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Energy Barrier (energy + barrier)
Kinds of Energy Barrier Selected AbstractsApplication of mean-separation-works method to protein crystal nucleationCRYSTAL RESEARCH AND TECHNOLOGY, Issue 3 2008Christo N. Nanev Abstract Using mean-separation-works method of Stranski and Kaischew calculations of nucleus form and energy barrier for its formation are performed for globular protein crystals. This is done on the basis of a simple model suggested for crystal nucleation of such proteins. The prerequisite for the model is the fact that strict selection of definite sticky patches on protein molecule surface is obligatory for forming crystal lattices. The calculation results are in agreement with experimental data. (© 2008 WILEY -VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The Role of Amine,B(C6F5)3 Adducts in the Catalytic Reduction of Imines with H2: A Computational StudyEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 15 2009Timofei Privalov Abstract This study thoroughly examines the potential energy surfaces (PESs) of two possible mechanisms for reduction of imines by B(C6F5)3 and H2. The key reaction steps of the first catalytic mechanism, which is the focus of our study, are: (i) the uptake of H2 by a thermally activated amine,B(C6F5)3 species; (ii) proton transfer from the NH2+ moiety of [RNH2CH2R,]+[HB(C6F5)3], to the imine; (iii) nucleophillic attack of the C-center of the iminium ion by the BH, group. The potential energy barriers of the latter, as determined by calculating the evolution of the H-bonded complex of an imine and [RNH2CH2R,]+[HB(C6F5)3], in toluene, are around 10 kcal,mol,1 each. In the second mechanism, only imines serve as basic partners of B(C6F5)3 in the H2 activation, which affords an [RN(H)CHR,]+[HB(C6F5)3], ion pair; direct reduction then proceeds via nucleophilic attack of the C-center by the BH, in [RN(H)CHR,]+[HB(C6F5)3],. This route becomes catalytic when the product amine is released into the solvent and B(C6F5)3 is re-used for H2 activation. Upon taking into account the association energy of an amine,B(C6F5)3 adduct [,9.5 kcal,mol,1 for tBuN(H)CH2Ph and B(C6F5)3 in toluene], the potential energy barrier for H2 uptake by an imine and B(C6F5)3 increases to 14.5 kcal,mol,1. We report a somewhat lower potential energy barrier for H2 uptake by thermally activated amine,B(C6F5)3 adducts [12.7 kcal,mol,1 for the B-N adduct of tBuN(H)CH2Ph and B(C6F5)3 in toluene], although the difference between the two H2 activationbarriers is within the expected error of the computational method. Two catalytic routes are compared based on B3LYP-computed PESs in solvent (toluene).(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] A Theoretical Study on the Reactivity of a Rhenium Hydroxo-Carbonyl Complex Towards ,-LactamsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 29 2008Violeta Yeguas Abstract The mechanism of the reaction between the complex [Re(OH)(CO)3(N2C2H4)] and azetidin-2-one or 3-formylamino- N -sulfonatoazetidin-2-one was investigated by using the B3LYP density functional theory methodology in conjunction with the PCM-UAHF model to take into account solvent effects. According to our calculations, the rate-determining energy barrier for the azetidin-2-one case of 38.8 kcal,mol,1, becomes 25.7 kcal,mol,1 in the case of the 3-formylamino- N -sulfonatoazetidin-2-one species. The presence of the sulfonato group is crucial for the cleavage of the ,-lactam N1,C2 bond by the Re complex thanks to the interaction of the sulfonato group with the hydroxy and bidentate ligands of the complex. This could be of interest for the synthesis of ,-amino acids and their derivatives from ,-lactams under mild conditions and in solvents of low polarity promoted by organometallic complexes. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Interactions of Cationic Palladium(II)- and Platinum(II)-,3 -Allyl Complexes with Fluoride: Is Asymmetric Allylic Fluorination a Viable Reaction?EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 7 2006Lukas Hintermann Abstract The complex cations [M(,3 -R2All)(PPFPz{3- tBu})]+ (M = PdII, R2All = 1,3-diphenylallyl, 1,3-dicyclohexylallyl, indenyl; M = PtII, R2All = 1,3-diphenylallyl; PPFPz-{3- tBu} = 3- tert -butyl-1-{1-[2-diphenylphosphanyl-ferrocenyl]ethyl}-1H -pyrazole)have been prepared as salts with PF6, or SbF6,. They have been characterized by NMR spectroscopy in solution and by X-ray crystallography in the solid state. Their reactions with sources of nucleophilic and "naked" fluoride have been investigated by multinuclear NMR spectroscopy. The PdII complexes did not undergo any nucleophilic substitution with concomitant release of allyl fluorides. The dicyclohexylallyl fragment was released as a 1,3-diene by elimination, but with other allyl complexes nonspecific decomposition reactions predominated. The complex [Pt(,3 -1,3-Ph2C3H3)(PPFPz{3- tBu})]PF6 underwent an anion exchange with Me4NF to give [Pt(1,3-Ph2C3H3)(PPFPz{3- tBu})]F which existed as a mixture of interconverting allyl isomers in solution at ambient temperature. For the bromide salt, [Pt(,3 -1,3-Ph2C3H3)(PPFPz{3- tBu})]Br, allyl isomerization was slow at ambient temperature. Precursors of Pt0 reacted with bromo-1,3-diphenylprop-2-ene to give [Pt2(,-Br)2(,3 -1,3-Ph2All)2] and precursors of Pd0 underwent oxidative additions with bromo- and fluoro-1,3-diphenyl-2-propene to give 1,3-diphenylallyl complexes of PdII. Therefore, the nucleophilic attack of fluoride on the allyl fragment of PdII complexes is endergonic, and the high energy barrier of this step is difficult to overcome in a catalytic allylic fluorination reaction. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Potential MRI Contrast Agents Based on Micellar Incorporation of Amphiphilic Bis(alkylamide) Derivatives of [(Gd,DTPA)(H2O)]2,EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 16 2003Kristof Kimpe Abstract DTPA-bisamide derivatives with alkyl chains containing 14, 16 and 18 carbon atoms were synthesized and complexes of various trivalent lanthanide ions (Ln = Gd, La, Pr, Eu) were formed. Variable temperature proton NMR spectroscopy of paramagnetic praseodymium(III) and europium(III) complexes revealed that long aliphatic substituents considerably increase the energy barrier for the intramolecular rearrangement around the lanthanide ion. The gadolinium(III) complexes were incorporated into mixed micelles, and photon correlation spectroscopy showed that the mean sizes of all the micelles were within the same range. The NMRD curves of all three DTPA-bisamide-gadolinium complexes incorporated in mixed micelles display higher relaxivity values than the commercially available Gd,DTPA contrast agent. The higher relaxivity obtained for the micellar DTPA-bisamide-gadolinium complexes with C14 and C16 chains relative to the micellar DTPA-bisamide-GdIII C18 chain complex could be attributable to the fact that the alkyl chain containing 18 carbon atoms is longer than the alkyl chain of the major component of the micelles, DPPC, in which it is inserted. This would allow increased mobility of the polar head and hence a lower relaxivity. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Alternative Mechanistic Paths in the Hetero-Diels,Alder Reaction of ,-Oxothiones: A Theoretical StudyEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 21 2007Laura Legnani Abstract DFT calculations at the B3LYP/6-311+G(d,p) level for the C, H, and O atoms and at the 6-311+G(2df,p) level for the S atom were used to study the hetero-Diels,Alder reactions between several ,-oxothiones and ethylene or methyl vinyl ether (MVE). All the transition states and the intermediates along the reaction pathways, as well as the reaction products, were located. The reactions with ethylene are all concerted though asynchronous whereas in the reactions with MVE the electron-releasing character of the methoxy substituent lowers the energy barriers and enhances the asynchronicity and the charge transfer process to such an extent that the reaction may become unconcerted and exhibit a two-step mechanism with a zwitterionic intermediate derived from nucleophilic attack of electron-rich MVE to the sulfur atom of the strongly electrophilically activated ,-oxothiones. The reactions are also favored by the conjugation of the newly formed C=C bond. Moreover, the geometric features of the diene exert a nonnegligible role, as dienes that are planar or almost planar in their ground state show a lower energy barrier. Thus, both geometric and electronic features of the dienes as well as of the dienophiles play a significant role in the easiness of the reactions and in their mechanism. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Comparison of the specificity, stability and individual rate constants with respective activation parameters for the peptidase activity of cruzipain and its recombinant form, cruzain, from Trypanosoma cruziFEBS JOURNAL, Issue 24 2001Wagner A. S. Judice The Trypanosoma cruzi cysteine protease cruzipain contains a 130-amino-acid C-terminal extension, in addition to the catalytic domain. Natural cruzipain is a complex of isoforms, because of the simultaneous expression of several genes, and the presence of either high mannose-type, hybrid monoantennary-type or complex biantenary-type oligosacharide chains at Asn255 of the C-terminal extension. Cruzipain and its recombinant form without this extension (cruzain) were studied comparatively in this work. S2 to S2, subsite specificities of these enzymes were examined using four series of substrates derived from the internally quenched fluorescent peptide Abz-KLRFSKQ-EDDnp (Abz, ortho -aminobenzoic acid; EDDnp, N -(2,4-dinitrophenyl)-ethylenediamine). Large differences in the kinetic parameters were not observed between the enzymes; however, Km values were consistently lower for the hydrolysis of most of the substrates by cruzain. No difference in the pH,activity profile between the two enzymes was found, but in 1 m NaCl cruzipain presented a kcat value significantly higher than that of cruzain. The activation energy of denaturation for the enzymes did not differ significantly; however, a negative entropy value was observed for cruzipain denaturation whereas the value for cruzain was positive. We determined the individual rate constants (k1, substrate diffusion; k,1, substrate dissociation; k2, acylation; k3, deacylation) and the respective activation energies and entropies for hydrolysis of Abz-KLRFSKQ-EDDnp determining the temperature dependence of the Michaelis,Menten parameters kcat/Km and kcat as previously described [Ayala, Y.M. & Di Cera, E. (2000) Protein Sci.9, 1589,1593]. Differences between the two enzymes were clearly detected in the activation energies E1 and E,1, which are significantly higher for cruzipain. The corresponding ,S1 and ,S,1 were positive and significantly higher for cruzipain than for cruzain. These results indicate the presence of a larger energy barrier for cruzipain relating to substrate diffusion and dissociation, which could be related to the C-terminal extension and/or glycosylation state of cruzipain. [source] Insight into Proton Conduction of Immobilised Imidazole Systems Via Simulations and Impedance Spectroscopy,FUEL CELLS, Issue 3-4 2008W. L. Cavalcanti Abstract The proton conduction in immobilised imidazole systems has been investigated in order to support the design of new membrane materials for polymer electrolyte membrane fuel cells (PEMFC). In the experimental part of this work, proton conductivities are measured via impedance spectroscopy. The simulation and modelling are performed combining molecular dynamics simulations and energy barrier calculations; the analysis is done via the proton jump energy barrier, collision ratio and radial distribution function. The dependence of the proton mobility on the temperature, spacer length and the density of conducting groups per area is presented. Donors and acceptors groups approach to each other within a distance from 2.8 to 3,Å where the energy barrier for a proton transfer is very low, which favours the proton jump under the studied conditions. The proton conductivity increases with increase in the spacer length. The simulation results are in good agreement with the proton conductivities presented. [source] A New Carbazole-Constructed Hyperbranched Polymer: Convenient One-Pot Synthesis, Hole-Transporting Ability, and Field-Effect Transistor PropertiesADVANCED FUNCTIONAL MATERIALS, Issue 16 2009Zhong'an Li Abstract A new hyperbranched polymer (HB-car), constructed fully by carbazole moieties, is successfully synthesized through a one-pot Suzuki coupling reaction. The resultant polymer is well-characterized, and its hole-transporting ability is studied carefully. The device, in which HB-car is utilized as a hole-transporting layer and tris-(8-hydroxyquinoline) aluminum as an electron-emitting layer as well as electron-transporting layer, gives a much higher efficiency (3.05,cd A,1), than that of a poly(N -vinylcarbazole) based device (2.19,cd A,1) under similar experimental conditions. The remarkable performance is attributed to its low energy barrier and enhanced hole-drifting ability in the HB-car based device. In addition, for the first time, a field-effect transistor (FET) based on the hyperbranched polymer is fabricated, and the organic FET device shows that HB-car is a typical p -type FET material with a saturation mobility of 1,×,10,5,cm2 V,1 s,1, a threshold voltage of ,47.1,V, and an on-to-off current ratio of 103. [source] Statistical theory of weak field thermoremanent magnetization in multidomain particle ensemblesGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2003Karl Fabian SUMMARY A non-equilibrium statistical theory of multidomain thermoremanent magnetization (TRM) is developed, which describes thermal magnetization changes as continuous inhomogeneous Markov processes. The proposed theory relies on three very general physical properties of TRM: (a) The probability that a magnetization state Sj is transformed during an infinitesimal temperature change into state Si depends only on external conditions and on Sj, but not on previously assumed states. (b) Due to time inversion symmetry of the Maxwell equations, the magnetic energies are invariant with respect to inversion of all spins in zero field. (c) The probability that an energy barrier between two magnetization states is overcome during a thermal process is governed by Boltzmann statistics. From these properties, the linearity of TRM with field is derived for generic multidomain particle ensembles. The general validity of Thellier's law of additivity of partial TRM's in weak fields is established and a method for proving a large class of similar additivity laws is developed. The theory allows consistent treatment of blocking and unblocking of remanence in multidomain particle ensembles and naturally explains apparent differences between blocking and unblocking temperatures. [source] Internal-rotation and inversion potential energy surfaces for methylamine and methylphosphineINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 5 2008Hae-Won Kim Abstract Using the HF, MP2, and DFT methods with a 6-311G** basis set, we examine the internal-rotation and inversion of the amino and phosphino groups in methylamine and methylphosphine. The resulting energy surfaces are reported as a function of rotation and inversion descriptors. Both surfaces are characterized by several special points: three minimum energy states, three first-order internal-rotation transition states, six first-order inversion transition states, and six second-order inversion transition states. The MP2 method gave better agreement with experiment. For methylamine, the MP2 energy barrier for internal-rotation is 8.73 kJ/mol; for first-order inversion it is 22.80 kJ/mol and for second-order inversion it is 22.41 kJ/mol. For methylphosphine, the MP2 energy barriers are 7.53, 149.88, and 149.65 kJ/mol, respectively. The energy barriers include ZPE correction contributions. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Theory of chemical bonds in metalloenzymes.INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4 2008Abstract A first principle investigation has been carried out for intermediate states of the catalytic cycle of a cytochrome P450. To elucidate the whole catalytic cycle of P450, the electronic and geometrical structures are investigated not only at each ground state but also at low-lying energy levels. Using the natural orbital analysis, the nature of chemical bonds and magnetic interactions are investigated. The ground state of the Compound 1 (cpd1) is calculated to be a doublet state, which is generated by the antiferromagnetic coupling between a triplet Fe(IV)O moiety and a doublet ligand radical. We found that an excited doublet state of the cpd1 is composed of a singlet Fe(IV)O and a doublet ligand radical. This excited state lies 20.8 kcal mol,1 above the ground spin state, which is a non-negligible energy level as compared with the activation energy barrier of ,E# = 26.6 kcal mol,1. The reaction path of the ground state of cpd1 is investigated on the basis of the model reaction: 3O(3p) + CH4. The computational results suggest that the reactions of P450 at the ground and excited states proceed through abstraction (3O-model) and insertion (1O-model) mechanisms, respectively. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Swift hopping gallium over [AlO4], tetrahedra in Ga/ZSM-5: A DFT studyINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 13 2007Ilya V. Kusmin Abstract Density functional theory calculations were carried out to investigate gallium species (Ga+, [GaH2]+, and [GaO]+) stabilization in Ga-exchanged HZSM-5, using cluster modeling approach. Three isomeric gallium positions over [AlO4], zeolite fragment at T12 position were found. These isomers are turning into each over with low activation energy barrier and gallium fragment revolves around [AlO4], tetrahedron by hopping between cationic positions. Activation energies of gallium fragment hopping were computed and compared for different gallium containing cations. Those barriers were found to be times less than the activation energies of catalytic processes on gallium-exchanged zeolite. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Theoretical study of the reaction of alkynes with furan catalyzed by AuCl3 and AuClINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 2 2007Hassan Rabaâ Abstract A general scheme for the endo- and exo-cyclization of furan reactivity with [L -AuIII, IClx] with (x = 3, 1 and L -acetylene and vinylidene) complexes is investigated using density functional theory (DFT) code. Two conceivable mechanisms via a [4 + 2] Diels,Alder process or carbene complex are analyzed. According to the activation energy values of the gold (III and I) catalyst, the first mechanism, which implies the Diels,Alder reaction of AuIII, is thermodynamically favored and gives more evidence of the intramolecular addition of the furan with the alkynes. The second mechanism, presumably assisted by the spontaneous formation of the exo-vinylidene complexes and intermediates of gold (III, I) by forming the carbene complex, is kinetically favored. Additionally, we compare our results with other structures with intramolecular additions that exhibit the quasi-similarity of gold analogue structures. Differences in activation energies are observed, according to the functional used. Finally, we probe the solvent effects, which decrease the energy barrier in the path. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Theoretical studies of mechanisms of cycloaddition reaction between difluoromethylene carbene and acetoneINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 2 2007Xiu Hui Lu Abstract Mechanisms of the cycloaddition reaction between singlet difluoromethylene carbene and acetone have been investigated with the second-order Møller,Plesset (MP2)/6-31G* method, including geometry optimization and vibrational analysis. Energies for the involved stationary points on the potential energy surface (PES) are corrected by zero-point energy (ZPE) and CCSD(T)/6-31G* single-point calculations. From the PES obtained with the CCSD(T)//MP2/6-31G* method for the cycloaddition reaction between singlet difluoromethylene carbene and acetone, it can be predicted that path B of reactions 2 and 3 should be two competitive leading channels of the cycloaddition reaction between difluoromethylene carbene and acetone. The former consists of two steps: (i) the two reactants first form a four-membered ring intermediate, INT2, which is a barrier-free exothermic reaction of 97.8 kJ/mol; (ii) the intermediate INT2 isomerizes to a four-membered product P2b via a transition state TS2b with an energy barrier of 24.9 kJ/mol, which results from the methyl group transfer. The latter proceeds in three steps: (i) the two reactants first form an intermediate, INT1c, through a barrier-free exothermic reaction of 199.4 kJ/mol; (ii) the intermediate INT1c further reacts with acetone to form a polycyclic intermediate, INT3, which is also a barrier-free exothermic reaction of 27.4 kJ/mol; and (iii) INT3 isomerizes to a polycyclic product P3 via a transition state TS3 with an energy barrier of 25.8 kJ/mol. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Theoretical study of the · H reaction with cytosineINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2007Hongyu Zhang Abstract We studied three possible reactions of H atom attacking the cytosine, using density functional theory (DFT) calculations. The results indicate that the H atom addition to the N3 site of cytosine is energetically more favorable than to the C5 or C6 site. The reaction of addition to the C6 site has an energy barrier of ,2.77 kcal/mol, which is ,2 kcal/mol higher than addition to C5. The energy of C5 H-adduct radical is also lower than that of C6 H-adduct radical. From the point of view of both energetics and reaction kinetics, the addition of the H atom to the C5 site is preferred to the addition to the C6 site. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Substituent effects on the physical properties and pKa of phenolINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4-5 2001Kevin C. Gross Abstract Substituent effects on the physical properties and pKa of phenol were studied using density functional theory [B3LYP/6-311G(d,p)] calculations. Substituents alter the physical properties of phenol such as the hydroxyl-group CO and OH bond lengths, the C OH bond angle, and the energy barrier to rotation about the C O bond, and also influence the hydroxyl-group pKa. Except for the rotational barrier, Hammett , constants showed strong correlation with these property changes. Several quantum chemical parameters, including the natural charge on the phenolic hydrogen Qn(H) and the natural charge on the phenoxide oxygen Qn(O,), the HF/6-311G(d,p) HOMO energy Ehomo, and the proton-transfer energy ,Eprot, outperformed the empirical Hammett constants in modeling changes in the pKa. All of these latter parameters yielded correlation coefficients ,r,>0.94 for the pKa. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001 [source] A phase-space method for arbitrary bimolecular gas-phase reactions: Theoretical descriptionINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 5 2001A. Gross Abstract A theoretical model for the calculation of rate constants for arbitrary bimolecular gas-phase reactions was developed. The method is based on the phase-space statistical method developed by Light and co-workers 1,6. In the present article this method is extended to arbitrary molecular systems. The new method requires knowledge of the molecular properties in the reaction and products channels of the chemical system. The properties are the vibrational frequencies, moments of inertia, and potential energy for the interacting species in their ground state equilibrium configuration. Furthermore, we have to calculate either the energy barrier or the long-range potential for the chemical system (if the reaction channel does not have an energy barrier). The usefulness of the method is that it can be applied to all bimolecular reactions, trimolecular reactions, and even reactions of higher orders. Therefore, it can be applied to cases where rate constants of complex chemical reactions are required, but reliable laboratory measurements or other means to estimate rate parameters are not yet possible. Even if spectroscopic data are not available for the reactants and products, it is possible to use electronic structure theory to calculate the required data. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001 [source] About the activation energies of the main and secondary relaxations in cured styrene butadiene rubberJOURNAL OF APPLIED POLYMER SCIENCE, Issue 4 2009Ada Ghilarducci Abstract This article studies the influence of the network structure on the activation energies of the , and , relaxations in vulcanized styrene butadiene rubber, SBR. A cure system based on sulphur and TBBS (N-t-butyl-2-benzothiazole sulfenamide) was used in the formulation of several compounds cured at 433 K. The activation energies were evaluated from internal friction (loss tangent) data of the compounds using an automated subresonant forced pendulum in a wide frequency range and between 80 K and 273 K. The internal friction data of the samples reveal two transitions, , and ,, characterized by the temperatures T, and T,, due to the glass transition and the phenyl group rotation of the copolymer, respectively. Although T, increases at higher crosslink density, it shows also a dependence with the amount of polysulphide and monosulphide linkages present in the samples. The highest activation energy for this process is obtained for the samples with high crosslink density and 30% of monosulphides in this structure. In the case of the ,-relaxation, there is a pronounced change in the activation energy between the uncured and the cured samples. The type of structure formed during vulcanization has an important effect in the activation energy of the segmental mode-process. In the case of the ,-process, the cis-trans isomerization that takes place during vulcanization in the butadiene part of the SBR, might be the cause of conformational changes in the surrounding of the phenyl rings that affect the energy barrier associated to the phenyl rotation. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009 [source] Rheological and electrical properties of bentonite in anionic polystyrene sulfonate and nonionic poly(vinyl alcohol)JOURNAL OF APPLIED POLYMER SCIENCE, Issue 6 2007Bassem A. Mostafa Abstract The flow properties of bentonite were studied with a combination of anionic polystyrene sulfonate (molecular weight = 70,000 g/mol) and nonionic poly(vinyl alcohol) (molecular weight = 50,000 g/mol). This combination had a significant effect on clay suspensions depending on the sequence of the polymer addition. The addition of 50 mg/L anionic polystyrene sulfonate followed by 200 mg/L nonionic poly(vinyl alcohol) improved the rheological properties with a 7% bentonite suspension, especially at 20°C and after 24 h of aging. However, by the reversal of this addition sequence, the rheological properties of bentonite suspensions were reduced. The , potential of bentonite suspensions of the individual polymers at the same concentration was ,42 mV for polystyrene sulfonate and ,63 mV for poly(vinyl alcohol). The combined effect of anionic polystyrene sulfonate followed by nonionic poly(vinyl alcohol) noticeably changed the , potential (,95 mV). With the reversal of the addition sequence, this parameter did not change. Potential energy profiles were constructed to investigate the suspension stability. Potential energy profiles of polystyrene sulfonate added to bentonite suspensions and followed by poly(vinyl alcohol) produced high repulsion potential energy between clay surfaces, reflecting high suspension stability. By the reversal of this sequence, a significant reduction of the energy barrier was observed. On the basis of the rheological and electrical properties of this system, the addition of polystyrene sulfonate followed by poly(vinyl alcohol) provides a promising tendency for a 7% clay concentration to meet desirable drilling mud properties. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 3886,3894, 2007 [source] Theoretical study on the gas-phase reaction mechanism between nickel monoxide and methane for syngas productionJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 6 2009Hua-Qing Yang Abstract The comprehensive mechanism survey on the gas-phase reaction between nickel monoxide and methane for the formation of syngas, formaldehyde, methanol, water, and methyl radical has been investigated on the triplet and singlet state potential energy surfaces at the B3LYP/6-311++G(3df, 3pd)//B3LYP/6-311+G(2d, 2p) levels. The computation reveals that the singlet intermediate HNiOCH3 is crucial for the syngas formation, whereas two kinds of important reaction intermediates, CH3NiOH and HNiOCH3, locate on the deep well, while CH3NiOH is more energetically favorable than HNiOCH3 on both the triplet and singlet states. The main products shall be syngas once HNiOCH3 is created on the singlet state, whereas the main products shall be methyl radical if CH3NiOH is formed on both singlet and triplet states. For the formation of syngas, the minimal energy reaction pathway (MERP) is more energetically preferable to start on the lowest excited singlet state other than on the ground triplet state. Among the MERP for the formation of syngas, the rate-determining step (RDS) is the reaction step for the singlet intermediate HNiOCH3 formation involving an oxidative addition of NiO molecule into the CH bond of methane, with an energy barrier of 120.3 kJ mol,1. The syngas formation would be more effective under higher temperature and photolysis reaction condition. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009 [source] A molecular mechanics force field for biologically important sterolsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 13 2005Zoe Cournia Abstract A parameterization has been performed of the biologically important sterols cholesterol, ergosterol, and lanosterol for the CHARMM27 all-atom molecular mechanics force field. An automated parameterization method was used that involves fitting the potential to vibrational frequencies and eigenvectors derived from quantum-chemical calculations. The partial charges were derived by fitting point charges to quantum-chemically calculated electrostatic potentials. To model the dynamics of the hydroxyl groups of the sterols correctly, the parameter set was refined to reproduce the energy barrier for the rotation of the hydroxyl group around the carbon connected to the hydroxyl of each sterol. The frequency-matching plots show good agreement between the CHARMM and quantum chemical normal modes. The parameters are tested in a molecular dynamics simulation of the cholesterol crystal structure. The experimental geometry and cell dimensions are well reproduced. The force field derived here is also useful for simulating other sterols such as the phytosterols sigmasterol, and campesterol, and a variety of steroids. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 1383,1399, 2005 [source] First-principle studies of intermolecular and intramolecular catalysis of protonated cocaineJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 10 2005Chang-Guo Zhan Abstract We have performed a series of first-principles electronic structure calculations to examine the reaction pathways and the corresponding free energy barriers for the ester hydrolysis of protonated cocaine in its chair and boat conformations. The calculated free energy barriers for the benzoyl ester hydrolysis of protonated chair cocaine are close to the corresponding barriers calculated for the benzoyl ester hydrolysis of neutral cocaine. However, the free energy barrier calculated for the methyl ester hydrolysis of protonated cocaine in its chair conformation is significantly lower than for the methyl ester hydrolysis of neutral cocaine and for the dominant pathway of the benzoyl ester hydrolysis of protonated cocaine. The significant decrease of the free energy barrier, ,4 kcal/mol, is attributed to the intramolecular acid catalysis of the methyl ester hydrolysis of protonated cocaine, because the transition state structure is stabilized by the strong hydrogen bond between the carbonyl oxygen of the methyl ester moiety and the protonated tropane N. The relative magnitudes of the free energy barriers calculated for different pathways of the ester hydrolysis of protonated chair cocaine are consistent with the experimental kinetic data for cocaine hydrolysis under physiologic conditions. Similar intramolecular acid catalysis also occurs for the benzoyl ester hydrolysis of (protonated) boat cocaine in the physiologic condition, although the contribution of the intramolecular hydrogen bonding to transition state stabilization is negligible. Nonetheless, the predictability of the intramolecular hydrogen bonding could be useful in generating antibody-based catalysts that recruit cocaine to the boat conformation and an analog that elicited antibodies to approximate the protonated tropane N and the benzoyl O more closely than the natural boat conformer might increase the contribution from hydrogen bonding. Such a stable analog of the transition state for intramolecular catalysis of cocaine benzoyl-ester hydrolysis was synthesized and used to successfully elicit a number of anticocaine catalytic antibodies. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 980,986, 2005 [source] A combined freeze-and-cut strategy for the description of large molecular systems using a localized orbitals approachJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 10 2005Stefano Borini Abstract A technique to reduce the computational effort in calculating ab initio energies using a localized orbitals approach is presented. By exploiting freeze strategy at the self-consistent field (SCF) level and a cut of the unneeded atomic orbitals, it is possible to perform a localized complete active space (CAS-SCF) calculation on a reduced system. This will open the possibility to perform ab initio treatments on very large molecular systems, provided that the chemically important phenomena happen in a localized zone of the molecule. Two test cases are discussed, to illustrate the performance of the method: the cis,trans interconversion curves for the (7Z)-13 ammoniotridec-7-enoate, which demonstrates the ability of the method to reproduce the interactions between charged groups; and the cisoid,transoid energy barrier for the aldehydic group in the C13 polyenal molecule. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 1042,1051, 2005 [source] Solubility and degradation of polyhydroxyalkanoate biopolymers in propylene carbonateAICHE JOURNAL, Issue 6 2010Christopher W. J. McChalicher Abstract New biobased materials and chemicals require different processing strategies than petroleum-derived commodities. The extraction and recovery of polyhydroxyalkanoate (PHA) biopolymers from the residual cellular biomass is particularly difficult because the polymer is accumulated within the cell. PHAs have low solubility in many classical polymer solvents and are most often dissolved using undesirable chlorinated solvents. The solubility kinetics is greatly diminished when these polymers are highly crystalline. Here, 1,2-propylene carbonate is used to dissolve highly crystalline polyhydroxybutyrate at ambient pressures and moderate temperatures. We have used kinetic studies of the dissolution of the crystalline material to determine the energy barrier for dissolution of the system. Further, the degradation of polyhydroxybutyrate and similarly prepared PHA block copolymers were studied during this extraction process using molecular weight characterization by gel permeation chromatography. Finally, we have used these findings to extract PHA block copolymers from dried biomass at the bench scale. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source] Intra- versus intermolecular hydrogen bonding equilibrium in 2-hydroxy- N,N -diethylbenzamide,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 2 2009P. Majewska Abstract Complex studies of the intramolecular versus intermolecular hydrogen bond equilibrium and internal rotation of the N,N -diethylamine group in 2-hydroxy- N,N -diethylbenzamide were conducted. The intramolecular versus intermolecular process in 2-hydroxy- N,N -diethylbenzamide was studied by UV,Vis, NMR, IR and Vapour Pressure Osmometric (VPO) methods as a function of temperature and concentration in non-polar, basic and protic solvents. The unequal positions of the ethyl groups were analysed and the energy barrier to the re-orientation was defined by the NMR method. This paper presents a study into a complicated nature of competitive interaction 2-hydroxy- N,N -diethylbenzamide with the environment by means of the aforesaid methods. Copyright © 2008 John Wiley & Sons, Ltd. [source] DFT study and NBO analysis of the mutual interconversion of cumulene compoundsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 5 2007Davood Nori-Shargh Abstract The B3LYP/6-31G* method was used to investigate the configurational properties of allene (1,2-propadiene) (1), 1,2,3-butatriene (2), 1,2,3,4-pentateriene (3), 1,2,3,4,5-hexapentaene (4), 1,2,3,4,5,6-heptahexaene (5), 1,2,3,4,5,6,7-octaheptaene (6), 1,2,3,4,5,6,7,8-nonaoctaene (7), and 1,2,3,4,5,6,7,8,9-decanonaene (9). The calculations at the B3LYP/6-31G* level of theory showed that the mutual interconversion energy barrier in compounds 1,8 are: 209.73, 131.77, 120.34, 85.00, 80.91, 62.19, 55.56, and 46.83,kJ,mol,1, respectively. The results showed that the difference between the average CC double bond lengths () values in cumulene compounds 1 and 2, is larger than those between 7 and 8, which suggest that with large n (number of carbon atoms in cumulene chain), the values approach a limiting value. Accordingly, based on the plotted data, the extrapolation to n,=,,, gives nearly the same limiting (i. e., ). Also, NBO results revealed that the sum of , -bond occupancies, , decrease from 1 to 8, and inversely, the sum of , -antibonding orbital occupancies, , increase from compound 1 to compound 8. The decrease of values for compounds 1,8, is found to follow the same trend as the barrier heights of mutual interconversion in compounds 1,8, while the decrease of the barrier height of mutual interconversion in compounds 1,8 is found to follow the opposite trend as the increase in the number of carbon atom. Accordingly, besides the previously reported allylic resonant stabilization effect in the transition state structures, the results reveal that the values, , ,(EHOMO,,,ELUMO), and the C atom number could be considered as significant criteria for the mutual interconversion in cumulene compounds 1,8. This work reports also useful predictive linear relationships between mutual interconversion energy barriers () in cumulene compounds and the following four parameters: , , ,(EHOMO,,,ELUMO), and CNumber. Copyright © 2007 John Wiley & Sons, Ltd. [source] Theoretical studies on the S,N interaction in sulfinamidesJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 4 2002Prasad V. Bharatam Abstract The potential energy surface of sulfinamide H(O)S,NH2 (1) was searched, using ab initio and density functional methods, to study the conformational preferences. High-accuracy G2MP2 calculations showed that the S,N rotational barrier in 1 is 7.0,kcal,mol,1. The inversion around N in 1 goes through a very low energy barrier. Charge analysis using the NPA method was performed to elucidate the electronic factors responsible for the observed trends in the S,N interactions. The strength of negative hyperconjugation in 1 was estimated using NBO analysis and by studying the substituent effect. The repulsions between the lone pairs on oxygen and nitrogen and the nN , ,*S,O negative hyperconjugation play an important role in the conformations. Copyright © 2002 John Wiley & Sons, Ltd. [source] Determination of the 2,3-pentadienedioic acid enantiomer interconversion energy barrier 1.JOURNAL OF SEPARATION SCIENCE, JSS, Issue 15 2006Classical kinetic approach Abstract A classical kinetic method was used to determine the energy barrier for the interconversion of 2,3-pentadienedioic acid enantiomers. Each individual enantiomer was isolated by collecting the appropriate peaks from the HPLC enantiomeric separation, of racemic 2,3-pentadienedioic acid. The isolated enantiomers were racemized at 22°C using various interconversion times. The ratio of enantiomers in each reaction solution was determined by HPLC at 22°C. The corresponding peak areas of the enantiomers and the interconversion times obtained from the HPLC chromatograms were used to calculate both the interconversion rate constants describing (+) , (,) and (,) , (+) interconversions as well as the energy barriers. It was confirmed that the interconversion of 2,3-pentadienedioic acid enantiomers is a first-order kinetic reaction. Both semiempirical and ab initio methods were used to explore the mechanism of the interconversion of 2,3-pentadienedioic acid enantiomers, and to calculate the interconversion energy barrier. Comparison of the interconversion energy barriers found by the ab initio method (,G# = 110.7 kJ/mol) and by classical kinetics in the mobile phase solution at 22°C (,Gapp = 93.9 ± 0.2 kJ/mol) shows a difference which may be attributed to the different conditions assumed in the theoretical calculation (i. e., a gaseous state) and the actual experimental conditions (i. e., liquid solution) and a possible catalytic effect of the solution composition. [source] Kinetic Studies of Mullite Synthesis from Alumina Nanoparticles and a Preceramic PolymerJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2008Flavio Griggio The crystallization kinetics of mullite formation in a diphasic precursor consisting of a silicone resin filled with commercial ,-alumina nanoparticles (15 nm mean particle size, specific surface area of 100 m2/g), heated in air from 1250° to 1350°C, was studied by X-ray diffraction. Transitional ,-alumina and amorphous silica from the pyrolysis of the preceramic polymer exhibited a remarkable reactivity, as demonstrated by a very low incubation time (from 500 s at 1250°C to 20 s at 1350°C), a high mullite yield (about 80 vol%, after 100 s at 1350°C), and a low activation energy for nucleation (677±60 kJ/mol). The activation energy values found were lower than those reported previously for other diphasic systems, including sol,gel precursors. Besides the high specific surface of nanosized ,-alumina particles, the low energy barrier could be attributed to the highly reactive silica deriving from the oxidation of Si,CH3 bonds in the silicone and to the homogeneous dispersion of the nanosized filler inside the preceramic polymer. Furthermore, the possibility of applying plastic shaping processing methods to the mixture of a preceramic polymer and nanosized filler makes this approach particularly valuable, in comparison, for instance, with sol,gel based alternatives. 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