2,2,2‐Trifluoroacetophenone as an Organocatalyst for the Oxidation of Tertiary Amines and Azines to N‐Oxides

A cheap, mild and environmentally friendly oxidation of tertiary amines and azines to the corresponding N‐oxides is reported by using polyfluoroalkyl ketones as efficient organocatalysts. 2,2,2‐Trifluoroacetophenone was identified as the optimum catalyst for the oxidation of aliphatic tertiary amines and azines. This oxidation is chemoselective and proceeds in high‐to‐quantitative yields utilizing 10 mol % of the catalyst and H₂O₂ as the oxidant.     

Selective Enzymatic Oxidation of Silanes to Silanols

Compared to the biological world's rich chemistry for functionalizing carbon, enzymatic transformations of the heavier homologue silicon are rare. We report that a wild‐type cytochrome P450 monooxygenase (P450_BM3 from Bacillus megaterium, CYP102A1) has promiscuous activity for oxidation of hydrosilanes to give silanols. Directed evolution was applied to enhance this non‐native activity and create a highly efficient catalyst for selective silane oxidation under mild conditions with oxygen as the terminal oxidant. The evolved enzyme leaves C?H bonds present in the silane substrates untouched, and this biotransformation does not lead to disiloxane formation, a common problem in silanol syntheses. Computational studies reveal that catalysis proceeds through hydrogen atom abstraction followed by radical rebound, as observed in the native C?H hydroxylation mechanism of the P450 enzyme. This enzymatic silane oxidation extends nature's impressive catalytic repertoire.     

On the mathematical formulation of the problem of reassembling fragmented objects: two new theorems

In this paper two new theorems are proved in association with the problem of matching three dimensional solid bodies. Rigorous mathematical criteria are given in order to test if two such bodies actually match in a certain position. Since this problem finds important application to the actual problem of reassembling fragmented objects e.g. archaeological, special care is taken to account for small gaps between matching fragments and fuzziness of the matching parameters.     

Sample restriction using radiofrequency field selective pulses in high-resolution solid-state NMR

In this article a method is suggested for restricting a sample (spatial localization) by preparing the magnetization with a phase-modulated radiofrequency pulse which inverts magnetization only over a very narrow range of radiofrequency field strengths. This is the most efficient method, in terms of sensitivity, of restricting the sample to improve rf homogeneity. The method is demonstrated by using it to improve the resolution obtained in a homonuclear dipolar decoupling experiment.     

Two-dimensional high-resolution NMR spectra in matched B(0) and B(1) field gradients

In a recent publication we presented a method to obtain highly resolved NMR spectra in the presence of an inhomogeneous B(0) field with the help of a matched RF gradient. If RF gradient pulses are combined with "ideal" 90 degrees pulses to form inhomogeneous z rotation pulses, the line broadening caused by the B(0) gradient can be refocused, while the full chemical shift information is maintained. This approach is of potential use for NMR spectroscopy in an inhomogeneous magnetic field produced by an "ex-situ" surface spectrometer. In this contribution, we extend this method toward two-dimensional spectroscopy with high resolution in one or both dimensions. Line narrowing in the indirect dimension can be achieved by two types of nutation echoes, thus leading to depth-sensitive NMR spectra with full chemical shift information. If the nutation echo in the indirect dimension is combined with a stroboscopic acquisition using inhomogeneous z-rotation pulses, highly resolved two-dimensional correlation spectra can be obtained in matched field gradients. Finally, we demonstrate that an INEPT coherence transfer from proton to carbon spins is possible in inhomogeneous B(0) fields. Thus, it is possible to obtain one-dimensional (13)C NMR spectra with increased sensitivity and two-dimensional HETCOR spectra in the presence of B(0) gradients of 0.4 mT/cm. These schemes may be of some value for ex-situ NMR analysis of materials and biological systems.     

Entropy and enthalpy of formation and migration of lattice defects in Na(I)

The self-diffusion coefficients of Na⁺ and I^?, as well as the ionic conductivity, are measured in Na(I) single crystals, pure and doped with Ca²⁺ ions. The computer analysis of the whole set of data shows that the atom transport in Na(I) is ensured essentially by the free vacancies with a very small contribution (～4%) of the vacancy pairs. The thermodynamic parameters of vacancy formation and migration are derived by taking the long range interactions into account (activity coefficients on the concentrations and relaxation factors on the mobilities). The respective entropies of Schottky defect formation, cation migration and anion migration are equal to 7.64, 3.23 and 2.91 k while the corresponding enthalpies are equal to 2.00, 0.58 and 0.77 eV.     

Tailoring the flow of soft glasses by soft additives

We examine the vitrification and melting of asymmetric star polymer mixtures by combining rheological measurements with mode coupling theory. We identify two types of glassy states, a single glass, in which the small component is fluid in the glassy matrix of the big one, and a double glass, in which both components are vitrified. Addition of small-star polymers leads to melting of both glasses, and the melting curve has a nonmonotonic dependence on the star-star size ratio. The phenomenon opens new ways for externally steering the rheological behavior of soft matter systems.     

The threshold for random -SAT is

Let be a random -CNF formula formed by selecting uniformly and independently out of all possible -clauses on variables. It is well known that if , then is unsatisfiable with probability that tends to 1 as . We prove that if , where , then is satisfiable with probability that tends to 1 as .

Our technique, in fact, yields an explicit lower bound for the random -SAT threshold for every . For our bounds improve all previously known such bounds.

     

PMMA/organomodified montmorillonite nanocomposites prepared by in situ bulk polymerization

Kinetics of the in situ bulk polymerization of methyl methacrylate in the presence of organomodified montmorillonite (MMT) was investigated using differential scanning calorimetry (DSC) and gravimetrically. Different amount and types of MMT under the trade names Cloisite were employed. Using DSC, the amount of heat released versus time, under isothermal conditions, was recorded, and eventually, the time evolution of polymerization rate and monomer conversion was calculated. Results on the variation of monomer conversion with reaction time were in good agreement to corresponding from the gravimetric measurements. The nanocomposites prepared were characterized with WAXD, TEM and FTIR, and their glass transition temperature, T _g, was measured with DSC. Depending on the added amount of nano-MMT, either exfoliated or intercalated structures were obtained. An enhancement of the polymerization rate with the presence of the nanoparticles was observed especially in the gel effect region. This was accompanied by a higher T _g and average molecular weight, as measured by GPC, of all nanocomposites compared to neat PMMA.