X- (X = O, S, Se) Ions in alkali halide lattices through density functional calculations. 2. Interstitial defect models

Density functional theory techniques are used to investigate the defect structure of X- (X = O, S, Se) ions in MZ (M = Na, K, Rb and Z = Cl, Br) alkali halides which exhibit monoclinic-I g-tensor symmetry, using cluster in vacuo, embedded cluster, and periodic embedding schemes. Although a perturbed interstitial defect model was suggested from electron paramagnetic resonance experiments (EPR), the nature of the perturbation is still unknown. An appropriate defect model is developed theoretically by comparing structural and energetical properties of various defect configurations. Further validation is achieved by cross referencing experimental and computed EPR data. On the basis of the computational results, the following defect model is proposed: the X- ion is located interstitially with a charge compensating halide vacancy in its first coordination shell.     

Some recent multi-frequency electron paramagnetic resonance results on systems relevant for dosimetry and dating

Electron Paramagnetic Resonance (EPR) applications like e.g. EPR dosimetry and dating, are usually performed at X-band frequencies because of practical reasons (cost, sample size, etc.). However, it is increasingly recognized that the radiation-induced EPR signals are strongly composite, what might affect dose/age estimates. A few recent examples from both the dosimetry and dating field, illustrating the problems, will be presented. The involved spectra are mainly due to carbonate-derived radicals (CO2-, CO3(3-), etc.). Measurements at higher microwave frequencies are often recommended to improve the insight into the spectra and/or the practical signal quantification. Recent results at Q- and W-band frequencies will show that a multi-frequency approach indeed opens many interesting perspectives in this field but also that each frequency may have specific (dis)advantages depending on the EPR probe and application involved. The discussion will concern carbonate-containing apatite single crystals, shells, modern and fossil tooth enamel.     

First example of an Ugi type reaction on phenylsulfinimine-avermectin B1 derivatives

A short synthesis of 4″-(S)-4″-deoxy-4″-trifluoroacetylamino-4″-alkylcarbamoyl-avermectin B₁4 has been developed through the diastereoselective Ugi reaction to an phenylsulfinimide intermediate.     

Experimental and theoretical study of the metallotropic migrations in cyclooctatetraeneosmiumtricarbonyl

Experimental verification of the mechanism of metallotropic migrations in cyclooctatetraeneosmiumtricarbonyl (3) by means of 2D EXSY NMR spectroscopy confirmed the mechanism of [1,2]-Os shifts with low activation barrier (E(A) = 5.9 +/- 0.2 kcal mol(-1), ln A = 32 +/- 1). Transition-state structure for this rearrangement obtained at the B3LYP/SDD level of theory testifies for the activation energy of 6.5 kcal mol(-1) and supports well the selective [1,2]-Os shifts observed for 3.     

Electron paramagnetic resonance study of S− and S− 3

Abstract

Two new sulphur are reported in monocristalline rubidiumchloride doped with rubidium and sulphur, and irradiated with X-rays at room temperature. By an extensive comparison with other experimental data on chalcogen centres in alkali halides an interstitial RbCl:S^? and a substitutional RbCl:S^? ₃ model is proposed for these paramagnetic defects. Theoretical calculations confirm the S^? ion model for the former.     

Induced Transparency for Gamma Radiation via Nuclear Level Mixing

A significant reduction of absorption of single gamma photons has been observed using the Mössbauer spectra of ⁵⁷Fe in a FeCO₃ crystal. The absorption deficit can be ascribed to partially destructive interference for absorption because of two indistinguishable absorption paths. The necessary coherence is created by means of level mixing produced by a suitable combination of a magnetic dipole and electric quadrupole interaction.