Use of scavenger agents in heterogeneous photocatalysis: truths, half-truths, and misinterpretations.

Even though a gap exists in understanding the behavior of radical scavengers and interfering species, they have been extensively employed to elucidate degradation mechanisms or to improve the degradation efficiency in heterogeneous photocatalysis. Focusing on the influence of different species, such as scavengers (t-butanol, formic acid, methanol, p-benzoquinone, oxalate, superoxide dismutase, and azide), interfering species (sulfite, dichromate, bromate, carbonate, chloride, and iodide) and inorganic ions (nitrate, sulfate, and phosphate), this work investigated the production of hydroxyl radicals and singlet oxygen during TiO2/UVA reactions. Electron paramagnetic resonance spectroscopy (EPR) was applied to investigate radicals formed in the presence of each interfering/scavenger species. Some scavengers and interfering species were studied during phenol degradation, chosen as a model substrate. All species, except bromate, hindered the degradation. para-Benzoquinone showed an increased hydroxyl radical production, attributed to the photo-reduction of quinones. Radicals other than hydroxyl radicals, such as carbon dioxide, hydroxymethyl, azide, and semiquinone, were identified in the presence of oxalate, methanol, azide, and para-benzoquinone, respectively. Some of these radicals can possibly interact with organic substrates due to their reduction potential; as a result, a critical interpretation must be done when these species are added to a heterogeneous photocatalysis process.

An Oxalate-Bridged Copper(II) Complex Combining Monodentate Benzoate, 2,2'-bipyridine and Aqua Ligands: Synthesis, Crystal Structure and Investigation of Magnetic Properties.

A dinuclear copper(II) complex of formula [{Cu(bipy)(bzt)(OH2)}2(µ-ox)] (1) (where bipy = 2,2'-bipyridine, bzt = benzoate and ox = oxalate) was synthesised and characterised by diffractometric (powder and single-crystal XRD) and thermogravimetric (TG/DTG) analyses, spectroscopic techniques (IR, Raman, electron paramagnetic resonance spectroscopy (EPR) and electronic spectroscopy), magnetic measurements and density functional theory (DFT) calculations. The analysis of the crystal structure revealed that the oxalate ligand is in bis(bidentate) coordination mode between two copper(II) centres. The other four positions of the coordination environment of the copper(II) ion are occupied by one water molecule, a bidentate bipy and a monodentate bzt ligand. An inversion centre located on the ox ligand generates the other half of the dinuclear complex. Intermolecular hydrogen bonds and π-π interactions are responsible for the organisation of the molecules in the solid state. Molar magnetic susceptibility and field dependence magnetisation studies evidenced a weak intramolecular-ferromagnetic interaction (J = +2.9 cm-1) between the metal ions. The sign and magnitude of the calculated J value by density functional theory (DFT) are in agreement with the experimental data.

DFT analysis of the linkage isomerism in penta(ammine)ruthenium(II/III) complexes of benzotriazole: Natural bond orbital method approach and a comprehensive energy decomposition analysis.

Penta(ammine)ruthenium benzotriazole complexes [RuII/III (NH3 )5 bta]+/2+ and [RuII/III (NH3 )5 btaH]2+/3+ (bta and btaH are the deprotonated and neutral form of the triazole ligand, respectively) can exhibit two linkage isomers κN1 and κN2. This system was investigated by density functional theory natural bond orbitals analysis and Su-Li energy decomposition analysis. Steric, electrostatic, exchange, repulsion, polarization, and dispersion energy components of the total metal-ligand interaction were quantitatively evaluated, and revealed that the overall metal-triazole ligand is comprised of donor-acceptor interactions like σ-donation and π-back-donation, which favors a specific isomer depending on the oxidation state of the ruthenium and the charge of the ligand. Further, activation energies (ΔG‡ ) for linkage isomerization reactions were calculated. Results were correlated with experimental chemical-electrochemical data and two plausible mechanisms are discussed. © 2019 Wiley Periodicals, Inc.

Metal complexes of a pentadentate N2O3bis(semicarbazone) Schiff-base. A case study of structure-spectroscopy correlation.

Schiff condensation of 2,6-diformyl 4-methylphenol with semicarbazide hydrochloride in 1:2 molar ratio produces the bis(semicarbazone) ligand, herein called H3L. A comprehensive spectroscopic analysis of the compound was performed by (1)H and (13)C NMR, FTIR and electronic spectroscopies. Assignments to the UV-vis spectrum of H3L were supported by semi-empirical quantum mechanics ZINDO/S calculations. The ligand H3L forms monoclinic crystals in the space group P21/c and its structure is stabilized by classic hydrogen bonds with propanone molecules. It promptly reacts with first row metal ions to produce the following coordination compounds: [Co2(L)(µ-NO3)]·DMF, [Ni2(H2L)(µ-CH3COO)(CH3COO)2]·2H2O, [Cu2(L)(µ-NO3)(H2O)2]·H2O, [Cu2(L)(µ-CH3COO)(H2O)2]·H2O and [Cu2(H2L)(µ-Cl)Cl2]·3H2O, that have different compositions, depending on the degree of deprotonation of the ligand upon coordination. Electronic and EPR spectroscopies as well as effective magnetic moment measurements of the complexes were used in an attempt to better understand their mode of coordination, the microsymmetry around the metal ions and magnetic properties.

Conserved histidine residues at the ferroxidase centre of the Campylobacter jejuni Dps protein are not strictly required for metal binding and oxidation.

Iron is an essential micronutrient for living organisms as it is involved in a broad variety of important biological processes. However, free iron inside the cell could be potentially toxic, generating hydroxyl radicals through the Fenton reaction. Dps (DNA-binding protein from starved cells) belongs to a subfamily of ferritins and can store iron atoms inside the dodecamer. The presence of a ferroxidase centre, composed of highly conserved residues, is a signature of this protein family. In this study, we analysed the role of two conserved histidine residues (H25 and H37) located at the ferroxidase centre of the Campylobacter jejuni Dps protein by replacing them with glycine residues. The C. jejuni H25G/H37G substituted variant showed reduced iron binding and ferroxidase activities in comparison with wt Dps, while DNA-binding activity remained unaffected. We also found that both CjDps wt and CjDps H25G/H37G were able to bind manganese atoms. These results indicate that the H25 and H37 residues at the ferroxidase centre of C. jejuni Dps are not strictly required for metal binding and oxidation.

Preparation, crystallography and spectroscopic properties of the polymeric {(1-(E)-2-pyridinylmethylidene)semicarbazone)(aqua)copper(II)} sulphate dihydrate complex: evidence of dynamic Jahn-Teller effect.

Pyridine-2-carbaldehyde semicarbazone ligand (HL) reacts with copper(II) sulphate in water solution to yield the coordination polymer [{Cu(II)(HL)(H(2)O)(SO(4))}(n)] (1). The crystals are triclinic with space group P(-1) and the metal ion is occupying a distorted octahedral geometry. EPR results show that a dynamic Jahn-Teller (J-T) effect is operative in water solutions and also support the stability of the polymeric chains as they continue to show a characteristic half-field Δm(S)=±2 transitions. UV-visible spectrum analysis allowed access to the J-T stabilization energy of 5995 cm(-1) in water. Thermogravimetric/differential thermal analysis curves showed a step-by-step decomposition of complex 1 with loss of water, release of SO(3) and oxidation of the semicarbazone ligand in the 30-422°C range.

The harmonic inversion of the field-swept fixed-frequency resonance spectrum.

When the spin Hamiltonian is a linear function of the magnetic field intensity the resonance fields can be determined, in principle, by an eigenfield equation. In this report, we show a new technical approach to the resonance field problem where the eigenfield equation leads to a dynamic equation or, more specifically, to a first order differential equation of a variable L(x), where x is associated with the magnetic field h. Such differential equation has the property that: its stationary solution is the eigenfield equation and the spectral information contained in L(x) is directly related to the resonance spectrum. Such procedure, known as the "harmonic inversion problem" (HIP), can be solved by the "filter diagonalization method" (FDM) providing sufficient precision and resolution for the spectral analysis of the dynamic signals. Some examples are shown where the resonance fields are precisely determined in a single procedure, without the need to solve eigenvalue equations.