On the modelling of solid state reactions.Synthesis of YAG

There is a model of yttrium aluminium garnet (YAG) synthesis presented in this article. The developed model is based on nonlinear reaction–diffusion partial differential equations. The solution was carried out numerically using finite difference techniques. We got dependability curves for diffusion and reaction rates and offered possible method to localize values of diffusion and reaction rate constants precisely enough.AMS subject classification: 35K57, 65M06     

Diffusion and Reaction Rates of the Yttrium Aluminium Garnet Synthesis using Different Techniques

The mathematical model of the yttrium aluminium garnet synthesis presented in this article. The model based on a system of non-stationary diffusion equations containing a non-linear term related to kinetics of reaction. Using computer-simulation tools and known experimental results we estimated the diffusion and reaction rates of the synthesis. Also it was shown that diffusion rate is a limited stage of the synthesis.     

Aqueous sol-gel synthesis route for the preparation of YAG: Evaluation of sol-gel process by mathematical regression model

Yttrium aluminium garnet (Y₃Al₅O₁₂, YAG) polycrystalline samples have been prepared by a simple aqueous sol-gel methodology. The influence of nineteen sol-gel processing variables on the formation of YAG has been investigated. Effects of different fabrication parameters on the phase purity and morphological properties of the compounds were studied by energy-dispersive spectrometry (EDS), X-ray powder diffraction (XRD) analysis and scanning electron microscopy (SEM). The parameters of the sol-gel processing such as pH of starting solution, concentration and nature of complexing ligand, temperature and duration of gelation, powder rehomogenization during annealing, duration and temperature of the final heat treatment were found to be the most significant. For the evaluation and verification of the experimental results the Brandon's model of a multiple regression was successfully used.     

Computational Modelling of the Behaviour of Potentiometric Membrane Biosensors

This paper presents a mathematical model of a potentiometric biosensor based on a potentiometric electrode covered with an enzyme membrane. The model is based on the reaction–diffusion equations containing a non-linear term related to theMichaelis–Menten kinetics of the enzymatic reaction. Using computer simulation the influence of the thickness of the enzyme membrane on the biosensor response was investigated. The digital simulation was performed using the finite difference technique. Results of the numerical simulation were compared with known analytical solutions.      

Formation of YAG from coprecipitated yttrium aluminium hydroxides

Thermal decomposition of the amorphous coprecipitate of yttrium and aluminium hydroxides forming yttrium aluminium garnet has been investigated employing thermal analyses, X-ray diffraction and IR spectroscopy. On heating, the coprecipitate progressively loses water forming a stable but highly disordered hydroxy garnet which crystallizes at 1180 K and decomposes to YAG at 1290 K. Nucleation of the crystalline phase appears to begin at 800 K.The authors thank Dr. P. V. Ravindran of the Analytical Chemistry and Shri N. K. Kulkarni of the Fuel Chemistry Divisions of BARC for their assistance in the thermoanalytical experiments.     

电感耦合等离子体发射光谱法测定钇铝石榴石中掺杂Yb(Ⅲ)离子

采用碳酸钠-硼砂(质量比1：1)做熔剂分解钇铝石榴石,并确定了熔剂的最优化条件．以电感耦合等离子体光谱法对样品中掺杂Yb(Ⅲ)离子作了定量检测考察了样品和熔剂中单个基体和混合基体对分析结果的影响．发现单个基体Y对Yb的测定有增强效应．基体A1对Yb的测定有一定的抑制作用;基体对Yb测定的干扰可通过基体匹配进行消除元素的俭出限为2μg／L,标准加入回收率为88％～100％实际样品7次检测的RSD为2．1％。本方法可以较好地满足实际样品分析的需要。     

Computational modeling of enzymatic keto-enol isomerization reactions

Catalysis of proton abstraction from nonacidic carbon atoms adjacent to a carbonyl or carboxylate group is a fundamental reaction in enzymology that has been extensively studied during the last few decades. Enzymes catalyzing these reactions, which normally involve labile enolic intermediates, need to overcome large pK _a differences between the reacting groups as well as high intrinsic free-energy barriers. Here, we present an overview of results from recent computer simulation studies of keto-enol isomerization reactions catalyzed by the enzymes glyoxalase I, triosephopsphate isomerase and ketosteroid isomerase. For all three enzymes it is found that electrostatic stabilization of the transient enolate intermediates, either by charge–charge interactions or by hydrogen bonding, accounts for the main part of the activation free-energy barrier reduction. Another catalytic effect observed in all cases is the reduction of the reorganization energy by the enzyme active site. Some other factors that have been proposed to be important for these reactions are also discussed and evaluated. Received: 3 January 2002 / Accepted: 13 May 2002 / Published online: 29 July 2002     

Sol‐Gel Preparation and Characterization of Codoped Yttrium Aluminium Garnet Powders

A sol‐gel method has been developed to prepare lanthanide‐codoped yttrium aluminium garnet (Y₃Al₅O₁₂, YAG) ceramic samples. The XRD patterns of the polycrystalline Y₃Al₅O₁₂:Nd,Ho and Y₃Al₅O₁₂:Nd,Er powders sintered at 1000 °C showed the formation of monophasic garnet materials. A homogeneous distribution of rare‐earth dopants in the YAG lattice was achieved in both cases. The morphological characterization of lanthanide‐codoped YAG by scanning electron microscopy (SEM) showed the formation of highly agglomerated nanocrystalline samples. The luminescence properties of sol‐gel derived doubly activated yttrium aluminium garnet samples were also investigated in the present study.     

Influence of a chelating agent on optical and morphological properties of YAG: Tb3+ phosphors prepared by the sol-gel process

Chemical modifications are widely used in sol-gel method in order to control the mechanisms involved during hydrolysis-condensation and polymerization processes. In this paper, the influence of acetylaceton (acac) used as a chelating agent in the synthesis of undoped and Tb³⁺-doped Y₃Al₅O₁₂ (YAG) powders and sols, has been investigated. Its effects on samples crystallization and morphology have been studied by means of high-temperature X-ray diffraction (HTXRD), thermal analysis (DTA-TG), infrared (IR) and Raman spectroscopies, transmission electron microscopy (TEM) and laser granulometry. It was shown that acetylaceton enhances the structural organization of YAG compound. Moreover, acac-modified powders exhibit much smaller particles than unmodified ones. Optical study has also been achieved on doped samples. Laser induced luminescence spectra and fluorescence decays of Tb³⁺ ions show that acac affects the optical properties of YAG: Tb³⁺ for any types of samples (sol, xerogel or crystallized powder). Powders demonstrate a better luminescence yield without acetylaceton, whereas stabilized sols are more efficient than unstabilized ones.     

白光LED用Ce∶YAG单晶的光学性能与掺杂浓度分析

采用提拉法生长Ce∶YAG单晶, 通过X射线衍射和激发发射光谱对其晶相结构和光谱特性进行了表征, 研究了Ce∶YAG单晶封装白光LED的最佳掺杂浓度. 在455 nm蓝光激发下, Ce∶YAG单晶的发射光谱可由中心波长526 nm(5d¹²E_gГ_8g→4f ¹²F_7/2Г_8u)的宽发射带(500~650 nm)组成; 激发光谱由343 nm(4f ¹²F_5/2Г_7u→5d^{1 2}E_gГ_7g)和466 nm(4f ¹²F_5/2Г_7u→5d^{1 2}E_gГ_8g)2个激发峰组成; Stokes位移为2448 cm^-1, Huang-Rhys因子为6.12. 研究结果表明, Ce∶YAG单晶中Ce离子掺杂浓度与封装的白光LED之间有对应关系, 在650 nm红粉调节下Ce离子最佳掺杂浓度范围为0.034~0.066.     

Computational insight into the effect of C(19) substituents on [1,7]-hydrogen shift in previtamin D

 B3LYP calculations in conjunction with natural bond orbital population analysis have been performed for a previtamin D model and corresponding transition structures for the [1,7]-hydrogen migration. In addition the 19,19-difluoro, 19-methoxy and 19-fluoro substituted analogs were investigated. The calculated activation barriers decrease in the following order: CHF₂>CH₃>CH₂OCH₃ (24.8, 23.5 and 20.1 kcal/mol). This is in qualitative agreement with experiments. It has been suggested that a decrease of the barrier by a 19-methoxy substituent and its increase by a 19,19-difluoro substituent are phenomena of different origin. In the case of 19-methoxy substitution, the effect is due to the charge redistribution in the triene system and the decrease of the C(19)–H bond energy. The effect of two fluorine substituents at C-19 on the activation barrier is suggested to originate from the combination and balance of several factors: electrostatic repulsion between the negative fluorine atom and the π-electron cloud over the conjugated system, an increase of the HOMO–LUMO gap, and geminal difluoro substitution affecting C–F and C–C bond energies. Received: 17 May 2002 / Accepted: 11 September 2002 / Published online: 14 February 2003     

Computational structural determination and energy landscape analysis of the hepatic carcinogen 2-(acetylamino)fluorene

 2-(Acetylamino)fluorene (AAF), a potent mutagen and a prototypical example of the mutagenic aromatic amines, forms covalent adducts to DNA after metabolic activation in the liver. A benchmark study of AAF is presented using a number of the most widely used molecular mechanics and semiempirical computational methods and models. The results are compared to higher-level quantum calculations and to experimentally obtained crystal structures. Hydrogen bonding between AAF molecules in the crystal phase complicates the direct comparison of gas-phase theoretical calculations with experiment, so Hartree–Fock (HF) and Becke–Perdew (BP) density functional theory (DFT) calculations are used as benchmarks for the semiempirical and molecular mechanics results. Systematic conformer searches and dihedral energy landscapes were carried out for AAF using the SYBYL and MMFF94 molecular mechanics force fields; the AM1, PM3 and MNDO semiempirical quantum mechanics methods; HF using the 3-21G*and 6-31G* basis sets; and DFT using the nonlocal BP functional and double numerical polarization basis sets. MMFF94, AM1, HF and DFT calculations all predict the same planar structures, whereas SYBYL, MNDO and PM3 all predict various nonplanar geometries. The AM1 energy landscape is in substantial agreement with HF and DFT predictions; MMFF94 is qualitatively similar to HF and DFT; and the MNDO, PM3 and SYBYL results are qualitatively different from the HF and DFT results and from each other. These results are attributed to deficiencies in MNDO, PM3 and SYBYL. The MNDO, PM3 and SYBYL models may be unreliable for compounds in which an amide group is immediately adjacent to an aromatic ring. Received: 26 May 2002 / Accepted: 12 December 2002 / Published online: 14 February 2003     

Thermal and mechanical-induced phase transformations during YAG and alumina–YAG syntheses

The preparation of pure Y₃Al₅O₁₂ (YAG) and 50 vol% Al₂O₃–YAG composite powders by a wet chemical route is presented. The role of the synthesis temperature during reverse-strike precipitation has been investigated, showing its relevant effect on the purity and homogeneity of YAG powder. The composite material was prepared by comparing two different synthesis routes. A composite powder was synthesized via reverse-strike temperature-controlled co-precipitation. In the latter case, a pure-alumina precursor was firstly reverse-strike precipitated and then doped with an yttrium salt solution. For both syntheses, the role of thermal and mechanical pre-treatments on the phase development was demonstrated.     

Adsorption and Structural Properties of Mineral–Carbon Sorbents

In the present work an attempt was made to obtain mineral–carbon sorbents by thermal decompositon. The mineral matrix for the sorbents (aluminium hydroxide) was based on petrochemical waste stream containing considerable amounts of aluminium chloride. Reference tests were carried out with a model solution prepared with the use of analytical grade AlCl₃. Atactic polypropylene and hydrocarbon mixtures obtained in the flotation of petrochemical waste waters were used as carbon-containing raw materials. The aim of this work was to determine the adsorption and structural characteristics of the complex sorbents and to check the possibility of evaluation of their hydrophobic-hydrophilic properties. This revised version was published online in August 2006 with corrections to the Cover Date.     

Mechanism and control of molecular energy flow: a modeling perspective

 Vibrational energy flow in organic molecules occurs by a multiple-time-scale mechanism that can be modeled by a single exponential only in its initial stages. The mechanism is a consequence of the hierarchical structure of the vibrational Hamiltonian, which leads to diffusion of vibrational wavepackets on a manifold with far fewer than the 3N−6 dimensions of the full vibrational state space. The dynamics are controlled by a local density of states, which does not keep increasing with molecular size. In addition, the number of vibrational coordinates severely perturbed during chemical reaction is small, leading to preservation of the hierarchical structure at chemically interesting energies. This regularity opens up the possibility of controlling chemical reactions by controlling the vibrational energy flow. Computationally, laser control of intramolecular vibrational energy redistribution can be modeled by quantum-classical, or by purely quantum-mechanical models of the molecule and control field. Received: 26 July 2002 / Accepted: 30 September 2002 / Published online: 2 December 2002 Electronic Supplementary Material to this paper can be obtained by using the Springer Link server located at http://dx.doi.org/10.1007/s00214-002-0394-2. Acknowledgements. This work was supported by NSF grant CHE 9986670. Correspondence to: M. Gruebele e-mail: gruebele@scs.uiuc.edu     

Computational NMR spectroscopy: reversing the information flow

Work on computational NMR recently carried out at our Laboratory in Padova is reviewed. We summarize our results concerning the calculation of NMR properties (chemical shifts and spin–spin coupling constants) in a variety of contexts, from the structure elucidation of complex organic molecules or molecules containing heavy atoms to weakly interacting species, such as those involved in hydrogen bonding or van der Waals CH-π interactions. We also present some original results, viz. the calculated ¹H and ¹³C spectra of the putative natural substance nimbosodione, the first examples of calculated ¹⁸¹Ta chemical shifts, spin–spin couplings in and through-space coupling constants involving ²⁰⁵Tl.     

Computation of the influence of chemical substitution on the pK a of pyridine using semiempirical and ab initio methods

The physical properties of chemicals are strongly influenced by their protonation state, affecting, for example, solubility or hydrogen-bonding characteristics. The ability to accurately calculate protonation states in the form of pK _as is, therefore, desirable. Calculations of pK _a changes in a series of substituted pyridines are presented. Computations were performed using both ab initio and semiempirical approaches, including free energies of solvation via reaction-field models. The selected methods are readily accessible with respect to both software and computational feasibility. Comparison of calculated and experimental pK _as shows the experimental trends to be reasonably reproduced by the computations with root-mean-square differences ranging from 1.22 to 4.14 pK _a units. Of the theoretical methods applied the best agreement occurred using the second-order M?ller–Plesset/6-31G(d)/isodensity surface polarized continuum solvation model, while the more computationally accessible Austin model 1/Solvent model 2 (SM2) approach yielded results similar to the ab initio methods. Analysis of component contributions to the calculated pK _as indicates the largest source of error to be associated with the free energies of solvation of the protonated species followed by the gas-phase protonation energies; while the latter may be improved via the use of higher levels of theory, enhancements in the former require improvements in the solvation models. The inclusion of alternate minimum in the computation of pK _as is also indicated to contribute to differences between experimental and calculated pK _a values. Received: 27 April 1999 / Accepted: 27 July 1999 / Published online: 2 November 1999     

Intramolecular proton transfer of serine in aqueous solution. Mechanism and energetics

Serine amino acid in aqueous solution is theoretically studied at the B3PW91/6-31+G^** level using a dielectric continuum solvent model. Neutral and zwitterionic structures in the gas phase and in solution are described and the proton-transfer mechanism is discussed. A neutral conformation in which the carboxyl hydrogen atom is already oriented toward the amino group seems to be the absolute energy minimum in the gas phase and the most stable neutral form in solution. The absolute energy minimum in solution is a zwitterionic form. The energy barrier for proton transfer is predicted to be very small, in particular when zero-point-energy contributions are added. Our calculations allow the dynamic aspects of the ionization mechanism to be discussed by incorporating nonequilibrium effects. Received: 28 June 1999 / Accepted: 13 October 1999 / Published online: 14 March 2000