Brillouin scattering study of phase transitions in LiK0.80(NH4)0.20SO4 mixed crystals

Brillouin spectroscopy was used to study the phase transitions of LiK_0.80(NH₄)_0.20SO₄ mixed crystals in the temperature range 10-300 K. The relevant elastic stiffness coefficients were evaluated at room temperature. The quasi-longitudinal γ₁₆ and the quasi-transverse γ₁₇ mode frequencies were measured in the above temperature range. From their frequency vs. temperature curve, three different phase transitions were determined. Two of the four phases presented by the crystal were found to be ferroelastic. The observed phases are tentatively assigned through a comparison with the phase transitions undergone by LiKSO₄ and LiK_0.96(NH₄)_0.04SO₄ crystals. An anomalous behavior of the Brillouin linewidth near the 260 K phase transition was observed.     

New sources of steroid sapogenins—XIX: 20S-hydroxyvespertilin,a new steroid lactone from Solanum vespertilio

From stems and leaves of Solanum vespertilio Ait. 20S-hydroxyvespertilin (1c), a new steroid lactone, has been isolated and the structure confirmed by synthesis of its dihydro derivative from tigogenin. 1c proved to be identical with a dihydroxy lactone obtained by Gashchenko et al as by-product of the conversion of diosgenin to 16-dehydropregnenolone. The stereochemistry at C₂₀ proposed by these authors is revised.     

Carbon-13 NMR study of (20,24)-epoxydammarane triterpenes

Assignments of the ¹³C NMR signals of the dammarane triterpenes, 3β,25,30-trihydroxy-(20R,24R)-epoxydammaran-16-one 3,30-diacetate (trevoagenin A diacetate) (2), its 20S-isomer (trevoagenin B diacetate) (3) and their related (20R)-3β,30-diacetoxy-16-oxo-25,26,27-trisnordammarane-24,20-lactone (4) and its 20S-isomer (5) have been achieved. Suitable tetrahydrofuran models have been synthesized in order to aid the ¹³C NMR assignments of the side-chain carbons of the above-mentioned compounds. The remarkable chemical shift differences observed for C-21 and C-22 between each pair of the C-20 epimers (2, 3 and 4, 5) allowed the confirmation of the C-20 stereochemistry of these ocotillol-type dammarane triterpenes.     

Pseudo-molecular approach for the elastic constants of nematic liquid crystals interacting via anisotropic dispersion forces

The bulk and the surface-like elastic constants of a nematic liquid crystal are calculated for an ensemble of particles interacting via anisotropic dispersion forces using the pseudo-molecular method. The geometrical anisotropy of the molecules is also taken into account in the calculations by choosing a molecular volume of ellipsoidal shape. Analytical expressions for the elastic constants are obtained as a function of the eccentricity in the molecular volume shape. The method allows one to explore the dependence on the molecular orientation with respect to the intermolecular vector by analyzing the magnitude and the behaviour of macroscopic elastic parameters defining the nematic phase.     

Phosphate group vibrational signatures of the osteoporosis drug alendronate

The infrared (IR) and Raman spectra of the osteoporosis drug alendronate in the monosodium trihydrate alendronate crystal were measured. In order to interpret them, density functional theory (DFT) calculations for the solvated alendronate molecule were performed following the structural features revealed by X‐ray data. A comparison between the DFT‐calculated IR and Raman of the converged species and the measured spectra unveils relevant phosphate group signatures in the 400–1400 cm⁻¹ wavenumber range, especially IR absorption bands at 1015, 1049, 1067, 1131, 1177, and 1235 cm⁻¹, which were related to CP and OP bond length stretching, and Raman lines at 449, 661, and 969 cm⁻¹, involving phosphate scissors and bond length vibrations. A comparison with experimental data of alendronate incorporated into hydroxyapatite (HAP) indicates that, for wavenumbers below 1500 cm⁻¹, the interaction of alendronate with HAP does not affect significantly the alendronate vibrational spectra, while for the 1600–3000 cm⁻¹ interval the interaction with HAP changes the normal mode wavenumbers by about −100 cm⁻¹. Copyright © 2014 John Wiley & Sons, Ltd.     

Divergence symmetries of critical Kohn-Laplace equations on Heisenberg groups

We show that every Lie point symmetry of semilinear Kohn-Laplace equations with a power-law nonlinearity on the Heisenberg group H ¹ is a divergence symmetry if and only if the corresponding exponent takes a critical value.     

Tuning of the excitation wavelength from UV to visible region in Eu(3+)-β-diketonate complexes: comparison of theoretical and experimental photophysical properties

A novel class of efficient visible light sensitized antenna complexes of Eu(3+) based on the use of a series of highly conjugated β-diketonates, namely, 1-(1-phenyl)-3-(2-fluoryl) propanedione, 1-(2-naphthyl)-3-(2-fluoryl)propanedione, 1-(4-biphenyl)-3-(2-fluoryl) propanedione, and 2,2'-bis(di-p-tolylphosphino)-1,1'-binaphthyl oxide as an ancillary ligand has been designed, synthesized, characterized and their photophysical properties investigated. The coordination geometries of the typical Eu(3+) complexes were calculated using the Sparkle/PM3 model. Photophysical properties of europium complexes benefit from adequate protection of the metal by the rigid phosphine oxide ligand against non-radiative deactivation and efficient ligand-to-metal energy transfer exceeding 50% as compared to precursor samples. The replacement of the phenyl group with the naphthyl or biphenyl groups in the 3-position of the fluoryl based β-diketonate ligand remarkably extends the excitation window of the corresponding Eu(3+) complexes towards the visible region (up to 500 nm). The highly conjugated β-diketonate ligands sensitize efficiently the luminescence of Eu(3+) ions with quantum yields ranging from 19 to 43 % in the solid state, which is among the highest reported for a visible sensitized Eu(3+)complex. The theoretical quantum efficiencies from the Sparkle/PM3 structures are in good agreement with the experimental values, clearly attesting to the efficacy of the theoretical models.