Electronic absorption spectra and the structure of the ligand in metal complexes with purpurin

Purpurin complexes with metals are formed due to metal coordination at the peri- or ortho-hydroxycarbonyl group involving anthraquinoid tautomers. The _l,* bands of metal purpurinates correspond to 1,10- and 1,4-anthraquinoid anions, 9,10-anthraquinoid structures are not typical of them.Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 3, 2005, pp. 237–240.Original Russian Text Copyright © 2005 by Fain, Zaitsev, Ryabov.This revised version was published online in April 2005 with a corrected cover date.This revised version was published online in April 2005 with a corrected cover date.     

Lattice spacing and isosteric heat of argon monolayers below 50K

The lattice constants of 2D solid Ar adsorbed on graphite are reported for many combinations of T and vapor pressure P, where 30 < T < 50 K and 10^?7 < P < 10^?4 Torr. Low-energy electron-diffraction patterns of high resolution provide accuracy to ±0.01 Å. Straight lines are fitted through points of constant lattice spacing on a log P versus T^?1 plot; their slopes and intercepts yield isosteric heats of adsorption as well as estimates of the 2D isothermal compressibility of the Ar solid. Comparison is made to calorimetric measurements of isosteric heat, and to a previous LEED compressibility measurement performed isothermally at 42 K.     

Diversity order gain for narrow-band multiuser communications withpre-combining group detection

In narrow-band multiuser communication systems with fading diversity, it is shown that pre-combining group detection can bridge the diversity-order gap in performance between the optimum and linear detectors. For a system with M diversity channels, the group detector diversity order is M-|G|, where |G| is the interfering group size, a design parameter. Group detection thus provides a more substantial improvement in performance in narrow-band channels over linear detection than in wide-band channels in which the diversity orders of the optimal and linear detectors are equal. Here, the complexity of the receiver is a new parameter which, in addition to the number of antennas, can be used to control the diversity order. Exact formulas for the pairwise-error probabilities and bounds for the bit-error rate are obtained, and numerical results are shown     

Determination of the sequence of tautomeric and conformational transformations of organic compounds

Russian Journal of General Chemistry -     

Overlayer-substrate spacing for argon and krypton on graphite determined by leed intensity analysis

Low-energy electron diffraction has been used to measure the spacing between monolayers of Ar or Kr and the outermost basal plane of a graphite substrate. The intensity of the (00) beam is recorded as a function of energy for graphite with and without the overlayer present. Using the analysis method of Cohen, Unguris and Webb, we find that the ratio of the two intensity curves shows an oscillation whose period is inversely proportional to the desired spacing. For Ar, we find a value of 3.2 ± 0.1 Å at 42 K. The results for Kr are somewhat ambiguous but suggest a value of 3.3 ± 0.1 A at 52 K.     

Quantum-chemical and correlation study of quinizarine protonation

Fine structure of quinizarine π _l ,π*-absorption in sulfuric acid is defined by isomerism of its protonated products. The monocation is an isomeric mixture, the isomers differ by the charge localization on various carbon atom: 1,4,9-trihydroxy-10-oxo-10H-anthracen-1-onium, 1,9,10-trihydroxy-4-oxo-4H-anthracen-1-onium, and 4,9,10-trihydroxy-10-oxo-10H-anthracen-1-onium. For the dication the structures found were exclusively the conformers of 1,4,9,10-tetrahydroxyanthrqacene 1,10-dication containing 0 to 2 intramolecular hydrogen bonds. The protonation leads to a shift of the tautomeric and conformational equilibria characteristic of quinizarine.     

Prediction of cyclohexane-water distribution coefficient for SAMPL5 drug-like compounds with the QMPFF3 and ARROW polarizable force fields

We present the performance of blind predictions of water—cyclohexane distribution coefficients for 53 drug-like compounds in the SAMPL5 challenge by three methods currently in use within our group. Two of them utilize QMPFF3 and ARROW, polarizable force-fields of varying complexity, and the third uses the General Amber Force-Field (GAFF). The polarizable FF’s are implemented in an in-house MD package, Arbalest. We find that when we had time to parametrize the functional groups with care (batch 0), the polarizable force-fields outperformed the non-polarizable one. Conversely, on the full set of 53 compounds, GAFF performed better than both QMPFF3 and ARROW. We also describe the torsion-restrain method we used to improve sampling of molecular conformational space and thus the overall accuracy of prediction. The SAMPL5 challenge highlighted several drawbacks of our force-fields, such as our significant systematic over-estimation of hydrophobic interactions, specifically for alkanes and aromatic rings.     

Discretization of electronic states in large InAsP/InP multilevel quantum dots probed by scanning tunneling spectroscopy

The topography and the electronic structure of InAsP/InP quantum dots are probed by cross-sectional scanning tunneling microscopy and spectroscopy. The study of the local density of states in such large quantum dots confirms the discrete nature of the electronic levels whose wave functions are measured by differential conductivity mapping. Because of their large dimensions, the energy separation between the discrete electronic levels is low, allowing for quantization in both the lateral and growth directions as well as the observation of the harmonicity of the dot lateral potential.