π-Bindungsbeitrag in Boranen, 2. Mitt.:Ab initio- und Kraftfeld-Rechnungen von Vinyldifluorboran

Ab initio calculations on vinyldifluoroborane yield a uj27766g3p8/xxlarge960.gif" alt="pgr" align="BASELINE" BORDER="0">-contribution of 23 kJ/mol (5.5 kcal/mol) mainly due to the C=C bond and not to the lone pairs of the fluorine atoms. The rotational barrier was also determined. The force field calculations favour the interpretation of the bond structure derived from the theab initio results.

     

Spectroscopic properties of the photoproducts of pyridoxal-5′-P irradiation: Catalytic site recognition of ribonuclease A

Photoproducts of pyridoxal-5uj5203628h1ku35h/xxlarge8242.gif" alt="prime" align="BASELINE" BORDER="0">-P, i.e., 4-pyridoxic-5uj5203628h1ku35h/xxlarge8242.gif" alt="prime" align="BASELINE" BORDER="0">-P and bis-pyridoxal-5uj5203628h1ku35h/xxlarge8242.gif" alt="prime" align="BASELINE" BORDER="0">-P, have been studied by spectroscopic methods. The spectroscopic properties of bis-pyridoxal-5uj5203628h1ku35h/xxlarge8242.gif" alt="prime" align="BASELINE" BORDER="0">-P (bis-PLP) resemble those of pyridoxal-5uj5203628h1ku35h/xxlarge8242.gif" alt="prime" align="BASELINE" BORDER="0">-P (PLP) under similar experimental conditions. The coupling of methylen hydrogens to the phosphorus atom has been shown by NMR spectroscopy. The singlet in the³¹P-NMR spectra and the triplet in¹H-undecoupled experiments confirm the presence of the phosphate group in the 5uj5203628h1ku35h/xxlarge8242.gif" alt="prime" align="BASELINE" BORDER="0"> position of the structure of the vitamin. The effect of pH and solvent composition on the relative distribution of species of bis-pyridoxine-5uj5203628h1ku35h/xxlarge8242.gif" alt="prime" align="BASELINE" BORDER="0">-P (bis-PNP) has been investigated by absorption and fluorescence spectroscopy. The acid-base dissociation of the phosphate group is easily detected by emission spectroscopy. Bis-PNP and bis-PLP bind to the enzyme RNase A and they behave as competitive inhibitors with respect to the substrate cytidine-2uj5203628h1ku35h/xxlarge8242.gif" alt="prime" align="BASELINE" BORDER="0">-3uj5203628h1ku35h/xxlarge8242.gif" alt="prime" align="BASELINE" BORDER="0">-cyclic phosphate. The natural forms of vitamin B₆, pyridoxine, and pyridoxine-5uj5203628h1ku35h/xxlarge8242.gif" alt="prime" align="BASELINE" BORDER="0">-P have no effect on the catalytic activity of the protein. Experimental evidence derived from fluorescence and inhibition experiments is consistent with the hypothesis that bis-PNP recognizes the catalytic site of RNase A.     

Synthesis and properties of functional derivatives ofN′-phosphoryl- andN′-phosphonoyldiazeneN-oxides; molecular structure ofN-(2,2-dimethyl-5-nitro-1,3-dioxan-5-yl)-N′-[methoxy(phenyl)phosphoryl]diazeneN-oxide

Methods for the synthesis of polyfunctionalNuj1q482/xxlarge8242.gif" alt="prime" align="BASELINE" BORDER="0">-phosphoryl- andNuj1q482/xxlarge8242.gif" alt="prime" align="BASELINE" BORDER="0">-phosphonoyldiazeneN-oxides containing hydroxyl, acetoxyl, and nitrate groups, and dibromoallyl and dibromopropyl fragments have been developed. The molecular structure ofN-(2,2-dimethyl-5-nitro-1,3-dioxan-5-yl)-Nuj1q482/xxlarge8242.gif" alt="prime" align="BASELINE" BORDER="0">-[methoxy(phenyl)phosphoryl)diazeneN-oxide was established by X-ray structural analysis.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1284–1289, July, 1994.     

Substituted 1,2-Thiazetidine 1,1-Dioxides. Synthesis of (<Emphasis Type="Italic">RS</Emphasis>)- and (<Emphasis Type="Italic">S</Emphasis>)-1,2-Thiazetidine-3-acetic Acid 1,1-Dioxide and its Reactions with Amino Acids and Dipeptides

Summary. (RS)-2-tert-Butyldimethylsilyl-1,2-thiazetidine-3-acetic acid 1,1-dioxide prepared from (RS)-S-benzyl-uj8c/xxlarge946.gif" alt="beta" align="MIDDLE" BORDER="0">-homocysteine was condensed via DCC/NHS with various L-amino acid esters or dipeptide esters yielding N-silylated uj8c/xxlarge946.gif" alt="beta" align="MIDDLE" BORDER="0">-sultam peptides. A uj8c/xxlarge946.gif" alt="beta" align="MIDDLE" BORDER="0">-sultam active ester was isolated as an intermediate. Desilylation with TBAF in THF yielded stable N-unsubstituted products, and deprotection of the benzyl esters was achieved by catalytic hydrogenation. (S)-S-Benzyl-uj8c/xxlarge946.gif" alt="beta" align="MIDDLE" BORDER="0">-homocysteine was obtained by fractional crystallization of the brucine salt of the racemate and transformed into benzyl (S)-1,2-thiazetidine-3-acetate, which was on the other hand synthesized by an enantiospecific route from uj8c/xxlarge969.gif" alt="ohgr" align="BASELINE" BORDER="0">-benzyl Boc-L-aspartate. Some uj8c/xxlarge946.gif" alt="beta" align="MIDDLE" BORDER="0">-sultam peptides were prepared from the (S)-enantiomer, and finally some uj8c/xxlarge946.gif" alt="beta" align="MIDDLE" BORDER="0">-sultam peptides containing D-Ala units were obtained.     

Modifications in structural and optical properties of Mn-ion implanted CdS thin films

In this paper, we report on modifications in structural and optical properties of CdS thin films due to 190 keV Mn-ion implantation at 573 K. Mn-ion implantation induces disorder in the lattice, but does not lead to the formation of any secondary phase, either in the form of metallic clusters or impurity complexes. The optical band gap was found to decrease with increasing ion fluence. This is explained on the basis of band tailing due to the creation of localized energy states generated by structural disorder. Enhancement in the Raman scattering intensity has been attributed to the enhancement in the surface roughness due to increasing ion fluence. Mn-doped samples exhibit a new band in their photoluminescence spectra at 2.22 eV, which originates from the d-d (⁴T₁ → ⁶A₁) transition of tetrahedrally coordinated Mn²⁺ ions.     

An investigation of the oxidized Ni/InAs interface

Ni was resistively deposited onto bulk InAs and subsequently oxidized in an O₂ atmosphere. The anneal temperature and time were 450 °C and 2.5 h, respectively. X-ray diffraction of the oxidized Ni/InAs sample revealed the formation of In₃Ni₂ and In₂O₃ on the front suggesting inter diffusion of In, Ni and O. NiO was not detected by X-ray diffraction. In a preliminary study, using glass as a substrate, NiO readily formed when using these oxidation parameters. Conductivity measurements of the oxidized Ni/InAs surface revealed a conducting front and insulating rear surface while TEM of the Ni/InAs interface revealed an intermediate amorphous diffusion zone between the “oxidized” Ni layer and the bulk InAs. A closer investigation of the intermediate layer supports the X-ray diffraction results, suggesting compound formation due to diffusion of oxygen and nickel into the substrate, and out-diffusion of In and As from the bulk of the sample. AES was used to further elucidate these results.     

The role of substrate surface termination in the deposition of?(111)?CdTe on?(0001) sapphire

The small lattice mismatch and sixfold symmetry offered by the (0001) planes of sapphire make it an ideal substrate candidate for the deposition of (111) CdTe films. There, however, exists a wide disparity in film quality among various researchers with both single crystal and highly twinned, multidomain films being reported. We have developed a pulsed laser deposition process that enables us to deposit nearly single-domain (111) CdTe films exhibiting excellent surface morphology. Such films are deposited on as-received sapphire substrates in vacuum conditions where oxygen is readily available. If, however, film deposition is preceded by the deposition of a submonolayer of aluminum prior to film growth then a secondary CdTe domain emerges with an in-plane orientation having a 180°-in-plane offset from the first domain. These multidomain films show poor crystallographic and morphological properties, similar to what has been reported elsewhere. It is concluded that the singly terminated (0001) sapphire substrates are a prerequisite for the deposition of high-quality (111) CdTe films.     

Growth of thick GaInN on grooved (1 0 1¯ 1¯) GaN/(1 0 1¯ 2¯) 4H-SiC

We succeeded in growing high-crystalline-quality thick (1 0 1¯ 1¯) Ga_0.92In_0.08N films on a grooved (1 0 1¯ 1¯) GaN/(1 0 1¯ 2¯) 4H-SiC underlying layer. We also fabricated GaInN/GaN multiple quantum wells (MQWs) with a peak wavelength of 580 nm on a high-crystalline-quality thick GaInN film. The photoluminescence intensity of the MQWs is about six times higher than that of MQWs grown on planar GaN and twice as high as that of MQWs grown on a GaN underlying layer having the same grooved structure.     

Structural, chemical and magnetic investigations of polycrystalline Zn1−xMnxO

Present study reports the detailed structural and magnetic, as well as chemical analysis of polycrystalline Zn_1−xMn_xO (where x=0, 0.005, 0.01, 0.03, 0.05 and 0.1) samples synthesized by the high-temperature solid state reaction route. X-ray diffraction studies reveal the presence of secondary phase for higher Mn-doping concentrations (x≥0.03). Secondary phase formation having spinel structure is confirmed and reported as an evidence for the first time using transmission electron microscopy study. Chemical investigations using X-ray photoelectron spectroscopy showed the presence of Mn in two valence states. From the observed results we are of the opinion that Zn²⁺ ions, mainly present at or near grain boundaries, diffuse into manganese oxide to form a stable spinel phase Zn_xMn_3−xO₄ at or near the grain boundaries of ZnO/Zn_1−xMn_xO. Magnetization measurements did not show any magnetic transition down to 5 K.     

λ-Divergence of the Fourier Series of Continuous Functions of Several Variables

In this paper, we consider the behavior of rectangular partial sums of the Fourier series of continuous functions of several variables with respect to the trigonometric system. The Fourier series is called uj45r/xxlarge955.gif" alt="lambda" align="BASELINE" BORDER="0">-convergent if the limit of rectangular partial sums over all indices uj45r/11006_2004_Article_455724_TeX2GIFIE1.gif" alt=" $$\vec M = (M_1 ,...,M_n )$$ " align="middle" border="0"> for which uj45r/11006_2004_Article_455724_TeX2GIFIE2.gif" alt=" $$1/\lambda \leqslant M_j /M_k \leqslant \lambda $$ " align="middle" border="0"> for all j and k exists. In the space of arbitrary even dimension 2m we construct an example of a continuous function with an estimate of the modulus of continuity uj45r/11006_2004_Article_455724_TeX2GIFIE3.gif" alt=" $$\omega (F,\delta ) = \mathop O\limits_{\delta \to + 0} ({\text{ln}}^{ - m} (1/\delta ))$$ " align="middle" border="0"> such that its Fourier series is uj45r/xxlarge955.gif" alt="lambda" align="BASELINE" BORDER="0">-divergent everywhere for any uj45r/11006_2004_Article_455724_TeX2GIFIE4.gif" alt=" $$\lambda >1$$ " align="middle" border="0"> .