New synthesis of flavanones catalyzed by l-proline

l-Proline is utilized as an efficient organocatalyst for the synthesis of substituted flavanones and chalcones in good yields. The efficiency of the catalyst was proved with a variety of substrates ranging from electron-deficient to electron-rich aryl aldehydes and 2-hydroxyacetophenones.     

NDDO MO calculations

Three valence-only schemes based on the Roby version of NDDO MO theory, which differ only in the core-valence treatment and the choice of the basis set, have been applied to the study of molecules containing first-row atoms. Orbital energies, charge distribution, dipole moments, field gradients, and a few other one-electron molecular properties are calculated to satisfactory accuracy. The schemes appear unreliable in the prediction of barriers to internal rotation in molecules, presumably due to errors in the core contribution to the total energy. An alternative treatment is suggested.     

The superconducting penetration depth from the semiclassical model

The semiclassical approach of Onsager and Pippard has been very successful in relating the electronic properties of a normal metal to its band structure and fermi surface. This paper extends the method to a superconductor. A generalized London equation relating the supercurrent density to the vector potential is obtained, in terms of band and fermi surface parameters and an energy gap which may be anisotropic. The results allow an interpretation of measured penetration depths directly in terms of the electronic and gap structures of the superconductor. They lead to easy physical visualization of the origin of anisotropies and other features of measured penetration depths.     

Alternative mechanisms for cyanide anion exchange with acetonitrile

Mechanisms for the recently observed exchange of cyanide anion in acetonitrile have been investigated by means of semi-empirical MNDO molecular orbital calculations. A prototropic type mechanism $\text{via}$ a cyclopropane intermediate is the most likely candidate. The previously proposed concerted migration is indicated to be the least favourable reaction pathway.     

Evidence for laser-induced formation of solvated electrons in room temperature ionic liquids

The photolytic generation of solvated electrons was observed for the first time in two room temperature ionic liquids (RTILs), trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide (IL) and 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide (IL). A 70 fs UV-pulse was used to excite the RTILs, while the transient response was monitored in the visible and near-infrared spectral regions. Immediately after excitation, a pulse duration limited rise of the induced absorption indicated the formation of solvated electrons suggesting the existence of pre-formed traps in RTILs. A broad transient absorption spectrum with a full width at half maximum of about 0.9 eV, typical for solvated electrons, was reconstructed from the transient profiles. Wavelength-independent relaxation dynamics at longer delay times suggest a lifetime of solvated electrons in the ns regime in agreement with results from pulse radiolysis studies. Adding 1,1-dimethylpyrrolidinium iodide to IL led to an increase of the UV absorbance and consequently, to an increase of the yield of solvated electrons. Furthermore, this solute is an efficient electron scavenger causing the transients to decay within about 40 ps.     

Phase-incoherent superconducting pairs in the normal state of Ba(Fe(1-x)Co(x))₂As₂

The normal state properties of the recently discovered ferropnictide superconductors might hold the key to understanding their exotic superconductivity. Using point-contact spectroscopy we show that Andreev reflection between an epitaxial thin film of Ba(Fe(0.92)Co(0.08))?As? and a silver tip can be seen in the normal state of the film up to temperature T～1.3T(c), where T(c) is the critical temperature of the superconductor. Andreev reflection far above T(c) can be understood only when superconducting pairs arising from strong fluctuation of the phase of the complex superconducting order parameter exist in the normal state. Our results provide spectroscopic evidence of phase-incoherent superconducting pairs in the normal state of the ferropnictide superconductors.     

The structure and energetics of defects in liquid crystals

The study of defects in ordered media has become a subject of considerable interest to condensed matter physicists in recent years. This article presents a systematic account of the structures, energies and interactions of defects in the nematic, smectic, cholesteric and discotic phases of liquid crystals. Relevant experimental observations are also described.