Relationship of charge to antiaromaticity in bis-fluorenyl dications and fluorenyl monocations: experimental support for theoretical calculations

The antiaromaticity of fluorenyl cations is dependent on the magnitude of the charge of the system. Theoretical assessments of antiaromaticity and charge were supported by experimental NMR chemical shifts. Delocalization was related to antiaromaticity, and evaluated through the standard deviation of the charges on carbons of the fluorenyl systems.     

Al(x)Ga(1-x)N-based deep-ultraviolet 320×256 focal plane array

We report the synthesis, fabrication, and testing of a 320×256 focal plane array (FPA) of back-illuminated, solar-blind, p-i-n, Al(x)Ga(1-x)N-based detectors, fully realized within our research laboratory. We implemented a pulse atomic layer deposition technique for the metalorganic chemical vapor deposition growth of thick, high-quality, crack-free, high Al composition Al(x)Ga(1-x)N layers. The FPA is hybridized to a matching ISC 9809 readout integrated circuit and operated in a SE-IR camera system. Solar-blind operation is observed throughout the array with peak detection occurring at wavelengths of 256 nm and lower, and falling off three orders of magnitude by ~285 nm. By developing an opaque masking technology, the visible response of the ROIC is significantly reduced; thus the need for external filtering to achieve solar- and visible-blind operation is eliminated. This allows the FPA to achieve high external quantum efficiency (EQE); at 254 nm, average pixels showed unbiased peak responsivity of 75 mA/W, which corresponds to an EQE of ~37%. Finally, the uniformity of the FPA and imaging properties are investigated.     

Influence of transition metal doping on tris(thiourea)zinc(II) sulphate crystals

Optical, thermal and dielectric properties of Ni(II)-doped tris(thiourea)zinc(II) sulphate single crystals grown by slow evaporation solution growth technique has been investigated. The lattice parameters of the as-grown crystals were obtained by single crystal X-ray diffraction analysis. The modes of vibrations of different functional groups present were identified by FT-IR studies. The surface morphological changes are observed in the doped specimen. The structure and the crystallinity of the material were confirmed by powder X-ray diffraction analysis. The UV–vis optical absorption spectrum shows the lower optical cut-off at ∼284 nm and the crystals are transparent in the entire visible region. The relative second harmonic generation efficiency measurements reveal the enhancement of efficiency by doping with small quantities of Ni(II).     

Dications of 3-phenyl-indenylidene dibenzo[a.d]cycloheptene: the role of charge in the antiaromaticity of cationic systems

Dications of 9-(3-phenyl-1H-inden-1-ylidene)-5H-dibenzo[a,d]cycloheptene, 5(2+), were prepared by oxidation with SbF(5) in SO(2)ClF, and their magnetic behavior was compared to dications of 9-(3-phenyl-1H-inden-1-ylidene)-9H-fluorene, 2(2+). The good correlation between the experimental (1)H NMR shifts for the dications that were oxidized cleanly and the chemical shifts calculated by the GAIO method supported the use of the nucleus independent chemical shifts, NICS, to evaluate the antiaromaticity of the indenyl systems of 2(2+)/5(2+) and their unsubstituted parent compounds, 6(2+) and 7(2+), as well as the antiaromaticity of the fluorenyl system of 2(2+)/7(2+) and the aromaticity of the dibenzotropylium system of 5(2+)/6(2+). Antiaromaticity was shown to be directly related to the amount of charge in the antiaromatic systems, with the antiaromatic systems more responsive to changes in the calculated NBO charge than the aromatic systems. The antiaromaticity was also shown to be directly related to the amount of delocalization in the ring system. The aromaticity of the dibenzotropylium system was much less responsive to changes in the amount of charge in the tropylium system, because the aromatic system was much more completely delocalized. Thus, antiaromatic species are more sensitive probes of delocalization than aromatic ones.     

A class of convolution integral equations involving a generalized polynomial set

The aim of this paper is to derive a solution of a certain class of convolution integral equation of Fredholm type whose kernel involves a generalized polynomial set. Our main result is believed to be general and unified in nature. A number of (known or new) results follow as special cases, simply by specializing the coefficients and parameters involved in the generalized polynomial set. For the sake of illustration, some special cases are mentioned briefly.     

Palmerolide A, a cytotoxic macrolide from the antarctic tunicate Synoicum adareanum

Palmerolide A, a 20-membered macrocyclic polyketide bearing carbamate and vinyl amide functionality, was isolated from the tunicate Synoicum adareanum collected from the vicinity of Palmer Station on the Antarctic Peninsula. Palmerolide A displays potent and selective cytotoxicity toward melanoma (UACC-66 LC50 = 0.018 muM) and appears to operate via inhibition (IC50 = 2 nM) of V-ATPase.     

Stabilization of coordinated carbonate in aqueous acidic solution: steric inhibition of protonation in Co(III) complexes containing chelated carbonate

The synthesis, characterization, X-ray crystal structures, and reactivity in aqueous acidic solution of the Co(III) carbonate complexes [Co(tpa)(O2CO)]ClO4.H2O, [Co(Me-tpa)(O2CO)]ClO4.0.5H2O, [Co(Me2-tpa)(O2CO)]ClO4.0.5H2O, and [Co(Me3-tpa)(O2CO)]ClO4 are reported (tpa = tris(2-pyridylmethyl)amine; Me-tpa, Me2-tpa, and Me3-tpa are derivatives of tpa containing one, two, and three 6-methylpyridyl rings, respectively). The complexes display very different spectroscopic and 59Co NMR properties, consistent with the decreasing ligand field strength of the tripodal amine ligands in the order tpa > Me-tpa > Me(2)-tpa > Me3-tpa. X-ray structural data show an increase in the average Co-N bond distances as the number of methyl groups on the tripodal amine ligand increases, and this is the result of steric interactions between the methyl groups and the carbonate ligand and between the methyl groups themselves. Rate data for the acid hydrolysis of [Co(tpa)(O2CO)]+ (I = 1.0 M (NaClO4), 25.0 degrees C) over the [HClO4] range of 0.10-1.0 M are consistent with a previously proposed mechanism involving protonation of the carbonate ligand prior to ring-opening, but the equilibrium constant for protonation is smaller in this case than those obtained previously, as is the equilibrium constant for proton transfer from the exo to the endo O atoms. Comparative rate data ([HCl] = 6.0 M, 25.0 degrees C) for the four complexes show that those containing methylated ligands undergo acid hydrolysis between 25 and 90 times more slowly than [Co(tpa)(O2CO)]+ under the same conditions, and it is proposed that this rate difference is a result of steric factors. Inspection of space-filling diagrams shows that one of the endo oxygen atoms is significantly sterically hindered by the methyl groups of the tripodal amine ligands, thus inhibiting protonation at this site and leading to slower observed rates of hydrolysis. The results obtained in this study are consistent with the endo oxygen atoms being the mechanistically important site of protonation in the acid hydrolysis of metal complexes containing chelated carbonate.