Paulo Fernando Bruno Gonçalves

Insights on recyclable catalytic system composed of task-specific ionic liquids for the chemical fixation of carbon dioxide

A series of imidazolium-based ionic liquids (ILs) were synthesised and used as single component and metal-free homogeneous catalysts to convert a renewable, inexpensive and non-toxic CO2 feedstock into useful products. The cycloaddition of carbon dioxide to epoxides to produce cyclic carbonate was evaluated. A detailed investigation was carried out on a variety of factors that affected the reactivity and selectivity, such as the catalyst structures (nature of cation and anion). The effect of reaction parameters (temperature, reaction time, CO2 uptake and catalyst amount) on the catalytic performance was also investigated in detail. High conversions and selectivities could be achieved under mild pressure condition (5 bar) using 1-n-butyl-3-methylimidazolium bromide. A synergetic effect of the acidic and basic sites as well as suitable hydrogen-bonding strength is considered crucial for the reaction to proceed smoothly. This protocol was found to be applicable to a disubstituted epoxide. Furthermore, the straightforward synthesis of cyclic carbonates by direct oxidative carboxylation from olefins was achieved using only 1-n-butyl-3-methylimidazolium bromide as a catalyst.

Novel selenoesters fluorescent liquid crystalline exhibiting a rich phase polymorphism

A simple and efficient procedure for the synthesis of a new class of selenoesters 4a and 4b was developed. Polarized-light optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction showed that the selenoester 4a with a shorter alkyl chain displayed a wide nematic range (ΔT = 110.7 °C), while 4b with a longer alkyl chain possesses a rich phase polymorphism: Cr → SmI → SmC → N → I mesophase sequence during heating cycle and I → N → SmC → SmI → SmX → Cr transition on cooling. For 4b, a large temperature range (135.7 °C) from crystal to isotropic phase can be observed. In addition, UV-Vis, steady-state fluorescence emission and excitation spectra in solution were also applied in order to characterize their photophysical behaviour. Compounds 4a and 4b are fluorescent in the blue region and present a Stokes shift at around 80 nm and 39 nm in dichloromethane and dioxane, respectively.

Calculation of the free energy of solvation from molecular dynamics simulations

Molecular dynamics simulation has been employed in the computation of the free energy of solvation for a large number of solute molecules with different chemical functionalities in the solvents water, acetonitril, dimethyl sulfoxide, tetrahydrofuran, and carbon disulfide. The free solvation energy has been separated into three contributions: the work necessary to create a cavity around the solute in the solvent, the electrostatic contribution, and the free energy containing the short-range interactions between solute and solvent molecules. The cavitational contribution was computed from the Claverie-Pierotti model applied to excluded volumes obtained from nearest-neighbor solute-solvent configurations treating the solvent molecules as spherical. The electrostatic term was calculated from a dielectric continuum approach with explicitly incorporating the solvents partial charges. The short-range contribution to the free solvation energy was obtained from the force field employed in the simulations. For solutions with available experimental data for the free energy of solvation, we found a satisfactory agreement of the computed free solvation energies and the experimental data set.

Semi-empirical study of a set of 2- 2 -hydroxyphenyl benzazoles using the polarizable continuum model

Abstract A set of molecules (2-(2′-hydroxyphenyl)benzoxazol (HBO), 2-(2′-hydroxyphenyl benzothiazole (HBT) and 2-(2′-hydroxyphenyl)benzimidazole (HBI)) exhibiting excited-state intramolecular proton transfer was studied using the polarizable continuum model in low-polar, non-protic solvents (chloroform and carbon tetrachloride), combined with the AM1 semi-empirical molecular orbital method in both gaseous and condensed phase. The heats of formations (Δ H f ) are lowered by the solvent effect, especially in chloroform. The increase in the solvent polarity causes an enlargement of the Stokes shift between absorption and emission.

A DFT study on the insertion of CO2 into styrene oxide catalyzed by 1‐butyl‐3‐methyl‐Imidazolium bromide ionic liquid

Green systems able to capture or fix CO2 are becoming more important specially to reduce environmental impacts. In this work, the mechanism of insertion of CO2 into styrene oxide (STYO) both in the absence and presence of the catalyst 1‐butyl‐3‐methyl‐imidazolium bromide (BMIm Br) was investigated through calculations based on density functional theory in the ωB97X‐D level. Two different routes were considered and it was shown they are energetically available and compete against each other. For both routes, the rate‐determinant step is the ring opening of STYO resulting from the nucleophilic attack of the Br− on the C atom from STYO and is associated mainly to the participation of the cation and the anion from the catalyst in the reaction. Reactive indices and noncovalent interaction analysis were used as a tool to investigate this reason. This work allowed a better comprehension of the underlying mechanism and the supplied data provide valuable support for the design of new more efficient ionic liquid catalyst.

New approach to free energy of solvation applying continuum models to molecular dynamics simulation.

A new approach to the calculation of the free energy of solvation from trajectories obtained by molecular dynamics simulation is presented. The free energy of solvation is computed as the sum of three contributions originated at the cavitation of the solute by the solvent, the solute‐solvent nonpolar (repulsion and dispersion) interactions, and the electrostatic solvation of the solute. The electrostatic term is calculated based on ideas developed for the broadly used continuum models, the cavitational contribution from the excluded volume by the Claverie‐Pierotti model, and the Van der Waals term directly from the molecular dynamics simulation. The proposed model is tested for diluted aqueous solutions of simple molecules containing a variety of chemically important functions: methanol, methylamine, water, methanethiol, and dichloromethane. These solutions were treated by molecular dynamics simulations using SPC/E water and the OPLS force field for the organic molecules. Obtained free energies of solvation are in very good agreement with experimental data.

Free energy of solvation from molecular dynamics simulation applying Voronoi-Delaunay triangulation to the cavity creation

The free energy of solvation for a large number of representative solutes in various solvents has been calculated from the polarizable continuum model coupled to molecular dynamics computer simulation. A new algorithm based on the Voronoi-Delaunay triangulation of atom-atom contact points between the solute and the solvent molecules is presented for the estimation of the solvent-accessible surface surrounding the solute. The volume of the inscribed cavity is used to rescale the cavitational contribution to the solvation free energy for each atom of the solute atom within scaled particle theory. The computation of the electrostatic free energy of solvation is performed using the Voronoi-Delaunay surface around the solute as the boundary for the polarizable continuum model. Additional short-range contributions to the solvation free energy are included directly from the solute-solvent force field for the van der Waals-type interactions. Calculated solvation free energies for neutral molecules dissolved in benzene, water, CCl4, and octanol are compared with experimental data. We found an excellent correlation between the experimental and computed free energies of solvation for all the solvents. In addition, the employed algorithm for the cavity creation by Voronoi-Delaunay triangulation is compared with the GEPOL algorithm and is shown to predict more accurate free energies of solvation, especially in solvents composed by molecules with nonspherical molecular shapes.

Optimization of molecular cavities in the PCM of neutral molecules using charge dependent atomic radii: applications to the semi-empirical AM1 and MNDO/PM3 methods

Abstract An improvement in the results from the polarizable continuum model (PCM), when coupled to the AM1 and MNDO/PM3 semi-empirical methods, has been obtained by an optimization of the molecular cavities using charge-dependent atomic radii. A set of neutral molecules, containing different functional groups, was used to parametrize functions relating Mulliken atomic net charges to atomic radii. Another set of neutral molecules was utilized to test these functions. The RMS errors in free energy of hydration were 0.47 kcal mol−1 for AM1 and 0.74 kcal mol−1 for MNDO/PM3.

Bis-arylsulfenyl- and bis-arylselanyl-benzo-2,1,3-thiadiazoles: synthesis and photophysical characterization

Bis-arylsulfenyl- and bis-arylselanyl-benzo-2,1,3-thiadiazoles were synthesized in good yields by copper-catalysed cross-coupling reaction of arylthiols or diaryl diselenides with the commercially available 4,7-dibromobenzo[c][1,2,5]thiadiazole. The arylsulfenyl derivatives present absorptions in the visible region (∼420 nm) with molar absorptivity coefficient and radiative rate constant values ascribed to spin and symmetry allowed π–π* electronic transitions, with almost complete absence of solvatochromic effect. An emission located in the cyan green to green region (514–570 nm), with a large Stokes shift (90–146 nm) was observed, probably associated to the charge transfer character of the S1 state. Theoretical calculations were also performed in order to study the geometry, charge distribution and photophysical properties of the molecules in their ground and excited electronic states. TD-DFT calculations were performed using the PBE1PBE and CAM-B3LYP functionals with cc-pVDZ basis set for geometrical optimisations in the S0 and S1 states and jun-cc-pVTZ basis set to obtain vertical transition energies and electronic properties. Solvent effects were included by IEF-PCM formalism using solvents with different dielectric constants. The computationally predicted transition energies calculated with CAM-B3LYP are in good agreement with the experimental results. No substantial solvatochromic effect was found in the absorption maxima, but in the emission from S1 state a redshift was observed on increasing the solvent polarity. This fact, combined with higher dipole moment in the first excited state and some spatial separation of HOMO and LUMO orbitals could indicate an intramolecular charge transfer character of the S1 state.

Structure-property relationships of smectic liquid crystalline polyacrylates as revealed by SAXS

A influencia da estrutura quimica dos grupos mesogenicos e do tamanho dos grupos espacadores, no comportamento de fase de uma serie de cristais liquidos polimericos de cadeia lateral (SCLCP), foram estudados utilizando-se espalhamento de raios-X a Baixo Ângulo (SAXS) e Microscopia Otica de Luz Polarizada (POM). Analises do arranjo das mesofases em amostras nao orientadas e orientadas por acao do campo magnetico sao descritas. O papel do tamanho do espacador lateral no empacotamento local e na largura da camada esmetica determinados nas mesofases SmA e SmC e elucidado. Os ângulos θ formados entre os grupos mesogenicos e a normal as camadas nas mesofases SmC foram determinados. Um estudo a respeito do grau de ordem em funcao da temperatura, para os polimeros esmeticos foi possivel atraves de medidas de SAXS. Uma ordenacao particular em um dos SCLCPs estudados e relacionada com a coexistencia de duas fases distintas. The influence of the chemical structure of the mesogenic groups and the length of the spacer groups on the phase behavior in a series of side-chain liquid crystalline polyacrylates (SCLCP) have been studied using Small Angle X-ray Scattering (SAXS) and Polarized Optical Microscopy (POM). Analyses of the mesophase arrangement in unaligned and aligned samples by magnetic field are reported. The role of the spacer length on the local packing and on the thickness of the layers encountered in the SmA and SmC mesophases is elucidated. The tilt angles θ of the mesogenic cores related to the normal of the layers in the SmC mesophases are measured. A study about the degree of order as a function of temperature for the smectic polymers was possible using SAXS measurements. A particular arrangement in one of the studied SCLCPs is related to the coexistence of two different phases.