The lock-in and reorientational phase transitions in metallic superlattices

We show that in the metallic magnetic superlattices, along with the bilinear RKKY-like exchange, there is significant biquadratic contribution arising from intrinsic and fluctuational mechanisms. Within the phenomenological approach, we postulate a particular form of free energy density functional and study the thermodynamic behaviour of the system under consideration. The model permits a transparent interpretation of the phase transitions, observed in metallic multilayers, with multiple oscillation periods and biquadratic exchange effects taken into account.     

The impact of a carbon nanotube on the cholesterol domain localized on a protein surface

The influence of a single walled carbon nanotube on the structure of a cholesterol cluster (domain) developed over the surface of the endothelial protein 1LQV has been investigated using the classical molecular dynamics (MD) simulation technique. We have observed a substantial impact of carbon nanotube on the arrangement of the cholesterol domain. The carbon nanotube can drag out cholesterol molecules, remarkably reducing the volume of the domain that settles down on the protein.     

Fluorescence Anisotropy Controlled by Light Quenching*

We demonstrated that fluorescence anisotropy can be effectively decreased or increased in the presence of light quenching, depending on relative polarizations of excitation and quenching pulses. For parallel light quenching, anisotropy decreases to 0.103 and z-axis symmetry is preserved. In the presence of perpendicular light quenching, the steady-state anisotropy of a pyridine-2-glycerol solution increases from 0.368 for an unquenched sample to 0.484 for a quenched one. We show that the angular distribution of transition moments loses z-axis symmetry in the presence of perpendicular light quenching. In these cases we used more general definitions of anisotropy. Induced by light quenching, anisotropy can be applied in both steady-state and time-resolved measurements. In particular, the systems with low or no anisotropy can be investigated with the proposed technique.     

Distillation preconcentration and clean-up of aqueous samples for direct aqueous injection-gas chromatographic determination of volatile polar organic pollutants

The applicability of distillation to concentrate and clean up heavily loaded aqueous samples for the analysis of volatile polar organic compounds by means of direct aqueous injection-gas chromatography was studied. Recoveries for acetone, acetonitrile, acrolein, acrylonitrile, butanone, 1,4-dioxane, ethyl acetate, and 3-pentanone were in the range of 60.6 to 73.4% with relative standard deviations of 3.6 to 5.5%. The corresponding enrichment factors were in the order of 200. The recovery did not depend on the concentration in the studied range of 0.70 to 89 μg/kg. The detection limits with the mass selective detector operating in the selected ion monitoring mode were in the order of 0.1 μg/kg. The method was successfully applied to treated waste water from a pharmaceutical factory. The content of the above analytes in the real sample ranged from below 0.1 to ca. 83 μg/kg. Received: 16 December 1996 / Revised: 9 April 1997 / Accepted: 16 April 1997     

2′-Deoxyisoguanosine and Base-Modified Analogues: Chemical and Photochemical Synthesis

The synthesis of 2′-deoxyisoguanosine ( 2 ), and the pyrrolo[2,3-d]pyrimidine and pyrazolo[3,4-d]pyrimidine 2′-deoxyribonucleosides 3 and 4 is described. Condensation of the imidazole precursor 5 with benzoyl isocyanate followed by reaction with ammonia gave 2 . Its N(7) regioisomer was obtained from 6 . Compound 2 was also prepared by the photochemically induced conversion of 2-Chloro- and 2-bromopurine 2′-deoxyribofuranosides 9a and 10 , respectively, in aqueous solution, The photo reaction was further used for the synthesis of the compounds 3 and 4 starting with the amino-chloro-2′-deoxynucleosides 9b and 9c , respectively.     

Molecular and crystal structure of bis-quinolizidine immonium salts,III: Δ1(6)-dehydrosparteinium diperchlorate

¹⁽⁶⁾-Dehydrosparteinium diperchlorate, [C₁₅H₂₆N₂]²⁺·2ClO ₄ ^– , is tetragonal; space groupP4₃,a=9.368(1),c=21.719(3) Å,Z=4,V _c =1906.1(5) ų,D _x =1.51g cm^–3,(CuK)=35.0 cm^–1. The finalR=0.055 for 1259 independent reflections. Atoms C(3) and C(4), which are trans-annular to the immonium bond in ringA, are disordered, as was observed for other bis-quinolizidine immonium salts. RingsB, C, andD have sofa, chair and chair conformations, respectively. The quinolizidinium and quinolizidine moieties have planar and cis configurations. The title compound was obtained by mercuric acetate dehydrogenation of sparteine to ⁵-dehydrosparteine and protonation of the latter with methanolic solution of perchloric acid to pH=2.0. From¹³C NMR measurements in DMSO-²H₆ and in²H₂O it is evident that in solution the chair/chair cis-ring fusion is present within the protonated quinolizidine moiety-identical to that found in the crystalline state by X-ray analysis.     

Spectroscopic Investigation of the Electronic Structure of YN: The XΣ Ground State and the New B1, C1 and D1 Excited Electronic States

YN molecules were produced in a free jet molecular beam apparatus by a laser vaporizing yttrium metal in the presence of He doped with NH₃. Laser excitation spectra were observed in the range 18 250-19 850 cm⁻¹. The ground state was confirmed to have ¹Σ⁺ symmetry. The fundamental vibration in the ground state was measured to be 650.6(1) cm⁻¹. Three new electronic states, B1, C1, and D1, were observed at 18 974.7(1), 19 023.3(1), and 19 824.0(1) cm⁻¹, respectively. The fundamental vibrations and equilibrium internuclear distances were found to be 718.3(1) cm⁻¹ and 1.939(8) for the B1 state and 723.5(1) cm⁻¹ and 1.9194(3) for the C1 state. Two additional electronic states were identified with the help of a deperturbation procedure, one of which is either the ¹Σ⁺ or the ³Σ₀⁻ state. The newly observed electronic states cannot be accounted for based on the existing ab initio results. We expect that these states correlate with the excited asymptote Y(4d¹5s²²D)+N(²D).     

Surface-plasmon-coupled emission of quantum dots

We studied surface plasmon-coupled emission (SPCE) of semiconductor quantum dots (QDs). These QDs are water-soluble ZnS-capped CdSe nanoparticles stabilized using lysine cross-linked mercaptoundecanoic acid. The QDs were spin-coated from 0.75% PVA solution on a glass slide covered with 50 nm of silver and a 5-nm protective SiO(2) layer. Excited QDs induced surface plasmons in a thin silver layer. Surface plasmons emitted a hollow cone of radiation into an attached hemispherical glass prism at a narrow angle of 48.5 degrees. This directional radiation (SPCE) preserves the spectral properties of QD emission and is highly p-polarized irrespective of the excitation polarization. The SPCE spectrum depends on the observation angle because of the intrinsic dispersive properties of SPCE phenomenon. The remarkable photostability can make QDs superior to organic fluorophores when long exposure to the intense excitation is needed. The nanosize QDs also introduce a roughness near the metal layer, which results in a many-fold increase of the coupling of the incident light to the surface plasmons. This scattered incident illumination transformed into directional, polarized radiation can be used simultaneously with SPCE to develop devices based on both quantum dot emission and light scattered from surface plasmons on a rough surface.     

Interference of surface plasmon resonances causes enhanced depolarized light scattering from metal nanoparticles

We show that the strongly depolarized light scattering from noble metal particles is a result of interference of two surface plasmon resonances on the same particle. The maximum depolarization occurs between two resonances. Under favorable conditions the anisotropy of the scattering light can be much lower than what is possible for dielectric particles. This explanation is discussed in relation to earlier published experimental measurements. Comparison of experimental results with theoretical calculations provides information on the shape distribution of metallic particles in the suspension.     

Substituent Reactivity and Tautomerism of Isoguanosine and Related Nucleosides

The substituent reactivity and tautomerism of isoguanine nucleosides is studied. Benzoylation or tosylation of isoguanine nucleosides (pyridine, room temperature) yields the 2-benzoyl derivatives 7c, 11 , and 12 or the 2-tosyl compounds 13 and 14 . The isobutyrylation of the 6-amino group which did not occur under these conditions was induced in the presence of Me₃SiCl. In the absence of Me₃SiCl, the reactivity of isoguanine substituents decreases in the order from 2-oxo → 5′-OH → 3′-OH → 6-NH₂. From isoguanine nucleosides, the N¹-( 2b ), N³-( 17 ), N⁶-( 15a,b ), and 2-O-alkylated ( 3b ) derivatives were prepared. Their pK_a values were determined and the UV and ¹³C-NMR spectra compared with regard to the alkylation position. Also the tautomeric forms of isoguanine nucleosides were determined UV-spectrophotometrically in aqueous and nonaqueous solution. Isoguanosine ( 1a ), its 2′-deoxy analogue 1b as well as the N⁶-methyl- and 8-substituted derivatives form lactam tautomers in aqueous solution, whereas the lactim form is present in dioxane.