Kinetics of the solution polymerization of dioxolane in the presence of water

Water acts as a cocatalyst in the polymerization of dioxolane initiated by the ion pair ～Si^⊕HSO₄^?. The dependence of the reaction rate on the water concentration exhibits a maximum, the width of which strongly depends on the concentration of dioxolane. The change of the coordinates of the maximum and its shape with the decrease of the monomer concentration causes the reaction rate to decrease with increasing conversion. The point at which the slope of the conversion curve changes is a function of initial concentration of water. There is a very fast decrease of the concentration of free water in the polymerizing system. The consumption of water is associated with some peculiarities. The amount of free water which remains in the system is a function of the concentration of the original initiator. The ratio [H₂O]_∞/[initiator] is constant over a rather broad range of initial concentration of water.     

Structure Elucidator: a versatile expert system for molecular structure elucidation from 1D and 2D NMR data and molecular fragments

StrucEluc is an expert system that allows the computer-assisted elucidation of chemical structures based on the inputs of a series of spectral data including 1D and 2D NMR and mass spectra. The system has been enabled to allow a chemist to utilize fragments stored in a fragment database as well as user-defined fragments submitted by the chemist in the structure elucidation process. The association of fragments in this way has been shown to dramatically speed up the process of structure generation from 2D NMR data and has helped to minimize or eliminate the need for user intervention thereby further enabling the vision of automated elucidation. The use of fragments has frequently transformed very difficult 2D NMR elucidation challenges into easily solvable tasks. A strategy to utilize molecular fragments has been developed and optimized based on specific challenging examples. This strategy will be described here using real world examples. Experience gained by solving more than 150 structure elucidation problems from a variety of literature sources is also reviewed in this work.     

One-dimensional magnetism and crystal structure ofcatena-di-bromobis(3,5-dimethylpyridine)copper(II)

Summary Crystals ofcatena-di--bromobis(3,5-dimethylpyridine)copper(II) are monoclinic, space group P2₁/a. The unit cell constants area=13.900(2),b=14.416(2),c=4.097(1) Å,=93.49(2)°, V=819.4 ų and Z=2. The structure was determined from powder data using a Guinier-Johansson focussing powder camera. The structure was solved using a simplex method for function minimization to a conventional R-value of 0.13.The structure consists of infinite linear chains parallel toc in which the copper coordination is distorted elongated octahedral. Cu-Br distances were found to be 2.449(7) and 3.286(7) Å, whereas the Cu-N bond length is 2.02(2) Å. All distances are in the range usually observed for this type of compounds.The antiferromagnetic superexchange interactions between adjacent Cu^II ions (J = –21 cm^–1) has been compared with those observed in structural similar CuBr₂L₂ compounds. The differences in observed J-values are discussed briefly, in relation to the structural variations. It appears that very small changes in structural parameters strongly affect the magnetic exchange.     

Purification and analysis of partially alkylated cyclodextrins by liquid and gas chromatography

Complex mixtures of partially alkylated cyclodextrins can be analyzed by both HPLC and high temperature capillary GC. Because of the limited efficiency of LC, suitable analytical and preparative separations can be achieved only with systems of carefully optimized selectivity. Using LC it has been possible to isolate and purify single cyclodextrin species from very complex mixtures of components which contain unreacted hydroxyl groups in addition to the alkoxy groups. Analysis of the reaction mixtures and of fractions taken from LC separations can be performed with advantage by high resolution capillary GC at high temperatures between 300 and 400 °C. The thermal stability of partially alkylated cyclodextrins in high temperature GC is considerably increased by trimethylsilylation of the free hydroxyl groups. Fast atom bombardment mass spectrometry and proton NMR were used to identify species isolated from the preparative LC separations.     

Artificial neural networks for quantification in unresolved capillary electrophoresis peaks

The application of the combination of experimental design (ED) and artificial neural networks (ANNs) for the quantification of overlapped peaks in capillary zone electrophoresis is described. When the total separation cannot be achieved by separation techniques, the use of ED-ANN can be a suitable approach. The unstability of EOF causes peak shift that has to be corrected in order to apply ED-ANN methods. In this work, normalization procedure of electropherograms with consequent application of ANNs for quantification purpose was developed. Both, spectra and electropherograms can be used as multivariate data. In general, both kinds of data were found to be suitable for unresolved peaks quantification by ED-ANN approach.     

Plastic deformation and performance of engineering polymer materials

The plastic deformation mechanism operating in polymer glasses is analyzed. The whole process consists of two main stages: nucleation of special shear defects, called PSTs (plastic shear transformations), and their disappearance. The important feature of plastic deformation of glasses is the storage of a large amount of internal energy ΔU_def upon straining. Such energy storage is the critical issue for mechanical performance of polymeric material: if the amount of stored energy is high, the appearance of macroscopic failure is very probable while glassy materials collecting a small amount of stored deformation energy are quite ductile. It is proposed that the rate of disappearance of PSTs is a key factor in dissipation of stored deformation energy. A parameter describing the dissipation ability of material upon deformation is introduced.     

Quantification of analytes in overlapping peaks from capillary electrophoresis using multivariate curve resolution-alternating least squares methods

Application of multivariate curve resolution with alternating least squares (ALS) methods to second-order data from capillary electrophoresis diode array detector (CE-DAD) is shown. Second-order data are easily obtained by setting individual data matrix of CE run one in top of the other. Initial qualitative solutions obtained by evolving factor analysis can be further optimized by simultaneous analysis of multiple electrophoresis run data with ALS regression. Quantification is achieved by the comparison of the analyte peak areas with that of pure standards. During the ALS regression procedure, the following constraints were applied: (i) both concentrations and unit pure spectra of the resolved components must be positive; (ii) elution profiles have an unimodal shape; (iii) correspondence exists between common species in the different data matrices; (iv) the pure spectrum of each species is the same in all runs where it is present. The above methods were applied for the determination of dinitrotoluene (DNT) isomeric compounds in overlapping peaks from CE.     

Multiparametric curve fitting-VII Determination of the number of complex species by factor analysis of potentiometric data

The number of complex species in solution may be determined by a computer-assisted factor analysis of a set of potentiometric titration curves, by finding the rank of the normalized data matrix. An application of the program SPECIES is demonstrated for some examples of titration data. The method is limited in that it can discriminate only between species with differing degrees of polymerization.     

On the tautological ring of ℳ g

We prove that any product of tautological classes of _g of degreed (in the Chow ring of _g with rational coefficients) vanishes ford>g–2 and is proportional to the class of the hyperelliptic locus in degreeg–2.Oblatum 28-I-1195 & 15-IV-1995In memory of Nicolaas H. Kuiper (1920–1994)     

Ion solvation by channel carbonyls characterized by 17O solid-state NMR at 21 T

Recently available ultrahigh magnetic fields offer new opportunities for studies of quadrupole nuclei in biological solids because of the dramatic enhancement in sensitivity and resolution associated with the reduction of second-order quadrupole interactions. Here, we present a new approach for understanding the function and energetics of ion solvation in channels using solid-state 17O NMR spectroscopy of single-site 17O-labeled gramicidin A. The chemical shift and quadrupole coupling parameters obtained in powder samples of lyophilized material are similar to those shown in the literature for carbonyl oxygens. In lipid bilayers, it is found that the carbonyl 17O anisotropic chemical shift of Leu10, one of the three carbonyl oxygens contributing to the ion binding site in gramicidin A, is altered by 40 ppm when K+ ion binds to the channel, demonstrating a high sensitivity to such interactions. Moreover, considering the large breadth of the carbonyl 17O chemical shift (>500 ppm), the recording of anisotropic 17O chemical shifts in bilayers aligned with respect to magnetic field B0 offers high-quality structural restraints similar to 15N and 13C anisotropic chemical shifts.