Substituent-dependent reactivity in the photodimerization of N-substituted dibenz[b,f]azepines

The photoprocesses of a series of N-substituted dibenz[b,f]azepines (iminostilbenes) were studied by absorption and emission spectroscopy, by laser flash photolysis, and by preparative irradiation with NMR analysis. In solutions, 2pi+2pi photodimers of N-cyano and N-acyl dibenzazepines are formed via the triplet state upon acetone- or benzophenone-sensitized energy transfer. T-T absorption spectra were measured and absorption coefficients were determined. The triplet energy transfer is equally efficient for N-alkyl dibenzazepines, which do not dimerize. Excited states of npi* character in the latter cases are discussed to rationalize the different reactivities. In spite of negligible intersystem crossing of 21 dibenzazepine derivatives, photodimers of N-acyl and N-cyano dibenzazepines are formed upon direct excitation in concentrated solutions (0.01-0.1 mol dm(-3)) as well as in the solid state. A selective anti-configuration of the photodimers was found throughout.     

Contributions of restructuring and oxidation to the aging of the surface of plasma polymers containing heteroatoms

The properties and composition of plasma polymer surfaces stored in air can change considerably over time, especially as a result of oxidative reactions. When plasma polymers contain an element other than O, it is possible to probe for mechanisms in addition to oxidation that contribute to the aging of the surface. Plasma polymers containing N were fabricated from either 1,3-diaminopropane (DAP),n-heptylamine (nHA), or allylamine (AA), and studied by X-ray photo-electron spectroscopy (XPS) and air/water contact angles (CA). For each of the plasma polymers, a multiexponential increase in the O/C ratio was observed over time using XPS. The N/C ratios remained constant (AA) or decreased somewhat (nHA and DAP). In contrast, the trends in CA values differed, declining for the nHA surfaces, rising for the AA, and changing little for the DAP. Surface roughness, assessed by scanning tunnelling or atomic force microscopy, did not change over time. The diverse adjustments in the polarity of each surface and the similar compositional changes between them are reconcilable if the aging of the plasma polymer surface is a manifestation of the superposition of concurrent oxidative reactions and partial surface reorientation; the former introduce polar groups and the latter transports then from the surface to deeper regions beyond the CA probe depth but within the XPS analysis depth. These processes vary between different plasma polymers. Data for the alkylamine plasma polymers is also compared with that for two plasma polymers fabricated from methanol. The change in composition, but not polarity, of the DAP surface after 4 days of storage demonstrates the importance of using multiple techniques to characterize the aging of plasma polymer surfaces.     

Strukturen neuer SeII- und TeII-Komplexe mit 2,2-Dicyanethylen-1,1-dithiolat, 2,2-Dicyanethylen-1,1-thioselenolat und 2,2-Dicyanethylen-1,1-diselenolat

Structures of New Se^II and Te^II Complexes Containing 2,2-Dicyanethylene-1,1-dithiolate, 2,2-Dicyanethylene-1,1-thioselenolate, and 2,2-Dicyanethylene-1,1-diselenolate (NBu₄)₂{Se[S₂C?C(CN)₂]₂} ( I ), (AsPh₄)₂ · {Te[SSeC?C(CN)₂]₂} ( II ), and (NBu₄)₂{Te[Se₂C?C(CN)₂]₂} ( III ) containing the bidentate chelate ligands 2,2-dicyanethylene-1,1-dithiolate i-mnt , 2,2-dicyanethylene-1,1-thioselenolate i-mnts , and 2,2-dicyanethylene-1,1-diselenolate i-mns have been prepared and characterized by X-ray structure analysis. The central units consist of [M(X? X)₂E₂]^2? (M = Se, Te; X? X = ligand; E = lone-pair) with fourfold coordinated Se^II and Te^II, respectively. The complex anions [Se(i-mnt)₂E₂]^2? as well as [Te(i-mnts)₂E₂]^2? show a trapezoide distortion with d(Se? S) = 2.276(5); 2.287(5); 2.803(5); 2.789(5) Å and d(Te? Se) = 2.611(2); 2.617(3); d(Te? S) = 2.889(5); 2.935(4) Å. In III there are centrosymmetric complex anions [Te(i-mns)₂E₂]^2? with nearly identical Te? Se-bond-lengths: 2.674(3) and 2.692(2) Å. These Te? Se bonds are elongated compared to usual Te? Se bonds.     

ULTRAVIOLET RADIATION - INDUCED MALIGNANT MELANOMA IN Monodelphis domestica

Several lines of evidence support the hypothesis that ultraviolet radiation (UVR) is involved in the etiology of cutaneous melanoma in humans. However, progress in understanding the mechanisms involved in induction of melanotic tumors by UVR has been hindered by lack of a suitable animal model. During the course of multiple exposures (3 times/wk for 70 wk) of the South American opossum, Monodelphis domestica, to UVR, we first observed the appearance of areas of dermal melanocytic hyperplasia (MH) on the exposed skin. Post-UVR exposure to photoreactivating light (320-500 nm) suppressed the occurrence of MH. We also observed at 100 weeks from first exposure that 10 of 46 surviving animals had developed melanotic tumors which arose, presumably, from areas of MH. Tumors on three of the 10 animals have been classified as malignant melanomas based on metastasis to lymph nodes. We conclude from these results that UVR can act as a complete carcinogen for melanoma induction and, based on the photoreactivation of MH induction, that DNA damage is involved in melanoma formation.     

Effect of Doping and Solution Redox Relays on the Efficiency of Photocathodic Processes at the p-InP/Water Interface

Light-induced interfacial electron transfer from two p-InP electrodes differing in the amount of majority carrier doping to a number of electron relays ( R ) dissolved in aqueous solution was investigated. The material with the lower carrier density (0.71 × 10¹⁸ cm^?3) exhibited much better wavelength response and quantum yield for electron transfer than the electrode doped with 2.3 × 10¹⁸ cm^?3 charge carriers. Using cobalt (III) sepulcrate, Co (sep)³⁺, as an electron relay a polychromatic light to electrical energy conversion efficiency of 18% was obtained. The potential of this relay for use in a regenerative photoelectrochemical cell is briefly discussed.     

Efficient palladium-catalyzed nucleophilic addition of triorganoindium reagents to carbocyclic derivatives

Palladium (0)-catalyzed allylic substitution reactions employing triorganoindium reagents have been investigated. In situ generated vinyl- and arylindiums react with substituted and unsubstituted cyclohex-2-enyl esters in the presence of 1-3 mol % Pd(2)(dba)(3) to produce vinyl- or arylcyclohexenes in moderate to excellent yields. The stereoselectivity of this process was also examined, and evidence is presented that the reaction proceeds with inversion of stereochemical configuration.     

Locally enhanced relative humidity for scanning probe nanolithography

The formation of a water meniscus between a sharp tip and a solid surface is one of the prevailing requirements for scanning probe microscope (SPM)-based lithographies, such as dip-pen nanolithography (DPN) and conductive tip induced oxidation. The water meniscus functions as a medium for the oxidation of or mass transfer to the solid surface. Here we report a simple, efficient, and effective approach to enhance the local relative humidity and thus increase the size of the water meniscus by bringing a water-containing capillary tube to the proximity of the tip-surface contact area. The enhancement in local relative humidity is confirmed via an increase in the measured tip-surface adhesion forces and the widths of DPN generated parallel lines. Compared to the global control of relative humidity for the whole lithography system, the short distance between the "water reservoir" and the tip-surface contact area enables rapid increase in the local vapor pressure of water, less perturbation, and minimal erosion to the state-of-the-art electronics. As a result, most scanning probe lithography experiments at high relative humidity can now be performed in a reasonable time frame.     

Different approaches toward an automatic structural alignment of drug molecules: Applications to sterol mimics,thrombin and thermolysin inhibitors

Summary A relative comparison of the binding properties of different drug molecules requires their mutual superposition with respect to various alignment criteria. In order to validate the results of different alignment methods, the crystallographically observed binding geometries of ligands in the pocket of a common protein receptor have been used. The alignment function in the program SEAL that calculates the mutual superposition of molecules has been optimized with respect to these references. Across the reference data set, alignments could be produced that show mean rms deviations of approximately 1 Å compared to the experimental situation. For structures with obvious skeletal similarities a multiple-flexible fit, linking common pharmacophoric groups by virtual springs, has been incorporated into the molecular mechanics program MOMO. In order to combine conformational searching with comparative alignments, the optimized SEAL approach has been applied to sets of conformers generated by MIMUMBA, a program for conformational analysis. Multiple-flexible fits have been calculated for inhibitors of ergosterol biosynthesis. Sets of different thrombin and thermolysin inhibitors have been conformationally analyzed and subsequently aligned by a combined MIMUMBA/SEAL approach. Since for these examples crystallographic data on their mutual alignment are available, an objective assessment of the computed results could be performed. Among the generated conformers, one geometry could be selected for the thrombin and thermolysin inhibitors that approached reasonably well the experimentally observed alignment.     

Hydrogenation without a transition-metal catalyst: on the mechanism of the base-catalyzed hydrogenation of ketones

The hydrogenation of unsaturated organic substrates such as olefins and ketones is usually effected by homogeneous or heterogeneous transition-metal catalysts. On the other hand, a single case of a transition-metal-free and purely base-catalyzed hydrogenation of ketones was reported by Walling and Bollyky some 40 years ago. Unfortunately, the harsh reaction conditions (ca. 200 degrees C, >100 bar H(2), potassium tert-butoxide as base) limit the substrate spectrum of this reaction to robust, nonenolizable ketones such as benzophenone. We herein present a mechanistic study of this process as a basis for future rational improvement. The base-catalyzed hydrogenation of ketones was found to be irreversible, and it shows first-order kinetics with respect to the substrate ketone, hydrogen, and catalytic base. The rate of the reaction depends on the type of alkali ion present (Cs > Rb - K > Na > Li). Using D(2) instead of H(2) revealed a rapid base-catalyzed isotope exchange/equilibration between the gas phase and the solvent as a concomitant reaction. The degree of deuteration of the product alcohols did not indicate a significant kinetic isotope effect. It is proposed that both ketone reduction and isotope exchange proceed via similar six-membered cyclic transition states involving the H(2)(D(2))-molecule, the alkoxide base, and the ketone (solvent alcohol in the case of isotope exchange). Mechanistic analogies are pointed out which apparently exist between the base-catalyzed hydrogenation of ketones studied here and the Ru-catalyzed asymmetric ketone hydrogenation developed by Noyori. In both cases, heterolysis of the hydrogen molecule appears to be assisted by a Br?nsted-base (i.e., alkoxide), the latter being bound to the substrate ketone or the catalyst ligand, respectively, by a bridging Lewis-acidic alkali ion.     

LiTiCl3: Synthesis and thermal behaviour

LiTiCl₃ is obtained as one example within an ample solid solution, Li_24–2nTi_nCl₂₄ (?4?n?10), by synpropotionation of TiCl₃ and Ti in the presence of LiCl (2:1:3 molar ratio) in sealed tantalum tubes at 750°C. It crystallizes with the inverse spinel-type structure according to (Li_0.67)^[4](Li_0.67Ti_1.33)^[4]Cl₄ with, at 25°C, a = 1048.62(4) pm, space group Fd3m. Thermal expansion is linear with α = 4.85 × 10^?5K^?1 up to about 300°C and thereafter, when the migration of Li⁺ from tetrahedral to octahedral interstices becomes increasingly important, it exhibits a relative decrease resulting, finally, in the phase transition to a NaCl-type structure that is observed for the first time at about 575°C.