Lúcia K. Noda

TiO2 with a high sulfate content—thermogravimetric analysis, determination of acid sites by infrared spectroscopy and catalytic activity

Sulfated metal oxides have a very high acidity, being considered by some as superacids, although it is still a controversial matter. There has been considerable investigation into sulfated ZrO2 in recent years, but sulfated TiO2 has been relatively neglected. There is evidence that the Bronsted acid sites would be responsible for the high levels of acidity and catalytic activity in sulfated ZrO2, since these sites would be present in samples with higher sulfate content. In order to verify whether for sulfated TiO2, a high level of acidity also occurs in samples with a higher sulfate content, samples were prepared with elevated levels of sulfate. Characterization was carried out through thermogravimetric analysis, determination of acid sites by infrared spectroscopy, through adsorption of pyridine and measurement of the catalytic activity in the isomerization reaction of n-hexane. It was verified that samples with elevated sulfate content were actually those which had Bronsted acid sites and a higher catalytic activity in the isomerization reation of n-hexane. It was also verified that the sulfate loss in the thermogravimetric analysis occurred at different temperatures, depending on the sulfate content of the sample.

Resonance Raman and crystallographic studies on the complex [Fe2(bbpnol)2]·2DMF (bbpnol=N,N′-bis(2-hydroxybenzyl)-2-ol-1,3-propanediamine)

Abstract The ligand N,N′-bis(2-hydroxybenzyl)-2-ol-1,3-propanediamine (H3bbpnol) reacts with iron(III) perchlorate forming two dinuclear bis-μ-alkoxo complexes, a ‘cis–trans’ isomer (complex 1) previously reported and a ‘cis–cis’ isomer (complex 2) characterized in this work. The main differences found in complex 2 structure are, (a) the four phenolic oxygens are trans to the alkoxo bridges; (b) each ligand is shared by the two Fe(III) ions occupying two coordination positions at each center. The Fe(III) centers in the resulting centrosymmetric structure in complex 2 have a distorted-octahedral geometry with the equatorial plane occupied by the phenolic and alcoholic oxygen atoms and the apical positions are filled by the aminic nitrogen atoms. The resonance Raman (RR) spectra of these two isomeric complexes are somewhat different with the intensity of some low-frequency modes increasing in the less symmetric core. The electronic spectra of both complexes are similar, but the molar absorptivities are substantially increased in complex 2, indicating the presence of an electronic coupling between the phenolate moieties trans in relation to the alkoxo bridge, and that phenolates coordinated cis to the alkoxo bridge do not seem to contribute to LMCT oscillator strength. This is directly reflected in the Raman excitation profiles (REP) of the chromophore modes, with the vibrational modes of the ‘cis–cis’ isomer showing a greater intensification compared with the ‘cis–trans’ isomer.

Vibrational and resonance Raman study of dithiosquarate

Abstract The resonance Raman excitation profiles of the dithiosquarate in the solid state, as its thallium salt, and in aqueous solution as its sodium salts were obtained. The assignment of its vibrational frequencies to the normal modes was carried out on basis of ab initio calculations, using the Lee–Yang–Parr correlation functional and the 6-31+G* basis, and of the potential energy distribution for the calculated frequencies. A correlation between the frequencies of the dithiosquarate, squarate and tetrathiosquarate anions shows good agreement with that obtained using the enhancement factors calculated from the excitation profiles. In the case of dithiosquarate the intensification of non-totally symmetric modes both for the thallium and for the sodium salt suggests the participation of the B-term mechanism, with more than one excited electronic state contributing to the Raman intensities.

Resonance Raman spectroscopy of FeII FeIII and FeIII FeIII model complexes containing an unsymmetrical dinucleating ligand: a biomimetic redox pair for uteroferrin

Raman excitation profiles for the complex [FeIIFeIII(bpbpmp)(C2 H3O2)2](ClO4 ) (1), and its oxidized form, the new compound [FeIIIFeIII(bpbpmp)(C2 H3O2)2](ClO4 )2 (2), are reported. H2bpbpmp is the proligand 2-bis[{(2-pyridylmethyl)-aminomethyl}-6-{(2-hydroxybenzyl)(2 -pyridylmethyl)}-aminomethyl]-4-methylphenol. For compound 1, the most enhanced vibrational mode in the Raman spectra is the n(Fe-Ophen-terminal), observed at 608 cm-1. For compound 2, the n(COphen-terminal), which corresponds to the band at 1276 cm-1, becomes the most enhanced one. These differences are ascribed to the changes in the electronic structure of the dinuclear phenolate bridged core upon oxidation. The phenolate bridge allows charge density transmission between the metal centers.