Comparative substoichiometric extraction of zinc with potassium salts of ethyl,propyl, butyl,pentyl and benzyl xanthates

The comparative extractability of zinc with potassium salts of ethyl, propyl, butyl, pentyl, and benzyl xanthates from the pH range of 3.5–9.0 into chloroform has been studied, employing a sensitive and rapid substoichiometric radiochemical method. The extent of reproducibility was tested in each case. The effect of associated ions on the extraction was studied. The amount of zinc present in the standard solutions was determined employing each xanthate separately. The zinc content present in geological water samples in and around Tirupati was determined by the method developed and compared with the values obtained by Atomic Absorption Spectrophotometry.     

Natural product hybrids as new leads for drug discovery

Natural products play an important role in the development of drugs, especially for the treatment of infections and cancer, as well as immunosuppressive compounds. However, the number of natural products is limited, whereas millions of hybrids as combinations of parts of different natural products can be prepared. This new approach seems to be very promising in the development of leads for both medicinal and agrochemical applications, as the biological activity of several new hybrids exceeds that of the parent compounds. The advantage of this concept over a combinatorial chemistry approach is the high diversity and the inherent biological activity of the hybrids.     

Aromaticity in azlactone anions and its significance for the Erlenmeyer synthesis

Azlactone anions—the key intermediates in the classical Erlenmeyer synthesis of amino acids—apparently possess aromatic stabilization, as indicated by the relative rate of base catalyzed deuterium exchange in the following analogs: 1-methyl-2-phenyl-5(4H)-imidazolone > 2-phenyl-5(4H)-oxazolone (azlactone) > 3,3-dimethyl-2-phenyl-4(3H)-pyrrolone. This is paralleled by the relative rate of condensation of these compounds with hexadeuteroacetone. Reported pK_a data also suggest that the azlactone products of the Erlenmeyer synthesis are analogs of the fulvenes.     

PDDG/PM3 and PDDG/MNDO: improved semiempirical methods

Two new semiempirical methods employing a Pairwise Distance Directed Gaussian modification have been developed: PDDG/PM3 and PDDG/MNDO; they are easily implemented in existing software, and yield heats of formation for compounds containing C, H, N, and O atoms with significantly improved accuracy over the standard NDDO schemes, PM5, PM3, AM1, and MNDO. The PDDG/PM3 results for heats of formation also show substantial improvement over density functional theory with large basis sets. The PDDG modifications consist of a single function, which is added to the existing pairwise core repulsion functions within PM3 and MNDO, a reparameterized semiempirical parameter set, and modified computation of the energy of formation of a gaseous atom. The PDDG addition introduces functional group information via pairwise atomic interactions using only atom-based parameters. For 622 diverse molecules containing C, H, N, and O atoms, mean absolute errors in calculated heats of formation are reduced from 4.4 to 3.2 kcal/mol and from 8.4 to 5.2 kcal/mol using the PDDG modified versions of PM3 and MNDO over the standard versions, respectively. Several specific problems are overcome, including the relative stability of hydrocarbon isomers, and energetics of small rings and molecules containing multiple heteroatoms. The internal consistency of PDDG energies is also significantly improved, enabling more reliable analysis of isomerization energies and trends across series of molecules; PDDG isomerization energies show significant improvement over B3LYP/6-31G* results. Comparison of heats of formation, ionization potentials, dipole moments, isomer, and conformer energetics, intermolecular interaction energies, activation energies, and molecular geometries from the PDDG techniques is made to experimental data and values from other semiempirical and ab initio methods.     

Phonons in TiO2---SiO2 glasses

We report the infrared and Raman studies of titanium doped vitreous silica glasses for a number of titanium concentrations. The vibrational modes associated with the randomly oriented chains of SiO₄ tetrahedra show broadenings and shifts. The LO-TO splitting of some Raman active modes decreases with increasing titanium concentration. This is attributed to the decrease in long-range coulomb fields associated with the chains of SiO₄ tetrahedra which are broken by the titanium atoms. The results are discussed in the context of random network models. An increase in the average intertetrahedral angle of the SiO₄ network is calculated from the data. This explains the anomalous decrease in the density of TiO₂---SiO₂ glasses with increasing titanium content. We identify two new modes associated with the distorted titanium tetrahedra. A polarized Raman mode at 1115 cm⁻¹ which is infrared inactive and an unpolarized Raman mode at 945 cm⁻¹ which is infrared active are observed.     

Conductance studies in a double-bend quantum structure

Transport phenomena in a double-bend quantum structure fabricated in the two-dimensional electron gas of a modulation doped GaAs/AlGaAs structure, are studied experimentally. The structure consists of an electrostatically defined quantum dot with two one-dimensional wires connected on opposite corners of the dot. The current–voltage characteristics of such devices exhibit quantized conductance breakdown (non-linear behavior), conductance variation with confinement, and non-linear and asymmetric behavior at high bias condition. Low temperature conductance of this structure shows evidence of resonant tunneling, while the peaks of the conductance vary with temperature.