Electronic structure and UV spectra of N-arylthio-1,4-benzoquinone imines

The electronic structure of N-arylthio-1,4-benzoquinone imines (II) was studied by quantum-chemical methods (CNDO/2). It was shown that the special characteristics of the reactivity of the compounds in reaction with chlorine compared with sulfenylketimines R₂C=N-S-Ar not containing a quinonoid ring may be due to the different nature of the lowest unoccupied molecular Orbitals (LUMO). The UV spectra of compounds (II) were investigated. In the visible region the spectra of all the compounds contain strong absorption (R₁ = R₂ = R₃ = R₄ = R₅ = H, _m = 433 nm, _m = 2.12·10⁴ liters/mole·cm), due to intramolecular charge transfer from the sulfur atom to the quinonoid fragment of the molecule. It was established that there is a linear relation between the energy of the transition and the ⁺ constants of the substituents in the aryl fragment. The assignment of the transitions was confirmed by calculations of the UV spectra of N-arylthio-1,4-benzoquinone imines by the PPP method. Comparison of the UV spectra of these compounds with the UV spectra of N-arylsulfonyl-1,4-benzoquinone imines makes it possible to conclude that the sulfur atom of the SO₂ group, unlike the divalent sulfur atom, is not capable of transmitting the electronic effects of the substituents from one part of the molecule to the other.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 4, pp. 407–417, July–August, 1985.     

Quenching of singlet oxygen (1deltag) by cobalt complexes with four nitrogen atoms in the equatorial plane

The quenching of singlet oxygen (1deltag) by cobalt(III)-bis-1,2-benzosemiquinone-diiminato complexes (general structure LCo(111)(BQDI)2ClO4) has been studied in different solvents by measuring the singlet oxygen phosphorescence decay in time-resolved experiments. The axial ligand (Ph3As, Ph3Sb, N-methyl-imidazole, pyrrolidine) has practically no influence on the quenching; however, the chlorinated benzosemiquinone-diiminato complex has a markedly lower quenching rate constant. The solvent effects can be fully explained by the difference in viscosity, which supports the assumption that the quenching is diffusion controlled. The reactive radius of the encounter pair has been estimated to be 0.3 nm, shorter than the radius of the complex itself, which points to the fact that singlet oxygen must approach the central cobalt atom at the (partially) open axial position. The significance of these results regarding the quenching of singlet oxygen by vitamin B12 derivatives is discussed.     

Quantitative multi-element mapping of ancient glass using a simple and robust LA-ICP-MS rastering procedure in combination with image analysis

The surface of two glass artefacts in mosaic style, probably fragments of conglomerate glass bowls dating back two millennia, was investigated by laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS). By rastering with the laser beam over a selected area of the surface of the glass artefacts, elemental oxide maps were generated. Quantification of the elemental oxides in the maps was achieved using a so-called sum normalization procedure, summating the elements—54 in total—as their oxides to 100% (w/w), without using an internal standard and applying only one external standard (NIST SRM glass 610). This results in a robust mapping procedure which automatically corrects for drift and defocusing issues. Sum normalization was applied to each pixel in the map separately and required a custom source code to process all the data in the tens of thousands of pixels to generate the elemental oxide concentration maps. The digital element maps generated upon rastering of the two glass artefacts are very compelling and are an excellent entry point to gain detailed insight into their fabrication and provenance using image analysis software for retrieval of localized elemental oxide concentrations and correlations.