Synthese und Struktur von γ-Halogenpropyl-octa(silasesquioxanen)

Synthesis and Structures of γ-Halopropyl-octa(silasesquioxanes) As a more rapid and versatile synthetic approach, we have studied the FeCl₃-catalyzed hydrolytic polycondensation of suited trichlorosilanes in a biphasic system which yields e.g. the new octa(silasesquioxane) (BrCH₂CH₂CH₂)₈Si₈O₁₂.     

Minimally generated Boolean algebras

We introduce the class of minimally generated Boolean algebras, i.e. those algebras representable as the union of a continuous well-ordered chain of subalgebras A ₁ where A _i+1 is a minimal extension of A _i. Minimally generated algebras are closely related to interval algebras and superatomic algebras.     

Comparison of localization and delocalization indices obtained with Hartree-Fock and conventional correlated methods: effect of Coulomb correlation

Atomic populations and localization [lambda(A)] and delocalization [delta(A,B)] indices (LIs and DIs) are calculated for a large set of molecules at the Hartree-Fock (HF), MP2, MP4(SDQ), CISD, and QCISD levels with the 6-311++G(2d,2p) basis set. The HF method and the conventional correlation methods [MP2, MP4(SDQ), CISD, and QCISD] yield distinct sets of LIs and DIs. Yet, within the four conventional correlation methods the differences in atomic populations and LIs and DIs are small. Relative to HF, the conventional correlation methods [MP2, MP4(SDQ), CISD, QCISD] yield virtually the same LIs and DIs for molecules with large charge separations while LIs and DIs that differ significantly from the HF values--the LIs are increased and DIs decreased--are obtained for bonds with no or small charge separations. Such is the case in the archetypal homopolar molecules HC(triple bond)CH, H2C=CH2, CH3-CH3, and "protonated cyclopropane" C(3)H(7) (+), in which case the bonding may be atypical. Relative to HF, the typical effect of the conventional correlation methods is to decrease the DI between atoms.     

Synthesis of a biotinated amphiphile

The synthesis of a biotinated amphiphile assembled from d-(+)-biotin, ethylene diamine as spacer, galactaric acid and 1-dodecylamine was achieved in six steps. The key step was the synthesis of a bisacetonide protected galactaric ester, the structure of which was determined by X-ray analysis. Aminolysis, spacer attachment, coupling with biotin and deprotection led to the amphiphilic galactaramide.     

High-temperature synthesis, single-crystal X-ray structure determination and solid-state NMR investigations of Ba7[SiO4][BO3]3CN and Sr7[SiO4][BO3]3CN

The novel alkaline earth silicate borate cyanides Ba₇[SiO₄][BO₃]₃CN and Sr₇[SiO₄][BO₃]₃CN have been obtained by the reaction of the respective alkaline earth metals M=Sr, Ba, the carbonates M^IICO₃, BN, and SiO₂ using a radiofrequency furnace at a maximum reaction temperature of 1350°C and 1450°C, respectively. The crystal structures of the isotypic compounds M^II₇[SiO₄][BO₃]₃CN have been determined by single-crystal X-ray crystallography (P6₃mc (no. 186), Z=2, a=1129.9(1) pm, c=733.4(2) pm, R₁=0.0336, wR₂=0.0743 for M^II=Ba and a=1081.3(1) pm, c=695.2(1) pm, R₁=0.0457, wR₂=0.0838 for M^II=Sr). Both ionic compounds represent a new structure type, and they are the first examples of silicate borate cyanides. The cyanide ions are disordered and they are surrounded by Ba²⁺/Sr²⁺ octahedra, respectively. These octahedra share common faces building chains along [001]. The [BO₃]³⁻ ions are arranged around these chains. The [SiO₄]⁴⁻ units are surrounded by Ba²⁺/Sr²⁺ tetrahedra, respectively. The title compounds additionally have been investigated by ¹¹B, ¹³C, ²⁹Si, and ¹H MAS-NMR as well as IR and Raman spectroscopy confirming the presence of [SiO₄]⁴⁻, [BO₃]³⁻, and CN⁻ ions.     

Structure elucidation of β-carotene isomers by HPLC-NMR coupling using a C30 bonded phase

The separation of cis/trans isomers of β-carotene has been performed with a C₃₀ stationary phase employing ¹H NMR spectroscopy as an on-line detection technique. 1D as well as 2D NMR spectra have been recorded in the stopped-flow mode for the predominant chromatographic peaks. Structural assignment of the five identified isomers was performed via comparison of simulated 1D ¹H NMR spectra on the basis of the structures of β-carotene cis/trans isomers with the experimental data, and also by the analysis of the proton-proton connectivities in the 2D NMR spectra of three isomers with the highest concentration. The chromatographic retention behaviour of the isomers agreed well with previously reported data. The advantage of the applied hyphenated coupling technique compared to conventional off-line techniques lies in the fact that chromatographic separation and NMR detection are performed in a closed system, so that reisomerization of the separated compounds is inhibited. Received: 29 May 1996 / Revised: 1 July 1996 / Accepted: 4 July 1996     

Resolution enhancement in multidimensional solid-state NMR spectroscopy of proteins using spin-state selection

We show that the resolution of homonuclear multidimensional solid-state NMR correlation experiments can be significantly improved using transition selection and spin-state-selective polarization transfer techniques. The selection and transfer of single states allow the removal of the J-coupling contribution from the line width in both the direct and indirect spectral dimensions. This is demonstrated with a new spin-state-selective CO-Calpha correlation experiment, applied to a microcrystalline 85-residue protein. With this new sequence, all four components of the CO-Calpha cross-peaks are separated into different spectra, obtained by linear combination of four recorded data sets. Line narrowing of up to 44% was obtained on the protein sample for the spin-state-selective CO-Calpha spectrum compared to a standard spin-diffusion experiment. The new technique also allows an easy distinction between "direct" and "relayed" transfer cross-peaks.