Anisotropic Charge Modulation in Ladder Planes of Sr_14-xCa_xCu_24O_41

The charge response of the ladders in Sr_14-xCa_xCu_24O_41 is characterized by dc resistivity, low frequency dielectric and optical spectroscopy in all three crystallographic directions. The collective charge-density wave screened mode is observed in the direction of the rungs for x=0, 3 and 6, in addition to the mode along the legs. For x=8 and 9, the charge-density-wave response along the rungs fully vanishes, while the one along the legs persists. The transport perpendicular to the planes is always dominated by hopping.Comment: 4 pages, 3 figures, submitted to PRB R

Comparison of two views regarding the nature of the X-H…phenyl interaction

There are two approaches in the analysis of the nature of the X-H…phenyl interactions. One is based on the assumption that atoms, bonds and the π plane belonging to the phenyl ring, are the points which can be involved in the interaction. The other states that center of the phenyl ring is the point acceptor. In this paper we compare two views using the directionality of the X-H vector and length/angle correlations relative to both assumed point acceptors. The results suggest that on the basis of this methodology there is no clear answer regarding the nature of the acceptor site in the phenyl ring.Physical chemistry 2004 : 7th international conference on fundamental and applied aspects of physical chemistry; Belgrade (Serbia); 21-23 September 200

Hole-depletion of ladders in Sr14_{14}Cu24_{24}O41_{41} induced by correlation effects

The hole distribution in Sr$_{14}$Cu$_{24}$O$_{41}$ is studied by low temperature polarization dependent O K Near-Edge X-ray Absorption Fine Structure measurements and state of the art electronic structure calculations that include core-hole and correlation effects in a mean-field approach. Contrary to all previous analysis, based on semi-empirical models, we show that correlations and antiferromagnetic ordering favor the strong chain hole-attraction. For the remaining small number of holes accommodated on ladders, leg-sites are preferred to rung-sites. The small hole affinity of rung-sites explains naturally the 1D - 2D cross-over in the phase diagram of (La,Y,Sr,Ca)$_{14}$Cu$_{24}$O$_{41}$Comment: 6 pages, 8 figure

Application of 4-Chlorobutyryl Group as Amino Protective Group in the Synthesis of 7-(D-2-Amino-2-phenylacetamido)-3-methyl-3-cephem-4-carboxylic Acid (Cephalexin)

4-Chlorobutyryl group was used as amino protective group for n(-)-alpha-phenylglycine (I). N-Hydroxysuccinimide ester III was prepared in high yield but partly racemic. Reaction of III or IV with V afforded epimeric mixtures of N,0-protected cephalexins (VI), which can be separated by crystallisation. The 4-chlorobutyryl group in VII was removed under very mild conditions giving cephalexin (IX) in good yield and purity

Molecular Mechanics and SCF MO Conformational Analysis of Indol-3-ylacetic Acid Phytohormone (Auxin)

Conformational analysis of indol-3-ylacetic acid (IAA)-plant growth hormone has been performed using molecular mechanics and the ab initio SCF MO theory. The equilibrium geometry of IAA has been determined. Relative energies of alternative conformations, their charge distribution, dipole moment and energy barriers between them have been calculated. The position of the carboxyl group relative to the indole ring depends on two torsion angles, Tl(C2-C3-C8-C9) and T2(C3-C8-C9=02). Rotational barriers for these two angles were explored and it emerged that both rotations (about the C3-C8and C8-C9 bonds) can be accomplished in a reasonable time period at room temperature (the barrier height is about 4.6-10.9 (TI) and 1.7-3.8 (T2) kJ/mol respectively, according to ab initio calculations. Ab initio (GAMESS)and molecular mechanics (DISCOVER (CVFF and cff91), SYBYL(TRIPOS) and MM2(87), calculations revealed qualitatively the same shape of potential energy surface (E =f(Tl, T2)). However, energy differences between various conformations depend on the basis set (ab initio calculations) and force field (molecular mechanics) used