Thiago C. Rozada

The conformational analysis of 2-halocyclooctanones.

The establishment of the most stable structures of eight membered rings is a challenging task to the field of conformational analysis. In this work, a series of 2-halocyclooctanones were synthesized (including fluorine, chlorine, bromine and iodine derivatives) and submitted to conformational studies using a combination of theoretical calculation and infrared spectroscopy. For each compound, four conformations were identified as the most important ones. These conformations are derived from the chair-boat conformation of cyclooctanone. The pseudo-equatorial (with respect to the halogen) conformer is preferred in vacuum and in low polarity solvents for chlorine, bromine and iodine derivatives. For 2-fluorocyclooctanone, the preferred conformation in vacuum is pseudo-axial. In acetonitrile, the pseudo-axial conformer becomes the most stable for the chlorine derivative. According to NBO calculations, the conformational preference is not dictated by electron delocalization, but by classical electrostatic repulsions.

Infrared and theoretical calculations in 2-halocycloheptanones conformational analysis

2-Halocycloheptanones (Halo=F, Cl, Br and I) were synthesized and their conformational analysis was performed through infrared spectroscopy data. The corresponding conformers geometries and energies were obtained by theoretical calculations at B3LYP/aug-cc-pVDZ level of theory in the isolated state and in solution. It was observed, by both approaches, that the conformational preferences were very sensitive to the solvent polarity, since its increase led to an increase in the population of the more polar conformer. An analysis of these conformational equilibria showed they suffer also the influence of stereoelectronic effects, like hyperconjugation and steric effects. These results were interpreted using natural bond orbital (NBO) analysis, which indicated that the electronic delocalization to the orbital π*(C=O) is directly involved in the stability increase of conformers I and II. The relative effect of the period of the halogen can also be noted, with changes in the conformational preferences and in the energies involved in the interactions of NBO.

Theoretical aspects of the unexpected regiospecific synthesis of pyrazole-5-carboxylates from unsymmetrical enaminodiketones

Pyrazoles are heterocycles with economic importance because of the wide applications of such compounds in the pharmaceutical and agrochemical industries. Pyrazoles can be prepared from the cyclocondensation of unsymmetrical enaminodiketones with hydrazines. However, this method often suffers from the formation of a regioisomeric mixture of pyrazoles with generally poor selectivity. Only in few cases, the regiospecific synthesis was possible. Nevertheless, up to now, the factors behind the regiospecificity of this reaction were unknown. Thus, in this work, we use density functional theory (M06-2X/6-31++G(d,p)) to analyze the reaction mechanism. Our results show that the activation-free energy leading to the possible products are similar, but the products present very different stabilities which explain the formation of a single regioisomer experimentally observed. In this way, the regiospecificity is governed by thermodynamical factors.

Rotational isomerism of some chloroacetamides: Theoretical and experimental studies through calculations, infrared and NMR

The geometries involved in the conformational equilibria of 2,2-dichloro-N-cyclohexyl-N-methyl-acetamide (DCCMA) and 2-chloro-N,N-dicyclohexylacetamide (CDCA) were investigated. Theoretical calculations at the B3LYP/cc-pVDZ level of theory showed that gauche forms (ClCCO) are the most stable and the predominant conformers in isolated phase. Both compounds had the conformational behavior in solvents of different polarities estimated from theoretical calculations with the PCM (Polarizable Continuum Model), at the same level of theory, using infrared data from deconvolution of the carbonyl absorption bands and (13)C NMR spectra. Their IR spectra showed two carbonyl absorptions and that the conformer with the highest dipole moment had its population increased when the most polar solvents were used, in accordance with the theoretical calculation in solution. (1)JCH coupling constants were obtained from their NMR spectra, and revealed that there was population variation of conformers with solvent exchange. Experimental data (NMR and IR) as well as calculations including the solvent effects followed the same trend.

Sol gel synthesis of 3- n -propyl(4-aminomethyl)pyridinium silsesquioxane chloride and the enhanced electrocatalytic activity of LbL films

AbstractA silsesquioxane based on a silica matrix and 4-(aminomethyl)pyridine group was successfully synthesized using the sol–gel process with the proposal of two different carbonic chain arrangments in the structure of the silica polymer, observed employing NMR techniques. The silsesquioxane proposed succeeds an improvement of the adsorption of monomer electroactive species on the surface of the electrode obtained through films assembly, this fact was attributed to the presence of two coordinating nitrogen in new silsesquioxane. In this study, the Layer-by-Layer (LbL) technique was shown suitable to be used to produce electroactive electrodes with indium tin oxide electrode (ITO) coated by tetrasulfonated nickel phthalocyanine (NiTsPc) alternated with the silsesquioxane. Voltammetric techniques were used to investigate the electrochemical behavior of the nitrite oxidation, which exhibited an enhanced current response at the modified electrode. The proposed electroddisplayed a current response which varied linearly with the concentration of nitrite in the range 0.113 to 0.860 mmol L−1 with a detection limit of 0.026 mmol L−1, a sensitivity of 32.06 µA (mmol L−1) and relative standard deviation of approximately 3%. HighlightsSynthesis of hybrid containing both ammoniummethyl pyridine and aminomethyl pyridinium groups.The new material allows a greater adsorption of metal phthalocyanine monomers.The electrode obtained allows the detection of 0.026 mmol L−1 nitrite.

Stereoelectronic effects of the glycosidic linkage on the conformational preference of D-sucrose

The stereoelectronic effects involved in the conformations shown by D-sucrose were analysed at the M06-2X/6-31++G(d,p) level, both under vacuum and in water with the continuum solvation model IEF-PCM, and employing the natural bond orbital theory (NBO) and non-covalent interactions (NCI). Different groups of conformations for D-sucrose in relation to the glycosidic linkage were evaluated and the results showed that not only were hydrogen bonds important to explain the relative energy observed, but it was also necessary to consider any stabilizing orbital interactions involving the glycosidic linkage. The most stable conformation observed in water had dihedral angles for the glycosidic linkage with values of 110.8° and −44.9° for ϕ and ψ, respectively.