Exploring Oxidative NHC-Catalysis as Organocatalytic Polymerization Strategy towards Polyamide Oligomers

The polycondensation of diamines and dialdehydes promoted by an N-heterocyclic carbene (NHC) catalyst in the presence of a quinone oxidant and hexafluoro-2-propanol (HFIP) is herein presented for the synthesis of oligomeric polyamides (PAs), which are obtained with a number-average molecular weight (M_n) in the range of 1.7–3.6 kg mol⁻¹ as determined by NMR analysis. In particular, the utilization of furanic dialdehyde monomers (2,5-diformylfuran, DFF; 5,5’-[oxybis(methylene)]bis[2-furaldehyde], OBFA) to access known and previously unreported biobased PAs is illustrated. The synthesis of higher molecular weight PAs (poly(decamethylene terephthalamide, PA10T, M_n = 62.8 kg mol⁻¹; poly(decamethylene 2,5-furandicarboxylamide, PA10F, M_n = 6.5 kg mol⁻¹) by a two-step polycondensation approach is also described. The thermal properties (TGA and DSC analyses) of the synthesized PAs are reported.     

Matrix algebras in Quasi-Newton methods for unconstrained minimization

Summary.  In this paper a new class of quasi-Newton methods, named ℒQN, is introduced in order to solve unconstrained minimization problems. The novel approach, which generalizes classical BFGS methods, is based on a Hessian updating formula involving an algebra ℒ of matrices simultaneously diagonalized by a fast unitary transform. The complexity per step of ℒQN methods is O(n log n), thereby improving considerably BFGS computational efficiency. Moreover, since ℒQN's iterative scheme utilizes single-indexed arrays, only O(n) memory allocations are required. Global convergence properties are investigated. In particular a global convergence result is obtained under suitable assumptions on f. Numerical experiences [7] confirm that ℒQN methods are particularly recommended for large scale problems. Received December 30, 2001 / Revised version received December 20, 2001 / Published online November 27, 2002 Mathematics Subject Classification (1991): 65K10 Correspondence to: P. Zellini     

Computational evaluation of ship manoeuvring performance based on scale model tests

Summary A brief review of the most important existing mathematical models for predicting the manoeuvring performance of a ship at the design stage is presented. A model based on the derivation of the hydrodynamic coefficients from force measurements on scale models is used to develop a computer program for the evaluation of the ship performance in some standard manoeuvres such as turning circle and zig-zag manoeuvres.

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Sommario Viene presentata una breve descrizione delle metodologie attuali più seguite per la identificazione di un modello matematico atto alla previsione delle caratteristiche di manovrabilità di una nave. Utilizzando coefficienti idrodinamici ricavati da prove su modelli in scala si è sviluppato un codice di calcolo che consente di ottenere la risposta della nave in alcune manovre standard quali quelle di evoluzione e zig-zag.

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Symbols G Center of gravity - g Acceleration due to gravity - I _ZZ Moment of inertia aboutz-axis - i _EP Effective moment of inertia about propeller axis - L Length between perpendiculars - m Ship mass - N Hydrodynamic moment aboutz-axis - n Rate of revolutions of propeller - O Origin of shipbound coordinate system - Q Propeller torque - Q _E Engine torque - q _F Engine fuel rate - R _T Total hull resistance - r Rate of turn aboutz-axis (yaw rate) - U Along-track velocity of0 - u, v Components ofU alongx, y-axes - X, Y Hydrodynamic forces alongx, y-axes - x,y,z Shipbound coordinate axes - x _G ,y _G ,z _G Coordinate of center of gravity in the shipbound system - x _o,y ₀,z ₀ Coordinate of 0 in the earthbound system, Fig. 1 - Drift angle - Rudder angle - Characteristic time - Heading angle Presented at the II Convegno AIMETA di Meccanica Computazionale, Rome, June 2–5, 1987.     

Fourfold [2]rotaxanes based on calix[4]arenes

Tetraurea calix[4]arenes with four loops form exclusively heterodimers with open-chain urea calix[4]arenes when they are dissolved in aprotic solvents. These assemblies can be considered as pseudorotaxanes. If open-chain tetraureas ending with maleic imide functions are used, their Diels-Alder reaction with 1,4,5,8-tetrapentoxyanthracene leads to tetra[2]rotaxanes which cannot be split into the single calixarene parts by hydrogen bond breaking solvents.     

Conformational studies by dynamic NMR. 100. Enantiomerization process of stereolabile atropisomers in pyridine-substituted adamantane derivatives

The barriers for interconverting the conformational enantiomers (stereolabile atropisomers) of pyridine-substituted adamantane derivatives have been determined by dynamic 13C NMR spectroscopy. The trend of these values parallels that anticipated by MM calculations. In at least one case, the computed structure was found to agree with that obtained by single-crystal X-ray diffraction. In addition, it has been possible to achieve a physical separation of a pair of these stereolabile atropisomers at -60 degrees C by means of the enantioselective cryogenic HPLC technique.     

Bisoxazolines with one and two sidearms: stereodirecting ligands for copper-catalysed asymmetric allylic oxidations of alkenes

A series of sidearm functionalized bisoxazoline ligands has been synthesized by reaction of the monolithiated methyl{bis(oxazolinyl)}methane with the appropriate electrophiles, and tested in the copper catalyzed asymmetric allylic oxidation of cyclohexene ("Kharasch-Sosnovski" reaction). The observed enantioselectivities were higher (up to 85% ee) than for the unfunctionalized bisoxazoline ("BOX") derivatives (ca. 60% ee). Regardless of the functional groups incorporated into the sidearm unit, the ee's obtained for the different derivatives were essentially indistinguishable. This implies that the sidearms do not interfere directly in this reaction and only play an indirect role by virtue of their steric demand. Three of the copper complexes have been characterized by X-ray diffraction, establishing a distorted octahedral coordination geometry around the copper atom in all three cases. In the elongated distorted CuN(2)O(4) octahedra, the two nitrogen atoms of the oxazolines and one oxygen atom of each acetate ligand occupy the 'equatorial' positions whereas the sidearms do not interact with the metal centres.     

Magnetic properties of Nd3+ in Nd−Ba−Cu−O-compounds

Neutron diffraction, neutron spectroscopy and magnetization measurements have been employed to study the structural, electronic, and magnetic behavior of eleven compounds with the general formula Nd_1+y Ca _v Ba_2–y–v Cu₃O _x (0y0.5; 0v0.25; 6x7). The structure turned out to react to oxygen reduction similar as other 123-compounds, yielding discontinuities close to the metal-insulator-transition and the well-known relations of bond lengths as a function ofT _c. The crystalline electric field (CEF) interaction, splitting the 10-fold degenerate ground-state J-multiplet of the Nd³⁺-ions into five doublet states, was investigated by neutron spectroscopy. The derived CEF parameters have been used to determine changes in the electronic surroundings of the Nd³⁺ ions. In addition, with the help of the CEF parameters the thermodynamic magnetic properties of these compounds were calculated which turn out to be in good agreement with the experimental data.