An ab initio calculation of the anisotropic hyperfine coupling constants in the low-lying vibronic levels of the X 2Pi electronic state of CCCH

The results of ab initio calculations of the vibronically averaged components of the anisotropic magnetic hyperfine tensor in the low-lying vibronic species of the X (2)Pi electronic state of CCCH and CCCD are reported. The electronically averaged hyperfine coupling constants for hydrogen and (13)C in (12)C (12)C (12)CH, (13)C (12)C (12)CH, (12)C (13)C (12)CH, (12)C (12)C (13)CH, and (12)C (12)C (12)CD are obtained as functions of two bending vibrational modes by the density functional theory method. The vibronic wave functions are calculated with help of a variational approach which takes into account the Renner-Teller effect and spin-orbit coupling. The results of the present study help to reliably interpret the experimental data previously published and predict the yet unobserved hyperfine structure in excited vibronic states of CCCH and CCCD.     

Molecular design of two sterol 14α-demethylase homology models and their interactions with the azole antifungals ketoconazole and bifonazole

Summary Sterol 14α-demethylase (CYP51) is one of the known major targets for azole antifungals. Therapeutic side effects of these antifungals are based on interactions of the azoles with the human analogue enzyme. This study describes for the first time a comparison of a human CYP51 (HU-CYP51) homology model with a homology model of the fungal CYP51 of Candida albicans (CA-CYP51). Both models are constructed by using the crystal structure of Mycobacterium tuberculosis MT-CYP51 (PDB code: 1EA1).The binding mode of the azole ketoconazole is investigated in molecular dynamics simulations with the GROMACS force field. The usage of special parameters for the iron azole complex binding is necessary to obtain the correct complex geometry in the active site of the enzyme models. Based on the dynamics simulations it is possible to explain the enantioselectivity of the human enzyme and also to predict the binding mode of the isomers of ketoconazole in the active site of the fungal model.     

Efficient transfer hydrogenation of ketones in the presence of ruthenium N-heterocyclic carbene catalysts

Novel ruthenium carbene complexes have been in situ generated and tested for the transfer hydrogenation of ketones. Applying Ru(cod)(methylallyl)₂ in the presence of imidazolium salts in 2-propanol and sodium-2-propanolate as base, turnover frequencies up to 346 h⁻¹ have been obtained for reduction of acetophenone. A comparative study involving ruthenium carbene and ruthenium phosphine complexes demonstrated the higher activity of ruthenium carbene complexes.     

Formation of large PEE domains in PEE212-PEO112 diblock copolymer monolayers: shift of the PEO-desorption transition

PEE212-PEO112 diblock copolymer monolayers are studied at the air/water interface. At large molecular areas, with X-ray reflectivity, PEE domains are observed, which are partly immersed into the water. The domain thickness increases on compression (28 to 40 A). With off-specular X-ray reflectivity, an average domain radius of 750 A is found, but there are also smaller domains. Due to these space constraints, most PEO blocks form a brush beneath the PEE domains. Only a few PEO blocks form a corona surrounding the domains and adsorb flatly onto the air/water interface. The PEO desorption transition is observed at the typical pressure of 9 mN/m, when the flatly adsorbed PEO is compressed at a domain fraction of 95%. It occurs at 6 A2/EO monomer, about half the value found for lipopolymers or diblock copolymers with NPEE approximately NPEO or NPEE < NPEO. Apparently, the thickness of the PEE domains is determined by the forces from the two interfaces, not by the PEO block, for which flat adsorption beneath the domain would be more favorable instead of formation of a PEO brush.     

Comparative analysis of a laser frequency stabilization method using a mobile and a fixed Lorentzian

Frequency stabilization using mobile resonances is compared with that employing frequency modulation. Analogy with radio-frequency experiments leads to an optimization of the error signal. Offsets occuring in each case are discussed and additional offsets due to asymmetry are evaluated.     

Die isotypen Verbindungen BaRh2Si2, BaIr2Si2 und BaPt2Ga2 – eine monokline Verzerrungsvariante der CaRh2B2-Struktur

The Isotypic Compounds BaRh₂Si₂, BaIr₂Si₂, and BaPt₂Ga₂ – a Monoclinic Distortion Variant of the CaRh₂B₂ Structure The new compounds BaRh₂Si₂ (monoclinic, P2₁/c, a = 792.6(1) pm, b = 664.5(7) pm, c = 767.9(4) pm, β = 91.2(1)°, Z = 4, 2867 reflexions, 47 parameters, R = 0.024), BaIr₂Si₂ (monoclinic, P2₁/c, a = 792.47(6) pm, b = 664.28(6) pm, c = 772.22(6) pm, β = 92.181(7)°, Z = 4, 1939 reflexions, 47 parameters, R = 0.037) and BaPt₂Ga₂ (monoclinic, P2₁/c, a = 850.4(1) pm, b = 647.3(1) pm, c = 819.8(1) pm, β = 95.97(1)°, Z = 4, 1506 reflexions, 47 parameters, R = 0.038) were prepared by reaction of the elements. Their structures were determined from single crystal data. The compounds crystallize isotypically with a distortion variant of the CaRh₂B₂ type of structure.     

Sensitivity of ammonia interaction with single-walled carbon nanotube bundles to the presence of defect sites and functionalities

Ammonia adsorption on single-walled carbon nanotubes (SWNTs) was studied by means of infrared spectroscopy at both cryogenic (approximately 94 K) and room (approximately 300 K) temperatures. At 94 K, vacuum-annealed SWNTs showed no detectable ammonia uptake. However, the ammonia adsorption was found to be sensitive to the functionalities and defects on the nanotube surfaces. NH3 adsorption was detected on HNO3-treated nanotubes, characterized by significant functionalities and defects, prior to vacuum annealing. NH3 desorbed from those nanotubes above 140 K, indicating a weak adsorbate-nanotube interaction (approximately 30 kJ/mol). Exposure of annealed samples to ambient air, which possibly regenerated functionalities and defects on nanotube surfaces, restored partially the ammonia uptake capacity. No ammonia adsorption on SWNTs was observed by infrared spectroscopy at room temperature with up to 80 Torr dosing pressure. This work suggests the influence of functionalities and/or defect densities on the sensitivity of SWNT chemical gas sensors. Our theoretical studies on NH3 adsorption on pristine and defective tubes, as well as oxidized tubes, corroborate these findings.     

Selective nucleation and growth of metal-organic open framework thin films on patterned COOH/CF3-terminated self-assembled monolayers on Au(111)

Metal-Organic open Framework coordination polymers (MOFs) are a fascinating class of materials. We demonstrate the selective growth of patterned thin films of MOF-5 and the subsequent loading with a palladium organic compound, which acts as a precursor for the deposition of palladium nanoclusters inside the cavities of the MOF-5 material.     

Novel antibacterial fibers of quaternized chitosan and poly(vinyl pyrrolidone) prepared by electrospinning

The preparation of continuous defect-free fibers from quaternized chitosan derivative (QCh) has been achieved by electrospinning of mixed aqueous solutions of QCh with poly(vinyl pyrrolidone) (PVP). The average fiber diameter significantly decreases from 2800 to 1500 nm on increasing the polyelectrolyte content. In order to impart to QCh/PVP electrospun fibers stability to water and water vapor, the fibers have been crosslinked by incorporation of photo-crosslinking additives into QCh/PVP spinning solutions and subsequent UV irradiation of the electrospun fibers. Photo-crosslinked QCh-containing electrospun mats show high antibacterial activity against the Gram-positive bacteria Staphylococcus aureus and Gram-negative bacteria Escherichia coli.     

Caloric properties of mixtures of refrigerants R22 / R114

The nonazeotropic refrigerant mixture chlorodifluoromethane (R22) and 1,2-dichlorotetrafluoroethane (R114) has been frequently suggested as a working fluid in cooling systems and heat pump applications. However, especially for mixtures exact and reliable measurements of the caloric properties are often missing, so that calculations with equations of state yield results of great uncertainty. In spite of the CFC-ozone problem of this mixture it can be considered as an exemplary mixture to set up accurate equations of state.

Therefore measurements with an isenthalpic throttle calorimeter were carried out for three different compositions of the mixture. The measured isenthalps could be reproduced within the experimental accuracy by polynomials. Together with the specific heat capacity of the pure components the measurements lead to several caloric properties in the liquid-, vapour- and critical region. The caloric properties can also be calculated by equations of state (EOS). It turned out that the results obtained from Bender's EOS with interaction parameters fitted to the experiments lead to a good agreement with the experimental data.