Effects of nitridation on the characteristics of silicon dioxide: dielectric and structural properties from ab initio calculations

By combining ab initio calculations and classical molecular dynamics, we determine how the inclusion of nitrogen in a silica matrix changes its dielectric constant, and elucidate the underlying mechanisms. We find that there is an entire range of nitrogen concentrations (up to approximately 25%) for which the structural pattern of the oxide is preserved in bulk SiON, and the dielectric constant increases mainly because of the variation of the ionic polarizability. This behavior is not sensitive to hydrogen passivation of nitrogen. The few defects, which are associated with electron states near the gap, are mainly centered on undercoordinated nitrogen and undercoordinated silicon, and tend to be removed by hydrogen.     

Synthesis and absolute configuration of (+) α-hydroxymethyltyrosine by X-ray analysis of its N-benzoyl derivative

-Hydroxymethyltyrosine has been synthesized by a route involving-hydroxymethylation. Dextrorotatory N-benzoyl--hydroxymethyltyrosine4 crystallizes in space group P2₁2₁2₁ with cell parameters:a=13.2266(9),b=16.1099(9),c=7.4475(5) Å,V=15.86.9(2) ų,Z=4. The structure was solved by direct methods and refined toR=0.047 andR _w =0.066 with 1549 independent and 956 reflections. The absolute configuration of 1 was determined asR by the application of Hamilton test and by the estimation of the Bijvoet coefficientB. Just as in N-benzoyl--methylcysteine (Wieczorek et al., 1989), the-amino acid residue adopts the C⁵ ring conformation similar to the fully extended form. The two side chains also adopt an extended conformation around the C atom.     

(−)-S-Benzyl-α-hydroxymethylcysteine: Confirmation of its (S)-configuration by x-ray analysis of itsN-benzoyl derivative

LaevorotatoryN-benzoyl-S-benzyl--hydroxymethylcysteine (1) crystallizes in space groupP2₁ with cell parameters:a=8.972(1),b=10.004(1),c=10.302(1)Å,=106.4(1)°,V _c=887.2(5)ų,Z=2. The structure was solved by direct methods and refined toR _w=0.054 with 1324 independent reflections. The absolute configuration of1 was determined asS by the application of Hamilton test and by the estimation of the Bijvoet coefficientB. The-amino acid residue adopts the C₅ ring conformation similar to the fully extended form. The two side chains also adopt an extended conformation around the C atom. X-Ray analysis of1 confirms theS configuration assigned previously for laevorotatoryS-benzyl--hydroxymethylcysteine (2) by chemical correlation.     

The induction of reentry in cardiac tissue. The missing link: How electric fields alter transmembrane potential

This review examines the initiation of reentry in cardiac muscle by strong electric shocks. Specifically, it concentrates on the mechanisms by which electric shocks change the transmembrane potential of the cardiac membrane and create the physiological substrate required by the critical point theory for the initiation of rotors. The mechanisms examined include (1) direct polarization of the tissue by the stimulating current, as described by the one-dimensional cable model and its two- and three-dimensional extensions, (2) the presence of virtual anodes and cathodes, as described by the bidomain model with unequal anisotropy ratios of the intra- and extracellular spaces, (3) polarization of the tissue due to changing orientation of cardiac fibers, and (4) polarization of individual cells or groups of cells by the electric field ("sawtooth potential"). The importance of these mechanisms in the initiation of reentry is examined in two case studies: the induction of rotors using successive stimulation with a unipolar electrode, and the induction of rotors using cross-field stimulation. These cases reveal that the mechanism by which a unipolar stimulation induces arrhythmias can be explained in the framework of the bidomain model with unequal anisotropy ratios. In contrast, none of the examined mechanisms provide an adequate explanation for the induction of rotors by cross-field stimulation. Hence, this study emphasizes the need for further experimental and theoretical work directed toward explaining the mechanism of field stimulation. (c) 1998 American Institute of Physics.     

Reactions of alkylalane diolates with water synthesis, characterisation and ε-caprolactone polymerisation activity of novel alane benzopinacolates

1,1,2,2-Tetraphenylethane-1,2-diol (benzopinacole) reacts with R₃Al to yield the trinuclear complexes {R₅Al₃[OC(C₆H₅)₂C(C₆H₅)₂O]₂} [where R = Me (1), R = Et (2)]. Reactions of compounds 1 and 2 with water results in an elimination of R₃Al and formation of unusual binuclear products {R₂Al₂(THF)[OC(C₆H₅)₂C(C₆H₅)₂O]₂} [3 (R = Me), 4 (R = Et)] and a complicated mixture of R₃Al hydrolysis products. Compounds were characterised by spectroscopy and crystal structures of 1-3 have been determined by single crystal X-ray diffraction. Binuclear complexes 3 and 4 demonstrate efficient catalytic activity toward ring-opening polymerisation of ε-caprolactone.     

Synthesis and X-ray study of hexahydro- and tetrahydrophospholo-[2,3-d]isoxazoles,a new class of heterocycles of potential fungicidal activity

Hexahydro-, 5b-1 and 6a,f,1 and tetrahydrophospholo[2,3-d]isoxazoles 8, 9 and 10 were synthesized by 1,3-dipolar cycloaddition of nitrones 3b-1 and benzonitrile oxide ( 4 ) to 2,3-dihydro-1-phenyl-1H-phosphole 1-oxide ( 1 ) and 2,3-dihydro-1-ethoxy-1H-phosphole 1-oxide ( 2 ). The structural assignment to the compounds was confirmed by an X-ray study of two compounds of the series 5a and 5m . The compounds show a good activity as fungicides against Plasmopara viticola on vines and against Botrytis cinerea on apples. Compounds 5a-d showed weak to moderate activity as herbicides.     

Dependence of transition state structure on substrate: the intrinsic C-13 kinetic isotope effect is different for physiological and slow substrates of the ornithine decarboxylase reaction because of different hydrogen bonding structures

Ornithine decarboxylase is the first and the rate-controlling enzyme in polyamine biosynthesis; it decarboxylates l-ornithine to form the diamine putrescine. We present calculations performed using a combined quantum mechanical and molecular mechanical (QM/MM) method with the AM1 semiempirical Hamiltonian for the wild-type ornithine decarboxylase reaction with ornithine (the physiological substrate) and lysine (a "slow" substrate) and for mutant E274A with ornithine substrate. The dynamical method is variational transition state theory with quantized vibrations. We employ a single reaction coordinate equal to the carbon-carbon distance of the dissociating bond, and we find a large difference between the intrinsic kinetic isotope effect for the physiological substrate, which equals 1.04, and that for the slow substrate, which equals 1.06. This shows that, contrary to a commonly accepted assumption, kinetic isotope effects on slow substrates are not always good models of intrinsic kinetic isotope effects on physiological substrates. Furthermore, analysis of free-energy-based samples of transition state structures shows that the differences in kinetic isotope effects may be traced to different numbers of hydrogen bonds at the different transition states of the different reactions.     

Complexes of Y,La, and lanthanides withm-aminobenzoic acid

Summary m-Aminobenzoates of Y, La and lanthanides prepared in the reaction of the hydroxides of metal withm-aminobenzoic acid in solution have the general formulaLn(m-C₆H₄NH₂COO)₃·nH₂O wheren=4 for Ho, Tm,n=5 for Y, Sm, Dy, Er, Lu, andn=6 for La-Nd, Eu, Gd, Tb, Yb. The water molecules in the hydrated compounds are in the outer coordination sphere. On heating in air at 350–410 K dehydration occurs and anhydrousm-aminobenzoatesLn(m-C₆H₄NH₂COO)₃ are formed. On the basis of the IR spectra it was found that the metal in hydratedm-aminobenzoate of lanthanides is simultaneously coordinated through amino- and carboxyl groups whereas in anhydrousm-aminobenzoates of lanthanides only trough the bidentate carboxyl group. From X-ray analysis it was stated that the hydratedm-aminobenzoates of lanthanides are isostructural in the whole range Y, La-Lu.

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Komplexe von Y, La und Lanthaniden mitm-Aminobenzoesäure

Zusammenfassung Zur Darstellung der Verbindungen des TypsLn(m-C₆H₄NH₂COO)₃·nH₂O (mitn=4 fürLn=Ho, Tm,n=5 fürLn=Y, Sm, Dy, Er, Lu undn=6 fürLn=La-Nd, Eu, Gd, Tb, Yb) wurde die berechnete Menge vonLn(OH)₃ undm-C₆H₄NH₂COOH-Lösung bei 363 K gemischt und zur Kristallisation gebracht. Die Produkte wurden abfiltriert, mit Alkohol gewaschen und bis zur Gewichtskonstanz getrocknet. Die VerbindungenLn(m-C₆H₄NH₂COO)₃·nH₂O sind isostrukturell, mit geringer Löslichkeit in Wasser bei Raumtemperatur. Beim Erhitzen erfolgt zunächst Entwässerung bei 350–410 K, später bei 600–1 050 K unter Zersetzung zu CeO₂, Pr₆O₁₁, Tb₄O₇ undLn ₂O₃. Die Infrarotspektren der Verbindungen wurden registriert. Es wurde festgestellt, daß die Koordination der Seltenerdmetalle mit den Liganden sowohl durch Amino- als auch mit Carboxylgruppen erfolgt.

     

Near infrared study of aqueous lanthanide perchlorates

The near infrared spectra of water in aqueous solutions of La(ClO₄)₃, Pr(ClO₄)₃, Nd(ClO₄)₃, Gd(ClO₄)₃, Er(ClO₄)₃, Yb(ClO₄)₃, Lu(ClO₄)₃, and NaClO₄ have been measured in the concentration range from 0.3 to 2.5 mol-dm^–3, at 25°C. The relative contents of free OH groups in the 1.0, 1.6, and 2.2M solutions have been calculated from extinction coefficients for water at 1160 nm. They increase with increasing salt concentration and are greater in solutions of the lighter lanthanide perchlorates at any fixed molarity. The results are discussed in terms of the stoichiometry and structure of hydrated cations of trivalent lanthanides.     

Spectrophotometric method for the determination of manganese with phenylfluorone in the presence of Triton X-100 and cetylpyridinium chloride in pharmacological preparations and vegetable fertilizers

A simple and very sensitive method for the spectrophotometric determination of manganese in pharmacological preparations and vegetable fertilizers is proposed. The method is based on the formation of a blue coloured complex of Mn (II) with 9-phenyl-2,3,7-trihydroxy-6-fluorone (PF) in the presence of cetylpyridinium chloride (CP) and Triton X-100. Optimum concentrations of PF, CP, Triton X-100 and pH ensuring maximum absorbance were defined. The complex Mn(II)-PF-CP-Triton X-100 shows maximum absorbance at 591 nm with the molar absorptivity value 1.77x10(5 )L mol(-1 )cm(-1). The detection limit of the method is 0.004 microg mL(-1). The Beer's law is obeyed for manganese concentrations in the range 0.02-0.2 microg mL(-1). The effect of foreign ions was elucidated. The statistical evaluation of the method was carried out for six determination using 5 microg Mn and the following results were obtained: standard deviation 0.021, confidence interval 5.05+/-0.05 microg Mn. The method has been applied for the determination of manganese in pharmacological preparations (Biovital, Kinder Biovital) and vegetable fertilizers (Hydrovit 100, Florovit).