Transesterification/Acylation of secondary alcohols mediated by N-heterocyclic carbene catalysts

N-Heterocyclic carbenes (NHC) are efficient catalysts for transesterification/acylation reactions involving secondary alcohols. The catalytic transformations are carried out employing low catalyst loadings in convenient reaction times at room temperature.     

Volume recovery of polystyrene/silica nanocomposites

Macroscopic volume as well as capacitive dilatometry (CD) measurements have been performed on polystyrene/silica nanocomposites during the course of physical aging below the glass transition temperature (T_g). Our results show that the macroscopic volume recovery during physical aging is not affected by the presence of nanofillers, whereas the CD measurements, delivering also information on the polymer matrix density, show acceleration of the recovery with increasing the silica content. Hence, the main outcome of the present work is that the evolution of macroscopic and matrix densities are markedly different in polymer nanocomposites. We interpret these results invoking an equilibration mechanism based on volume holes diffusion. According to this model, excess free volume migration at the polymer/filler interface only modifies the matrix density, thereby explaining the faster recovery detected by CD measurements in comparison to the macroscopic volume one. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys., 2013     

Spectral dependence of the excitation of the “forbidden” second optical harmonic in an aqueous suspension of purple membranes by femtosecond laser pulses under electronic resonance conditions

The spectral dependence of the efficiency of generation of the forbidden (in the electric-dipole approximation) second harmonic (FSH) and its energy on the energy of the excitation pulses is investigated experimentally in a highly disperse suspension of purple membranes containing bacteriorhodopsin (BR) under excitation by femtosecond laser pulses into the single-photon absorption band of BR. The experimental data for the case of resonance excitation attest to an interference character of the interaction of optical nonlinearities of different orders in the process leading to the formation of the FSH signal. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 4, 251–255 (25 February 1998)     

Out-of-phase coupled periodic waveguides: a “couplonic” approach

We investigate the properties of out-of-phase coupled periodic waveguides (CPW) in the frame of a rigorous coupled-mode theory (CMT), with special emphasis on internal symmetries at the scale of a unit cell. Specifically, we demonstrate an exact mathematical equivalence between the continuously distributed system and its discrete counterpart, made of four-port networks interconnected by segments of transmission lines.     

Effects of structure on exchange interactions in crystalline and amorphous alloys GdxY50−xAg50

Magnetic interactions and effects of dilution with nonmagnetic Y on the magnetic properties of crystalline and amorphous alloys Gd_xY_50−xAg₅₀ (10 ≤ x ≤ 50) have been investigated by measurements of bulk magnetization and susceptibility and by Mössbauer spectroscopy with ¹⁵⁵Gd. The crystalline alloys order antiferromagnetically for all Gd concentrations with a noncollinear arrangement of Gd moments induced by negative biquadratic exchange interactions. In amorphous alloys, ferromagnetic order is found for large Gd concentrations (x ≥ 40). Below the critical concentration x_cr, in the range 30<x_cr<40, properties typical for magnetic cluster glasses are observed. Magnetic hyperfine fields B_hf at ¹⁵⁵Gd nuclei vary with x in opposite directions in amorphous and in crystalline alloys. In crystalline alloys, the variation is due to a positive transferred hyperfine field. In amorphous alloys, a reduction of |B_hf| with decreasing Gd concentration is caused by a reduction of the frozen Gd moments in the cluster glass phase.     

Anomalous Light-Induced Spin-State Switching for Iron(II) Spin-Crossover Molecules in Direct Contact with Metal Surfaces

Light-induced spin-state switching is one of the most attractive properties of spin-crossover materials. In bulk, low-spin (LS) to high-spin (HS) conversion via the light-induced excited spin-state trapping (LIESST) effect may be achieved with a visible light, while the HS-to-LS one (reverse-LIESST) requires an excitation in the near-infrared range. Now, it is shown that those phenomena are strongly modified at the interface with a metal. Indeed, an anomalous spin conversion is presented from HS state to LS state under blue light illumination for Fe^II spin-crossover molecules that are in direct contact with metallic (111) single-crystal surfaces (copper, silver, and gold). To interpret this anomalous spin-state switching, a new mechanism is proposed for the spin conversion based on the light absorption by the substrate that can generate low energy valence photoelectrons promoting molecular vibrational excitations and subsequent spin-state switching at the molecule–metal interface.     

Aminoboronic acids and esters: from synthetic challenges to the discovery of unique classes of enzyme inhibitors

Although boronic acids have attracted considerable interest as versatile intermediates in organic synthesis, their contributions in chemical biology and drug discovery programs have long been underestimated. This situation is changing since the beginning of the 2000s, mainly due to significant advances in modern organoborane chemistry and the recent FDA approval of Velcade?, a boropeptide used for multiple myeloma treatment. There is now a significant renewed interest in the design and synthesis of new boron-containing compounds. Due to their close analogy to their carbon counterparts, aminoboronic acids, alone or incorporated at the C-terminal position of a peptide, represent one of the major classes of organoboranes evaluated as potential drug candidates. This critical review aims to provide an overview of the current state of the art in their synthesis and their most relevant biological properties (156 references).     

Predicting mixture coadsorption in soft porous crystals: experimental and theoretical Study of CO2/CH4 in MIL-53(Al)

We present a synergistic experimental and theoretical study of CO(2)/CH(4) mixture coadsorption in breathing metal-organic framework MIL-53(Al). Mixture adsorption experiments were performed and their results were analyzed by comparing them to predictions made from pure-component adsorption data using the Osmotic Framework Adsorption Solution Theory (OFAST). This analytical model, fully validated for the first time, was then used to predict coadsorption properties as a function of temperature, pressure, and mixture composition. The phase diagrams obtained show a surprising non-monotonic behavior.