Home About us Contact
|
Home About us Contact | ||
|
|
||
Experimental Values (experimental + value)
Selected AbstractsUV Index Experimental Values During the Years 2000 and 2001 from the Spanish Broadband UV-B Radiometric Network,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 2 2002José A. Martínez-Lozano ABSTRACT An analysis is made of experimental ultraviolet erythemal solar radiation data measured during the years 2000 and 2001 by the Spanish UV-B radiation evaluation and prediction network. This network consists of 16 Robertson,Berger type pyranometers for evaluating solar erythemal radiation and five Brewer spectroradiometers for evaluating the stratospheric ozone. On the basis of these data the Ultraviolet Index (UVI) was evaluated for the measuring stations that are located either in coastal regions or in the more densely populated regions inland on the Iberian Peninsula. It has been checked that in most cases the maximum irradiance values corresponded to solar noon, although there were exceptions that could be explained by cloudiness. The maximum experimental values of the UVI were around 9 during the summer, though frequently passing this value at the inland measurement stations. The annual accumulated dose of irradiation on a horizontal plane has also been studied, as well as the evolution through the year in units of energy, standard erythemal doses and minimum erythemal doses, according to different phototypes. [source] Theoretical framework for the distribution of trace metals among the operationally defined speciation phases of a sedimentENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 4 2001René A. Nome Abstract The use of a model based on Langmuir's isotherm to evaluate the metal associated with separate geochemical phases of a sediment is proposed and its validity tested with sediments of certified composition. The model takes into account a standard procedure for a certified reference material (CRM601), which defines, experimentally, a set of sequential extractions that divide the sediment into four operational fractions. The derived equations allow the treatment of data from sediment of Flumendosa Lake, Italy, and certified material CRM601 and also allow the computation of corrected concentrations, i.e., the metal affinities for each fraction. Experimental values for Ni show its low sensitivity and an equal distribution among different phases, which suggests a similar adsorption mechanism in all cases. In the case of Cd, the corrected concentration in the Fe/Mn oxide phase is nine times higher than for the residual fraction. For sediment of the Bèsos River, Spain, results show the percentage distribution of Ni over different fractions. Affinity values for Ni on a Flumendosa Lake sediment have also been calculated. The present model is simple to apply and shows satisfactory agreement with experimental data. [source] CFD modelling and experimental investigation of an ejector refrigeration system using methanol as the working fluidINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 2 2001S. B. Riffat Abstract This paper presents results of computational fluid dynamic (CFD) analysis and experimental investigation of an ejector refrigeration system using methanol as the working fluid. The CFD modelling was used to investigate the effect of the relative position of the primary nozzle exit within the mixing chamber on the performance of the ejector. The results of the CFD were used to obtain the optimum geometry of the ejector, which was then used to design, construct and test a small-scale experimental ejector refrigeration system. Methanol was used as the working fluid, as it has the advantage of being an ,environmentally friendly' refrigerant that does not contribute to global warming and ozone layer depletion. In addition, use of methanol allows the ejector refrigeration system to produce cooling at temperatures below the freezing point of the water, which of course would not be possible with a water ejector refrigeration system. CFD results showed that positioning the nozzle exit at least 0.21 length of the mixing chamber throat's diameter upstream of the entrance of the mixing chamber gave better performance than pushing it into the mixing chamber. Experimental values of coefficient of performance (COP) between 0.2 and 0.4 were obtained at operating conditions achievable using low-grade heat such as solar energy and waste heat. Copyright © 2001 John Wiley & Sons, Ltd. [source] Assessment of a semi-quantitative method for estimation of the rejection of organic compounds in aqueous solution in nanofiltrationJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 7 2006Bart Van der Bruggen Abstract A large number of different mechanisms describing the retention of dissolved organic compounds in nanofiltration have been proposed. A recent review identified the parameters possibly involved in the separation performance and suggested a qualitative classification of dissolved compounds. Continuing this approach, a semi-quantitative assessment of the observed rejections in nanofiltration is given in this paper, based on threshold values of key parameters such as molecular weight and molecular weight cut-off (MWCO), molecular size, pH and pKa, hydrophobicity (logKow) and membrane charge. Experimental values and literature data were used to provide a broad basis for comparison. It was concluded that (a) all categories that contain hydrophobic components are badly defined, in particular for small components, with rejections varying from low to very high, (b) all components that contain hydrophilic components have relatively high rejections and (c) all categories that contain charged components have well-defined, high rejections (intermediate for membranes with low surface charge). In all cases, the average rejection is higher when the component's molecular weight is larger than the MWCO of the membrane and when the molecular size is larger than the pore size of the membrane. Copyright © 2006 Society of Chemical Industry [source] RHEOLOGICAL PROPERTIES OF NONCOHESIVE APPLE DISPERSION WITH HELICAL AND VANE IMPELLERS: EFFECT OF CONCENTRATION AND PARTICLE SIZEJOURNAL OF FOOD PROCESS ENGINEERING, Issue 5 2000DENIS CANTÚ-LOZANO ABSTRACT The proportionality constant, ks, between shear rate, ,, and agitation velocity, N, for a helical ribbon-screw (HRS) agitator was 17.8. Using the HRS agitator, values of consistency index K and the flow behavior index n of 14 apple pulp suspensions at seven different solids concentrations and two average particle diameters 0.71 mm and 1.21 mm were determined; in addition, values of the Casson viscosity ,c and yield stress ,OC were also calculated. The magnitudes of K increased and of n decreased with increase in pulp concentration. Experimental values of the vane yield stress, ,O,, measured with a six-blade vane increased with increase in pulp content. The values of ,OC obtained using the Casson model were close to the experimental values ,O,. The effect of particle size on the relative viscosity, ,r, was correlated with Peclet number. [source] Critical diameters and temperature domains for MBE growth of III,V nanowires on lattice mismatched substratesPHYSICA STATUS SOLIDI - RAPID RESEARCH LETTERS, Issue 4 2009G. E. Cirlin Abstract We report on the growth properties of InAs, InP and GaAs nanowires (NWs) on different lattice mismatched substrates, in particular, on Si(111), during Au-assisted molecular beam epitaxy (MBE). We show that the critical diameter for the epitaxial growth of dislocation-free III,V NWs decreases as the lattice mismatch increases and equals 24 nm for InAs NWs on Si(111), 39 nm for InP NWs on Si(111), 44 nm for InAs NWs on GaAs(111)B, and 110 nm for GaAs NWs on Si(111). When the diameters exceed these critical values, the NWs are dislocated or do not grow at all. The corresponding temperature domains for NW growth extend from 320 °C to 340 °C for InAs NWs on Si(111), 330 °C to 360 °C for InP NWs on Si(111), 370 °C to 420 °C for InAs NWs on GaAs(111)B and 380 °C to 540 °C for GaAs NWs on Si(111). Experimental values for critical diameters are compared to the previous findings and are discussed within the frame of a theoretical model. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Effect of heat treatment on various physical properties in Zr80Ni20 metallic glassPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 8 2006I. Kokanovi Abstract The effect of heat treatment on various physical properties in Zr80Ni20 metallic glass has been investigated by means of differential scanning calorimetry and electrical resistivity measurements. Experimental values for the crystallisation temperature and activation energy of the crystallisation processes were derived by studying these processes at different heating rates. The electrical resistivity of the annealed samples up to a temperature slightly above the first crystallization exotherm decreases with decreasing heating rates and increasing annealing temperatures. The temperature dependence of the electrical resistivity of the samples has been interpreted in terms of the weak localization of electrons and contribution by electron,phonon scattering. The contribution by electron,phonon scattering increases with increasing the fraction of crystalline phases in the glass matrix. The superconducting transition temperature, Tc, of the Zr80Ni20 metallic glass annealed at heating rates of 60 K/min and 10 K/min up to annealing temperatures below its second crystallisation exotherm decreases with decreasing heating rates and increasing annealing temperatures. The homogeneity of the annealed Zr80Ni20 metallic glass is discussed by using the superconducting transition width as a criterion. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Heat transfer for pultrusion of a modified acrylic/glass reinforced compositePOLYMER COMPOSITES, Issue 1 2002Mariasun Sarrionandia Experimental values of the temperature on the wall and into the die were obtained for the pultrusion of a modified acrylic resin. The equation of continuity, and energy balance, coupled with a kinetic expression for the curing system, are solved using difference method to calculate the temperature and the conversion profiles in the thickness direction in a rectangular pultrusion die. The effects of the process variables (e.g. pulling rate, die temperature, die thickness and content of fibers) on the performance of the pultrusion are evaluated. [source] A Simple Experimental Technique to Measure Tortuosity in Packed BedsTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 6 2006J. M. P. Q. Delgado Abstract In this article, a simple and inexpensive experimental technique easy to build in laboratory, for the measurement of tortuosity of a packed bed of inert particles, is described. Experimental values of the tortuosity were obtained with four different packed beds of sand. The experimental results obtained are in good agreement with the theoretical values of tortuosity in a porous media; and the data reported showed the tortuosity to increase with decrease in the void fraction of the packed bed as expected. On décrit dans cet article une technique expérimentale simple et peu coûteuse facile à concevoir en laboratoire, pour la mesure de la tortuosité d'un lit garni de particules inertes. Des valeurs expérimentales de tortuosité ont été obtenues avec quatre lits différents garnis de sable. Les résultats expérimentaux obtenus montrent un bon accord avec les valeurs théoriques de la tortuosité dans un milieu poreux; et les données signalées montrent que la tortuosité augmente avec la diminution de la fraction de vide du lit garni tel qu'attendu. [source] Absorption Rate of Carbon Dioxide by K2CO3 -KHCO3 DEA Aqueous SolutionASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 1-2 2005K. Takahshi Gas absorption rates of carbon dioxide from CO2 -air mixtures into K2CO3, K2CO3 -KHCO3, DEA and K2CO3 -KHCO3 -DEA solutions have been measured by using a wetted wall column. The method to determine the enhancement factor of chemical absorption is confirmed by the absorption of carbon dioxide in NaOH solution. The enhancement factor was correlated with the potassium concentration [K+] (= 2[K2CO3 + [KHCO3]), the DEA concentration, and the CO2 -loading ratio of the absorbent. The chemical reaction for CO2, absorption has been expressed by an (m,n)th order irreversible-reaction model, where the reaction orders are m = 1 for CO2, n = 1.35 for DEA, n = 0.6 for K2CO3, and n = 0.6 and 1.35 for K2CO3 and DEA respectively. Experimental values of the enhancement factor were successfully reproduced by the calculations, where the reaction rate constants reflected the effect of loading ratio of the absorbent with carbon dioxide. [source] Acoustical Properties of Binary Mixtures of Heptane with Ethyl Acetate or Butyl AcetateCHINESE JOURNAL OF CHEMISTRY, Issue 3 2010Divya Shukla Abstract Mixed solvents rather than single pure liquids are of utmost practical importance in chemical and industrial processes as they provide an ample opportunity for the continuous adjustment of desired properties of the medium. Therefore, ultrasonic velocity (u) and density (,) were measured for the binary mixtures formed by heptane with ethyl acetate or butyl acetate at temperatures 293, 298 and 303 K over the entire composition range. Deviation in ultrasonic velocity (,u), deviation in isentropic compressibility (,,s), and excess intermolecular free length (LEf) have been evaluated using the ultrasonic velocity data and the computed results were fitted to the Redlich-Kister polynomial equation. The values of ,u, ,,s and LEf were plotted against the molar fraction of heptane. The observed positive and negative values of excess parameters were discussed in terms of molecular interaction between the components of the mixtures. Experimental values of ultrasonic velocity and density were compared with the results obtained by theoretical estimation procedures. The results were discussed in terms of average absolute deviation (AAD). [source] Transition States of the Asymmetric Michael Reactions of Aldehydes Catalyzed by Trimethylsilyl-Protected DiphenylprolinolEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 16 2009Jian-Qiang Zhao Abstract The asymmetric Michael reactions of aldehydes and nitroalkenes catalyzed by trimethylsilyl-protected diphenylprolinol were investigated by using density functional theory calculations. As a result of the stereospecific blockade of the bulky diphenylsiloxymethyl group on the pyrrolidine ring, the Re face of the enamine double bond is effectively shielded. For acetaldehyde, there are two different conformers of the enamine intermediate. On the basis of the two conformers of the enamine intermediate, four different reaction pathways were considered and four different transition states were searched for the enantioselective asymmetric Michael reaction of acetaldehyde and nitroalkene. The lowest- and second-lowest-energy transition states are both formed via the same intermediate IM2. The enantiomeric excess, calculated to be 96,%,ee, is in good agreement with the experimental value. For propanal, on the basis of the four different conformers of the prolinol,enamine intermediate, eight different reaction pathways were considered and eight transition states were searched for the enantioselective asymmetric Michael reaction. The calculated ee value is 99.5,%, which is in good agreement with the experimental ee value of 99,%. The lowest- and second-lowest-energy transition states are formed via different enamine intermediates, which is different from the case of acetaldehyde. The calculations also reveal that the intermediates play an important role in the reactions.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Using symbolic computing in building probabilistic models for atomsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2006Silviu Guiasu Abstract This article shows how symbolic computing and the mathematical formalism induced by maximizing entropy and minimizing the mean deviation from statistical equilibrium may be effectively applied to obtaining probabilistic models for the structure of atoms, using trial wave functions compatible with an average shell picture of the atom. The objective is not only to recover the experimental value of the ground state mean energy of the atom, but rather to better approximate the unknown parameters of these trial functions and to calculate both correlations between electrons and the amount of interdependence among different subsets of electrons of the atoms. The examples and numerical results refer to the hydrogen, helium, lithium, and beryllium atoms. The main computer programs, using the symbolic computing software MATHEMATICA, are also given. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source] Investigation of excited-state properties of fluorene,thiophene oligomers by the SAC-CI theoretical approachJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 10 2010Potjaman Poolmee Abstract Excited states of fluorene-ethylenedioxythiophene (FEDOT) and fluorene- S,S -dioxide-thiophene (FTSO2) monomers and dimers were studied by the symmetry-adapted cluster (SAC)-configuration interaction (CI) method. The absorption and emission peaks observed in the experimental spectra were theoretically assigned. The first three excited states of the optimized conformers, and the conformers of several torsional angles, were computed by SAC-CI/D95(d). Accurate absorption spectra were simulated by taking the thermal average for the conformers of torsional angles from 0° to 90°. The conformers of torsional angles 0°, 15°, and 30° mainly contributed to the absorption spectra. The full width at half-maximum of the FEDOT absorption band is 0.60 eV (4839 cm,1), which agrees very well with the experimental value of 0.61 eV (4900 cm,1). The maximum absorption wavelength is located at 303 nm, which is close to those of the experimental band (327 nm). The calculated absorption spectrum of FTSO2 showed two bands in the range of 225,450 nm. This agrees very well with the available experimental spectrum of a polymer of FTSO2, where two bands are detected. The excited-state geometries were investigated by CIS/6-31G(d). These showed a quinoid-type structure which exhibited a shortening of the inter-ring distance (0.06 Å for FEDOT and 0.04 Å for FTSO2). The calculated emission energy of FEDOT is 3.43 eV, which agrees very well with the available experimental data (3.46 eV). The fwhmE is about 0.49 eV (3952 cm,1), while the experimental fwhm is 0.43 eV (3500 cm,1). For FTSO2, two bands were also found in the emission spectrum. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010 [source] Gibbs energy of activation for thermal isomerization of (1Z)-acetaldehyde hydrazone and (1Z)-acetaldehyde N,N-dimethylhydrazone by Gaussian-4 theory and CCSD(T)/CBS computationsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 14 2009Shih-I Lu Abstract In this article, we examined the Gibbs energy of activation for the Z/E thermal isomerization reaction of (1Z)-acetaldehyde hydrazone and (1Z)-acetaldehyde N,N-dimethylhydrazone, at 298.15 K in the solvent of cyclohexane. We carried out computations employing both the Gaussian-4 (G4) theory and the coupled cluster method using both single and double substitutions and triple excitations noniteratively, CCSD(T). The CCSD(T) energy is extrapolated to the complete basis set (CBS). We compared the calculated results to the available experimental observation. It appeared that both G4 and CCSD(T)/CBS computations overestimated the experimental value by as much as about 6 and 12 kcal/mol in the present two cases. We discussed possible sources of error and proposed the experimental kinetic data could be questionable. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009 [source] Tl(I)-the strongest structure-breaking metal ion in water?JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 6 2007A quantum mechanical/molecular mechanical simulation study Abstract Structural and dynamical properties of the Tl(I) ion in dilute aqueous solution have been investigated by ab initio quantum mechanics in combination with molecular mechanics. The first shell plus a part of the second shell were treated by quantum mechanics at Hartree-Fock level, the rest of the system was described by an ab initio constructed potential. The radial distribution functions indicate two different bond lengths (2.79 and 3.16 Å) in the first hydration shell, in good agreement with large-angle X-ray scattering and extended X-ray absorption fine structure spectroscopy results. The average first shell coordination number was found as 5.9, and several other structural parameters such as coordination number distributions, angular distribution functions, and tilt- and ,-angle distributions were evaluated. The ion,ligand vibration spectrum and reorientational times were obtained via velocity auto correlation functions. The TlO stretching force constant is very weak with 5.0 N m,1. During the simulation, numerous water exchange processes took place between first and second hydration shell and between second shell and bulk. The mean ligand residence times for the first and second shell were determined as 1.3 and 1.5 ps, respectively, indicating Tl(I) to be a typical "structure-breaker". The calculated hydration energy of ,84 ± 16 kcal mol,1 agrees well with the experimental value of ,81 kcal mol,1. All data obtained for structure and dynamics of hydrated Tl(I) characterize this ion as a very special case among all monovalent metal ions, being the most potent "structure-breaker", but at the same time forming a distinct second hydration shell and thus having a far-reaching influence on the solvent structure. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007 [source] The arsenic clusters Asn (n = 1,5) and their anions: Structures, thermochemistry, and electron affinitiesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 7 2004Yi Zhao Abstract The molecular structures, electron affinities, and dissociation energies of the Asn/As (n = 1,5) species have been examined using six density functional theory (DFT) methods. The basis set used in this work is of double-, plus polarization quality with additional diffuse s - and p -type functions, denoted DZP++. These methods have been carefully calibrated (Chem Rev 2002, 102, 231) for the prediction of electron affinities. The geometries are fully optimized with each DFT method independently. Three different types of the neutral-anion energy separations reported in this work are the adiabatic electron affinity (EAad), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE). The first dissociation energies De(Asn,1 -As) for the neutral Asn species, as well as those De(As -As) and De (Asn,1 -As,) for the anionic As species, have also been reported. The most reliable adiabatic electron affinities, obtained at the DZP++ BLYP level of theory, are 0.90 (As), 0.74 (As2), 1.30 (As3), 0.49 (As4), and 3.03 eV (As5), respectively. These EAad values for As, As2, and As4 are in good agreement with experiment (average absolute error 0.09 eV), but that for As3 is a bit smaller than the experimental value (1.45 ± 0.03 eV). The first dissociation energies for the neutral arsenic clusters predicted by the B3LYP method are 3.93 eV (As2), 2.04 eV (As3), 3.88 eV (As4), and 1.49 eV (As5). Compared with the available experimental dissociation energies for the neutral clusters, the theoretical predictions are excellent. Two dissociation limits are possible for the arsenic cluster anions. The atomic arsenic results are 3.91 eV (As , As, + As), 2.46 eV (As , As + As), 3.14 eV (As , As + As), and 4.01 eV (As , As + As). For dissociation to neutral arsenic clusters, the predicted dissociation energies are 2.43 eV (As , As2 + As,), 3.53 eV (As , As3 + As,), and 3.67 eV (As , As4 + As,). For the vibrational frequencies of the Asn series, the BP86 and B3LYP methods produce good results compared with the limited experiments, so the other predictions with these methods should be reliable. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 907,920, 2004 [source] Nonlinear modeling of protein separation in a preparative-scale dynamic field gradient focusing instrumentAICHE JOURNAL, Issue 1 2009Noah I. Tracy Abstract Dynamic field gradient focusing (DFGF) uses an electric field gradient opposed by a counter-flow of buffer to separate milligrams of proteins according to their electrophoretic mobilities. A nonlinear model of protein separation in a preparative-scale DFGF device was developed to aid in refining the instrument's design and finding optimal run conditions prior to performing experiments. The model predicted the focal points of bovine serum albumin (BSA), and bovine hemoglobin (Hb) to within the 95% confidence intervals about the means of the experimental values. The resolution between the proteins in the model was 2.08, which was 3% less than the lower limit of the 95% confidence interval about the experimental value. The model predicted 67% more dispersion than was present in the experimental device, which made the simulated BSA peak 22% wider than the experimentally measured width. © 2008 American Institute of Chemical Engineers AIChE J, 2009 [source] Thermochemistry of cyclopentadienylidene (c-C5H4, C2v, 3B1), cyclopentadienyl radical (c-C5H5,, C2v, 2B1) and 1,3-cyclopentadiene (c-C5H6, C2v, 1A1): a theoretical study by the G2M(RCC,MP2) methodJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 3 2001Thanh Lam Nguyen Abstract The thermochemistry of c-C5H4 (3B1), c-C5H5, and c-C5H6 was investigated at the G2M(RCC,MP2) level. The heat of formation (,Hof,298), ionization potential (IP), electron affinity (EA) and proton affinity (PA) are respectively, 125.1, 204.4, 42.5 and 230.7,kcal,mol,1 for c-C5H4 (3B1), 63.5, 195.2, 44.9 and 197.2,kcal,mol,1 for c-C5H5, and 32.1, 199.9, 26.0 and 195.2,kcal,mol,1 for c-C5H6. The computed values for the last two molecules are in excellent agreement with available experimental data, with the errors of ±1 and ±2,kcal,mol,1 for the heat of formation and the other parameters, respectively. In addition, the heat of formation of c-C5H4,, is evaluated as 82.0,kcal,mol,1, about 10,kcal,mol,1 higher than the experimental value. This leads to the suggestion that the experimental ,Hof,298(c-C5H4,,) value needs to be re-examined. Copyright © 2001 John Wiley & Sons, Ltd. [source] Resolving an apparent discrepancy between theory and experiment: spin,spin coupling constants for FCCFMAGNETIC RESONANCE IN CHEMISTRY, Issue 11 2008Janet E. Del Bene Abstract Ab initio equation of motion coupled cluster singles and doubles (EOM,CCSD) and second-order polarization propagator approximation (SOPPA) calculations have been performed to evaluate spin,spin coupling constants for FCCF (difluoroethyne). The computed EOM-CCSD value of 3J(FF) obtained at the experimental geometry of this molecule supports the previously reported experimental value of 2.1 Hz, thereby resolving an apparent discrepancy between theory and experiment. This coupling constant exhibits a strong dependence on the CC and CF distances, and its small positive value results from a sensitive balance of paramagnetic spin-orbit (PSO) and spin-dipole (SD) terms. The three other unique FCCF coupling constants 1J(CC), 1J(CF), and 2J(CF) have also been reported and compared with experimental data. While 1J(CF) is in agreement with experiment, the computed value of 2J(CF) is larger than our estimate of the experimental coupling constant. Copyright © 2008 John Wiley & Sons, Ltd. [source] Ab-initio investigation of structural, electronic and optical properties for three phases of ZnO compoundPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 9 2007Z. Charifi Abstract The complex density-functional theory (DFT) calculations of structural, electronic and optical properties for the three phases: wurtzite (B4), zincblende (B3) and rocksalt (B1) of ZnO compound have been reported using the full-potential linearized-augmented plane-wave (FP-LAPW) method as implemented in the WIEN2k code. We employed both the local-density approximation (LDA) and the generalized-gradient approximation (GGA), which is based on exchange,correlation energy optimization to calculate the total energy. Also, we have used the Engel,Vosko GGA formalism, which optimizes the corresponding potential for band-structure calculations. The 3d orbitals of the Zn atom were treated as the valence band. The calculated structural properties (equilibrium lattice constant, bulk modulus, etc.) of the wurtzite and rocksalt phases are in good agreement with experiment. The B4 structure of ZnO is found to transform to the B1 structure with a large volume collapse of about 17%. The phase transition pressure obtained by using LDA is about 9.93 in good agreement with the experimental data. B1-ZnO is shown to be an indirect bandgap semiconductor with a bandgap of 1.47 eV, which is significantly smaller than the experimental value (2.45 ± 0.15 eV). While B3 and B1 phases have direct bandgap semiconductors with bandgaps 1.46 and 1.57 eV, respectively. Also, we have presented the results of the effective masses. We present calculations of the frequency-dependent complex dielectric function , (,) and it zero-frequency limit ,1(0). The optical properties of B4 phase show considerable anisotropic between the two components. The reflectivity spectra has been calculated and compared with the available experimental data. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The long-wavelength limit of the structure factor of amorphous silicon and vitreous silicaACTA CRYSTALLOGRAPHICA SECTION A, Issue 1 2010Adam M. R. De Graff Liquids are in thermal equilibrium and have a non-zero structure factor = = in the long-wavelength limit where is the number density, is the temperature, is the scattering vector and is the isothermal compressibility. The first part of this result involving the number (or density) fluctuations is a purely geometrical result and does not involve any assumptions about thermal equilibrium or ergodicity, so is obeyed by all materials. From a large computer model of amorphous silicon, local number fluctuations extrapolate to give = 0.035 0.001. The same computation on a large model of vitreous silica using only the silicon atoms and rescaling the distances gives = , which suggests that this numerical result is robust and perhaps similar for all amorphous tetrahedral networks. For vitreous silica, it is found that = , close to the experimental value of = obtained recently by small-angle neutron scattering. Further experimental and modeling studies are needed to determine the relationship between the fictive temperature and structure. [source] Calculation of the Energy of Explosives with a Partial Reaction Model.PROPELLANTS, EXPLOSIVES, PYROTECHNICS, Issue 1 2006Comparison with Cylinder Test Data Abstract The energy delivered by explosives is described by means of the useful expansion work along the isentrope of the detonation products. A thermodynamic code (W-DETCOM) is used, in which a partial reaction model has been implemented. In this model, the reacted fraction of the explosive in the detonation state is used as a fitting factor so that the calculated detonation velocity meets the experimental value. Calculations based on such a model have been carried out for a number of commercial explosives of ANFO and emulsion types. The BKW (Becker-Kistiakowsky-Wilson) equation of state is used for the detonation gases with the Sandia parameter set (BKWS). The energy delivered in the expansion (useful work) is calculated, and the values obtained are compared with the Gurney energies from cylinder test data at various expansion ratios. The expansion work values obtained are much more realistic than those from an ideal detonation calculation and, in most cases, the values predicted by the calculation are in good agreement with the experimental ones. [source] Optimization by response surface methodology (RSM) for toluene adsorption onto prepared acid activated clayTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 6 2008Abdelfattah Amari Abstract Adsorption of toluene onto acid activated clay was carried out. Modified clay was prepared by acid attack (H2SO4) on raw material. Response surface methodology based on a 2-level, 4-variables central composite orthogonal design was used to evaluate the effects of important parameters on the adsorption of toluene on to activated clay. Temperature (53.8,96.2°C), contact time (0.57,6.93 h), mass ratio of liquid/solid (3.38,7.62) and strength of acid (7.75,57.24%) were chosen as process variables for the optimization. Of these parameters, temperature reaction and time had greater impact on toluene adsorption than did the other parameters. Analysis of variance (ANOVA) shows a good agreement between theoretical analysis and experimental data. The validity of model is verified by an experiment at the optimum conditions. The optimum conditions for the maximum adsorption of toluene onto activated clay are: temperature of 96.2°C, a contact time of 6.93 h, a liquid/solid ratio of 5.98 and strength of acid of 32.94%. Since the predicted values and the actual experimental value obtained for the maximum adsorption of toluene are within 95% confidence intervals, the final model is considered valid and has satisfactory predictive ability. L'adsorption du toluène sur l'argile activée par l'acide est réalisée. L'argile modifiée a été préparée par attaque acide (H2SO4) de la matière première. On a eu recours à la méthodologie de surface de réponse basée sur un plan d'expériences orthogonal composite centré à 2 niveaux et 4 variables afin d'évaluer les effets des paramètres importants pour l'adsorption du toluène sur l'argile activée. La température (53,8°C,96,2°C), le temps de contact (0,57,6,93 h), le rapport massique liquide/solide (3,38,7,62) et la force de l'acide (7,75,57,24%) ont été choisis comme variables de procédé pour l'optimisation. Parmi ces paramètres, le temps de contact et la température ont le plus grand impact sur l'adsorption du toluène. L'analyse de la variance (ANOVA) montre un bon accord avec l'analyse théorique et les données expérimentales. La validité du modèle est vérifiée par une expérience dans les conditions optimales. Les conditions optimales pour l'adsorption du toluène sur l'argile activée sont: la température de 96,2°C, le temps de contact de 6,93 h, le rapport liquide/solide de 5,98 et la force de l'acide de 32,94%. Étant donné que la valeur prédictée et la valeur expérimentale réelle obtenue par l'adsorption maximale du toluène se trouvent dans un intervalle de confiance de 95%, le modèle final est considéré comme valide et possède une capacité de prédiction satisfaisante. [source] Packing of ruthenium sensitizer molecules on mostly exposed faces of nanocrystalline TiO2: crystal structure of (NBu4+)2[Ru(H2tctterpy)(NCS)3]2,·0.5,DMSOAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 11 2002V. Shklover Abstract An X-ray crystal study of the new ,black dye' sensitizer tri(thiocyanato)(4,4,,4,-tricarboxy-2,2,:6,,2,-terpyridine)ruthenium(II) is reported. In the crystal, strong hydrogen bonds form chains of ruthenium complex dianions with the O···O distances of 2.48,2.54,Å. From the molecular geometry of the dianions, structural models of their close packing on the (101) and (001) crystal surfaces of TiO2 (anatase) have been built. The maximum possible density of molecular packing noticeably exceeds the experimental value. The hydrogen bonding between the anions in monolayers, located on the TiO2 surface, is discussed. Copyright © 2002 John Wiley & Sons, Ltd. [source] Study of liquid droplets impact on dry inclined surfaceASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 5 2009Jie Cui Abstract The impact of droplets on the surface is a common phenomenon. The outcome of a droplet impacting on a solid surface depends on the properties of the liquid, the surface conditions and the kinematics parameters, i.e. velocity and momentum. During the impact process, the phenomenons, such as spread, rebound, often appear. This paper presents the results of an experimental investigation of droplets impacting on inclined solid surface at low velocity. The effects of the impact parameters on the droplet impingement are studied. Measures were performed using a high-speed camera. It has been shown that the impacting droplets spread on the surface until liquid surface tension and viscosity overcame inertial forces, after which they recoiled off the surface. The maximum diameter of a droplet spread was measured. In addition, a further forecasting expression has been obtained through energy model when a droplet impacts on an inclined surface without splashing. It is found that it is in good agreement with experimental value and can well predict the maximum spread diameter. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd. [source] Accurate calculation of the density of proteinsACTA CRYSTALLOGRAPHICA SECTION D, Issue 7 2000Michael L. Quillin On the basis of theoretical calculations, Andersson & Hovmöller have recently suggested that the long-established value of 1.35,g,cm,3 for the mean density of proteins should be revised to 1.22,g,cm,3[Andersson & Hovmller (2000), Acta Cryst. D56, 789790]. To substantiate their assertion, these authors used the Voronoi algorithm to calculate the mean atomic volume for 30 representative protein structures. The Voronoi procedure requires that atoms of interest be bounded on all sides by other atoms. Volume calculations for surface atoms that are not surrounded or are only sparsely surrounded by other atoms either are not possible or may be unreliable. In an attempt to circumvent this problem, Andersson & Hovmöller rejected atoms with calculated volumes that were indeterminate or were greater than 50,Å3. In the present study, it is shown that this criterion is not sufficiently restrictive to ensure accurate volume determinations. When only strictly buried atoms are included in the volume calculations using the Voronoi algorithm, the mean density is found to be 1.47 ± 0.05,g,cm,3. In addition, an alternate procedure based on the Connolly algorithm that permits all protein atoms to be included in volume calculations gives 1.43 ± 0.03,g,cm,3 for the mean density of the same set of proteins. The latter two calculated values are mutually consistent and are in better agreement with the experimental value. [source] Bis(trifluoromethyl)phosphinous Acid (CF3)2P-O-H: An Example of a Thermally Stable Phosphinous Acid,Synthesis, Gas-Phase Structure, and Rotational IsomersCHEMISTRY - A EUROPEAN JOURNAL, Issue 13 2006Berthold Hoge Priv.-Doz. Abstract The bis(trifluoromethyl)phosphinous acid, (CF3)2POH, is the only known example of a thermally stable phosphinous acid. Although this compound has been known since 1960, little is known about the chemistry of this extraordinary compound; this might be due to the tedious, and in some part risky, synthesis that was originally published. An improved, simple, and safe synthesis that is based on the treatment of the easily accessible (CF3)2PNEt2, with at least three equivalents of p -toluene sulfonic acid, is presented. The reaction results in a complete conversion to the phosphinous acid, which is isolated in almost 90,% yield. The compound exists in an equilibrium of two POH rotational isomers, a fact which is supported by quantum chemical calculations. The relative enthalpy difference of 6.4 kJ,mol,1, calculated at the B3PW91/6-311G(3d,p) level, is in excellent agreement with the experimental value of 5.9 kJ,mol,1, which was determined from the temperature dependence of the (OH) bands of the two rotational isomers. The complete experimental vibrational spectra of both rotamers, their predicted vibrational spectra obtained by using quantum chemical calculations, and an attempt at photoinduced isomerization of matrix-isolated (CF3)2POH is presented. The experimental structure, obtained from an electron-diffraction study in the gas phase, is reproduced very well by ab initio and density functional theory (DFT) methods. [source] Analysis of Classical and Quantum Paths for Deprotonation of Methylamine by Methylamine DehydrogenaseCHEMPHYSCHEM, Issue 12 2007Kara E. Ranaghan Abstract The hydrogen-transfer reaction catalysed by methylamine dehydrogenase (MADH) with methylamine (MA) as substrate is a good model system for studies of proton tunnelling in enzyme reactions,an area of great current interest,for which atomistic simulations will be vital. Here, we present a detailed analysis of the key deprotonation step of the MADH/MA reaction and compare the results with experimental observations. Moreover, we compare this reaction with the related aromatic amine dehydrogenase (AADH) reaction with tryptamine, recently studied by us, and identify possible causes for the differences observed in the measured kinetic isotope effects (KIEs) of the two systems. We have used combined quantum mechanics/molecular mechanics (QM/MM) techniques in molecular dynamics simulations and variational transition state theory with multidimensional tunnelling calculations averaged over an ensemble of paths. The results reveal important mechanistic complexity. We calculate activation barriers and KIEs for the two possible proton transfers identified,to either of the carboxylate oxygen atoms of the catalytic base (Asp428,),and analyse the contributions of quantum effects. The activation barriers and tunnelling contributions for the two possible proton transfers are similar and lead to a phenomenological activation free energy of 16.5±0.9 kcal,mol,1 for transfer to either oxygen (PM3-CHARMM calculations applying PM3-SRP specific reaction parameters), in good agreement with the experimental value of 14.4 kcal,mol,1. In contrast, for the AADH system, transfer to the equivalent OD1 was found to be preferred. The structures of the enzyme complexes during reaction are analysed in detail. The hydrogen bond of Thr474,(MADH)/Thr172,(AADH) to the catalytic carboxylate group and the nonconserved active site residue Tyr471,(MADH)/Phe169,(AADH) are identified as important factors in determining the preferred oxygen acceptor. The protein environment has a significant effect on the reaction energetics and hence on tunnelling contributions and KIEs. These environmental effects, and the related clearly different preferences for the two carboxylate oxygen atoms (with different KIEs) in MADH/MA and AADH/tryptamine, are possible causes of the differences observed in the KIEs between these two important enzyme reactions. [source] Ionization-Induced Proton Transfer in Model DNA Base Pairs: A Theoretical Study of the Radical Ions of the 7-Azaindole DimerCHEMPHYSCHEM, Issue 12 2004Hsing-Yin Chen Dr. Abstract Proton-transfer reactions of the radical anion and cation of the 7-Azaindole (7AI) dimer were investigated by means of density functional theory (DFT). The calculated results for the dimer anion and cation were very similar. Three equilibrium structures, which correspond to the non-proton-transferred (normal), the single-proton-transferred (SPT) and the double-proton-transferred (tautomeric) forms, were found. The transition states for proton-transfer reactions were also located. The calculations showed that the first proton-transfer reaction (normal,SPT) is exothermic and almost barrier-free; therefore, it should occur spontaneously in the period of a vibration. In contrast, the second proton-transfer reaction (SPT,tautomer) was found to be far less-probable in terms of reaction energy and barrier. Hence, it was concluded that both (7AI)2and (7AI)2exist in the SPT form. The conclusion was further confirmed by the calculated electron vertical detachment energy (VDE) of the SPT form of (7AI)2, 1.33 eV, which is very close to the experimental measurement of 1.35 eV. The calculated VDEs of the normal and tautomeric (7AI)2forms were too small compared to the experimental value. The proton transfer process was found to be multidimensional in nature involving not only proton motion but also intermolecular rocking motion. In addition, IR spectra were calculated and reported. The spectra of the three structures showed very different features and, therefore, can be considered as fingerprints for future experimental identifications. The implications of these results to biology and spectroscopy are also briefly discussed. [source] | ||||||||||||||||||||||||||||||||||||||||||||||