Interfacial interactions between urea formaldehyde and cellulose nanofibrils (CNFs) of varying chemical composition and their impact on particle board (PB) manufacture

Cellulose - Wood-based panels are commonly used as building materials for interior and exterior purposes. Their production and utilization have increased in recent decades due to the useful...     

Discrete Nonholonomic Lagrangian Systems on Lie Groupoids

This paper studies the construction of geometric integrators for nonholonomic systems. We develop a formalism for nonholonomic discrete Euler–Lagrange equations in a setting that permits to deduce geometric integrators for continuous nonholonomic systems (reduced or not). The formalism is given in terms of Lie groupoids, specifying a discrete Lagrangian and a constraint submanifold on it. Additionally, it is necessary to fix a vector subbundle of the Lie algebroid associated to the Lie groupoid. We also discuss the existence of nonholonomic evolution operators in terms of the discrete nonholonomic Legendre transformations and in terms of adequate decompositions of the prolongation of the Lie groupoid. The characterization of the reversibility of the evolution operator and the discrete nonholonomic momentum equation are also considered. Finally, we illustrate with several classical examples the wide range of application of the theory (the discrete nonholonomic constrained particle, the Suslov system, the Chaplygin sleigh, the Veselova system, the rolling ball on a rotating table and the two wheeled planar mobile robot). This work was partially supported by MEC (Spain) Grants MTM 2006-03322, MTM 2007-62478, MTM 2006-10531, project “Ingenio Mathematica” (i-MATH) No. CSD 2006-00032 (Consolider-Ingenio 2010) and S-0505/ESP/0158 of the CAM.     

Tracking the Light-Induced Excited-State Dynamics and Structural Configurations of an Extraordinarily Long-Lived Metastable State at Room Temperature

Time-resolved X-ray (Tr-XAS) and optical transient absorption (OTA) spectroscopy on the pico-microsecond timescale coupled with density functional theory calculations are applied to study the light-induced spin crossover processes of a Fe-based macrocyclic complex in solution. Tr-XAS analysis after light illumination shows the formation of a seven-coordinated high-spin quintet metastable state, which relaxes to a six-coordinated high-spin configuration before decaying to the ground state. Kinetic analysis of the macrocyclic complex reveals an unprecedented long-lived decay lifetime of approximately 42.6 μs. Comparative studies with a non-macrocyclic counterpart illustrate a significantly shortened approximately 568-fold decay lifetime of about 75 ns, and highlight the importance of the ligand arrangement in stabilizing the reactivity of the excited state. Lastly, OTA analysis shows the seven-coordinated high-spin state to be formed within approximately 6.2 ps. These findings provide a complete understanding of the spin crossover reaction and relaxation pathways of the macrocyclic complex, and reveal the importance of a flexible coordination environment for their rational design.     

Peroxidase activity of new mixed‐valence cobalt complexes with ligands derived from pyridoxal

New mixed‐valence cobalt complexes with ligands derived from pyridoxal were synthesized and characterized, and their application as mimetics of the peroxidase enzyme was investigated. Single‐crystal X‐ray diffraction was used to analyze all complex structures in the solid state and their electrochemical behavior was investigated. A reactivity pattern was observed in the complex synthesis regarding the cobalt compounds from which analogous zwitterionic derivatives were obtained. The importance of these compounds lies in understanding their behavior in an oxidizing environment and evaluating whether they can activate hydrogen peroxide to oxidize phenolic compounds. In nature, enzymes called peroxidases, which efficiently oxidize phenolic compounds, trigger many reactions involving the activation of hydrogen peroxide to oxidize organic substrates. However, these enzymes present several disadvantages, including denaturation and elevated costs. Therefore, these limitations can be overcome by expanding research into the study of synthetic catalysts for the oxidation of phenolic compounds using hydrogen peroxide, which is a highly relevant field of bioinorganic chemistry.     

New Pterosin-Precursor Hydrindacenes

Two new structures of hydrindacenes are prepared with good yield and free of isomers. These structures could be the precursors of several pterosins.     

Importance of Outer‐Sphere and Aggregation Phenomena in the Relaxation Properties of Phosphonated Gadolinium Complexes with Potential Applications as MRI Contrast Agents

A series composed of a tetra‐, a tris‐ and a bisphosphonated ligand based on a pyridine scaffold ( L⁴ , L³ and L² , respectively) was studied within the frame of lanthanide (Ln) coordination. The stability constants of the complexes formed with lanthanide cations (Ln=La, Nd, Eu, Gd, Tb, Er and Lu) were determined by potentiometry in aqueous solutions (25.0 °C, 0.1 M NaClO₄), showing that the tetraphosphonated complexes are among the most stable Ln^III complexes reported in the literature. The complexation of L⁴ was further studied by different titration experiments using mass spectrometry and various spectroscopic techniques including UV/Vis absorption, and steady state and time‐resolved luminescence (Ln=Eu and Tb). Titration experiments confirmed the formation of highly stable [Ln L⁴ ] complexes. ³¹P NMR experiments of the Lu L⁴ complex revealed an intramolecular interconversion process which was studied at different temperatures and was rationalized by DFT modelling. The relaxivity properties of the Gd^III complexes were studied by recording their ¹H NMRD profiles at various temperatures, by temperature dependent ¹⁷O NMR experiments (Gd L⁴ ) and by pH dependent relaxivity measurements at 0.47 T (Gd L³ and Gd L² ). In addition to the high relaxivity values observed for all complexes, the results showed an important second‐sphere contribution to relaxivity and pH dependent variations associated with the formation of aggregates for Gd L² and Gd L³ . Finally, intravenous injection of Gd L⁴ to a mouse was followed by dynamic MRI imaging at 1.5 T, which showed that the complex can be immediately found in the blood stream and rapidly eliminated through the liver and in large part through the kidneys.     

Insights into the mechanism of the site-selective sequential palladium-catalyzed cross-coupling reactions of dibromothiophenes/dibromothiazoles and arylboronic acids. Synthesis of PPARbeta/delta agonists

A reactivity study, aided by NMR spectroscopy, allowed a mechanistic rationale to be postulated for the palladium-catalyzed regioselective coupling of arylboronic acid (and arylstannane where feasible) at the position next to the sulfur atom in functionalized dibromothiophenes and dibromothiazoles. The analysis of the NMR spectra (using 19F from the boronic acid CF3 group and 31P from the phosphine of the catalyst as probes) of the entire reaction starting from the dibromoheterocycles allowed the qualitative proposal that the transmetalation is the rate-limiting step for both sequential substitution processes. The extremely facile oxidative addition at the C-Br bond next to the sulfur atom of the heterocycle instead determines the positional selectivity. An additional Stille reaction then replaced the second halogen, providing the trisubstituted heterocyclic scaffolds of PPAR ligands, which displayed PPARbeta/delta agonist activity, as revealed by reporter assays in living cells.