Página 1 dos resultados de 2901 itens digitais encontrados em 0.077 segundos

Solvent assisted decomposition of the tetrahedral intermediate of the transesterification reaction to biodiesel production. A density functional study

LIMA, Emmanuela F. de; CARNEIRO, Jose Walkimar de M.; FENOLLAR-FERRER, C.; MIERTUS, Stanislav; ZINOVIEV, Sergey; TAPANES, Neyda C. Om; ARANDA, Donato A. G.
Fonte: ELSEVIER SCI LTD Publicador: ELSEVIER SCI LTD
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
45.94%
B3LYP/6-31 + G(d) calculations were employed to investigate the mechanism of the transesterification reaction between a model monoglyceride and the methoxide and ethoxide anions. The gas-phase results reveal that both reactions have essentially the same activation energy (5.9 kcal mol(-1)) for decomposition of the key tetrahedral intermediate. Solvent effects were included by means of both microsolvation and the polarizable continuum solvation model CPCM. Both solvent approaches reduce the activation energy, however, only the microsolvation model is able to introduce some differentiation between methanol and ethanol, yielding a lower activation energy for decomposition of the tetrahedral intermediate in the reaction with methanol (1.1 kcal mol(-1)) than for the corresponding reaction with ethanol (2.8 kcal mol(-1)), in line with experimental evidences. Analysis of the individual energy components within the CPCM approach reveals that electrostatic interactions are the main contribution to stabilization of the transition state. (C) 2009 Elsevier Ltd. All rights reserved.; EFL; EFL; ANP/PETROBRAS; ANP/PETROBRAS; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq); CNPq; FAPERJ; FAPERJ

Structure and electronic properties of hydrated mesityl oxide: a sequential quantum mechanics/molecular mechanics approach

Damasceno, Marcus Vinicius Araujo; Costa Cabral, Benedito J.; Coutinho, Kaline Rabelo
Fonte: SPRINGER; NEW YORK Publicador: SPRINGER; NEW YORK
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
46.14%
The hydration of mesityl oxide (MOx) was investigated through a sequential quantum mechanics/molecular mechanics approach. Emphasis was placed on the analysis of the role played by water in the MOx syn-anti equilibrium and the electronic absorption spectrum. Results for the structure of the MOx-water solution, free energy of solvation and polarization effects are also reported. Our main conclusion was that in gas-phase and in low-polarity solvents, the MOx exists dominantly in syn-form and in aqueous solution in anti-form. This conclusion was supported by Gibbs free energy calculations in gas phase and in-water by quantum mechanical calculations with polarizable continuum model and thermodynamic perturbation theory in Monte Carlo simulations using a polarized MOx model. The consideration of the in-water polarization of the MOx is very important to correctly describe the solute-solvent electrostatic interaction. Our best estimate for the shift of the pi-pi* transition energy of MOx, when it changes from gas-phase to water solvent, shows a red-shift of -2,520 +/- 90 cm(-1), which is only 110 cm(-1) (0.014 eV) below the experimental extrapolation of -2,410 +/- 90 cm(-1). This red-shift of around -2,500 cm(-1) can be divided in two distinct and opposite contributions. One contribution is related to the syn -> anti conformational change leading to a blue-shift of similar to 1...

Fotofísica e propriedades dinâmicas de sistemas moleculares; Photophysics and dynamical properties of molecular systems

González, Yoelvis Orozco
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 31/10/2012 Português
Relevância na Pesquisa
46.16%
A fotodinâmica de sistemas moleculares representa um dos principais tópicos atuais da físico-química molecular. O conhecimento das propriedades dos estados eletrônicos excitados tem permitido desenvolver áreas de vital importância como das energias renováveis, da fotomedicina, dos sensores fluorescentes, entre outras. O objetivo desta tese está orientado a estudar teoricamente a influência do meio (ou efeito de solvente) na fotofísica e nas propriedades dos estados eletrônicos excitados de sistemas moleculares. Nesta tese, primeiramente foi feito um estudo em fase gasosa da superfície de energia potencial do sistema molecular HSO2 e do efeito da energia rotacional na reação OH+SO. Na superfície de energia potencial foram caracterizadas um grande número de estruturas estacionárias e foi encontrado um estado de transição que liga a região mais energética da superfície com a menos energética. Em relação ao papel da energia rotacional na reação mencionada, foi realizado um estudo de trajetórias quase-clássicas, onde foi observado um decréscimo da reatividade com o aumento da energia rotacional total depositada nos reagentes. Posteriormente, foi estudado o efeito do solvente nas propriedades dos estados eletrônicos excitados e nos mecanismos de decaimento de três sistemas moleculares...

Estudos dos efeitos de solventes no espectro de absorção eletrônica da merocianina de Brooker e derivados; Studies of Solvent Effects in the Electronic Absorption Spectrum of the Brooker's merocyanine and Derivatives.

Damasceno, Marcus Vinicius Araujo
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 25/02/2015 Português
Relevância na Pesquisa
46%
Nesta tese estudamos o espectro de absorção da Merocianina de Brooker (MB) e três derivados em solventes com diferentes polaridades. O interesse no estudo desses sistemas se dá pela presença de algumas propriedades particulares das merocianinas, por exemplo, apresentam um grande deslocamento solvatocrômico devido à mudança na polaridade do meio. Nós apresentamos os resultados teóricos obtidos para o espectro de absorção das moléculas considerando diferentes estruturas para o estado fundamental: formas trans, cis, zwiteriônica (zw), protonada (MBH+) e dímero(MB2). O efeito do solvente foi considerado utilizando diferentes modelos de solvatação: (i) modelo contínuo polarizável (PCM), (ii) através de uma configuração eletrostática média do solvente (ASEC), (iii) incluindo algumas moléculas explícitas do solvente juntamente com um ambiente eletrostático gerado pelas demais moléculas do solvente. Como uma investigação adicional, apresentamos nossos resultados de medidas experimentais para o espectro de absorção da MB em vários solventes variando acidez e concentração. Vimos, através de cálculos quânticos, que as formas cis/trans apresentam a excitação eletrônica na mesma região, e que a deformação estrutural gerada pela forma zw provoca um deslocamento para o vermelho na excitação eletrônica. Os nossos resultados teóricos e experimentais mostram que a forma MBH+ apresenta um solvatocromismo pequeno...

Evaluation of solvent effect on the extraction of phenolic compounds and antioxidant capacities from the berries: application of principal component analysis

Boeing, Joana Schuelter; Barizao, Erica Oliveira; Costa e Silva, Beatriz; Montanher, Paula Fernandes; Almeida, Vitor de Cinque; Visentainer, Jesui Vergilio
Fonte: Biomed Central Ltd Publicador: Biomed Central Ltd
Tipo: Artigo de Revista Científica Formato: 9
Português
Relevância na Pesquisa
46.15%
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES); Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq); Background: This study evaluated the effect of the solvent on the extraction of antioxidant compounds from black mulberry (Morus nigra), blackberry (Rubus ulmifolius) and strawberry (Fragaria x ananassa). Different extracts of each berry were evaluated from the determination of total phenolic content, anthocyanin content and antioxidant capacity, and data were applied to the principal component analysis (PCA) to gain an overview of the effect of the solvent in extraction method.Results: For all the berries analyzed, acetone/water (70/30, v/v) solvent mixture was more efficient solvent in the extracting of phenolic compounds, and methanol/water/acetic acid (70/29.5/0.5, v/v/v) showed the best values for anthocyanin content. Mixtures of ethanol/water (50/50, v/v), acetone water/acetic acid (70/29.5/0.5, v/v/v) and acetone/water (50/50, v/v) presented the highest antioxidant capacities for black mulberries, blackberries and strawberries, respectively.Conclusion: Antioxidants extractions are extremely affected by the solvent combination used. In addition, the obtained extracts with the organic solvent-water mixtures were distinguished from the extracts obtained with pure organic solvents...

Descomposicion termica del diperoxido de pinacolona (3,6-diterbutil-3,6-dimetil-1,2,4,5-tetraoxaciclohexano) en solución de 2-metoxietanol

Eyler,Gladys N.; Cañizo,Adriana I.; Nesprias,Rosa K.
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/05/2002 Português
Relevância na Pesquisa
45.87%
The thermal decomposition reaction of pinacolone diperoxide (DPP; 0.02 mol kg-1) in 2-methoxyethanol solution studied in the temperature range of 110.0-150.0 °C, follows a first-order kinetic law up to at least 50% DPP conversion. The organic products observed were pinacolone, methane and tert-butane. A stepwise mechanism of decomposition was proposed where the first step is the homolytic unimolecular rupture of the O-O bond. The activation enthalpy and activation entropy for DPP in 2-methoxyethanol were calculated (deltaH# = 43.8 ± 1.0 kcal mol-1 and deltaS# = 31.9 ± 2.6 cal mol-1K-1) and compared with those obtained in other solvents to evaluate the solvent effect.

Efecto de solvente sobre la descomposición térmica de trans-3,6-dimetil-3,6-difenil-1,2,4,5-tetraoxaciclohexano en solución

Eyler,Gladys N.; Cañizo,Adriana I.; Mateo,Carmen M.
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/12/2004 Português
Relevância na Pesquisa
45.87%
The thermal decomposition reaction of trans-3,6-dimethyl-3,6-diphenyl-1,2,4,5-tetraoxacyclohexane (acetophenone cyclic diperoxide, DPAF), in different solvents (methanol, 1,4-dioxane, acetonitrile and 2-propanol/benzene mixtures) in the initial concentration and temperature ranges of (4.2-10.5) x 10-3 M and 140.0 to 185.0 ºC, respectively, follows a pseudo first order kinetic law up to at least 70% DPAF conversion. An important solvent effect on the rate constant values, activation parameters (DH# and DS#) and reaction products obtained in different solvents is detected, showing that the reaction is accelerated in alcohols.

Estrutura e propriedades da difenil-4-amina sulfonato de sódio: um dopante de filmes condutores

Pires,José Maria; Jorge,Francisco Elias; Machado,Luiz Carlos; Marins,Antonio Augusto Lopes
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/10/2006 Português
Relevância na Pesquisa
45.87%
This work contains the theoretical simulation of the conformation of diphenyl-4-amine sodium sulphonate (DASNa) and correlates its geometry with conductivity, showing that the conductivity increases as the molecule becomes more planar. The solvent effect was also evaluated, using water and dimethylsulfoxide. Some properties, such as bond distance, vibration al frequency and effective charge were calculated. The utilization of diphenyl-4-amine sodium sulphonate (DASNa) as dopant of aniline was investigated in view of the HOMO-LUMO energy gap.

On the cis -> trans isomerization of the square-planar [Pt(Cl)(SnCl3)(PH3)2] compound: ab initio gas phase reaction mechanism and solvent effects using continuum models

Rocha,Willian R.; Almeida,Wagner B. de
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/04/2000 Português
Relevância na Pesquisa
45.96%
The cis -> trans isomerization of the d8 square planar [Pt(Cl)(SnCl3)(PH3)2 ] compound was investigated at the MP4(SDQ)//MP2 level of theory. The optimized structures located on the gas-phase potential energy surface indicate that this reaction proceeds through a quasi-tetrahedral transition state. The influence of electronic effects of the ligands on the reaction mechanism was investigated with the Charge Decomposition Analysis (CDA) method, which gave support to understand the strong trans effect of the SnCl3 ligand. The solvent effect on the gas phase energy reaction was evaluated using the SCRF and IPCM continuum models. In both cases, an increase on the energy barrier for the process was observed and, the thermodynamical stability of the cis and trans isomers was changed upon solvation.

Molecular Structure of Heterocycles: 6. Solvent Effects on the 17O Nmr Chemical Shifts of 5-Trichloromethylisoxazoles

Martins,Marcos A. P.; Freitag,Rogério A.; Zimmermanna,Nilo E. K.; Sinhorin,Adilson P.; Cúnico,Wilson; Bastos,Giovani P.; Zanatta,Nilo; Bonacorso,Helio G.
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/12/2001 Português
Relevância na Pesquisa
46.02%
A multilinear-regression analysis using the Kamlet-Abboud-Taft (KAT) solvatochromic parameters in order to elucidate and quantify the solvent effects on the 17O chemical shifts of three 5-trichloromethylisoxazoles [(1a) non-, (1b) 3-methyl- and (1c) 4-methyl-substituted] is reported. The chemical shifts of ring oxygen atom, O1, of compounds 1a-c show dependencies (in ppm) on the solvent polarity-polarizability of -4.8pi*, -3.2pi*, -8.9pi*, on the solvent hydrogen-bond-donor (HBD) acidities 0.9alpha, -0.2alpha, -2.7alpha and the solvent hydrogen-bond-acceptor (HBA) basicities -0.4beta, 1.9beta, 0.9beta, respectively. The data of net charges of O1 and dipole moment, obtained from MO calculations (AM1), are compared with the solvent effect parameters obtained for compounds 1a-c.

The role of nonelectrostatic solvation to chemical reactions in liquid phase

Pliego Jr.,Josefredo R.
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/04/2005 Português
Relevância na Pesquisa
45.96%
The relative importance of cavity formation and dispersion-repulsion contributions to the activation free energy of chemical reactions in liquid phase is discussed, taken as example the reaction of one and two H2O molecules with CCl2 in aqueous solution. The solvent effect on these systems was investigated by free energy perturbation in previous publications. In the present report, were used the Pierotti scaled particle theory and the Claverie-Pierotti method to compute the free energy of cavity formation, while the dispersion-repulsion contribution was determined by the Floris and Tomasi method. We have found that the cavity term is the most important nonelectrostatic contribution of the solvent to the activation free energy, while the dispersion-repulsion contribution accounts for 25% to 35% of that term. In addition, we have observed that the direct Pierotti formula is more accurate than the Claverie-Pierotti method for the present system.

Dual bifunctional catalysis and the α-effect in the reaction of hydroxylamine with phenylacetate

Mazera,Deise J.; Pliego Jr.,Josefredo R.; Almerindo,Gizelle I.; Gesser,José C.
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/11/2011 Português
Relevância na Pesquisa
55.95%
The reaction of hydroxylamine with phenyl acetate was theoretically investigated to shed light on the role of catalysis and the origin of the α-effect in this system. Calculations at the B3LYP/6-311+G(2df,2p)//HF/6-31G(d) level and the solvent effect included at the PCM/HF/6-31G(d) level predict that the direct attack of hydroxylamine to the carbonyl centre has a high ΔG‡ barrier, close to 50 kcal mol-1. A second hydroxylamine molecule can catalyse the process through bifunctional catalysis using both the NH2 and OH groups simultaneously. This dual bifunctional catalysis decreases the ΔG‡ to 19 kcal mol-1 and is able to explain the experimentally observed kinetics and product ratio.

Effect of solvents on the morphological characterization of enteric nanoparticles

Dupeyrón Martell,Danay Rosa; Rieumont Briones,Jacques; González Hurtado,Mayra; del Real López,Alicia; Castaño Meneses,Víctor Manuel
Fonte: ABM, ABC, ABPol Publicador: ABM, ABC, ABPol
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/01/2009 Português
Relevância na Pesquisa
56%
Size and external and internal morphologies of nanoparticles and microparticles are very important on the design of drug devices for controlled release. Random enteric copolymers such as poly (methacrylic acid-co-ethyl acrylate) and poly (methacrylic acid-co-methyl methacrylate) were used to produce nanoparticles, which contain a model drug and could be employed as drug carriers for proteins. The solvent effect on re-dispersion of such nanoparticles was studied by Scanning Electron Microscopy (SEM) and revealed not only differences in size, but also several shapes, depending on the chemical nature of the polymer matrix and the non-solvent used. Acrylate containing copolymers in acidic aqueous dispersions lead to spheroidal particles. However for the copolymer containing methyl methacrylate, spheroidal particles collapsed in a "grenade" type morphology and besides some cubic structures are also formed. Dynamic Light Scattering (DLS) studies of the re-dispersed nanoparticles showed the strong tendency to form agglomerates not only in acidic water but also in hexane and the presence of bimodal size distributions.

Thorpe-Ingold Acceleration of Oxirane Formation is Mostly a Solvent Effect

Kostal, Jakub; Jorgensen, William L.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em 30/06/2010 Português
Relevância na Pesquisa
45.91%
The Thorpe-Ingold hypothesis for the gem-dimethyl effect in the cyclization reactions of 2-chloroethoxide derivatives has been investigated computationally in the gas phase and in aqueous solution. Ab initio MP2/6-311+G(d,p) and CBS-Q calculations reveal little intrinsic difference in reactivity with increasing α-methylation for the series of reactants 1 – 3. However, inclusion of continuum hydration or of explicit hydration through mixed quantum and statistical mechanics (MC/FEP) simulations does reproduce the substantial, experimentally observed rate increases with increasing α-methylation. Analysis of the MC/FEP results provides clear evidence that the rate increases stem primarily from increased steric hindrance to hydration of the nucleophilic oxygen atom with increasing α-methylation. Thus, the gem-dimethyl acceleration of oxirane formation for 1 – 3 is found to be predominantly a solvent effect.

Molecular insights on the solvent effect of methanol additive in glycine polymorph selection

Patala, Srikanth
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 85 p.
Português
Relevância na Pesquisa
46.11%
In an effort to improve control and design in organic crystallization, the effect of solvent on polymorph selection has gained tremendous interest in recent years. In this thesis, molecular simulation techniques are used to gain insight into the solvent effect on glycine crystallization in water-methanol mixtures. We report the validation of the Optimized Potential for Liquid Simulations (OPLS) force field and parameters with modified Lennard-Jones parameters for hydrogens attached to a-carbon in glycine zwitterion. Solution and interface simulations in water and 50% v/v water-methanol solutions reveal the mechanism through which methanol additive results in the crystallization of the least stable [Beta]-glycine polymorph. Free energy calculations through the Umbrella Sampling method show an increased stability of the centrosymmetric dimer structure ([alpha]-glycine growth unit) in the presence of the methanol additive. Even though the dimer structure is more stable in water-methanol mixtures, a higher fraction of glycine monomers were observed in water-methanol mixtures. It is revealed through thermodynamic arguments that a drastic decrease in solubility results in a higher fraction of glycine monomers in water-methanol mixtures. It was hypothesized in previous studies that the presence of monomer units docking onto the (010) interface of [alpha]-glycine inhibits further growth due to exposed ammonium groups at the interface. The effect of solvent on crystal growth inhibition is explored by the interface simulations of a-glycine in water-methanol mixtures. When the monomer units are docked onto the interface...

A kinetic method for the study of solvent environments of thiol groups in proteins involving the use of a pair of isomeric reactivity probes and a differential solvent effect. Investigation of the active centre of ficin by using 2,2'- and 4,4'- dipyridyl disulphides as reactivity probes.

Malthouse, J P; Brocklehurst, K
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em 01/01/1980 Português
Relevância na Pesquisa
46.11%
1. Whereas the second-order rate constants for the reaction of the thiolate ion of 2-mercaptoethanol with 4,4'-dipyridyl disulphide (k4PDS) and with 5,5'-dithiobis-2-nitrobenzoate dianion increase with decreasing dielectric constant of the solvent, or remain unchanged, the rate constant for the analogous reaction with 2,2'-dipyridyl disulphide (k2PDS) decreases. This anomalous solvent effect and other unusual physicochemical properties of 2,2'-dipyridyl disulphide are discussed. 2. The differential effect of solvent on the reactions of thiolate ion with the 2,2'- and 4,4'-dipyridyl disulphides is shown to provide a method of characterizing solvent environments of thiol groups in proteins by a reactivity-probe method that should not suffer from the usual drawback associated with the existence of steric or binding effects of unknown magnitude. Application of the method to ficin (EC 3.4.22.3) suggests that its active-centre thiol group resides in a relatively hydrophobic environment. 3. The pH-k profile for the reaction of ficin with 4,4'-dipyridyl disulphide is reported.

Linear free energy relationship analysis of solvent effect on singlet oxygen reactions with mono and disubstituted anthracene derivatives

Lemp Miranda, Else Mónica; Günther, Germán; Zanocco, Antonio L.; Castro Olivares, Rafael
Fonte: Universidade do Chile Publicador: Universidade do Chile
Tipo: Artículo de revista
Português
Relevância na Pesquisa
46.05%
Detection of O2(1 g) emission, max = 1270 nm, following laser excitation and steady-state methodswere employed to measure the total reaction rate constant, kT, and the reactive reaction rate constant, kR, for the reaction between singlet oxygen and seven mono and disubstituted anthracenes. In most solvents, the rate constants for the quenching of singlet oxygen by monosubstituted anthracenes are in the order of 106M−1 s−1, whereas for the disubstituted compounds the values are in the order of 107M−1 s−1 indicating that both, mono and disubstituted anthracenes are good quenchers of singlet oxygen. The reactive rate constants were very close to the total rate constants, implying that quenching is essentially a chemical process. The analysis of solvent effect on kT and kR by using LSER equations indicates that singlet oxygen deactivation by the anthracene derivatives is accelerated by solvents with large values of both, dipolarity/polarizability and Hildebrand solvent parameters, being inhibited by hydrogen bond donor solvents in some cases. Solvent effect analysis support the formation of a loose, zwitterionic like exciplex for 9-methylanthracene and 9-methoxyanthracene, a loose exciplex with partial charge separation for di-substitutes anthracene and a compact exciplex with partial charge separation for anthracene and 9-phenylanthracene.

The solvent effect in the hydrogenation of citral over Ir and Ir-Fe/TiO2 catalysis

Rojas, Hugo; García Fierro, José Luis; Reyes, P.
Fonte: Sociedad Chilena de Química Publicador: Sociedad Chilena de Química
Tipo: Artículo
Português
Relevância na Pesquisa
46.08%
The solvent effect in the citral hydrogenation over iridium catalysts has been studied. The catalysts used in this research were, the monometallic Ir/ TiO2 reduced at low or high temperature (LT y HT) and the bimetallic Ir-Fe/TiO2 (LT y HT) catalysts. As solvent n-heptane, an equimolar mixture n-heptane-1-propanol and 1-propanol were used. The reactions were carried out at a hydrogen pressure of 8.27 bar and at 363 K. In the LT catalyst, the catalytic activity increases upon the addition of Fe3+. This positive effect was obtained keeping constant the selectivity to the unsaturated alcohol, being 100% selective to the products obtained by hydrogenation of the C=O bond. In the HT catalysts, both the addition of Fe and the presence of TiOx moieties generated by the strong metal support interaction (SMSI) effect, lead to an increase of surface acidity. In LT and HT catalysts, an increase in the polarity of the solvent increases the catalytic activity, however the reaction pathway is modified by the presence of acid sites which in polar solvents allow the formation of acetals, mainly in the HT catalysts.; The authors thank MIDEPLAN, Grants Milenio ICM 99-92 for their financial support.; Peer reviewed

Effect of solvent on the hydrogenation of 4-phenyl-2-butanone over Pt based catalysts

McManus, J.; Daly, H.; Thompson, J. M.; Connor, E.; Hardacre, C.; Wilkinson, S. K.; Sedaie Bonab, N.; ten Dam, J.; Simmons, M. J. H.; Stitt, E. H.; D'Agostino, C.; McGregor, J.; Gladden, L. F.; Delgado, J. J.
Fonte: Elsevier Publicador: Elsevier
Tipo: Article; published version
Português
Relevância na Pesquisa
46.05%
This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.jcat.2015.06.008; In part I of this study, experimental data were presented for the hydrogenation of 4-phenyl-2-butanone with a 4% Pt/TiO2 catalyst where the reaction rate and selectivity (ketone vs. aromatic ring) varied with solvent. In this paper, a rigorous kinetic model is presented utilising these data, incorporating statistical analysis methods to strengthen the foundations of mechanistically sound kinetic models. A fundamental kinetic model for the system is presented and a 2-site model was determined to be most appropriate, describing aromatic hydrogenation (postulated to be over a platinum site) and ketone hydrogenation (at the platinum titania interface). Solvent choice has little impact on the ketone hydrogenation rate constant but strongly impacts aromatic hydrogenation due to solvent-catalyst interaction. Reaction selectivity is also correlated to a fitted product adsorption constant parameter. This kinetic analysis method is the first of its kind demonstrating the role of solvents in influencing reactant adsorption and reaction selectivity.; We acknowledge EPSRC for funding as part of the CASTech grant (EP/G011397/1) and the Department of Employment and Learning for a studentship (IM). NSB was funded by a PhD scholarship from the University of Birmingham. SKW was supported by an Engineering Doctorate Studentship in Formulation Engineering at the University of Birmingham sponsored by the EPSRC (EP/G036713/1) and Johnson Matthey.

Solvent effect on the 1H NMR spectra of a pyridinocalix(4)arene derivative and its protonation constants in methanol

Danil de Namor,A. F.; Aguilar Cornejo,A. F.; Chahine,S.; Nolan,K.
Fonte: Anales de la Asociación Química Argentina Publicador: Anales de la Asociación Química Argentina
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/07/2004 Português
Relevância na Pesquisa
46.09%
The solvent effect on the 1H NMR spectra of 5, 11, 17, 23 tetra-tert-butyl-25, 26, 27, 28-tetra-[2-(4-pyridyl) methoxy]calix(4) arene, 1c in a wide range of solvents with different dielectric constants was investigated and the results are discussed. It is shown that the 1H NMR spectrum of this macrocycle is sensitive to the nature of the solvent. In all solvents the ligand shows a distorted ‘cone’ conformation..The aromatic protons show the most significant deshielding effect in acetonitrile, nitromethane and pyridine relative to chlroform, which may be attributed to the interaction of these solvents (acetonitrile,nitromethane) through their methyl groups with the hydrophobic cavity of the ligand. For pyridine, π-π interactions between the pyridyl groups and this solvent may occur. The protonation constants of this ligand in methanol were derived from potentiometric titration data in this solvent. The results are compared with those for structural isomers of 1c as well as with those previously reported for lower rim calix(4)arene derivatives containing aliphatic and alicyclic amines as pendant arms. Protonation constant data show that the affinity of the latter to interact with the proton is greater than that of the former macrocycle in methanol. Final conclusions are given.