Glaucio B. Ferreira

Vibrational spectra of bis(dmit) complexes of main group metals: IR, Raman and ab initio calculations

This work reports a theoretical-experimental investigation of the infrared/Raman vibrational spectra of several metal 1,3-dithole-2-thione-4,5-dithiolate (dmit) complexes; [NEt4]2[Zn(dmit)2], [NEt4][Sb(dmit)2] and [NEt4][Bi(dmit)2]. IR/Raman spectra of all the solids and the solution IR spectrum of [NEt4]2[Zn(dmit)2] were recorded from 4000 to 100 cm-1. Two regions were clearly identified: below 380 cm-1, the modes presented significant metal-ligand contributions, and above, the modes indicated major ligand contributions. IR/Raman spectra of NEt4Br have also been recorded. The vibrational bands have been assigned starting from the analysis of related compounds, of previous published spectra and compared to several ECPs (SBK, Stuttgart), basis sets (with and without functions d) and methodologies (RHF, MP2 and DFT). Geometries, frequencies and intensities were calculated and compared to the experimental frequencies seeking secure assignment. Thus, an excellent agreement was obtained for several experimental bands.

Oxidative dehydration of glycerol to acrylic acid over vanadium-impregnated zeolite beta

The oxidative dehydration of glycerol to acrylic acid was studied over vanadium-impregnated zeolite Beta. Catalysts were prepared by wet impregnation of ammonium metavanadate over ammonium-exchanged zeolite Beta, followed by air calcination at 823 K. Impregnation reduced the specific surface area, but did not significantly affected the acidity (Bronsted and Lewis) of the zeolites. The catalytic evaluation was carried out in a fixed bed flow reactor using air as the carrier and injecting glycerol by means of a syringe pump. Acrolein was the main product, with acetaldehyde and hydroxy-acetone (acetol) being also formed. Acrylic acid was formed with approximately 25% selectivity at 548 K over the impregnated zeolites. The result can be explained by XPS (X-ray photoelectron spectroscopy) measurements, which indicated a good dispersion of the vanadium inside the pores.

Rearrangement, Nucleophilic Substitution, and Halogen Switch Reactions of Alkyl Halides over NaY Zeolite : Formation of the Bicyclobutonium Cation Inside the Zeolite Cavity

Rearrangement and nucleophilic substitution of cyclopropylcarbinyl bromide over NaY and NaY impregnated with NaCl was observed at room temperature. The first-order kinetics are consistent with ionization to the bicyclobutonium cation, followed by internal return of the bromide anion or nucleophilic attack by impregnated NaCl to form cyclopropylcarbinyl, cyclobutyl, and allylcarbinyl chlorides. The product distribution analysis revealed that neither a purely kinetic distribution, similar to what is found in solution, nor the thermodynamic ratio, which favors the allylcarbinyl halide, was observed. Calculations showed that bicyclobutonium and cyclopropylcarbinyl carbocations are minimal over the zeolite structure, and stabilized by hydrogen bonding with the framework structure. A new process of nucleophilic substitution is reported, namely halogen switch, involving alkyl chlorides and bromides of different structures. The reaction occurs inside the zeolite pores, due to the confinement effects and is an additional proof of carbocation formation on zeolites. The results support the idea that zeolites act as solid solvents, permitting ionization and solvation of ionic species.

An experimental and theoretical study of the electronic spectra of tetraethylammonium [bis(1,3-dithiole-2-thione-4,5-dithiolato)M(III)] and tetraethylammonium [bis(1,3-dithiole-2-one-4,5-dithiolato)M(III)] (M = Sb or Bi)

Metal complexes of dmit and related ligands, such as dmio, have been studied extensively over the last few decades: [dmit](2-) = bis(1,3-dithiole-2-thione-4,5-dithiolato); [dmio](2-) = bis(1,3-dithiole-2-one-4,5-dithiolato). While much effort has been placed on determining structural properties, very few vigorous spectroscopic studies of these metal complexes have been undertaken. The spectroscopic features of the infrared, Raman and UV-vis spectra, previously reported, have largely been used to characterize compounds. In particular, many details of the electronic structure are still to be revealed. We now report a detailed analysis of the UV-vis spectra of the [M(dmit)](-1) and [M(dmio)](-1) anions, where M = Sb(III) or Bi(III). Experimental spectra were deconvoluted and analysed by several theoretical methodologies, including ab initio CI, TD and CISD calculations. The results led to the assignments of several MLCT/LMCT bands, occurring between 390 and 300 nm, and the confirmation of metal orbital contributions to the HOMO-LUMO boundary.

An insight into carvedilol solid forms: effect of supramolecular interactions on the dissolution profiles

Carvedilol, a β-blocker drug used to treat hypertension, is known to exhibit polymorphism. Thus far, the crystal structure of two polymorphs (I and II) and one hydrate have been reported. In this study, three crystal modifications of carvedilol were obtained from crystallization experiments. The structure of another polymorph (III) was elucidated for the first time and the crystal structure of the hydrate was also determined from single-crystal diffraction data. Carvedilol structures are characterized by variations in their molecular conformations, with different orientations of the carbazole moiety. Further conformational details also differentiate them. Carvedilol structures II, III and hydrate were characterized by thermal and microscopic methods. A higher melting temperature polymorph (III), compared to form II, was identified when carvedilol was crystallized from methanol. This polymorph showed a higher intrinsic dissolution rate than polymorph II in hydrochloric acid at pH 1.4 containing sodium lauryl sulfate (0.1% w/v). Although it was not expected due to its greater stability, the higher dissolution rate of polymorph III could be explained by its structural features. The dissolution of carvedilol in acid medium is related to its ability to form protonated molecules. DFT calculations with the WB97XD functional were carried out for evaluation of the classical and non-classical hydrogen bonds between the molecules in carvedilol structures. The energies involved in the intermolecular interactions explain some experimental observations. This result showed that intermolecular interactions influence the solid-state properties of carvedilol. Moreover, polymorph III exhibits a higher dissolution rate than II, showing great potential for formulation strategies of this poorly water soluble drug.

Molecular structure, natural bond analysis, vibrational, and electronic spectra of aspartateguanidoacetatenickel(II), [Ni(Asp)(GAA)]·H2O: DFT quantum mechanical calculations

The aspartateguanidoacetatenickel (II) complex, [Ni(Asp)(GAA)], was synthesized and structural analysis was performed by means of the experimental methods: determination of the C, H, N and O contents, thermogravimetry, infrared and Raman spectroscopy. DFT:B3LYP/6-311G(d, p) calculations have been performed giving optimized structure and harmonic vibrational wavenumbers. Second derivative of the FT-infrared, FT-Raman and Surface Raman Enhanced Scattering (SERS) spectra, and band deconvolution analysis were also performed. Features of the FT-infrared, FT-Raman and SERS confirmed theoretical structure prediction. Full assignment of the vibrational spectrum was also supported by a carefully analysis of the distorted geometries generated by the normal modes. The Natural Bond Orbital analysis (NBO) was also carried out as a way to study the Ni (II) hybridization leading to the pseudo planar geometry of the framework, and the extension of the atomic N and O hybrid orbital of the different amino acids in the bond formation. Bands of charge transfer and d-d transitions were assigned in the UV-Vis spectrum.

DFT studies of imino and thiocarbonyl ligands with the pentaaqua Mg2+ cation: affinity and associated parameters

AbstractThe affinity of the pentaaqua Mg2+ cation for a set of para-substituted imino [HN = CHC6H4(R)] and thiocarbonyl [S = CHC6H4(R)] ligands (R = H, F, Cl, Br, OH, OCH3, CH3, CN, NH2 and NO2) was analyzed with DFT (B3LYP/6-31+G(d)) and semi-empirical (PM6-DH2) methods. The interaction enthalpy was calculated to quantify the affinity of the Mg2+ cation for the ligands. Additionally, geometric and electronic parameters were correlated with the intensity of the metal-ligand interaction. The imino ligands have stronger interaction with the pentaaqua Mg2+ cation than the thiocarbonyl derivatives. The electronic nature of the substituent is the main parameter that determines the interaction enthalpy. Ligands with electron donor substituents have more exothermic interaction enthalpies than those with electron withdrawing groups. The HSBA analysis showed that the interaction between the Mg2+ cation and hard bases (imino ligands) is stronger than with soft bases (thiocarbonyl derivatives). The EDA analysis showed that the electrostatic, covalent and repulsion components of the interaction are the most affected by the substituent, whereas the dispersion and exchange components are almost constant. The affinity of the Mg2+ cation for a set of para-substituted imino and thiocarbonyl ligands was evaluated in terms of interaction enthalpy, Gibbs free energy, geometric and electronic parameters, showing that the nature of the substituent directly modulates the electrostatic, covalent and repulsion components of the interaction.

Molecular structure, natural bond analysis, vibrational and electronic spectra, surface enhanced Raman scattering and Mulliken atomic charges of the normal modes of [Mn(DDTC)2] complex

Theoretical and experimental bands have been assigned for the Fourier Transform Infrared and Raman spectra of the bis(diethyldithiocarbamate)Mn(II) complex, [Mn(DDTC)2]. The calculations have been based on the DFT/B3LYP method, second derivative spectra and band deconvolution analysis. The UV-vis experimental spectra were measured in acetonitrile solution, and the calculated electronic spectrum was obtained using the TD/B3LYP method with 6-311G(d, p) basis set for all atoms. Charge transfer bands and those d-d spin forbidden were assigned in the UV-vis spectrum. The natural bond orbital analysis was carried out using the DFT/B3LYP method and the Mn(II) hybridization leading to the planar geometry of the framework was discussed. Surface enhanced Raman scattering (SERS) was also performed. Mulliken charges of the normal modes were obtained and related to the SERS enhanced bands.

The Effect of Gamma-Al2O3 Support on the NO Adsorption on Pd4 Cluster

The effect of γ-Al2O3 support on the NO adsorption on Pd4 clusters was investigated by means of density functional theory (DFT) calculations. Pd4 adsorbed on γ-Al2O3 (represented by a Al14O24H6 cluster) changes its preferential geometry from tetrahedral to a distorted planar structure. The alumina support promotes a higher dispersion in the palladium catalyst and reduces the NO adsorption energy to -25.6 kcal mol-1 (computed at B3LYP/LANL2DZ/6-311+G(d)), in close agreement with the experimental value of -27.2 kcal mol-1. On the bare planar Pd4 cluster the NO molecule adsorbs in a bridge arrangement, with adsorption energy of -41.2 kcal mol-1. Adsorption on the tetrahedral Pd4 cluster occur preferentially in an atop mode, with adsorption energy of -30.6 kcal mol-1. Charge density analysis show that the electron flux between the NO molecule and Pd4 depends on the adsorption form, with back-donation being stronger in the bridge adsorption mode.

Vibrational and electronic spectroscopy of neutral antimony coordination compounds of the 1,3-dithiole-2-thione-4,5-dithiolate (dmit).

The S 1s X-ray absorption near edge structure (XANES) and X-ray photoelectron spectra (XPS) of the neutral complexes [SbL(dmit)] (L = Br or I; dmit =1,3-dithiole-2-thione-4,5-dithiolate) have been measured using tunable synchrotron radiation. The valence shell electronic excitation by ultraviolet-visible (UV-vis) spectroscopy and the infrared vibrational spectra are presented and analyzed. The UV-vis results lead to an assignment of bands at 400 nm as π(Sm) → π*(C═S), where S(m) is the thiolate sulfur. The corresponding S 1s → π*(C═S) transition was identified at 2468.3 eV. Ab initio calculations, within the improved virtual orbital (IVO) method, carried out with the GSCF3 program, were applied to establish a complete and accurate spectral assignment. It has been the first attempt to apply such methodology for dmit coordination compounds, and very consistent results were obtained.