Bildung und Schwingungsspektrum des Pentachloro-oxo-niobat(V)-Ions. Die Kristallstruktur von [As(C6H5)4]2NbOCl5 · 2 CH2Cl2

Formation and Vibrational Spectrum of the Pentachloro-oxo-niobate(V) Ion. Crystal Structure of [As(C₆H₅)₄]₂NbOCl₅ · 2 CH₂Cl₂ [As(C₆H₅)₄]₂NbOCl₅ is formed by reaction of NbOCl₃ with [As(C₆H₅)₄]Cl in CH₂Cl₂; form the solution it crystallizes with two molecules of CH₂Cl₂. The i.r. and Raman spectrum is in accord with a NbOCl₅ ion of symmetry 4mm. The crystals are triclinic (space group P1 ) with the lattice constants a = 1043, b = 1187, c = 2269 pm, α = 97.4, β = 106.3 and γ = 101.4°. The crystal structure was determined with X-ray diffraction data and refined to a residual index of R = 0.042. In the structure there are two of each crystallographic independent As(C₆H₅)₄⁺ ions and CH₂Cl₂ molecules. The NbOCl₅ ion has an octahedral shape with a Nb? O distance of 197 pm.     

The CNDO approximation in the Green's function method for VIP calculation

By introducing a CNDO adapted approximation into an appropriate irreducible interaction part a simple formula for calculation of vertical ionization potentials (VIP's) is derived. The method is applied to the molecules F₂, HF, C₂F₄, CH₂F₂, BF₃, CH₃F, CF₄.     

Tuning octopolar NLO chromophores: synthesis and spectroscopic characterization of persubstituted 1,3,5-tris(ethynylphenyl)benzenes

The synthesis of a series of octopolar 1,3,5-tris(ethynylphenyl)benzenes via Sonogashira coupling is described, varying the substituents on both the central benzene core as well as the acetylenic periphery. In particular, systems bearing an electron-rich core and an electron-poor periphery are obtained that display advanced optical properties. The linear (by UV-vis and fluorescence spectroscopy) and second-order nonlinear optical properties are studied, the latter by hyper-Rayleigh scattering (HRS) (Hendrickx, E.; Clays, K.; Persoons, A. Acc. Chem. Res. 1998, 31, 675-683). The influence of different core and periphery substitution is revealed by the optical properties and confirms the possibility of fine-tuning those. Because of the presence of (one-photon) fluorescence for all compounds, femtosecond hyper-Rayleigh scattering has been applied (Olbrechts, G.; Strobbe, R.; Clays, K.; Persoons, A. Rev. Sci. Instrum. 1998, 69, 2233-2241). The implementation of the deconvolution in the frequency domain allows for a demodulation and a phase shift between immediate (nonlinear) scattering and time-delayed (multiphoton) fluorescence for high modulation frequencies (Wostyn, K.; Binnemans, K.; Clays, K.; Persoons, A. Rev. Sci. Instrum. 2001, 72, 3215-3220). In accordance with the linear optical properties, the second-order NLO properties can also be tuned by varying the core and peripheric substitutents.     

Identification of oligo(p-phenylene vinylene)-naphthalene diimide heterocomplexes by scanning tunneling microscopy and spectroscopy at the liquid-solid interface

Co-assembly of a melamine derivative covalently equipped with two oligo(p-phenylene vinylene) chromophores and a naphthalene diimide dye results in the formation of heterocomplexes at the liquid-solid interface which was shown using bias dependent imaging and scanning tunneling spectroscopy, despite the disordered nature of the assemblies.     

Second-harmonic generation in GFP-like proteins

The second-order nonlinear optical properties of green fluorescent proteins (GFPs), such as the photoswitchable Dronpa and enhanced GFP (EGFP), have been studied at both the theoretical and experimental levels. In the case of Dronpa, both approaches are consistent in showing the rather counterintuitive result of a larger second-order nonlinear polarizability (or first hyperpolarizability, beta) for the protonated state, which has a higher transition energy, than for the deprotonated, fluorescent state with its absorption at lower energy. Moreover, the value of beta for the protonated form of Dronpa is among the highest reported for proteins. In addition to the pH dependence, we have found a wavelength dependence in the beta values. These properties are essential for the practical use of Dronpa or other GFP-like fluorescent proteins as second-order nonlinear fluorophores for symmetry-sensitive nonlinear microscopy imaging and as nonlinear optical sensors for electrophysiological processes. An accurate value of the first hyperpolarizability is also essential for any qualitative analysis of the nonlinear images.     

Synthesis of aminocyclopentanols: alpha-D-galacto configured sugar mimics

Four aminocyclopentanols, as mimics of putative intermediates in the hydrolysis of alpha-d-galactosides, have been synthesized through a number of stereoselective transformations using the cis-fused cyclopentane-1,4-lactone (1R, 5S, 7R, 8R)-7,8-dihydroxy-2-oxabicyclo[3.3.0]oct-3-one as a chiral building block. The compounds were tested towards various glycosidases but showed no anomer selectivity in the inhibition of alpha- and beta-galactosidases.     

Experimental investigation of stresses in a ceramic-to-metal brazed joint

There is increasing use of joined ceramic and metallic material combinations exposed to a widening range of temperature. However, difficulties arise from the interaction of the different thermal expansion and ductility properties of component materials when a brazed joint is cooled from brazing to room temperature. Reliability demands on equipment employing ceramicto-metal bonds in structural and sealing applications justify the efforts made for a more complete understanding of the mechanics invelved. For this investigation, specimens were made by copper brazing the ID of a thin-wall metal ring to the OD of a ceramic disk. Residual stresses were induced as the bond metal solidified at the interface of the ring and disk. Electrical-resistance strain gages were fastened to the OD of the metal ring to determine change in stress as the braze material was removed. An apparatus and technique were developed for chemical removal of the braze material while protecting the specimen materials and instrumentation from corrosion. Stress-strain curves of the metals involved were obtained up to 1500°F. This data was used to predict the magnitude of the stresses. The calculated and measured stresses were in reasonable agreement.     

Unexpected Silaazetidine Formation in the Thermolysis of thf‐Supported Silanimine Et2Si=N–SitBu3·thf

The silyl amide Et₂SiCl‐NLi‐SitBu₃ can be cleanly prepared from precursor silylamine Et₂SiCl‐NH‐SitBu₃ and Li[nBu]. The CF₃SO₃SiMe₃ induced LiCl elimination of Et₂SiCl‐NLi‐SitBu₃ in thf afforded a 2‐silaazetidine derivative by [2+2] cycloaddition of Et₂Si=N–SitBu₃ with Et₂Si(OCH=CH₂)–NH–SitBu₃. X‐ray quality crystals of this 2‐silaazetidine derivative (triclinic, space group P$\bar{1}$ ) were grown from benzene at room temperature. The starting material for this approach, Et₂SiCl–NH–SitBu₃, is water‐sensitive. Hydrolysis of Et₂SiCl‐NH‐SitBu₃ gave [tBu₃SiNH₃]Cl along with (Et₂SiO)_n oligomers. The hydro chloride [tBu₃SiNH₃]Cl could be isolated and was characterized by X‐ray crystallography (trigonal, space group P$\bar{3}$ ).     

Structure of 1,5-Anhydro-D-fructose: X-ray Analysis of Crystalline Acetylated Dimeric Forms

Abstract

Acetylation of 1,5-anhydro-D-fructose under acidic conditions gave two crystalline acetylated dimeric forms, which by X-ray analysis were shown to be diastereomeric spiroketals formed between C-2 and C-2/C-3. The structures of the compounds differed only at the configuration at C-2. Acetylation or benzoylation of 1,5-anhydro-D-fructose in pyridine yielded 3,6-di-O-acetyl-1,5-anhydro-4-deoxy-D-glycero-hex-3-enos-2-ulopyra-nos or crystalline 1,5-anhydro-3,6-di-O-benzoyl-4-deoxy-D-glycero-hex-3-enos-2-ulo-pyranose.