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Kinetic Factors (kinetic + factor)
Selected Abstracts[2,+,3] Cycloaddition of C,C,N -triphenylnitrone to trans -substituted nitroethylenes in the light of an AM1 and AM1/COSMO computational study,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 5 2003Andrzej Baranski Abstract AM1 calculations suggest that the [2,+,3] cycloaddition of triphenylnitrone 1 to nitroalkenes 2a,c in the gas phase takes place in a concerted manner. However, the azolidine ring bonds C3,C4 and O1,C5 are not formed fully synchronously. Kinetic factors favor the formation of cycloadducts with nitro group in position C-4 of the azolidine ring (path A). Introduction of toluene, acetone or acetonitrile as a reaction medium increases the activation barriers for both regioisomeric paths. Simultaneously, the energy profiles for path A undergo qualitative change. In this case, two transition states and an intermediate with zwitterionic character were localized by means of the AM1/COSMO method. Copyright © 2003 John Wiley & Sons, Ltd. [source] Influence of complex formation upon inclusion of Mn(II), Co(II), Ni(II), and Cu(II) in ZnC2O4·2H2OCRYSTAL RESEARCH AND TECHNOLOGY, Issue 3 2004B. Donkova Abstract The inclusion of 3d-impurities Mn(II), Co(II), Ni(II) and Cu(II) in a crystalline precipitate of ZnC2O4·2H2O is investigated. This study is a part of the systematic one deal with the mechanism of inclusion of 3d-ions in sparingly soluble oxalate systems. The experiments are carried out in bi- end multi-component systems at two different mediums , one with deficiency of oxalate ions, another with excess. The insertion of 3d-ions upon mass crystallization of ZnC2O4·2H2O does not proceed by a simple ionic substitution. The results show that the inserted amount of impurity depends on some physicochemical characteristics of the neutral monooxalato complexes [MnC2O4]o, [CoC2O4]o, [NiC2O4]o and [CuC2O4]o. Good agreement between included impurity and the concentration of its complex in the solution is established. The stability constant of monooxalato complex affects the impurity inclusion. This effect depends on the medium nature. In the deficiency of oxalate ions the factor determining the inclusion is thermodynamic one , stability of monooxalato complexes. In the excess of oxalate ions inserted amount depends on kinetic factor , the formation rate of these complexes. In the term of that the insertion of Mn(II) is definitely different in the two mediums while that of the Ni (II) does not depend on the medium. The copper shows deviation from overall dependence in the two mediums due to the Jahn-Teller distortion. Its double decreasing insertion in the excess of oxalate ions is related with stabilization of [Cu(C2O4)2]2- . The conclusions presume that by varying the background medium and taking in view the ions present in the solution, the amount of inserted impurities can be predicted and controlled. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Crystallization of Highly Supercooled Silicate MeltsADVANCED ENGINEERING MATERIALS, Issue 12 2006M. Roskosz Crystallization of liquids in the system CaO-MgO-Al2O3 -SiO2 at one atmosphere has been studied at temperatures between the glass transition (Tg) and the solidus. To determine the textures, compositions and unit-cell parameters of the crystalline phases, the authors have characterized the experimental charges over a wide range of length scales by scanning and transmission electron microscopy, electron microprobe analyses, X-ray diffraction, and Raman spectroscopy. With increasing temperature, crystals tend to reach the equilibrium composition, but the relative importance of thermodynamic and kinetic factors is a single function of T - Tg, regardless of liquid composition. This feature is of considerable practical interest as it provides the possibility, not only to predict, but also to control the composition of the crystallizing phases. [source] Towards Efficient Dispersion of Carbon Nanotubes in Thermotropic Liquid CrystalsADVANCED FUNCTIONAL MATERIALS, Issue 19 2010Stefan Schymura Abstract Motivated by numerous recent reports indicating attractive properties of composite materials of carbon nanotubes (CNTs) and liquid crystals (LCs) and a lack of research aimed at optimizing such composites, the process of dispersing CNTs in thermotropic LCs is systematically studied. LC hosts can perform comparably or even better than the best known organic solvents for CNTs such as N -methyl pyrrolidone (NMP), provided that the dispersion process and choice of LC material are optimized. The chemical structure of the molecules in the LC is very important; variations in core as well as in terminal alkyl chain influence the result. Several observations moreover indicate that the anisotropic nematic phase, aligning the nanotubes in the matrix, per se stabilizes the dispersion compared to a host that is isotropic and thus yields random tube orientation. The chemical and physical phenomena governing the preparation of the dispersion and its stability are identified, taking into account enthalpic, entropic, as well as kinetic factors. This allows a guideline on how to best design and prepare CNT,LC composites to be sketched, following which tailored development of new LCs may take the advanced functional material that CNT,LC composites comprise to the stage of commercial application. [source] Amorphous Calcium Carbonate is Stabilized in ConfinementADVANCED FUNCTIONAL MATERIALS, Issue 13 2010Christopher J. Stephens Abstract Biominerals typically form within localized volumes, affording organisms great control over the mineralization process. The influence of such confinement on crystallization is studied here by precipitating CaCO3 within the confines of an annular wedge, formed around the contact point of two crossed half-cylinders. The cylinders are functionalized with self-assembled monolayers of mercaptohexadecanoic acid on gold. This configuration enables a systematic study of the effects of confinement since the surface separation increases continuously from zero at the contact point to macroscopic (mm) separations. While oriented rhombohedral calcite crystals form at large (>10,µm) separations, particles with irregular morphologies and partial crystallinity are observed as the surface separation approaches the dimensions of the unconfined crystals (5,10,µm). Further increase in the confinement has a significant effect on the crystallization process with flattened amorphous CaCO3 (ACC) particles being formed at micrometer separations. These ACC particles show remarkable stability when maintained within the wedge but rapidly crystallize on separation of the cylinders. A comparison of bulk and surface free-energy terms shows that ACC cannot be thermodynamically stable at these large separations, and the stability is attributed to kinetic factors. This study therefore shows that the environment in which minerals form can have a significant effect on their stability and demonstrates that ACC can be stabilized with respect to the crystalline polymorphs of CaCO3 by confinement alone. That ACC was stabilized at such large (micrometer) separations is striking, and demonstrates the versatility of this strategy, and its potential value in biological systems. [source] Unusual atmospheric pressure chemical ionization conditions for detection of organic peroxidesJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 9 2003David Rondeau Abstract Organic peroxides such as the cumene hydroperoxide I (Mr = 152 u), the di- tert -butyl peroxide II (Mr = 146 u) and the tert -butyl peroxybenzoate III (Mr = 194 u) were analyzed by atmospheric pressure chemical ionization mass spectrometry using a water,methanol mixture as solvent with a low flow-rate of mobile phase and unusual conditions of the source temperature (,50 °C) and probe temperature (70,200 °C). The mass spectra of these compounds show the formation of (i) an [M + H]+ ion (m/z 153) for the hydroperoxide I, (ii) a stable adduct [M + CH3OH2]+ ion (m/z 179) for the dialkyl peroxide II and (iii) several protonated adduct species such as protonated molecules (m/z 195) and different protonated adduct ions (m/z 227, 389 and 421) for the peroxyester III. Tandem mass spectrometric experiments, exact mass measurements and theoretical calculations were performed for characterize these gas-phase ionic species. Using the double-well energy potential model illustrating a gas-phase bimolecular reaction, three important factors are taken into account to propose a qualitative interpretation of peroxide behavior toward the CH3OH2+, i.e. thermochemical parameters () and two kinetic factors such as the capture constant of the initial stable ion,dipole and the magnitude of the rate constant of proton transfer reaction into the loose proton bond cluster. Copyright © 2003 John Wiley & Sons, Ltd. [source] Sorption dynamics in fixed-beds of inert core spherical adsorbents including axial dispersion and Langmuir isothermAICHE JOURNAL, Issue 7 2009M. Khosravi Koocheksarayi Abstract The effects of axial dispersion and Langmuir isotherm on transient behavior of sorption and intraparticle diffusion in fixed-beds packed with monodisperse shell-type/inert core spherical sorbents are studied. The system of partial differential equations of the mathematical model is solved numerically using finite difference methods. Results are presented in the form of breakthrough curves for adsorption and desorption processes. Results reveal that the shape of the breakthrough curves is influenced by both hydrodynamic and kinetic factors. Hydrodynamic factor is governed by axial dispersion and is controlled by changes of Peclet number. Simulation results reveal that when linear adsorption isotherm is used, the effect of axial dispersion on breakthrough curves of the system is important for Peclet numbers smaller than 50, whereas, for Langmuir isotherm axial dispersion is considerable for Peclet numbers less than 80. In addition, effects of type of adsorption isotherms and size of adsorbents on breakthrough curves are investigated, and results are compared with existing reports in the pertinent literature. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Role of thermodynamic, molecular, and kinetic factors in crystallization from the amorphous stateJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 4 2008Chandan Bhugra Abstract Though there is an advantage in using the higher solubility amorphous state in cases where low solubility limits absorption, physical instability poses a significant barrier limiting its use in solid oral dosage forms. Unlike chemical instability, where useful accelerated stability testing protocols are common, no methodology has been established to predict physical instability. Therefore, an understanding of the factors affecting crystallization from the amorphous state is not only important from a scientific perspective but also has practical applications. Crystallization from the amorphous matrix has been linked to the molecular mobility in the amorphous matrix and recent research has focused on developing the link between these two fundamental properties of glass forming materials. Although researchers have been actively working in this area for some time, there is no current review describing the present state of understanding of crystallization from the amorphous state. The purpose of this review therefore is to examine the roles of different factors such as molecular mobility, thermodynamic factors, and the implication of different processing condition, in crystallization from the amorphous state. We believe an increased understanding of the relative contributions of molecular mobility and processing conditions are vital to increased usage of the amorphous state in solid oral dosage forms. © 2007 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 97:1329,1349, 2008 [source] Nanoencapsulation of a hydrophobic compound by a miniemulsion polymerization processJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 9 2004Yingwu Luo Abstract The nanoencapsulation of hydrophobic compounds by miniemulsion polymerization, a convenient one-step encapsulation technique for nanocapsules, was investigated in terms of the thermodynamics and kinetics. The encapsulation was achieved by polymerization inducing phase separation within minidroplets dispersed in an aqueous phase. Thermodynamic factors (the level and type of surfactant, the level of the hydrophilic comonomer, and the monomer/paraffin ratio), kinetic factors (the level of the crosslinking agent or chain-transfer agent), and nucleation modes were all found to have a great influence on the latex morphology. Specifically, for a styrene/paraffin system, there were optimum levels of sodium dodecyl sulfate (1.0 wt %), the hydrophilic comonomer (1.0 wt % methyl acrylate acid), and the chain-transfer agent (0.2 wt % n -dodecanethiol) for obtaining well-defined nanocapsules of paraffin with a styrene/paraffin ratio of 1:1. When the styrene/paraffin ratio was reduced, however, it was more difficult to achieve a fully encapsulated particle morphology. Homogeneous nucleation could compete with encapsulation, and this resulted in a pure polymer particle and a half-moon morphology. Conditions were also found under which complete encapsulation could be observed with a water-soluble initiator (potassium persulfate), contrary to certain reports. Replacing potassium persulfate with an oil-soluble initiator (2,2-azobisisobutyronitrile) had little influence on the morphology under those conditions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2145,2154, 2004 [source] A new polymorph of triphenylmethylamine: the effect of hydrogen bondingACTA CRYSTALLOGRAPHICA SECTION C, Issue 2 2009Victor N. Khrustalev Crystallization of the hexane reaction mixture after treatment of LiGe(OCH2CH2NMe2)3 with Ph3CN3 gives rise to a new triclinic (space group P) polymorph of triphenylmethylamine, C19H17N, (I), containing dimers formed by N,H...N hydrogen bonds, whereas the structure of the known orthorhombic (space group P212121) polymorph of this compound, (II), consists of isolated molecules. While the dimers in (I) lie across crystallographic inversion centres, the molecules are not truly related by them. The centrosymmetric structure is due to the statistical disordering of the amino H atoms participating in the N,H...N hydrogen-bonding interactions, and thus the inversion centre is superpositional. The conformations and geometric parameters of the molecules in (I) and (II) are very similar. It was found that the polarity of the solvent does not affect the capability of triphenylmethylamine to crystallize in the different polymorphic modifications. The orthorhombic polymorph, (II), is more thermodynamically stable under normal conditions than the triclinic polymorph, (I). The experimental data indicate the absence of a phase transition in the temperature interval 120,293,K. The densities of (I) (1.235,Mg,m,3) and (II) (1.231,Mg,m,3) at 120,K are practically equal. It would seem that either the kinetic factors or the effects of the other products of the reaction facilitating the hydrogen-bonded dimerization of triphenylmethylamine molecules are the determining factor for the isolation of the triclinic polymorph (I) of triphenylmethylamine. [source] Mechanism and exo -Regioselectivity of Organolanthanide-Mediated Intramolecular Hydroamination/Cyclization of 1,3-Disubstituted Aminoallenes: A Computational StudyCHEMISTRY - A EUROPEAN JOURNAL, Issue 9 2006Sven Tobisch Priv.-Doz. Abstract The complete catalytic reaction course for the organolanthanide-assisted intramolecular hydroamination/cyclization (IHC) of 4,5-heptadien-1-ylamine by a prototypical [(,5 -Me5C5)2LuCH(SiMe3)2] precatalyst has been critically scrutinized by employing a reliable DFT method. A computationally verified mechanistic scenario for the IHC of 1,3-disubstituted aminoallene substrates has been proposed that is consistent with the empirical rate law determined by experiment and accounts for crucial experimental observations. It involves kinetically rapid substrate association and dissociation equilibria, facile and reversible intramolecular allenic CC insertion into the LnN bond, and turnover-limiting protonation of the azacycle's tether functionality, with the amine-amidoalleneLn adduct complex representing the catalyst's resting state. This mechanistic scenario bears resemblance to the mechanism that has been recently proposed in a computational exploration of aminodiene IHC. The unique features of the IHC of the two substrate classes are discussed. Furthermore, the thermodynamic and kinetic factors that control the regio- and stereoselectivity of aminoallene IHC have been elucidated. These achievements have provided a deeper insight into the catalytic structure,reactivity relationships in organolanthanide-assisted cyclohydroamination of unsaturated CC functionalities. [source] | |||||||||||||