Analysis of solvent‐accessible voids and proton‐coupled electron transfer of 2,6‐bis(1H‐imidazol‐2‐yl)pyridine and its hydrochloride

The crystal structures of the solid form of solvated 2,6‐bis(1H‐imidazol‐2‐yl)pyridine (H₂dimpy) trihydrate, C₁₁H₉N₅·3H₂O·[+solvent], I , and its hydrate hydrochloride salt 2‐[6‐(1H‐imidazol‐2‐yl)pyridin‐2‐yl]‐1H‐imidazol‐3‐ium chloride trihydrate, C₁₁H₁₀N₅⁺·Cl^?·3H₂O, II , are reported and analysed in detail, along with potentiometric and spectrophotometric titrations for evaluation of the acid–base equilibria and proton‐coupled electron‐transfer reactions. Compound I crystallizes in the high‐symmetry trigonal space group P3₂21 with an atypical formation of solvent‐accessible voids, as a consequence of the 3₂ screw axis in the crystallographic c‐axis direction, which are probably occupied by uncharacterized disordered solvent molecules. Additionally, the trihydrated chloride salt crystallizes in the conventional monoclinic space group P2₁/c without the formation of solvent‐accessible voids. The acid–base equilibria of H₂dimpy were studied by potentiometric and spectrophotometric titrations, and the results suggest the formation of H₃dimpy⁺ (pK_a1 = 5.40) and H₄dimpy²⁺ (pK_a2 = 3.98), with the electrochemical behaviour of these species showing two consecutive irreversible proton‐coupled electron‐transfer reactions. Density functional theory (DFT) calculations corroborate the interpretation of the experimental results and support the assignment of the electrochemical behaviour.     

具有生物活性的有机硅化合物的研究——Ⅶ.1-芳酰氧基-2,8,9-三氧杂-5-氮杂-1-硅杂三环[3.3.3.0~(1,5)]十一碳烷的研究

本文用新的方法合成了九个1-芳酰氧基-2,8,9-三氧杂-5-氮杂-1-硅杂三环[3.3.3.0~(1,5)]十一碳烷(简称1-芳酰氧基杂氮硅三环)。本方法具有产率高,后处理简便等优点。X-射线晶格衍射确定了该类化合物分子内 N→Si 键键长较短,说明配键强度较大。报道该类化合物的 IR,1~H NMR 及 MS,并讨论了质谱断裂机理。     

Theoretical estimates of the IR spectrum of formamide intercalated into kaolinite

Calculations at PM3 and PBE/6‐31G levels of part of the IR spectrum of the formamide‐kaolinite intercalatation compound based on a 110‐atom cluster of kaolinite with one formamide molecule are reported. Frequencies and intensities for the formamide vibrations and stretchings of four cluster hydroxyls were calculated through partial hessian matrices and polar tensors obtained by numerical differentiation of energy gradients and dipole moment. The formamide molecule attaches to the kaolinite inner surfaces in multiple conformations with its CN bond vector parallel to the surfaces. Hydrogen bonds are formed between the formamide hydrogen atoms (both from NH₂ or CH groups) and the siloxane surface and between the formamide oxygen and nitrogen atoms and the aluminol hydroxyls. The general features of the experimental assignment of the spectrum are confirmed, but the observed splitting of formamide bands is attributed to vibrations from differently attached molecules. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Thio-Pybox and Thio-Phebox complexes of chromium, iron, cobalt and nickel and their application in ethylene and butadiene polymerisation catalysis

A series of bis(thiazolinyl)- and bis(thiazolyl)pyridine Thio-Pybox ligands and their metal complexes of chromium(III), iron(II), cobalt(II) and nickel(II) has been prepared, as well as a nickel(II) complex containing a monoanionic bis(thiazolinyl)phenyl Thio-Phebox ligand. These new metal complexes have been characterised and used as catalysts, in combination with the co-catalyst MAO, for the polymerisation of ethylene and for the polymerisation of butadiene. In the case of ethylene polymerisation, the Thio-Pybox and Thio-Phebox metal complexes have shown relatively low polymerisation activities, much lower compared to the related bis(imino)pyridine complexes of the same metals. In the polymerisation of butadiene, several Thio-Pybox cobalt(II) complexes show very high activities, significantly higher than the other metal complexes with the same ligand. It is the metal, rather than the ligand, that appears to have the most profound effect on the catalytic activity in butadiene polymerisation, unlike in the polymerisation of ethylene, where bis(imino)pyridine ligands provide highly active catalysts for a range of 1st row transition metals.     

Anomalous magnetic behavior of superconducting thin films in small magnetic fields [4] close to the transition temperature

Using a specially designed SQUID magnetometer we measured the temperature dependence of the critical current density in a ring patterned thin film for magnetic fields parallel to the c-axis. In addition, the temporal relaxation of the remanent state as prepared by field cooling in an external field of 100 Oe at different temperatures is determined. The j c ( T ) data show a field-dependent anomalous kink close to T_c pointing to reduced dissipation with increasing temperature allowing to construct a corresponding H-T borderline. A similar behavior is observed for the normalized relaxation rate S ( T ) as extracted from the temporal behavior of the remanent state, which, at low temperatures, exhibits the expected increase for increasing T-values, while an anomalous decrease of S ( T ) is found for temperatures above 85 K. While the low-T regime is attributed to creep of 2D pinned single vortex lines, the high-T behavior is suggested to be dominated by collective motion with a more sluggish dynamics. This change in dynamics is also reflected by the activation barriers for flux creep U ( j ), which show a corresponding crossover in μ from 0.06 to 0.99. An additional scaling analysis of the E-j characteristics for according to vortex glass theory reveals quasi-2D collective creep behavior with . Received: 8 April 1998 / Revised: 15 July 1998 / Accepted: 2 September 1998     

Ultra-high-performance liquid chromatography using a fused-core particle column for fast analysis of propolis phenolic compounds

Propolis is a bee product with a complex chemical composition formed by several species from different geographical origins. The complex propolis composition requires an accurate and reproducible characterization of samples to standardize the quality of the material sold to consumers. This work developed an ultra-high-performance liquid chromatography with a photodiode array detector method to analyze propolis phenolic compounds based on the two key propolis biomarkers, Artepillin C and p-Coumaric acid. This choice was made due to the complexity of the sample with the presence of several compounds. The optimized method was hyphenated with mass spectrometry detection allowing the detection of 23 different compounds. A step-by-step strategy was used to optimize temperature, flow rate, mobile phase composition, and re-equilibration time. Reverse-phase separation was achieved with a C18 fused-core column packed with the commercially available smallest particles (1.3 nm). Using a fused-core column with ultra-high-performance liquid chromatography allows highly efficient, sensitive, accurate, and reproducible determination of compounds extracted from propolis with an outstanding sample throughput and resolution. Optimized conditions permitted the separation of the compounds in 5.50 min with a total analysis time (sample-to-sample) of 6.50 min.     

Tetra-tert-butyl-s-indacene is a Bond-Localized C2h Structure and a Challenge for Computational Chemistry

Whether tetra-tert-butyl-s-indacene is a symmetric D_2h structure or a bond-alternating C_2h structure remains a standing puzzle. Close agreement between experimental and computed proton chemical shifts based on minima structures optimized at the M06-2X, ωB97X-D, and M11 levels confirm a bond-localized C_2h symmetry, which is consistent with the expected strong antiaromaticity of TtB-s-indacene.