Configurações centrais planares do tipo pipa

Neste artigo estudamos configurações centrais planares do tipo pipa para o problema de quatro corpos. Mostramos a existência de tais configurações para as pipas côncavas, quando uma das massas está no interior do triângulo formado pelas outras três massas, e para as pipas convexas, quando uma das massas está no exterior do triângulo formado pelas outras três massas.

Missing levels in acoustic resonators

It is shown that the deviations of the experimental statistics of six chaotic acoustic resonators from Wigner-Dyson random matrix theory predictions are explained by a recent model of random missing levels. In these resonatorsa made of aluminum plates a the larger deviations occur in the spectral rigidity (SRs) while the nearest-neighbor distributions (NNDs) are still close to the Wigner surmise. Good fits to the experimental NNDs and SRs are obtained by adjusting only one parameter, which is the fraction of remaining levels of the complete spectra. For two Sinai stadiums, one Sinai stadium without planar symmetry, two triangles, and a sixth of the three-leaf clover shapes, was found that 7%, 4%, 7%, and 2%, respectively, of eigenfrequencies were not detected.

Electromagnetic Wave Scattering by Schwarzschild Black Holes

We analyze the scattering of a planar monochromatic electromagnetic wave incident upon a Schwarzschild black hole. We obtain accurate numerical results from the partial wave method for the electromagnetic scattering cross section and show that they are in excellent agreement with analytical approximations. The scattering of electromagnetic waves is compared with the scattering of scalar, spinor, and gravitational waves. We present a unified picture of the scattering of all massless fields for the first time.

Scattering of massless scalar waves by Reissner-Nordstrom black holes

We present a study of scattering of massless planar scalar waves by a charged nonrotating black hole. Partial wave methods are applied to compute scattering and absorption cross sections, for a range of incident wavelengths. We compare our numerical results with semiclassical approximations from a geodesic analysis, and find excellent agreement. The glory in the backward direction is studied, and its properties are shown to be related to the properties of the photon orbit. The effects of the black hole charge upon scattering and absorption are examined in detail. As the charge of the black hole is increased, we find that the absorption cross section decreases, and the angular width of the interference fringes of the scattering cross section at large angles increases. In particular, the glory spot in the backward direction becomes wider. We interpret these effects under the light of our geodesic analysis.

Scattering of sound waves by a canonical acoustic hole

This is a study of a monochromatic planar perturbation impinging upon a canonical acoustic hole. We show that acoustic hole scattering shares key features with black hole scattering. The interference of wave fronts passing in opposite senses around the hole creates regular oscillations in the scattered intensity. We examine this effect by applying a partial wave method to compute the differential scattering cross section for a range of incident wavelengths. We demonstrate the existence of a scattering peak in the backward direction, known as the glory. We show that the glory created by the canonical acoustic hole is approximately 170 times less intense than the glory created by the Schwarzschild black hole, for equivalent horizon-to-wavelength ratios. We hope that direct experimental observations of such effects may be possible in the near future.

Experimental and theoretical study of two-photon absorption in nitrofuran derivatives: Promising compounds for photochemotherapy

We report experimental and theoretical studies of the two-photon absorption spectrum of two nitrofuran derivatives: nitrofurantoine, (1-(5-nitro-2-furfurilideneamine)-hidantoine) and quinifuryl, 2-(5`-nitro-2`-furanyl) ethenyl-4-{N-[4`-(N,N-diethylamino)-1`-methylbutyl]carbamoyl} quinoline. Both molecules are representative of a family of 5-nitrofuran-ethenyl-quinoline drugs that have been demonstrated to display high toxicity to various species of transformed cells in the dark. We determine the two-photon absorption cross-section for both compounds, from 560 to 880 nm, which present peak values of 64 GM for quinifuryl and 20 GM for nitrofurantoine (1 GM = 1 x 10(-50) cm(4).s.photon(-1)). Besides, theoretical calculations employing the linear and quadratic response functions were carried out at the density functional theory level to aid the interpretations of the experimental results. The theoretical results yielded oscillator strengths, two-photon transition probabilities, and transition energies, which are in good agreement with the experimental data. A higher number of allowed electronic transitions was identified for quinifuryl in comparison to nitrofurantoine by the theoretical calculations. Due to the planar structure of both compounds, the differences in the two-photon absorption cross-section values are a consequence of their distinct conjugation lengths. (c) 2011 American Institute of Physics. [doi:10.1063/1.3514911]

cis-Bis{1,1-dibenzyl-3-[(furan-2-yl)carbonyl]thioureato-kappa 2 O,S}nickel(II)

In the title compound, [Ni(C(20)H(17)N(2)O(2)S)(2)], the NiII atom is coordinated by the S and O atoms of two 1,1-dibenzyl-3-[(furan-2-yl)carbonyl]thioureate ligands in a distorted square-planar geometry. The two O and two S atoms are mutually cis to each other. The Ni-S and Ni-O bond lengths lie within the range of those found in related structures. The dihedral angle between the planes of the two chelating rings is 20.33 (6)degrees.

1-furfuryl-3-furoylthiourea

The title compound, C11H10N2O3S, was synthesized from furoyl isothiocyanate and furfurylamine in dry acetone. The thiourea group is in the thioamide form. The trans-cis geometry of the thiourea group is stabilized by intramolecular hydrogen bonding between the carbonyl and cis-thioamide and results in a pseudo-S(6) planar ring which makes dihedral angles of 2.5 (3) and 88.1 (2)degrees with the furoyl and furfuryl groups, respectively. There is also an intramolecular hydrogen bond between the furan O atom and the other thioamide H atom. In the crystal structure, molecules are linked by two intermolecular N-H center dot center dot center dot O hydrogen bonds, forming dimers. These dimers are stacked within the crystal structure along the [010] direction.

3-Nitrobenzoic acid-3-methyl-4-nitropyridine 1-oxide (1/1)

The title adduct, C(7)H(5)NO(4)center dot C(6)H(6)N(2)O(3), forms part of an ongoing study of the design of non-centrosymmetric systems based on 3-methy-4-nitropyridine 1-oxide. The components of the adduct are linked by intermolecular O-H center dot center dot center dot O hydrogen bonds. The rings of the two components are nearly planar, with a dihedral angle of 11.9 (2)degrees between the planes. The supramolecular structure shows that molecules of the title complex are linked into sheets by a combination of strong O-H center dot center dot center dot O and weak C-H center dot center dot center dot O hydrogen bonds.

4-{2-[4-(Dimethylamino)phenyl]ethylidene}benzonitrile

In the crystal of the title compound, C(17)H(16)N(2), molecules are linked by C-H center dot center dot center dot N hydrogen bonds, forming rings of graph-set motifs R(2)(1) (6) and R(2)(2) (10). The title molecule is close to planar, with a dihedral angle between the aromatic rings of 0.6 (1)degrees. Torsion angles confirm a conformational trans structure.

cis-Bis[1,1-dibenzyl-3-(furan-2-ylcarbonyl)thioureato-kappa(2)O,S]copper(II)

In the title compound, [Cu(C(20)H(17)N(2)O(2)S)(2)], the Cu(II) atom is coordinated by the S and O atoms of two 1,1-dibenzyl-3-(furan-2-ylcarbonyl)thioureate ligands in a distorted square-planar geometry. The two O and two S atoms are mutually cis to each other. The Cu-S and Cu-O bond lengths lie within the ranges of those found in related structures. The dihedral angle between the planes of the two chelating rings is 26.15 (6)degrees.

1-(o-tolyl)thiourea

In the title compound, C(8)H(10)N(2)S, the o-tolyl group and the thiourea core are planar. The mean planes of the two groups are almost perpendicular [82.19 (8)degrees]. The thiourea group is in the thioamide form, in which resonance is present. In the crystal structure, molecules are linked by intermolecular N-H center dot center dot center dot S hydrogen bonds, forming two infinite chains parallel to the (110) and (110) planes.

cis-bis(N-benzoyl-N ',N '-dibenzylthio-ureato-kappa O-2,S)nickel(II)

In the title compound, [Ni(C22H19N2OS)(2)], the Ni-II atom is coordinated by the S and O atoms of two N-benzoyl-N',N'-dibenzylthioureate ligands in a slightly distorted square-planar geometry. The two O atoms are cis, as are the two S atoms.

Contrasting photoelectrochemical behaviour of two isomeric supramolecular dyes based on meso-tetra(pyridyl)porphyrin incorporating four (mu(3)-oxo)- triruthenium(III) clusters

A saddle shaped tetracluster porphyrin species containing four [Ru(3)O(OAc)(6)(py)(2)](+) clusters coordinated to the N-pyridyl atoms of 5,10,15,20-tetra(3-pyridyl)porphyrin, H(2)(3-TCPyP), has been investigated in comparison with the planar tetra(4-pyridyl) porphyrin analogue H(2)(4-TCPyP). The steric effects from the bulky peripheral complexes play a critical role in the H(2)(3-TCPyP) species, determining a non-planar configuration around the porphyrin centre and precluding any significant pi-electronic coupling, in contrast with the less hindered H(2)(4-TCPyP) species. Both systems exhibit a photoelectrochemical response in the presence of nanocrystalline TiO(2) films, involving the porphyrin excitation around 450 nm. However, only in the H(2)(4-TCPyP) case do the cluster moieties also contribute to the photoinduced electron injection process at 670 nm, reflecting the relevance of the electronic coupling between the porphyrin centre and the peripheral complexes.

Biomass burning in Brazil`s Amazonian ""arc of deforestation"": Burning efficiency and charcoal formation in a fire after mechanized clearing at Feliz Natal, Mato Grosso

Estimates of greenhouse-gas emissions from deforestation are highly uncertain because of high variability in key parameters and because of the limited number of studies providing field measurements of these parameters. One such parameter is burning efficiency, which determines how much of the original forest`s aboveground carbon stock will be released in the burn, as well as how much will later be released by decay and how much will remain as charcoal. In this paper we examined the fate of biomass from a semideciduous tropical forest in the ""arc of deforestation,"" where clearing activity is concentrated along the southern edge of the Amazon forest. We estimated carbon content, charcoal formation and burning efficiency by direct measurements (cutting and weighing) and by line-intersect sampling (LIS) done along the axis of each plot before and after burning of felled vegetation. The total aboveground dry biomass found here (219.3 Mg ha(-1)) is lower than the values found in studies that have been done in other parts of the Amazon region. Values for burning efficiency (65%) and charcoal formation (6.0%, or 5.98 Mg C ha(-1)) were much higher than those found in past studies in tropical areas. The percentage of trunk biomass lost in burning (49%) was substantially higher than has been found in previous studies. This difference may be explained by the concentration of more stems in the smaller diameter classes and the low humidity of the fuel (the dry season was unusually long in 2007, the year of the burn). This study provides the first measurements of forest burning parameters for a group of forest types that is now undergoing rapid deforestation. The burning parameters estimated here indicate substantially higher burning efficiency than has been found in other Amazonian forest types. Quantification of burning efficiency is critical to estimates of trace-gas emissions from deforestation. (C) 2009 Elsevier B.V. All rights reserved.