Mechanism of ketone hydrosilylation using NHC–Cu(I) catalysts: a computational study

The porous MIL-47 material shows a selective adsorption behavior for para-, ortho-, and meta-isomers of xylenes, making the material a serious candidate for separation applications. The origin of the selectivity lies in the differences in interactions (energetic) and confining (entropic). This paper investigates the xylene–framework interactions and the xylene–xylene interactions with quantum mechanical calculations, using a dispersion-corrected density functional and periodic boundary conditions to describe the crystal. First, the strength and geometrical characteristics of the optimal xylene–xylene interactions are quantified by studying the pure and mixed pairs in gas phase. An extended set of initial structures is created and optimized to sample as many relative orientations and distances as possible. Next, the pairs are brought in the pores of MIL-47. The interaction with the terephthalic linkers and other xylenes increases the stacking energy in gas phase (?31.7?kJ/mol per pair) by roughly a factor four in the fully loaded state (?58.3?kJ/mol per xylene). Our decomposition of the adsorption energy shows various trends in the contributing xylene–xylene interactions. The absence of a significant difference in energetics between the isomers indicates that entropic effects must be mainly responsible for the separation behavior.     

Combined cation-π and anion-π interactions for zwitterion recognition

Brothers and enemies: Anion-π and cation-π interactions act in a synergistic way when gathered in the molecular cavity of a hemicryptophane host, affording an efficient contribution (-170?kJ?mol(-1)) in zwitterion recognition. NMR titration experiments and calculations reveal the positioning of the guest in the cavity of the heteroditopic receptor. This study emphasizes the importance of anion-π bonds in host-guest chemistry.     

Measurement of Aluminum–Carbon Distances Using S‐RESPDOR NMR Experiments

It is demonstrated that reliable aluminum–carbon distances can be measured in samples with ¹³C natural abundance by NMR spectroscopy. Overcoupled resonators, with only one radio‐frequency synthesizer and one amplifier, are used to irradiate in the same pulse sequence ²⁷Al and ¹³C nuclei, which differ by only 3.6 % in Larmor frequencies. The combination of ²⁷Al saturation pulse with heteronuclear dipolar recoupling yields dipolar dephasing of the ¹³C signal, which only depends on the Al? C distance and the efficiency of the saturation pulse. Therefore, reliable distances can be obtained by rapid fitting of experimental data to an analytical expression. It is demonstrated that with natural isotopic abundance this approach allows recovery of Al? C distances of 216 pm for the covalent bond in lithium tetraalkyl aluminates, commonly used as a co‐catalyst in olefin polymerization processes, and which range from 274 to 381 pm for the three carbon atoms in aluminum lactate. The accuracy of the measured internuclear distances is carefully estimated.     

Reversible formation of a PdC(x) phase in Pd nanoparticles upon CO and O2 exposure

The structure and chemical composition of Pd nanoparticles exposed to pure CO and mixtures of CO and O(2) at elevated temperatures have been studied in situ by a combination of X-ray Diffraction and X-ray Photoelectron Spectroscopy in pressures ranging from ultra high vacuum to 10 mbar and from room temperature to a few hundred degrees celsius. Our investigation shows that under CO exposure, above a certain temperature, carbon dissolves into the Pd particles forming a carbide phase. Upon exposure to CO and O(2) mixtures, the carbide phase forms and disappears reversibly, switching at the stoichiometric ratio for CO oxidation. This finding opens new scenarios for the understanding of catalytic oxidation of C-based molecules.     

The first-principles treatment of the electron-correlation and spin-orbital effects in uranium mononitride nuclear fuels

The DFT+U calculations were employed in a detailed study of the strong electron correlation effects in a promising nuclear fuel-uranium mononitride (UN). A simple method for solving the multiple minima problem in DFT+U simulations and insure obtaining the correct ground state is suggested and applied. The crucial role of spin-orbit interactions in reproduction of the U atom total magnetic moment is demonstrated. Basic material properties (the lattice constants, the spin- and total magnetic moments on U atoms, the magnetic ordering, and the density of states) were calculated varying the Hubbard U-parameter. By varying the tetragonal unit cell distortion, the meta-stable states have been carefully identified and analyzed. The difference in the magnetic and structural properties obtained for the meta-stable and ground states is discussed. The optimal effective Hubbard parameter U(eff) = 1.85 eV reproduces correctly the UN anti-ferromagnetic ordering, and only slightly overestimates the experimental total magnetic moment of the U atom and the unit cell volume.     

Ultrafast fluorescence studies of dye sensitized solar cells

Time-resolved fluorescence spectra from the RuN719 dye exhibit very short lifetimes (<30 fs) in solutions, on non-injecting substrates and on injecting ones. This reveals <10 fs intramolecular energy redistribution competing with the injection. We conclude that injection proceeds on a sub-10 fs time scale from non-thermalized levels of the dye.     

17 e- rhenium dicarbonyl CO-releasing molecules on a cobalamin scaffold for biological application

Cyanocobalamin (B(12)) offers a biocompatible scaffold for CO-releasing 17-electron dicarbonyl complexes based on the cis-trans-[Re(II)(CO)(2)Br(2)](0) core. A Co-C≡N-Re conjugate is produced in a short time and high yield from the reaction of [Et(4)N](2)[Re(II)Br(4)(CO)(2)] (ReCORM-1) with B(12). The B(12)-Re(II)(CO)(2) derivatives show a number of features which make them pharmaceutically acceptable CO-releasing molecules (CORMs). These cobalamin conjugates are characterized by an improved stability in aqueous aerobic media over the metal complex alone, and afford effective therapeutic protection against ischemia-reperfusion injury in cultured cardiomyocytes. The non-toxicity (at μM concentrations) of the resulting metal fragment after CO release is attributed to the oxidation of the metal and formation in solution of the ReO(4)(-) anion, which is among the least toxic of all of the rare inorganic compounds. Theoretical and experimental studies aimed at elucidating the aqueous chemistry of ReCORM-1 are also described.     

A slowly relaxing rigid biradical for efficient dynamic nuclear polarization surface-enhanced NMR spectroscopy: expeditious characterization of functional group manipulation in hybrid materials

A new nitroxide-based biradical having a long electron spin-lattice relaxation time (T(1e)) has been developed as an exogenous polarization source for DNP solid-state NMR experiments. The performance of this new biradical is demonstrated on hybrid silica-based mesostructured materials impregnated with 1,1,2,2-tetrachloroethane radical containing solutions, as well as in frozen bulk solutions, yielding DNP enhancement factors (ε) of over 100 at a magnetic field of 9.4 T and sample temperatures of ~100 K. The effects of radical concentration on the DNP enhancement factors and on the overall sensitivity enhancements (Σ(?)) are reported. The relatively high DNP efficiency of the biradical is attributed to an increased T(1e), which enables more effective saturation of the electron resonance. This new biradical is shown to outperform the polarizing agents used so far in DNP surface-enhanced NMR spectroscopy of materials, yielding a 113-fold increase in overall sensitivity for silicon-29 CPMAS spectra as compared to conventional NMR experiments at room temperature. This results in a reduction in experimental times by a factor >12,700, making the acquisition of (13)C and (15)N one- and two-dimensional NMR spectra at natural isotopic abundance rapid (hours). It has been used here to monitor a series of chemical reactions carried out on the surface functionalities of a hybrid organic-silica material.