Strategies for the synthesis of bi- and triarylic materials starting from commercially available phenols

A series of arylstannanes have been synthesized, through an S_RN1 mechanism, in good to excellent yields (74%-99%) by the photostimulated reaction of trimethyl stannyl ion with substrates supporting different nucleofugal groups. The arylstannanes thus obtained were suitable intermediates for Stille cross-coupling reactions leading to asymmetric bi- and triaryl compounds in acceptable global yields. An attractive feature of this route is that simple commercially available benzenediols, chloro- and methoxy phenols might be useful starting substrates, leading the latter to higher global yields of products in fewer steps. The strategies proposed open a broad synthetic tool.     

Synthesis of tetracyclic hydroxyquinones by cycloaddition reactions with dienols.

Cycloaddition of naphthazarin and derivatives to dienols, generated thermnally from benzocyclobuenol or by irradiation of 2-methylbenzaldehyde or 2-methylacetophenone, leads to tetracyclic systems related to those present in anthracycline and tetracycline antibiotics.     

Precipitation chromatography fractionation: The effect of the ratio support weight/sample size

The effect of the gravimetric ratio, packing beads/polymer (r), in the chromatographic fractionation of polystyrene has been studied. Anionic polystyrene of bimodal molecular weight distribution was fractionated in three experiments with r values of 25, 50 and 130; a value of about 50 provided the most efficient separation. A GPC self-consistency analysis of this fractionation has been done with very satisfactory results. On the other hand, GPC analysis could not show any significant broadening of the molecular weight distributions of the fractions in going from r values of 130 down to 25.     

A geometric representation of spectral and temporal vowel features: quantification of vowel overlap in three linguistic varieties

A geometrical method for computing overlap between vowel distributions, the spectral overlap assessment metric (SOAM), is applied to an investigation of spectral (F1, F2) and temporal (duration) relations in three different types of systems: one claimed to exhibit primary quality (American English), one primary quantity (Jamaican Creole), and one about which no claims have been made (Jamaican English). Shapes, orientations, and proximities of pairs of vowel distributions involved in phonological oppositions are modeled using best-fit ellipses (in F1 x F2 space) and ellipsoids (F1 x F2 x duration). Overlap fractions computed for each pair suggest that spectral and temporal features interact differently in the three varieties and oppositions. Under a two-dimensional analysis, two of three American English oppositions show no overlap; the third shows partial overlap. All Jamaican Creole oppositions exhibit complete overlap when F1 and F2 alone are modeled, but no or partial overlap with incorporation of a factor for duration. Jamaican English three-dimensional overlap fractions resemble two-dimensional results for American English. A multidimensional analysis tool such as SOAM appears to provide a more objective basis for simultaneously investigating spectral and temporal relations within vowel systems. Normalization methods and the SOAM method are described in an extended appendix.     

Sub-nanometer Au monolayer-protected clusters exhibiting molecule-like electronic behavior: quantitative high-angle annular dark-field scanning transmission electron microscopy and electrochemical characterization of clusters with precise atomic stoichiometry

The synthesis and characterization of the clusters Au13[PPh3]4[S(CH2)11CH3]2Cl2 (1) and Au13[PPh3]4[S(CH2)11CH3]4 (2) are described. These mixed-ligand, sub-nanometer clusters, prepared via exchange of dodecanethiol onto phosphine-halide gold clusters, show enhanced stability relative to the parent. The characterization of these clusters features the precise determination of the number of gold atoms in the cluster cores using high-angle annular dark-field scanning transmission electron microscopy, allowing the assignment of 13 gold atoms (+/-3 atoms) to the composition of both cluster molecules. Electrochemical and optical measurements reveal discrete molecular orbital levels and apparent energy gaps of 1.6-1.7 eV for the two cluster molecules. The electrochemical measurements further indicate that the Au13[PPh3]4[S(CH2)11CH3]2Cl2 cluster undergoes an overall two-electron reduction. The electrochemical and spectroscopic properties of the two Au13 cluster molecules are compared with those of a secondary synthetic product, which proved to be larger Au thiolate-derivatized monolayer-protected clusters with an average core of Au180. The latter shows behavior fully consistent with the adoption of metallic-like properties.     

Chemo-differentiating MCRs based on alpha-ketoesters and terminal alkynoates. A homoaldol-based ABB' system

A novel ABB' 3 component reaction (3-CR) system based on the organocatalyzed homoaldolic condensation of alpha-ketoesters in the presence of terminal conjugated alkynoates is described.     

Three-center-two-electron and four-center-four-electron bonds. A study by electron charge density over the structure of methonium cations

We study the electronic density charge topology of CH(5)(+) species 1 (C(s)()), 2 (C(s)()), and 3 (C(2)(v)) at ab initio level using the theory of atoms in molecules developed by Bader. Despite the reports of previous studies concerning carbocationic species, the methane molecule is protonated at the carbon atom, which clearly shows its pentacoordination. In addition to the fact that hydrogen atoms in the methonium molecule behave in a very fluxional fashion and that the energy difference among the species 1, 2, and 3 are very low, is important to point out that two different topological situations can be defined on the basis of our study of the topology of the electronic charge density. Then, the species 1 and 2 present a three-center-two-electron (3c-2e) bond of singular characteristics as compared with other carbocationic species, but in the species 3, the absence of a 3c-2e bond is noteworthy. This structure can be characterized through the three bond critical points found, corresponding to saddle points on the path bonds between the C-H(2,3,5) that lie in the same plane. These nuclei define a four-center interaction where the electronic delocalization produced among the sigma(C-H) bonds provide a stabilization of the three C-H bonds involved in this interaction (the remaining two C-H bonds are similar to those belonging to the nonprotonated species). Our results show that bonding situations with a higher number of atom arrays are possible in protonated hydrocarbons.     

Efficient three-step iterative methods with sixth order convergence for nonlinear equations

In this paper, we present two new three-step iterative methods for solving nonlinear equations with sixth convergence order. The new methods are obtained by composing known methods of third order of convergence with Newton’s method and using an adequate approximation for the derivative, that provides high order of convergence and reduces the required number of functional evaluations per step. The first method is obtained from Potra-Pták’s method and the second one, from Homeier’s method, both reaching an efficiency index of 1.5651. Our methods are comparable with the method of Parhi and Gupta (Appl Math Comput 203:50–55, 2008). Methods proposed by Kou and Li (Appl Math Comput 189:1816–1821, 2007), Wang et al. (Appl Math Comput 204:14–19, 2008) and Chun (Appl Math Comput 190:1432–1437, 2007) reach the same efficiency index, although they start from a fourth order method while we use third order methods and simpler arithmetics. We prove the convergence results and check them with several numerical tests that allow us to compare the convergence order, the computational cost and the efficiency order of our methods with those of the original methods.     

A modified Newton-Jarratt’s composition

A reduced composition technique has been used on Newton and Jarratt’s methods in order to obtain an optimal relation between convergence order, functional evaluations and number of operations. Following this aim, a family of methods is obtained whose efficiency indices are proved to be better for systems of nonlinear equations.