Counting nucleon pairs

A simple method is used to count average numbers of different kinds of nucleon pairs within the Sp(4) seniority-like model which describesT=1 pairing modes of protons and neutrons. These numbers as well as the total strengths of pair-transfer processes clearly indicate that pn configurations are preferred in theN _p=N _n odd-odd systems. Presented at Workshop on calculation of double-beta-decay matrix elements (MEDEX’97), Prague, May 27–31, 1997.     

Common Tangents to Spheres in ℝ3

We prove that four spheres in ℝ³ have infinitely many real common tangents if and only if they have aligned centers and at least one real common tangent.     

Pt(IV)-catalyzed cyclization of arene-alkyne substrates via C-H bond functionalization

We herein report that PtCl₄ has proven to be a hydroarylation catalyst with an efficiency and substrate scope superior to previously known methods. This catalyst demonstrated consistent performance with arene-yne substrates of diverse structural features, including propargyl ethers, propargylamines, and alkynoate esters, providing good to excellent yields of the 6-endo products (chromenes, dihydroquinolines, and coumarins). In contrast, Pt(II), Pd(II), and Ga(III) salts were shown to be sensitive to the substitution on the alkyne moiety. PtCl₄ is compatible with both terminal and disubstituted alkynes, as well as with various functionalities on the arene ring, including methyl, methoxyl, hydroxyl, protected amine, and halide.     

Site-specific phenylation of pyridine catalyzed by phosphido-bridged ruthenium dimer complexes: a prototype for C-H arylation of electron-deficient heteroarenes

Direct and selective catalytic arylation of alpha-C-H bond in pyridine with iodobenzene was achieved in up to 70% yield. Phosphido-bridged bisruthenium complexes 6a and 6b arising from Ru3(CO)12 and PPh3 were identified as active catalysts. The formation of complexes 6a and 6b was investigated, a sequence of C-H and C-P bond cleavage, cluster fragmentation, and disproportionation was established, and the intermediate ruthenium complexes lying on this pathway were isolated and fully characterized.     

Selective C-arylation of free (NH)-heteroarenes via catalytic C-H bond functionalization

A new system for palladium-catalyzed arylation of a broad spectrum of free (NH)-heteroarenes has been developed (indole, pyrrole, pyrazole, 2-phenylimidazole, imidazole, benzimidazole, and purine). Remarkable selectivity has been achieved in the presence of MgO base, providing single C-arylation products, while no N-arylation and no bis-arylation products have been detected. In the case of free imidazole, exclusive C-4 arylation may be switched to exclusive 2-arylation by the addition of CuI to the Pd/Ph3P/MgO system. When free aryl-(NH)-azoles are desired, direct arylation eliminates three steps in comparison to standard methods, including N-protection, stoichiometric metalation or halogenation, and N-deprotection.     

A theoretical study of the large Hg-Hg spin-spin coupling constants in Hg(2)(2+), Hg(3)(2+), and Hg(2)(2+)-crown ether complexes

Nuclear spin-spin coupling constants (1)J(Hg-Hg) in the systems Hg(2)(2+) and Hg(3)(2+) represent the largest coupling constants so far observed in NMR experiments. We have performed a computational study on these ions, on Hg(2)(2+) complexes with 18-crown-6 and 15-crown-5, and on Hg(3)(2+) with solvent molecules and counterions. The results obtained with our recently developed program for the density functional computation of heavy nucleus spin-spin coupling constants are in good agreement with experiments. The data reveal that the bare ions Hg(2)(2+) and Hg(3)(2+) would afford much larger coupling constants than those experimentally observed, with an upper limit of approximately 0.9 MHz for Hg(2)(2+). This limit is much larger than that previously estimated by Hückel theory. It is demonstrated that in solution or due to complexation the experimentally determined values are much smaller than the free ion's coupling constants. With the help of intuitive MO arguments, it is illustrated how the environment strongly reduces the coupling constants in Hg(2)(2+) and Hg(3)(2+). The two-bond coupling constant (2)J(Hg-Hg) in Hg(3)(2+) is also examined.     

Selective and catalytic arylation of N-phenylpyrrolidine: sp3 C-H bond functionalization in the absence of a directing group

We herein describe our studies on arylation of N-phenylpyrrolidine, which led to the development of a new transformation for the direct and selective arylation of sp3 C-H bonds in the absence of a directing group. In this method, Ru(H)2(CO)(PCy3)3 4 was used as the catalyst, and preliminary mechanistic studies suggested that Ru(Ph)(I)(CO)(PCy3)2 5 is the key intermediate of the catalytic cycle. A large kinetic isotope effect (kH/kD = 5.4) was observed, which supports the proposal that C-H bond metalation is the slow step. Preliminary examination of the substrate scope showed that in addition to N-phenylpyrrolidine, N-methyl- and N-benzylpyrrolidine, as well as N-benzoylpyrrolidine, were arylated under the reaction conditions.     

Multivariate Time Series Imputation: An Approach Based on Dictionary Learning

The problem addressed by dictionary learning (DL) is the representation of data as a sparse linear combination of columns of a matrix called dictionary. Both the dictionary and the sparse representations are learned from the data. We show how DL can be employed in the imputation of multivariate time series. We use a structured dictionary, which is comprised of one block for each time series and a common block for all the time series. The size of each block and the sparsity level of the representation are selected by using information theoretic criteria. The objective function used in learning is designed to minimize either the sum of the squared errors or the sum of the magnitudes of the errors. We propose dimensionality reduction techniques for the case of high-dimensional time series. For demonstrating how the new algorithms can be used in practical applications, we conduct a large set of experiments on five real-life data sets. The missing data (MD) are simulated according to various scenarios where both the percentage of MD and the length of the sequences of MD are considered. This allows us to identify the situations in which the novel DL-based methods are superior to the existing methods.     

HPLC column-switching technique for sample preparation and fluorescence determination of propranolol in urine using fused-core columns in both dimensions

A new and fast high-performance liquid chromatography (HPLC) column-switching method using fused-core columns in both dimensions for sample preconcentration and determination of propranolol in human urine has been developed. On-line sample pretreatment and propranolol preconcentration were performed on an Ascentis Express RP-C-18 guard column (5?×?4.6 mm), particle size, 2.7 μm, with mobile phase acetonitrile/water (5:95, v/v) at a flow rate of 2.0 mL min^?1 and at a temperature of 50 °C. Valve switch from pretreatment column to analytical column was set at 4.0 min in a back-flush mode. Separation of propranolol from other endogenous urine compounds was achieved on the fused-core column Ascentis Express RP-Amide (100?×?4.6 mm), particle size, 2.7 μm, with mobile phase acetonitrile/water solution of 0.5 % triethylamine, pH adjusted to 4.5 by means of glacial acetic acid (25:75, v/v), at a flow rate of 1.0 mL min^?1 and at a temperature of 50 °C. Fluorescence excitation/emission detection wavelengths were set at 229/338 nm. A volume of 1,500 μL of filtered urine sample solution was injected directly into the column-switching HPLC system. The total analysis time including on-line sample pretreatment was less than 8 min. The experimentally determined limit of detection of the method was found to be 0.015 ng mL^?1.