The pK a values of over 41 organic acids in dimethyl sulfoxide (DMSO) solution were calculated using ab initio electronic structure methods at MP2 and MP4 levels of electron correlation and including basis set of 6-31+G(d) and 6-311+G(2df,2p) quality. The solvation was included through the polarizable continuum model (PCM), using the recent parametrization of Pliego and Riveros. The root-mean-square (RMS) error over this set of molecules having different functional groups is only 2.2 units. A linear fit on this data set decreases this error by only 0.2 units, indicating that this empirical correction is not necessary. The major error in the calculated pK a value was −5.3 units for the CH3SO3H solute. Halogenated carboxylic acids have also presented a high deviation (∼4 units). An explanation for these high deviations is the possibility of strong hydrogen-bond formation involving the neutral acid molecule and DMSO. The pK a values were also calculated using a combination of theoretical solvation data with experimental gas-phase data. In this case, the RMS error increased to 2.3 units for a set of 36 acids. Our results show that the performance of the PCM model with a fixed atomic radius in DMSO solution is very superior to its performance in aqueous solution, which is a behavior that can be attributed to the presence of strong and specific solute−solvent interactions of ionic solutes with water molecules. In addition, no extensive parametrization of the PCM model is needed to describe the solvation of anions in DMSO solution.
Al 3+ -Mg 2+ mixed oxides were prepared by coprecipitation and characterized with scanning electron microscopy (SEM), energy dispersive Xray fluorescence (EDXRF), temperature programmed desorption of CO 2 (CO 2 -TPD), and N 2 adsorption/desorption isotherms (BET and BJH methods). By increasing the MgO concentration up to 31.8% (w/w), X-ray diffraction (XRD) measurements suggested an incipient magnesium aluminate spinel (MgAl 2 O 4 ) phase. However, the spinel crystalline structure was obtained only after calcination at 950 °C. These materials were tested as catalysts in the propanolysis reaction of methyl paraoxon. This reaction in the presence of the more efficient incipient MgAl 2 O 4 spinel is of the order of 2.5 × 10 5 -fold faster than the spontaneous propanolysis reaction and results in the formation of a product that is structurally related to a family of flame retardants. The different products of propanolysis and hydrolysis were identified by electrospray ionization mass spectrometry (ESI(+)-MS), ESI(+)-MS/MS) and liquid chromatography mass spectrometry (LC-MS/MS).
A simple method to prepare magnesium oxide catalysts for biodiesel production by transesterification reaction of soybean oil with ethanol is proposed. The method was developed using a metal-chitosan complex. Compared to the commercial oxide, the proposed catalysts displayed higher surface area and basicity values, leading to higher yield in terms of fatty acid ethyl esters (biodiesel). The deactivation of the catalyst due to contact with CO 2 and H 2 O present in the ambient air was verified. It was confirmed that the active catalytic site is a hydrogenocarbonate adsorption site.
A hidrólise neutra da formamida em solução aquosa foi investigada por métodos ab initio de alto nível, incluindo o efeito do solvente pelo modelo contínuo polarizável (PCM). Considerou-se até quatro moléculas explícitas de água, sendo analisados os mecanismos envolvendo a formação do intermediário tetraédrico (stepwise mechanism) e o mecanismo de formação direta do ácido carboxílico via expulsão de amônia (concerted mechanism), estes com a participação de moléculas de água agindo como um catalisador bifuncional. Também foi investigado um mecanismo de catálise básica geral, com uma molécula de água agindo como base. Os cálculos em nível CCSD(T)/6-311+G(2df,2p)/ /MP2/6-31G(d) predizem que o mecanismo stepwise com duas moléculas de água no estado de transição é o mais favorável. Porém, a barreira de energia livre de 48,7 kcal mol-1 indica que a cinética é extremamente lenta e a reação não seria observada. Também analisamos o efeito do solvente sobre a geometria do estado de transição e notamos que esse efeito é de pouca importância na barreira energética. Testes com a teoria do funcional da densidade, usando o funcional B3LYP, mostram que conjuntos pequenos de funções de base como 6-31G(d) leva a barreiras de ativação extremamente subestimadas, enquanto que o uso do conjunto de funções de base 6-311+G(2df,2p) prevê barreiras próximas ao nosso melhor nível de cálculo. O presente estudo levanta dúvidas com relação a barreira experimental de 31 kcal mol-1 e sugerimos que a constante cinética k w reportada na literatura seja apenas um artefato matemático oriundo do ajuste de curvas. The neutral hydrolysis of formamide in aqueous solution involving up to four explicit solvent molecules was theoretically investigated using high level ab initio methods and including the bulk solvent effect through the Polarizable Continuum Model (PCM). We have analyzed cyclic transition states structures involving bifunctional catalysis and a general base catalysis structure. Both a stepwise mechanism with tetrahedral intermediate formation and a concerted mechanism were investigated. Our calculations at CCSD(T)/6-311+G(2df,2p)//MP2/6-31G(d) level predict an observable activation free energy barrier of 48.7 kcal mol-1 , corresponding to a stepwise water catalyzed mechanism with two water molecules into the transition state. Liquid phase geometry optimization was also performed, but the effect on the activation free energy is modest. Tests with density functional theory were carried out. The B3LYP/6-31G(d) calculation underestimates the barrier by 13 kcal mol-1 , whereas the B3LYP/6-311+G(2df,2p) method predicts an accurate barrier. The present study raises important questions about the reliability of the experimental activation free energy of 31.0 kcal mol-1 and suggests that the neutral hydrolysis of formamide does not take place at all.
[reaction: see text] Reliable theoretical calculations predict a free energy barrier for nitrile formation from the reaction between the cyanide ion and ethyl chloride in DMSO solvent of 24.1 kcal/mol, close to the experimental value of 22.6 kcal/mol. We have also predicted that the isonitrile formation is less favorable by 4.7 kcal/mol, while the elimination mechanism is less favorable by more than 10 kcal/mol. These results indicate that isonitrile formation and bimolecular elimination are not significant side reactions for primary alkyl chloride reactions.
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