Persistent spectral-hole burning in the wide-gap semiconductor SiC doped with vanadium

Persistent spectral-hole burning was performed in the gamma line of V(4+) in the wide-gap semiconductor 6HSiC. Spectral holes burned at 11 K were stable to temperatures of at least 320 K for several days. The hole-burning mechanism consists of two-step photoionization of V(4+) (self-gated spectral-hole burning). The spectral holes could be erased optically, either by pumping of electrons back from stable traps or, presumably, by a charge-transfer transition from the valence band to the V(5+) ions.     

Off-line pyrolysis and compound-specific stable carbon isotope analysis of lignin moieties: a new method for determining the fate of lignin residues in soil

Off-line pyrolysis was used to liberate lignin moieties from dung and soil and, after trimethylsilylation, the delta(13)C values of these derivatives were determined by gas chromatography-combustion-isotope ratio mass spectrometry. Initial delta(13)C values determined for 4-vinylphenol, syringol, 4-vinylguaiacol, 4-acetylsyringol, 4-vinylsyringol, 4-(2-Z-propenyl)syringol, 4-(2-E-propenyl)syringol and 4-(2-propenone)syringol pyrolysis products of the lignin polyphenol structure from C(4) (delta(13)C(bulk) = -12.6%) and C(3) (delta(13)C(bulk) = -30.1 per thousand) dung confirmed the robust and reproducible nature of the off-line preparation technique. C(4) dung was used as a treatment in a randomised field experiment to assess the short-term sequestration of dung carbon in managed grasslands. Since lignin was on average 3.5 per thousand depleted in (13)C compared with bulk dung delta(13)C values, this may have resulted in an under-estimation of dung C incorporation based on bulk delta(13)C values. Therefore, an investigation of the compound-specific delta(13)C values of dung-derived lignin moieties extracted from soils sampled up to 372 days was undertaken. Delta(13)C values between lignin moieties extracted from treated and untreated soils showed that dung-derived lignin was not especially resistant to degradation and suggested that individual moieties of the lignin macromolecule must: (i) move into soil, (ii) be degraded, or (iii) be transformed diagenetically at different rates. This adds to a gathering body of evidence that lignin is not particularly stable in soils, which has considerable significance for the perceived role of different biochemical components in the cycling of C in soils.     

Synthesis and characterization of [Cu(NHC)2]X complexes: catalytic and mechanistic studies of hydrosilylation reactions

The preparation of two series of [Cu(NHC)2]X complexes (NHC=N-heterocyclic carbene, X=PF6 or BF4) in high yields from readily available materials is reported. These complexes have been spectroscopically and structurally characterized. The activity of these cationic bis-NHC complexes in the hydrosilylation of ketones was examined, and both the ligand and the counterion showed a significant influence on the catalytic performance. Moreover, when compared with related [Cu(NHC)]-based systems, these cationic complexes proved to be more efficient under similar reaction conditions. The activation step of [Cu(NHC)2]X precatalysts towards hydrosilylation was investigated by means of 1H NMR spectroscopy. Notably, it was shown that one of the N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) ligands in [Cu(IPr)2]BF4 is displaced by tBuO(-) in the presence of NaOtBu, producing the neutral [Cu(IPr)(OtBu)]. This copper alkoxide is known to be a direct precursor of an NHC-copper hydride, the actual active species in this transformation. Furthermore, reagent loading and counterion effects have been rationalized in light of the species formed during the reaction.     

The Linear KdV Equation with an Interface

The interface problem for the linear Korteweg–de Vries (KdV) equation in one-dimensional piecewise homogeneous domains is examined by constructing an explicit solution in each domain. The location of the interface is known and a number of compatibility conditions at the boundary are imposed. We provide an explicit characterization of sufficient interface conditions for the construction of a solution using Fokas’s Unified Transform Method. The problem and the method considered here extend that of earlier papers to problems with more than two spatial derivatives.     

Oxidation of some cage hydrocarbons by dioxiranes. Nature of the transition structure for the reaction of C-H bonds with dimethyldioxirane: a comparison of B3PW91 density functional theory with experiment

Dimethyl- (DMD) and methyl(trifluoromethyl)-dioxiranes were used for oxyfunctionalization of spiro{1',7-cyclopropan-(E)-2-methylbicyclo[2.2.1]heptane} (), tricyclo[3.2.2.0(2,4)]nonane (), exo-endo-endo- () and exo-exo-exo- () heptacyclo[9.3.1.0(2,10).0(3,8).0(4,6).0(5,9).0(12,14)]pentadecane, yielding tertiary alcohols as the main products. The rate constants for oxidation of by DMD were measured and the Arrhenius parameters determined. The DFT theory (B3LYP and B3PW91) using restricted and unrestricted methods was employed to study the oxidation reaction of the C-H bond of cage hydrocarbons , adamantane, and acetone with DMD. The kinetic isotopic effect calculated using unrestricted methods agreed with experiment. The reaction mechanism in terms of the concerted oxygen insertion vs. the radical part is discussed.     

Diphenyl(dipyrazolyl)methane complexes of Ni: synthesis, structural characterization, and chromotropism of NiBr2 derivatives

The reaction of NiBr2 with the bidentate ligand diphenyl(dipyrazolyl)methane (dpdpm) gives the pentacoordinated complexes [(dpdpm)Ni(mu-Br)Br]2 (1), [(dpdpm)NiBr2(H2O)] (2a), and [(dpdpm)NiBr(H2O)2]Br (2b), or the octahedral complexes [(dpdpm)NiBr(H2O)2(CH3CN)]Br (3), [(dpdpm)2NiBr2] (4), and [(dpdpm)2NiBr(H2O)]Br (5). All of these complexes are paramagnetic, both in the solid state and in solution, and have been characterized by spectroscopic (IR, NMR, and UV-vis-NIR) and X-ray diffraction studies. The unoccupied coordination site in the pentacoordinated compounds allows long-range interactions, in the solid state, between the Ni center and a Ph substituent of the dpdpm ligand. These weak interactions are replaced by Ni-solvent interactions, both in the solid state and in solution, facilitating the interconversion of these compounds under various reaction conditions and leading to interesting solvato-, vapo-, and thermochromic properties. UV-vis-NIR spectroscopy has been used to study these phenomena. Absorption spectra for the room-temperature methanol or acetonitrile solutions of the pentacoordinate or octahedral compounds show three main bands in the region of 350-1000 nm that represent spin-allowed (d-d) transitions from the ground state 3A2g to the excited states 3T2g, 3T1g(3F), and 3T1g(3P). A weak shoulder was also detected on the middle peak in most spectra (700-800 nm), representing the spin-forbidden 3A2g-->1Eg transition. On the other hand, the spectra of high-temperature CH2Cl2 or acetone solutions of all complexes show four main bands at ca. 490, 650-660, 860, and 1000 nm, in addition to a shoulder on the first or second band.     

The effectiveness of three multi-component binding models in describing the binary competitive equilibrium adsorption of two cytochrome b(5) mutants

Competitive adsorption isotherms for two conservative surface charge-neutralizing mutants of cytochrome b(5), E11Q and E44Q, previously measured with competitor concentration held constant over the range of the isotherm, were used to test three widely-used multi-component isotherm models. The extended Langmuir-Freundlich, Langmuir and Jovanovic-Freundlich models each adequately described the weaker infinite dilution adsorption of the E44Q protein in the presence of the strong binding E11Q. The extended Langmuir-Freundlich model generally gave the lowest errors at higher concentrations, and the Jovanovic-Freundlich model gave the best fits when using empirically optimized maximal loading values based on multi-component as well as pure-component isotherm data.     

Highly active/selective and adjustable zirconium polymerization catalysts stabilized by aminopyridinato ligands

This paper describes a substantial enhancement of the aminopyridinato ligand stabilized early transition metal chemistry by introducing the sterically very demanding 2,6-dialkylphenyl substituted aminopyridinato ligands derived from (2,6-diisopropylphenyl)-[6-(2,6-dimethylphenyl)-pyridin-2-yl]-amine (1a-H, ApH) and (2,6-diisopropylphenyl)-[6-(2,4,6-triisopropylphenyl)-pyridin-2-yl]- amine (1b-H, Ap^∗H). The corresponding bis aminopyridinato zirconium dichloro complexes, [Ap₂ZrCl₂] (3a) and [Ap₂^∗ZrCl₂] (3b) and the dimethyl analogues, [Ap₂ZrMe₂] (4a) and [Ap₂^∗ZrMe₂] (4b) (Me = methyl) were synthesized, using standard salt metathesis routes. Single-crystal X-ray diffraction was carried out for the dichloro derivatives. Both zirconium metal centers have a distorted octahedral environment with a cis-orientation of the chloride ligands in 3a and a closer to trans-arrangement in 3b. The dimethyl derivatives are proven to be highly active ethylene polymerization catalysts after activation with [R₂N(Me)H][B(C₆F₅)₄] (R = C₁₆H₃₃-C₁₈H₃₇). During attempted co-polymerizations of α-olefins (propylene) and ethylene high activity and selectivity for ethylene and nearly no co-monomer incorporation was observed. Increasing the steric bulk of the ligand going from (2,6-dimethylphenyl) to (2,4,6-triisopropylphenyl) substituted pyridines, switches the catalyst system from producing long chain α-olefins to polymerization of ethylene in a living fashion. In contrast to the dimethyl complexes only [Ap₂^∗ZrCl₂] in the presence of MAO at elevated temperature gave decent polymerization activity. NMR investigations of the reaction of dichloro complexes with 25 equiv. of MAO or AlMe₃ at room temperature revealed, that [Ap₂ZrCl₂] decomposes under ligand transfer to aluminum and formation of [ApAlMe₂], while [Ap₂^∗ZrCl₂] remains almost unreacted under the same conditions. The aminopyridinato dimethyl aluminum complexes, [ApAlMe₂] (5a) and [Ap^∗AlMe₂] (5b) were synthesized independently and structurally characterized. The aluminum complexes 5a and b show no catalytic activity towards ethylene, when “activated” with[R₂N(Me)H][B(C₆F₅)₄].