Characterization of iron-silicon thin films by means of electron diffraction and X-ray spectroscopy

Iron-silicon thin films have been characterized by means of analytical transmission electron microscopic methods. Under certain conditions — composition and annealing temperature — these films exhibit thermoelectric behavior. In particular, the morphology and phase formation which results from annealing of these films, and doping with oxygen and nitrogen, are of interest. The thermoelectric phase -FeSi₂ is formed at temperatures above 500°C. This phase is transformed into electrically conducting phases at about 1000°C. A small oxygen content does not influence this crystallization process. If the oxygen content is higher than 15 atom-% the electrically conducting phases exist even at 500°C. The presence of a small nitrogen content inhibits the formation of the -FeSi₂ phase. The development of silicon and iron nitrides is possible.     

Redetermination of the Crystal Structure of the 1,2,4,7-anti-tetramethylbicyclo[2.2.1]heptan-2-yl cation at 110 K

The structure of the 1,2,4,7-anti-tetramethylbicyclo[2.2.1]heptan-2-yl cation ( 1 ) was redetermined by X-ray crystal structure analysis of its Sb₂F₁₁ salt at 110 K (P2₁/c, R1 = 5.76%). The most important structural features of 1 are: C(1)? C(2) = 1.409(9), C(1)? C(6) = 1.710(8), and C(2)…C(6) = 2.113(9) Å and C(2)? C(1)? C(6) = 84.7(4)°. These results agree with those obtained earlier by other methods for the rapidly equilibrating, partially s?-delocalized 1,2-dimethylbicyclo[2.2.1]heptan-2-yl cation ( 2 ). The detailed experimental procedure for the preparation of crystalline 1 · Sb₂F₁₁ and the crystal selection and mounting are described.     

A Photoelectron-spectroscopic investigation of the Homoconjugative Interaction between π- and Walsh-Orbitals in endo- and exo-Cyclopropano-norbornene

Photoelectron spectra of endo- and exo-cyclopropano-norbornene ( = endo- and exo-tricyclo[3.2.1.0^2.4]octa-6-ene) show that a significant homoconjugation exists between the π-orbital of the double bond and the symmetric Walsh-es-orbital of the cyclopropane ring in the exo-isomer, whereas the interaction is negligeable in the endo-derivative.     

Quantum chemistry with an attached processor

The Floating Point Systems, Inc. Model 164 Attached Processor (FPS-164) is a high-speed, pipelined, parallel processor designed for large-scale scientific computation. Benchmark studies of operations common in quantum chemistry codes are discussed and the performance of the FPS-164 is compared with other commonly available computers. A complete system of electronic structure codes has been implemented on the FPS-164 using the Fortran-77 cross-compiler and calls to optimized vector and matrix routines. The conversion of a generalized valence bond (GVB ) code illustrates the strategy adopted to adapt Fortran codes to the FPS-164. A typical production example, a large scale (GVB ) and configuration interaction calculation on the vinyl radical, shows a net throughput equivalent to nearly nine VAX 11/780 computers.     

Theoretical investigations of Fe porphyrins. V. Low-lying electronic states of bisammineporphinatoiron(III)

Ab initio CI calculations are reported on the lowest doublet, quartet, and sextet states of [Fe^III(P)(NH₃)₂]⁺. The low-spin ground state is calculated as (d_xy² (d_π)³ with d_xy(d_π)⁴ higher by 0.15 eV. The near-ir bands at ～1 eV observed in low-spin ferriheme proteins are attributed to (π → d_π) transitions. The lowest high-spin state is ⁶A_1g, and the near-ir transitions of the high-spin ferriheme proteins observed at ～1.2 eV are attributed to higher ⁶[tripsextet] excited states [i.e., ring triplet, metal sextet]. The 30-ps “triplet” transient populated with low quantum yield observed in laser-flash studies on Fe^III(TPP)CI [TPP = tetrapbenylporphyrin] may be an ¹[tripsextet] state.     

Rhodopsin exhibits a preference for solvation by polyunsaturated docosohexaenoic acid

An all-atom molecular dynamics simulation of rhodopsin in a membrane environment has been carried out with lipid composition similar to that of the retinal membrane. The initial conformation of the protein was taken from the X-ray crystallographic structure (1F88), while those of the lipids came from a previous molecular dynamics simulation. During the course of the 12.5 ns simulation, the initially randomly placed lipids adopt an anisotropic solvation structure around the protein. The lipids, having one saturated stearic acid chain and one polyunsaturated docosohexaenoic acid chain with a zwitterionic phosphatidylcholine headgroup, arrange themselves to maximize contact between the polyunsaturated chain and the protein surface. This organization is driven by energetically favorable interactions between the transmembrance helices and the docosohexaenoyl chains that are largely of the van der Waals type. These observations are consistent with various experimental studies on rhodopsin and other G-protein coupled receptors and with the picture of extreme flexibility in polyunsaturated fatty acid chains that has arisen from recent NMR and computational work.     

Sequentially photocleavable protecting groups in solid-phase synthesis

[reaction: see text] A sequential solid-phase peptide synthesis was developed using both photolabile linker and protecting groups. The chromatic sequential lability between a tert-butyl ketone-derived linker (sensitive to irradiation at 305 nm) and a nitroveratryloxycarbonyl (NVOC) group (sensitive at 360 nm) was exploited to prepare Leu-Enkephalin in a 55% overall yield. This new strategy allows the preparation of peptides in essentially neutral medium, by avoiding the use of common deprotection reagents such as trifluoroacetic acid or piperidine.     

Studies on the Chemistry of Thienoannelated O , N - and S , N -Containing Heterocycles, 28 [1]. Synthesis of Imidazo[1,5- d ]thieno[2,3- b ][1,4]thiazine Derivatives as GABA -Receptor Ligands

Summary.  GABA-receptor-ligands are still very interesting in drug-development. Usually benzodiazepines are used in the treatment but they have serious side-effects. Thus, a recently synthesized quioxaline derivative which showed reduced side-effects in an animal model was used as a model-substance. The cyclus was modified to optimize the pharmacological profile. Accordingly, a series of imidazo-thieno-thiazines was synthesized starting from 5-acetyl-2-chloro-3-nitrothiophene to yield 6-ethyl-2,3-dihydro-1H-thieno[2,3-b][1,4]thiazine-2-one. Reaction with potassium tert-butoxide and diethylchlorophosphate gave an intermediate, which resulted in the desired ring system after adding the corresponding isocyanides and potassium tert-butoxide. Corresponding author. E-mail: thomas.erker@univie.ac.at Received August 6, 2002; accepted August 13, 2002     

Tuning the band gap in hybrid tin iodide perovskite semiconductors using structural templating

Structural distortions within the extensive family of organic/inorganic hybrid tin iodide perovskite semiconductors are correlated with their experimental exciton energies and calculated band gaps. The extent of the in- and out-of-plane angular distortion of the SnI4(2-) perovskite sheets is largely determined by the relative charge density and steric requirements of the organic cations. Variation of the in-plane Sn-I-Sn bond angle was demonstrated to have the greatest impact on the tuning of the band gap, and the equatorial Sn-I bond distances have a significant secondary influence. Extended Hückel tight-binding band calculations are employed to decipher the crystal orbital origins of the structural effects that fine-tune the band structure. The calculations suggest that it may be possible to tune the band gap by as much as 1 eV using the templating influence of the organic cation.