Synthesis and X-ray crystal structure of [1,3-bis(η5-cyclopentadienyl)-1,1,3,3-tetramethyldisiloxane]hafnium(IV) dichloride

Summary The tetramethyldisiloxane-bridged hafnocene complex [HfCl₂{-(⁵-C₅H₄)SiMe₂OSiMe₂(²-C₅H₄)}] (1) has been prepared by the reaction of HfCl₄ with the dilithiatedbis(cyclopentadienyl)disiloxane (LiC₅H₄SiMe₂)₂O in a molar ratio of 1:2. The new compound was characterized by spectroscopic and X-ray diffraction methods. The crystals are monoclinic of space groupP2₁/c and isostructural with the corresponding complexes of titanium and zirconium. The unit cell dimensions area=13.51(1) Å,b=8.672(7) Å,c=15.41(1) Å, =97.15(2)°, andZ=4.

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Darstellung und Röntgenkristallstruktur von [1,3-Bis(⁵-cyclopentadienyl)-1,1,3,3-tetramethyldisiloxan]hafnium(IV)dichlorid

Zusammenfassung Der tetramethyldisiloxan-überbrückte Hafnocenkomplex [HfCl₂{-(⁵-C₅H₄)-SiMe₂OSiMe₂(²-C₅H₄)}] (1) wurde durch Umsetzung von HfCl₄ mit dem lithiiertenBis(cyclopentadienyl)disiloxan (LiC₅H₄SiMe₂)₂O im Molverhältnis 1:2 dargestellt. Die neue Verbindung wurde spektroskopisch und röntgenographisch charakterisiert. Sie kristallisiert in der monoklinen RaumgruppeP2₁/c und ist isostrukturell mit den entsprechenden Titan-und Zirkoniumkomplexen. Die Dimensionen der Einheitszelle sinda=13.51(1) Å,b=8.672(7) Å,c=15.41(1) Å, =97.15(2)° undZ=4.

     

Selecting variables for k-means cluster analysis by using a genetic algorithm that optimises the silhouettes

The goal of present work is to analyse the effect of having non-informative variables (NIV) in a data set when applying cluster analysis and to propose a method computationally capable of detecting and removing these variables. The method proposed is based on the use of a genetic algorithm to select those variables important to make the presence of groups in data clear. The procedure has been implemented to be used with k-means and using the cluster silhouettes as fitness function for the genetic algorithm.The main problem that can appear when applying the method to real data is the fact that, in general, we do not know a priori what the real cluster structure is (number and composition of the groups).The work explores the evolution of the silhouette values computed from the clusters built by using k-means when non-informative variables are added to the original data set in both a literature data set as well as some simulated data in higher dimension. The procedure has also been applied to real data sets.     

Multivariate calibration techniques applied to the spectrophotometric analysis of one-to-four component systems

The UV spectrophotometric analysis of a multicomponent mixture containing paracetamol, caffeine, tripelenamine and salicylamide by using multivariate calibration methods, such as principal component regression (PCR) and partial least-squares regression (PLS), was described. The calibration set was based on 47 reference samples, consisting of quaternary, ternary, binary and single-component mixtures, with the aim to develop models able to predict the concentrations of unknown samples containing as many as one-to-four components. The calibration models were optimized by an appropriate selection of the number of factors as well as wavelength ranges to be used for building up the data matrix and excluding any information about the interfering excipients included in pharmaceutics. The PCR and PLS models were compared and their predictive performance was inferred by a successful application to the assays of synthetic mixtures and pharmaceutical formulations.     

Liner as the Key of Injection Optimization in Pesticide Analysis

Regulations for pesticide residue analysis in food require very low detection limits; thus requiring maximum sensitivity in the gas chromatographic determination. This is accomplished by an overall method optimisation, which includes optimisation of injector parameters. Here we study the effect of the inlet liner design on the optimisation by comparing five liner designs in splitless and pulsed splitless injection modes, using a test mixture of fifteen pesticides analyzed by GC-ECD. Possible links between the injection parameters and liner types were evaluated, with the result that, accurate choice of inlet liner and injection parameters can reduce detection limits by up to 300%. Revised: 25 October 2005 and 9 January 2006     

Transformation of dynamic DSC results into isothermal data for the curing kinetics study of the resol resins

Kinetics of thermosetting polymers curing is difficult to study by isothermal methods based on the differential scanning calorimetry (DSC) technique. The difficulty is due to the low sensitivity of the equipment for total reaction heat measurements during high temperature process. The aim of this paper is to display the equivalence between a dynamic model, the Ozawa method, and an isothermal isoconversional fit, which allows predicting the isothermal behavior of the resol resins cure through dynamic runs by DSC. In this work, lignin–phenol–formaldehyde and commercial phenol–formaldehyde resol resins were employed. In addition, the isothermal kinetic parameters for both resins were performed by means of transformation of the data obtained from the dynamic Ozawa method.     

Electroreduction of the antifouling agent TCMTB and its electroanalytical determination in tannery wastewaters

The electrochemical reduction of antifouling agent 2-thiocyanomethylthiobenzothiazole (TCMTB) was investigated by cyclic and pulse differential voltammetry. The irreversible electrode reduction of TCMTB proceeded by ECEC reaction mechanism by two electrons transfer with one irreversible wave. Upon the basis of electrochemical evidence, the electrodic reaction mechanism was suggested to formation of mercaptobenzothiazole (MTB) in solution.Subsequently, a pulse differential method is described for the formation of TCMTB based on this electroreduction. Having been obtained a detection limit of 1.0 × 10⁻⁷ mol L⁻¹ and recovery to 98% to concentration of 2.0 × 10⁻⁶ mol L⁻¹. Therefore, the proposed method in this study is practical, sensitive and accurate for the analysis of TCMTB in tannery wastewater samples.     

Some aspects of quality assessment of the k0-based method of NAA

Neutron activation analysis is one of the analytical techniques often used for certification of reference materials. The k₀-based method of instrumental neutron activation analysis can also be applied in intercomparison runs in the certification process and therefore it is desirable to know its accuracy in advance. Possible systematic errors related to the application of nuclear data at given neutron flux rate parameters, that can affect the uncertainties of the results obtained by this specific method, are elucidated and error propagation factors calculated for a typical irradiation position in the TRIGA Mark II reactor of the Jozef Stefan Institute. It was found that these uncertainties are at the level of 1–2% on the average.     

An implicit function theorem: Comment

In Ref. 1, Jittorntrum proposed an implicit function theorem for a continuous mappingF:R ⁿ ×R ^m R ⁿ, withF(x ⁰,y ⁰)=0, that requires neither differentiability ofF nor nonsingularity of _x F(x ⁰,y ⁰). In the proof, the local one-to-one condition forF(·,y):A R ⁿ R ⁿ for ally B is consciously or unconsciously treated as implying thatF(·,y) mapsA one-to-one ontoF(A, y) for ally B, and the proof is not perfect. A proof can be given directly, and the theorem is shown to be the strongest, in the sense that the condition is truly if and only if.     

Second crystal modification of 1,10-diazonia-18-crown-6 dichloride

The second crystal modification (II) of 1,10-diazonia-18-crown-6 dichloride [H₂DA18C6]²⁺·2Cl^?, where [H₂DA18C6]²⁺ is the 1,10-diaza-18-crown-6 dication (DA18C6) with two protonated nitrogen atoms, was investigated by XRD. The monoclinic modification (II) (space group P2₁/n, a = 9.600 Å, b = 5.689 Å, c = 15.749 Å, β = 91.03°, Z = 2) was solved by direct methods and refined by full-matrix least squares analysis in an anisotropic approximation to R = 0.032 for all 2494 independent reflections collected (CAD-4 automatic diffractometer, λMoK _α). In modification II, the conformation of the DA18C6 dication and the ion packing differ from those in triclinic modification I investigated previously.     

Unexpected solids from enantiopure cationic palladium complexes and racemic anions: A structural study of chiral non-discrimination

Enantiomerically pure cationic complexes were obtained via cyclopalladation of primary amines and subsequent addition of a chelating ethylendiamine ligand. No diastereomeric resolution was observed upon combining these cations with racemic mandelate or hydratropate anions, but four less popular products, namely three double salts and a solid solution, were obtained and structurally characterized. For one of the double salts, the alternative ionic compounds based on different stereoisomers of the same residues were synthesized independently: The conventional racemic solid and both diastereomeric salts formed by enantiopure cations and anions were studied by single crystal X-ray diffraction. Lattice energy calculations confirm that the diastereomeric salts differ significantly; formation of the partially racemic double salt, however, is energetically favourable and precludes resolution.