Use of factor analysis and liquid chromatography to determine the number of species in reactions of potassium ions with selected polyethers

Abstract factor analysis is used to determine the number of species in chemical systems from data obtained by high-performance liquid chromatography. Retention data for the systems involving the reaction between potassium ions and three polyethers, benzo-5-crown-5, dibenzo-18-crown-6, and dibenzo-24-crown-8, were obtained in methanol at room temperature. The experimental data were factor-analyzed in order to yield the number of species in the reacting system. In addition to the usual criteria, it is proposed that tree-dimensional graphs of calculated data be compared to three-dimensional graphs of raw data in order to evaluate the factor space. It is also suggested that the three-dimensional graphs showing the residual error matrix be examined as a tool in the evaluation of the factor space. Two species were found from the reaction between benzo-15-crown-5 and potassium, one from dibenzo-18-crown-6-, and there from dibenzo-24-crown-8 in methanol.     

Mean field theory-based calculation of FLC polarization

Mean field theory and Monte Carlo sampling are applied to the calculation of the spontaneous polarization density of ferroelectric liquid crystals by the ensemble averaging of single molecules confined in mean field potentials reflecting the SmC environment. Molecules are modelled with atomistic detail, using intramolecular interaction potentials derived from ab initio quantum mechanical calculations. This technique is applied to thirteen members of a family of novel fluoro ether-based compounds. Comparison with experiment shows that the observed variation of polarization density with chemical structure is well reproduced in most cases, but that the observed temperature dependence of polarization density is not captured by our model. The features of molecular organization responsible for the discrepancies between theory and experiment are discussed.     

The 3-21G(N*) basis set: An economical polarized basis set for ab initio studies on nitrogen-containing molecules

The 3-21G basis set shares with its older cousin, the 4-31G basis set, a tendency to overestimate valence angles at nitrogen atoms and to underestimate seriously barriers to inversion at such atoms. The 6-31G* basis set generally yields greatly improved results in these respects. It is here shown that, for a variety of molecules, supplementation of the 3-21G basis set at three- or two-coordinate nitrogen atoms with a set of six d-functions having exponent 1.0 leads to optimized geometries and inversion barriers at such nitrogen centers in good agreement with results obtained with the 6-31G* basis set. This supplemented basis set, designated as 3-21G(N*), also leads to calculated vibrational frequencies in good agreement with those calculated with the 6-31G* basis set. The 3-21G(N*) basis set offers an economical alternative to the 6-31G* basis set, particularly for molecules containing several first-row atoms other than nitrogen.     

Synthesis and characterization of polymaleimides containing 4-cyanobiphenyl–based side groups for nonlinear optical applications

A new homologous series of SCLCPs containing the 4-cyanobiphenyl mesogenic group attached to the polymaleimide backbone through paraffinic spacers of two to eight methylene units have been prepared. All the polymers exhibit liquid crystalline behavior; specifically S_Ad- (or S_C-) like and nematic phases are observed. The glass transition temperature decreases from 150 to 43°C on increasing spacer length. The isotropization temperatures exhibit an odd–even effect on varying the length and parity of the spacer, in which the odd members exhibit the higher values. This is attributed to the change in the average shape of the side chain as the parity of spacer is varied. The isotropization temperatures (>300–120°C) and the mesophase thermal stabilities (190–60°C) are high. Comparison is made with polymers containing the same mesogenic group attached to backbones of decreasing rigidity. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. A Polym. Chem. 36: 2531–2546, 1998     

Smectic liquid crystal alignment using mechanically rubbed n-octadecylsiloxane self-assembled monolayers

In recent years a variety of techniques has appeared for the fabrication and manipulation of self-assembled monolayers (SAMs). This development now offers new tools for the study and control at the molecular level of the interaction of liquid crystals (LCs) with solid surfaces, a research area of great importance for liquid crystal applications. In this paper we show that mechanically rubbed octadecylsiloxane SAMs generate a novel surface alignment of LCs in which the in-plane surface anisotropy usually accompanying rubbing is operative, but only for smectics in which the mean molecular long axis, [ncirc], is tilted from the layer normal. On our SAMs smectic phases align with the layers parallel to the SAM surface, and in tilted smectics the surface component of [ncirc] is along the rubbing direction. This anisotropy is absent in the nematic phases which align with [ncirc] strictly normal to the surface. This behaviour can be understood in terms of a rubbed SAM monolayer surface, which is low energy, molecularly smooth, and rendered anisotropic by the rubbing. UV irradiation of rubbed SAMs gave excellent planar alignment ([ncirc] parallel to the surface). This type of control over LC alignment has not been previously reported.     

Easy Access to Enantiomerically Pure Heterocyclic Silicon-Chiral Phosphonium Cations and the Matched/Mismatched Case of Dihydrogen Release

Phosphonium ions are widely used in preparative organic synthesis and catalysis. The provision of new types of cations that contain both functional and chiral information is a major synthetic challenge and can open up new horizons in asymmetric cation-directed and Lewis acid catalysis. We discovered an efficient methodology towards new Si-chiral four-membered CPSSi* heterocyclic cations. Three synthetic approaches are presented. The stereochemical sequence of anchimerically assisted cation formation with B(C₆F₅)₃ and subsequent hydride addition was fully elucidated and proceeds with excellent preservation of the chiral information at the stereogenic silicon atom. Also the mechanism of dihydrogen release from a protonated hydrosilane was studied in detail by the help of Si-centered chirality as stereochemical probe. Chemoselectivity switch (dihydrogen release vs. protodesilylation) can easily be achieved through slight modifications of the solvent. A matched/mismatched case was identified and the intermolecularity of this reaction supported by spectroscopic, kinetic, deuterium-labeling experiments, and quantum chemical calculations.     

A new smoothing-regularization approach for a maximum-likelihood estimation problem

We consider the problem min _i=1 ^m (aⁱ,x–b_iloga ⁱ, z) subject tox 0 which occurs as a maximum-likelihood estimation problem in several areas, and particularly in positron emission tomography. After noticing that this problem is equivalent to mind(b, Ax) subject tox 0, whered is the Kullback-Leibler information divergence andA, b are the matrix and vector with rows and entriesa ⁱ,b _i, respectively, we suggest a regularized problem mind(b, Ax) + d(v, Sx), where is the regularization parameter,S is a smoothing matrix, andv is a fixed vector. We present a computationally attractive algorithm for the regularized problem, establish its convergence, and show that the regularized solutions, as goes to 0, converge to the solution of the original problem which minimizes a convex function related tod(v, Sx). We give convergence-rate results both for the regularized solutions and for their functional values.The research of A. N. Iusem was partially supported by CNPq Grant No. 301280/86-MA.     

Preparation of Anatase Hollow TiO2 Spheres and Their Photocatalytic Activity in the Photodegradation of Chlorpyrifos

Hollow anatase titania (TiO₂) spheres were synthesized using fructose and tetrabutyl titanate (Ti(OC₄H₉)₄, TBT) as the precursors via the conventional hard template method. The morphological, structural and thermal properties of the products were characterized using scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, X‐ray diffraction (XRD), thermogravimetric and differential thermal analysis (TG‐DTA), Brunauer? Emmett? Teller (BET) surface area analysis and diffuse reflectance ultraviolet visible (DR UV? Vis) spectroscopy. XRD revealed that the hollow TiO₂ prepared was in the anatase phase and the BET surface area measured was about 22 m² g^?1. The photocatalytic activity of the synthesized hollow anatase TiO₂ in the photodecomposition of chlorpyrifos was 18.67 % higher than that obtained using commercial TiO₂.