Nora B. Okulik

Topological Insights into the Nature of the Halogen−Carbon Bonds in Dimethylhalonium Ylides and Their Cations

In this study the nature of the bonding in a series of dimethylhalonium ylides (fluoronium, chloronium, bromonium and iodonium) was analyzed by means of topological methodologies (AIM and ELF analysis), to document the changes in the nature of the C-X bonds (X = F, Cl, Br, I) upon the series. For the sake of comparison the same study was performed on the corresponding dimethylhalonium cations (XC 2H 6 (+)) and the XCH 3 series. The wave functions used for the topological analysis were obtained at B3LYP level using extended triple-zeta basis sets. The formation of the cationic XC 2H 6 (+) structures can be interpreted to arise from the interaction between the XCH 3 and CH 3 (+) moieties. The resultant structures can be explained in terms of the superposition of two electrostatically interacting and two dative mesomeric structures. The halogen-carbon bonds have all the characteristics of the charge-shift (CS) bonds. The analysis of the C-X bond in the XC 2H 5 series shows a progressive reinforcing of the CH 3X-CH 2 bond, from FC 2H 5 that can be considered as formed from two fragments, FCH 3 and CH 2, to IC 2H 5, in which the CH 3I-CH 2 bond has all the features of a multiple bond involving atoms bearing lone pairs. Particularly interesting is BrC 2H 5, in which a special type of bond (hybrid covalent-dative double bond) has been characterized. The energetic stability of the XC 2H 5 structures with respect to the dissociation into the XCH 2 + CH 3 and XCH 3 + CH 2 ground-state fragments was studied in detail.

The interaction between sulfathiazole and cobalt(II): potentiometric studies

Potentiometric studies of sulfathiazole (HST) in the presence and absence of cobalt(II) were performed. Equilibrium constants for the formation of the detected species, [Co(ST)]+ and [Co(ST)(OH)], are reported. UV-Vis spectrophotometric measurements suggest that the coordination Co(II)-sulfathiazole might be through a N atom, which, in agreement with MO calculations, could be a thiazolic one. In spite of sulfonamides being better ligands at pH >7, [Co(ST)]+ was found at pH » 3.

Improved QSAR Analysis of the Toxicity of Aliphatic Carboxylic Acids

The results of a QSPR study of the toxicity of carboxylic acids in aqueous solution are reported. The molecular set comprises 35 carboxylic acids with the corresponding pKa values in water. The set of molecular and topological parameters includes electrotopological state of the carboxy and methyl groups, molar refractivity, refractive index, n-octanol-water partition coefficient logKo/w, surface tension, and polarizability. Quite reasonable estimates are obtained, which improve the results of previous theoretical calculations.

A combined experimental and computational study of the esterification reaction of glycerol with acetic acid

AbstractThis work describes theoretical and experimental studies on glycerol esterification to obtain acetins focusing on the obtained isomers. The reaction of glycerol with acetic acid was carried out on Amberlyst 36 wet. Density functional theory calculations on the level of M06-2X functional and 6-311+G(d,p) basis set are carried out and the most stable structures of the reactants and products are located by considering a large number of conformers. The thermodynamics is discussed in terms of the calculated reaction Gibbs free energy. The AIM theory was used to characterize reactants and products. The glycerol esterification with acetic acid is found to be thermodynamically favored, with exothermal property. These agree well with experiments and allow us to explain the relative selectivity of products. FigureGlycerol esterification with acetic acid

Theoretical study of new pseudohalogen CS2N3 and some related compounds

Abstract We have performed a detailed theoretical study of four structures: (i) pseudohalide anion [CS 2 N 3 ] − ( 1 ) in the salt [Na + ][CS 2 N 3 ] − 4H 2 O, (ii) the hydracid HN 3 SCS ( 2 ), (iii) the interpseudohalogen CS 2 N 3 –CN ( 3 ) and the dipseudohalogen (CS 2 N 3 ) 2 ( 4 ), formally deriving from the radical CS 2 N 3 . The corresponding topological characterization was made by means of an analysis of the electronic charge resorting to the AIM topological theory. We have found satisfactory agreement among the geometrical parameters predicted by the theoretical methods and the experimental parameters available in the standard literature. The analysis of the topological properties calculated at the bond critical points allowed us to describe the chemical bonds in every structure as having a covalent nature, to assign a polar character to the exocyclic C–S and the delocalization of the electronic density over the ring surfaces.

Study of the Topological Properties of Some Pseudohalides.

The pseudohalide principle has been used extensively in nonmetal chemistry to predict the structure and stability of many molecular species. The 1,2,3,4-thiatriazole-5-thiolate anion, CS2N3(-), is of particular interest. In a short communication we have recently reported the topological study of some CS2N3(-)containing species reported by Crawford et al. Previous reports on these compounds showed that in covalent derivatives not only does the ring remain intact but also the site of attachment of the R group is most likely at the exocyclic sulfur atom in contrast to the previously suggested N-R connectivity. Therefore, the structure and bonding of derivatives of the CS2N3(-) moiety is clearly an important question. With that in our mind, we undertook a topological analysis, based on the AIM theory, to gain more insight into the bonding in covalent derivatives of the CS2N3(-) moiety, trying to find an explanation to the origin of the N-H and S-H connectivities. The question is which is the reason that makes all the covalent derivatives prefer the S-R connectivity while the hydracid has an N-H one.

Synthesis of bioadditives of fuels from biodiesel-derived glycerol by esterification with acetic acid on solid catalysts

ABSTRACT In this paper, glycerol esterification with acetic acid (AA) was studied on several solid acid catalysts: Al2O3, Al-MCM-41, HPA/SiO2, HBEA, Amberlyst 15 and Amberlyst 36 with the aim of determining the reaction conditions and the nature of the surface acid sites required to produce selectively triacetylglycerol (triacetin). The acidity of the catalysts (nature, density and strength of acid sites) was characterized by temperature-programmed desorption of NH3 and FTIR of adsorbed pyridine. Al2O3 (Lewis acidity) did not show any activity in the reaction. In contrast, highest activity and selectivity to the triacetylated product (triacetin) were obtained on catalysts with Brønsted acidity: Amberlyst 15 and Amberlyst 36. The effect of temperature and molar ratio of AA to glycerol was studied, and the results showed that both parameters have a significant impact on the production of the desired product. Glycerol conversion rate and selectivity to triacetin increased when temperature or AA to glycerol molar ratio were increased, reaching a triacetin yield on Amberlyst 36 of 44% at 393 K and AA to glycerol molar ratio of 6. Deactivation and reusability of Amberlyst 36 were evaluated by performing consecutive catalytic tests. The presence of some irreversible deactivation due to sulfur loss was observed. In addition, the feasibility of using crude glycerol from biodiesel production as reactant was also investigated. Conversion of crude pretreated glycerol yielded values of triacetin and diacetin similar to those obtained with the commercial pure glycerol although at a lower rate.

Topological properties of some PhSeX compounds

A theoretical study on the series of compounds “PhSeX”, where Ph = phenyl, Se = selenium and X = Cl, Br, I, CN or SCN, is reported and compared with previously reported experimental data. The molecular geometry for these PhSeX compounds was studied at the DFT/B3LYP level of calculation by means of the 6-311G(d,p) basis set. The equilibrium structures of the molecules were dependent on the method employed to compare the known solid structures. A topological study of the calculated PhSeX species, based on the AIM theory, was carried out to gain a deeper insight into the bonding nature and to find an explanation for the structural diversity exhibited by these PhSeX compounds. The results reported herein illustrate the subtle differences in the solid-state structures of PhSeX compounds.

Theoretical investigation of the conformational space of baicalin

Flavonoids are a large group of polyphenolic compounds ubiquitously present in plants. They are important components of human diet. They are recognized as potential drug candidates to be used in the treatment and prevention of a lot of pathological disorders, due to their protective effects. Baicalin (7-glucuronic acid 5, 6-dihydroxyflavone) is one of the main single active constituents isolated from the dried roots of Scutellaria baicalensis Georgi. The great interest on this flavonoid is due to its various pharmacological properties, such as antioxidant, antimicrobial, anti-inflammatory, anticancer and so on, and its high accumulation in the roots of S. baicalensis. The aim of our work was to analyze the geometric and electronic properties of baicalin conformers (BCL), thus performing a complete search on the conformational space of this flavonoid in gas phase and in aqueous solution. The results indicate that the conformational space of baicalin is formed by eight conformers in gas phase and five conformers in aqueous solution optimized at B3LYP/6-311++G** theory level. BCLa2TT and BCLa1TT conformers have low stability in gas phase and very high stability in aqueous solution. This variation is related to a modification in the τ1 angle that represents the relative position of the glucuronide unit respect to the central rings of the flavan nucleus (A and C). This modification was successfully explained by examining the changes in the hydrogen bond (HB) interactions that occur in the region around the hydroxyl group located in position 6 of ring A. Besides, the molecular electrostatic potential (MEP) and frontier molecular orbital (FMO) analyses indicate that BCLa2TT and BCLa1TT conformers are the most favorable conformers for interacting with positively charged species (such as metal ions) in aqueous media (such as biological fluids).