Determination of the 14N quadrupole coupling constant of nitroxide spin probes by W-band ELDOR-detected NMR

Nitroxide spin probe electron paramagnetic resonance (EPR) has proven to be a very successful method to probe local polarity and solvent hydrogen bonding properties at the molecular level. The g(xx) and the (14)N hyperfine A(zz) principal values are the EPR parameters of the nitroxide spin probe that are sensitive to these properties and are therefore monitored experimentally. Recently, the (14)N quadrupole interaction of nitroxides has been shown to be also highly sensitive to polarity and H-bonding (A. Savitsky et al., J. Phys. Chem. B 112 (2008) 9079). High-field electron spin echo envelope modulation (ESEEM) was used successfully to determine the P(xx) and P(yy) principal components of the (14)N quadrupole tensor. The P(zz) value was calculated from the traceless character of the quadrupole tensor. We introduce here high-field (W-band, 95 GHz, 3.5 T) electron-electron double resonance (ELDOR)-detected NMR as a method to obtain the (14)N P(zz) value directly, together with A(zz). This is complemented by W-band hyperfine sublevel correlation (HYSCORE) measurements carried out along the g(xx) direction to determine the principal P(xx) and P(yy) components. Through measurements of TEMPOL dissolved in solvents of different polarities, we show that A(zz) increases, while |P(zz)| decreases with polarity, as predicted by Savitsky et al.     

Crystal engineering via C—H⋯F and C—H⋯π interactions in two substituted indoles

The crystal structures of 1‐(4‐fluoro­phenyl)‐2‐phenyl‐4,5,6,7‐tetra­hydro‐1H‐indole, C₂₀H₁₈FN, and 1‐(4‐fluoro­phenyl)‐6,6‐di­methyl‐2‐phenyl‐4,5,6,7‐tetra­hydro‐1H‐indole, C₂₂H₂₂FN, have been determined in order to study the role of `organic fluorine' in crystal engineering. These mol­ecules pack in the crystal structure via different types of molecular motifs utilizing weak C—H⋯F and C—H⋯π interactions.     

Layer architecture in 4‐nitro­phenyl and 4‐chloro­phenyl 3‐nitro­benzene­sulfonate at 100 K

The structures of the title compounds, C₁₂H₈N₂O₇S and C₁₂H₈ClNO₅S, contain weak C—H⋯O interactions creating layers of mol­ecules which, taking the conformation of the mol­ecules into account, are arranged in an ABAB sequence. Both structures can be designated, therefore, as ordered racemates of rotameric species.     

High-pressure studies on electronic and mechanical properties of FeBO3 (B = Ti,Mn, Cr) ceramics – a first-principles study

Using the density functional theory (DFT) method, the electronic and mechanical properties of perovskites FeBO₃ (B = Ti, Mn, Cr) nanostructures were studied in the pressure range of 0–100 GPa. The band structure studies show the change in the band structure upon substitution of different B cation in FeBO₃ perovskite structure. The density of states spectrum gives the perception of change in the electronic properties of FeBO₃ with the substitution of B cation. The bulk, shear and Young's moduli were calculated and an increase in the moduli is noticed. Moreover, the hardness increases under high pressure. The high-pressure studies of FeBO₃ perovskite nanostructures are explored at atomistic level. The findings show that ductility and hardness of FeBO₃ get increased upon an increase in the applied pressure. The substitution of Ti, Mn and Cr on FeBO₃ shows a significant change in the electronic and mechanical properties.     

Synthesis of poly(ethylene glycol) block copolymers by a redox process and their spectral characterization

Novel block copolymers of poly(ethylene glycol) (PEG) with acrylamide (AAm) and methacrylic acid (MAA) were synthesized using a redox system consisting of ceric ions and PEG in aqueous acidic medium. The molecular weight of PEG in the redox system was varied to obtain a series of block copolymers with differing molecular weights of PEG segment. The polymerization proceeded via macroradical generation, which was substantiated by ESR spectroscopy. This macroradical acted as a redox macroinitiator for the block copolymerization of the vinyl monomers. The formation of the block copolymers was confirmed by fractional precipitation technique.     

Dependence of positron capture rate on dislocation density in solid solutions of KCl—KBr

Generally well-annealed alkali halide crystals exhibit three lifetime components. τ₁ arises due to annihilation with anion and τ₂ due to Bloch Ps. The origin of τ₃ is not well understood. Positron lifetime and etching techniques have been performed on solid solutions of KCl_100−xBr_x with mole % of x = 0.0, 15, 30, 37, 50, 70, and 100. The positron lifetime spectra have been analysed to resolve into three components. The positron capture rate K₃ for the longest lived component and the dislocation density shows a non-linear variation with composition. The positron capture rate varies linearly with the dislocation density of the samples. The origin of τ₃ is identified as due to positronium formation in the dislocations present in the samples. The trapping probability per dislocation has been estimated to be 2.2 × 10⁻¹²/s.