅(THF)₂ (DMAT=2-dimethylamino-α-trimethylsilylbenzyl). Eight different aldimines and the ketimine Ph₂C=NPh could be successfully reduced by PhSiH₃ at temperatures between 25–60 °C with catalyst loadings down to 2.5 mol %. Also, simple amides like KN(SiMe₃)₂ or Ae[N(SiMe₃)₂]₂ (Ae=Ca, Sr, Ba) catalyze this reaction. Activities increase with metal size. For most substrates the activity increases along the row K<Ca<Sr<Ba. Fastest conversion was found for imines with alkyl substituents at N and aryl rings at C, for example, PhC(H)=NtBu, while tBuC(H)=NtBu or PhC(H)=NPh react much slower. Reasonable functional group tolerance is observed. The proposed metal hydride mechanism is supported by stoichiometric reactions using a catalyst model system, isolation of intermediates and DFT calculations.     

Evaluation of select variables in the ion chromatographic determination of F-, Cl-, Br-, NO(-)3, SO(-2)4, and PO(-3)4 in serum samples

A full experimental design at two levels is applied for the estimation of the significance of select factors that may influence the ion chromatography (IC) determination of F-, Cl-, Br-, NO(-)3, SO(-2)4, and PO(-3)4 in serum samples. The factors studied are various sample deproteinization procedures, eluent composition, and flow rates. Deproteinization using either acetonitrile-NaOH or ultrafiltration can be used in order to obtain a significant protein removal before IC analysis; however, the former is recommended because it is less time-consuming and cheaper. Better resolution is obtained when a sodium hydroxide solution is used as the eluent. There is no influence of the sample's deproteinization procedures on the chromatographic resolution.     

Formulation of Three-Dimensional Joint Constraints Using the Absolute Nodal Coordinates

A wide variety of mechanical and structural multibody systems consist ofvery flexible components subject to kinematic constraints. The widelyused floating frame of reference formulation that employs linear modelsto describe the local deformation leads to a highly nonlinear expressionfor the inertia forces and can be applied to only small deformationproblems. This paper is concerned with the formulation and computerimplementation of spatial joint constraints and forces using the largedeformation absolute nodal coordinate formulation. Unlike the floatingframe of reference formulation that employs a mixed set of absolutereference and local elastic coordinates, in the absolute nodalcoordinate formulation, global displacement and slope coordinates areused. The nonlinear kinematic constraint equations and generalized forceexpressions are expressed in terms of the absolute global displacementsand slopes. In particular, a new formulation for the sliding jointbetween two very flexible bodies is developed. A surface parameter isintroduced as an additional new variable in order to facilitate theformulation of this sliding joint. The constraint and force expressionsdeveloped in this paper are also expressed in terms of generalizedCholesky coordinates that lead to an identity inertia matrix. Severalexamples are presented in order to demonstrate the use of theformulations developed in the paper.     

A theoretical study of the comonomer effect in the ethylene polymerization with zirconocene catalytic systems

The PM3(tm) semiempirical method has been used to optimize the structures for the reactants and transition states of the first and second ethylene insertion processes into zirconocene catalytic systems. The results obtained for these reactions are compared with calculations published in the literature performed at different ab-initio theoretical levels. The agreement between our calculations and those reported in the literature is satisfactory. Taking advantage of the reduced computational effort required in semiempirical calculations two additional processes related with the so-called comonomer effect were also studied: ethylene/1-hexene copolymerization, and chain termination reaction, both in the homopolymerization and in copolymerization of ethylene with 1-hexene comonomer. The calculated activation energies support some experimental findings such as the higher polymerization activities in the presence of comonomers and also the molecular weight reduction of the copolymers due to the more favorable β-elimination reactions. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1157–1167, 1998     

Effect of a second ethylene molecule on the insertion of ethylene in zirconocene catalyst systems: A QM semiempirical study

The PM3(tm) semiempirical method was used to show the effect of a second ethylene molecule in the backside position on the frontside ethylene insertion in the Cp₂Zr ⁿPr⁺ γ‐agostic resting state. The same calculations without a companion second ethylene molecule were performed to compare geometrical parameters, energies, and electrostatic charges. The results obtained show that the geometrical parameters for both cases were identical, but differences in the electrostatic charges were observed. The energy profile presented two barriers, the first corresponding to the alkyl‐chain rotation along the Zr Cα bond and the second relating to the insertion process itself. The presence of a companion second ethylene molecule affected the energetic profile by lowering the energy barrier of the first stage with respect to the process without the companion second ethylene molecule. These results provide some theoretical support to the well‐known trigger effect. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 571–582, 2000     

Synthesis of high surface area CoAl2O4 and NiAl2O4 spinels by an alkoxide route

The mixed metal oxides CoAl₂O₄ and NiAl₂O₄ were prepared in a high surface area form by using a sol-gel method which involves hydrolysis of cobalt-aluminium and nickel-aluminium isopropoxides. The resulting gels were calcined at 673 K to yield the corresponding spinel-type metal oxides. Powder X-ray diffraction was used to confirm spinel formation. Nitrogen adsorption-desorption measurements have shown the materials obtained to be highly porous. The BET surface area was found to be 240 m²g⁻¹ for CoAl₂O₄ and 255 m²g⁻¹ for NiAl₂O₄. Both materials showed a unimodal distribution of pore radii, where the most frequent pore radius is 5 nm for cobalt aluminate and 3 nm for nickel aluminate.     

The Nitro Ketal Approach to Furane Systems. Total Synthesis of Racehic Bilobanone

Continuing our studies with the chemistry of ketals of β-nitro ketones¹, we wish to describe a new furane synthesis exemplified by a total synthesis of the sesquiterpene bilobanone 1 ².