A theoretical investigation of the 1,2-additon and 1,4-addition reactions of benzylamine to crotonaldehyde in toluene solution is reported in this study, including the effect of trace amounts of acetic acid (AcOH) and methanesulfonic acid (CH3SO2OH). We have determined the reaction free energy profile and performed a detailed microkinetic analysis. Our results point out that this reaction system needs catalysis to take place and it was found that CH3SO2OH is a powerful catalyst, outperforming AcOH in lowering the free energy barrier for the 1,2-addition reaction, which leads to the formation of α,β-unsaturated imine. On the other hand, the 1,4-addition reaction has the direct nucleophilic attack of benzylamine to the β-carbon of s-cis conformation of crotonaldehyde as the rate-determining step, corresponding to slow kinetics. Our results suggest that the experimentally observed formation of the imine can be explained by the presence of hidden acid catalysts present in the reaction medium.

The use of metrics to evaluate scientists is widespread in the present time, with implications for hiring, fellowships, and research grants. Such fact requires that metrics must be constantly scrutinized to be improved. This work analyzes the use of metrics in the area of chemistry in Brazil and discusses its limitations and shortcomings. The main findings indicate that the use of the impact factor must be complemented by the cited half-life of the journals, and a composite metric named influence factor is proposed, similar to the R-impact. The h-index is not a good metric anymore because does not correct for authorship inflation. The individual h-index (hi), which takes into account fractional counting of citations, is more reliable than the h-index. An analysis on the use of hi-index with randomly selected 15 Brazilian chemists among the top 500 more productive shows an important effect on the ranking order.

The classical alkylation reaction of benzene with isopropyl chloride catalyzed by the species AlCl3 and Al2Cl6 was studied using reliable calculations at M06-2X/def2-TZVPP//X3LYP/def2/SVP level of theory and SMD model for solvent effect. We evaluated the formation of dimers, trimers, tetramers, and pentamers and showed that Al2Cl6 dimers exist in greater proportion, in agreement with experimental observations. The experimental solubility of Al2Cl6 in benzene was also included in the theoretical kinetics analysis. The reaction catalyzed by AlCl3 species presents the highest barrier, in part due to unfavorable dissociation of the Al2Cl6 species. The mechanism via Al2Cl6 catalysis is more effective and even considering its low solubility, the calculated observed DG‡ is only 20.6 kcal mol-1, indicating a fast reaction rate. The mechanism involves the formation of the CH3CHCH3+…Al2Cl7- ion pair, which reacts with benzene to form a Wheland intermediate and this carbon-carbon bond formation step corresponds to the rate-determining one.

The formation of azeotropes is related to the deviation of the ideality of liquid mixtures. This deviation is expressed by the activity coefficient of each component. However, the physical origin of this behavior is not well quantified in terms of molecular interactions. In this work, a regular solution model was used, relating the β parameter to the solvation free energy of pure liquids in themselves and in the other component. A mathematical expression was derived that indicates when an azeotrope is formed. This equation shows that the greater the difference in vapour pressure between components, the greater the solvation difference of each liquid in itself and in the other component is needed to form the azeotrope. A set of 10 mixtures were randomly chosen and investigated as a test, and the equation worked in all cases.

Finding the most efficient solvent for a chemical reaction can demand costly experimental procedure and the screening is usually limited to few solvents. The use of theoretical methods could accelerate the search for the best solvent, able to promote most effective kinetics and thermodynamics of a reaction. In this work, it was proposed an automated procedure that calculates the solvent effect for a chemical reaction using all the 179 solvents available in SMD (solvation model density). The reaction of 2-bromoacetophenone with pyridine was used as a test. The SMD model correctly predicts the reactivity trends for five solvents, which experimental data are available. We have found that sulfolane, a less usual solvent, is the best one for this reaction. The present study points out that computational screening of large set of solvents using the SMD model is a viable approach and could be useful for chemical reactions optimization.

Acid catalyzed decomposition of C5 and C6 sugars has been considered an important source of biomass derived chemicals. An essential step in this process is the extraction of furfural, hydroxymethylfurfural and levulinic acid from aqueous phase. In this work, a computational screening of 178 organic solvents for simultaneous extraction of these chemicals from aqueous phase has been done with the continuum SMD (solvation model based on density) model. Our analysis has taken in account the partition coefficient, water miscibility, boiling point and toxicity of the organic solvents. The present theoretical results indicate that C6 and C7 ketones are the most adequate solvents, and 4-heptanone is predicted to be particularly useful.

In the past seven years, the SMD (Solvent Model Density) method has been widely used by computational chemists. Thus, assessment on the reliability of this model for modeling chemical process in solution is worthwhile. In this report, it was investigated six anion-molecule nucleophilic substitution reactions in methanol and dipolar aprotic solvents. Geometry optimizations have been done at SMD/X3LYP level and single point energy calculations at CCSD(T)/TZVPP + diff level. Our results have indicated that the SMD model is not adequate for dipolar aprotic solvents, with a root of mean squared (RMS) error of 5.6 kcal mol-1, while the Polarizable Continuum Model (PCM) method with the Pliego and Riveros atomic cavities has led to RMS error of only 3.2 kcal mol-1. For methanol solvent, the SMD method has a RMS error of 3.0 kcal mol-1. The classical protic to dipolar aprotic solvent rate acceleration effect was not predicted by the SMD model for the tested systems.

Cubic equations of state are able to predict the liquid-vapour coexistence curve of volatile substances. However, this property is rarely used to teach Physical Chemistry because solving the resulting equations require numerical technics and many mathematical operations. In this report was shown how the interactive method for finding the root of an equation can be applied with the Excel to determine the vapour pressure of the liquid, as well as the molar volumes of the gas and liquid phases. The generated points was used to build the curves of phase transition in the P ´ V and P ´ T graphics.

<p>It is proposed a new expression for the β parameter in the regular solution theory, using the solvation free energy concept instead of the solubility parameter. This modified regular solution theory can be applied for any liquid mixture combined with any method for solvation free energy calculation. The new approach was tested in fifteen liquid mixtures using the solvation model density (SMD) continuum solvation model and has successfully predicted the phase behavior, with exception of water-acetonitrile mixture. This flaw can be attributed to the microheterogeneity of this system. The main advantage of the present approach is a fast prediction of phase separation.</p>

A reação da hidroxilamina com acetato de fenila foi investigada teoricamente para se obter uma melhor compreensão do efeito alfa nas reações de nucleófilos neutros. Os cálculos foram feitos em nível B3LYP/6-311+G(2df,2p)//HF/6-31G(d) incluindo o solvente (água) em nível PCM/HF/6-31G(d). Os resultados mostram que um ataque direto da hidroxilamina leva a uma barreira de energia livre de ativação ΔG‡ muito alta, em torno de 50 kcal mol-1. Uma segunda molécula de hidroxilamina pode catalisar o processo através de catálise bifuncional usando ambos os grupos NH2 e OH simultaneamente. Esta catálise bifuncional reduz o ΔG‡ para 19 kcal mol-1 e é capaz de explicar a relação de produtos cinético e termodinâmico observada experimentalmente.

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.

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.

Continuum solvation models are nowadays widely used in the modeling of solvent effects and the range of applications goes from the calculation of partition coefficients to chemical reactions in solution. The present work presents a detailed explanation of the physical foundations of continuum models. We discuss the polarization of a dielectric and its representation through the volume and surface polarization charges. The Poisson equation for a dielectric was obtained and we have also derived and discuss the apparent surface charge method and its application for free energy of solvation calculations.

A importância de cada termo de solvatação não eletrostática, mais especificamente, formação de cavidade e dispersão-repulsão, para reações químicas em fase líquida foi analisada tomando como exemplo a reação de uma e duas moléculas de H2O com CCl2. O cálculo do efeito do solvente para estas reações em solução aquosa, via perturbação de energia livre, está disponível na literatura e foi usado para comparação. Neste trabalho, utilizamos a fórmula de Pierotti e o método de Claverie-Pierotti para computar o valor da contribuição de formação de cavidade, enquanto que o termo de dispersão-repulsão foi calculado pelo método de Floris e Tomasi. Nossos resultados mostram que o termo de cavidade é o mais importante, sendo que o termo de dispersão-repulsão corresponde de 25% a 35% do termo de cavidade. Além disso, observamos que a aplicação direta da fórmula de Pierotti para a energia livre de cavidade é mais acurada do que o método de Claverie-Pierotti para este sistema.

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.

Os parâmetros termodinâmicos para a reação dos carbenos CH2 (simpleto e tripleto), CF2 e C(OH)2 com formaldeído, acetaldeído e acetona foram determinados através de cálculos ab initio de alto nível. O metileno simpleto forma ilídeos estáveis com os três compostos carbonílicos, tendo um deltaHº de reação de -41.6 kcal/mol, -40.0 kcal/mol (isômero E), -44.5 kcal/mol (isômero Z) e -40.8 kcal/mol, respectivamente. O metileno tripleto também forma ilídeo com formaldeído, com um deltaE de reação de -15.7 kcal/mol, resultando que a formação do ilídeo neste estado eletrônico é reversível. Já os carbenos CF2 e C(OH)2 formam ilídeos com formaldeído com um deltaHº de 13.0 kcal/mol e 22.0 kcal/mol, e um deltaGº de 25.3 kcal/mol e 34.0kcal/mol, respectivamente. Estes valores positivos de energia livre indicam que estes ilídeos não devem ser formados a partir de carbenos e carbonilas, e possivelmente estes carbenos reagem com aldeídos e cetonas por cicloadição 1,2.

Thermodynamic parameters for the reaction of CH2 (singlet and triplet), CF2 and C(OH)2 carbenes with formaldehyde, acetaldehyde and acetone, forming carbonyl ylides, were determined using high level ab initio molecular orbital calculations. Singlet methylene forms a stable ylide with the three carbonyl compounds, with a deltaHº of -41.6 kcal/mol, -40.0 kcal/mol (E isomer), -44.5 kcal/mol (Z isomer) and -40.8 kcal/mol, respectively. Triplet methylene also forms a y lide with formaldehyde with a deltaE of -15.7 kcal/mol, resulting in the triplet ylide being formed reversibly. For the reaction with formaldehyde, the CF2 and C(OH)2 carbenes have a positive deltaHº of 13.0 kcal/mol and 22.0 kcal/mol, and a deltaGº of 25.3 kcal/mol and 34.0 kcal/mol, respectively. Due to its unfavorable free energy of formation, and the potential competition of direct 1,2 cycloaddition of carbene to the C=O group, CF2 and C(OH)2 should not react with carbonyl compounds forming ylides.

Results of high level ab initio calculations of the intermolecular potentials and theoretical dispersion coefficients for the Ne2 and Ar2 dimers were utilized to build analytical potentials for these species. The obtained potentials were used in the calculation of the dimers rovibrationals levels, and their respective spectroscopic constants determined. A comparison of high level experimental data with our theoretical results shows a very good agreement for Ne2, and also a good agreement for the Ar2 dimer.