Quantum-chemical investigation of the influence of the vacant orbitals of the heteroatom on the shaping of the electronically excited states and luminescence-spectroscopic properties of triphenylgallium

The electronic structure of the triphenylgallium molecule in the ground and electronically excited states has been calculated in the framework of the CNDO/S-CI method in sp and spd bases. The electron density in the AO's and atoms, the first ionization potential the energies and matrices of the changes in the electron density of the S_n and T_n states, the matrix elements of the spin-orbit coupling operator $$ " align="middle" border="0"> and $$ " align="middle" border="0"> , the intensities and directions of polarization of the S_n S_o and T₂ T_n transitions, and the phosphorescence lifetimes have been calculated. The following spectroscopic parametrization has been proposed for the Ga atom in the sp and spd bases: 1/2(I_s + a_s) = 10.075 eV, 1/2(I_p + A_p) = 4.265 eV, 1/2(I_d + A_d)=0, _Ga ^p =5 eV, _Ga ^d =O, _Ga ^sp =–7 eV, _Ga ^spd =–10 eV, and _s,p,d=1.433. The influence of the d AO's on the shaping of the orbital nature of the S₁ and T₂ states has been analyzed.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 5, pp. 584–590, September–October, 1985.     

An evaluation of cellulose as a substrate for room-temperature phosphorescence

Cotton-linter pulps, wood pulps and several filter papers have been evaluated as substrates for room-temperature phosphorescence. A variety of chemical treatments of one filter paper is discussed in terms of reducing the background phosphorescence of the cellulose and in evaluating possible trace contaminants in cotton fibres. In order to account for uniformity of filter paper used in room temperature phosphorescence, a final evaluation of several different lots of one type of filter paper is presented.     

Determination of vanadium by solid-phase spectrophotometry after its preconcentration as an Eriochrome Cyanine R complex on a dextran-type exchanger

A method is described for the determination by solid phase spectrophotometry (SPS) of trace amounts of vanadium in natural water and crude petroleum samples. The procedure is based on fixation on a dextran-type anion exchanger of the complex V(IV)-Eriochrome Cyanine R. The absorbance of the gel, at 563 and 750 nm, packed in a 1 mm cell, is measured directly. Vanadium can be determined in the 0.6-25.0 mug l(-1) range with a relative standard deviation of 2.2%. The comparison of the SPS method and the gallic acid persulphate method shows that the linearity, analytical sensitivity and precision were better for the SPS method, and that the latter method has lower detection and quantification limits than the gallic acid persulphate method.     

Computational modeling of amorphous silica. 4. Modeling the initial structures. Aerogel

A qualitative classification algorithm for constructing large clusters of aerogel model structures is used. According to this algorithm, aerogel is classified with polysilicas characterized by a packing of long chains of silicon-oxygen tetrahedra. A model of the aerogel structure is suggested, according to which aerogel is described as a set of cross-linked chains forming double or more complex ribbons. This model is in agreement with the conclusions drawn from analysis of the vibration spectra of its components. Institute of Surface Chemistry, Ukrainian Academy of Sciences. Russian University of Peoples' Friendship. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 3, pp. 35–38, May–June, 1994. Translated by O. Kharlamova     

Iodine(V) reagents in organic synthesis. Part 3. New routes to heterocyclic compounds via o-iodoxybenzoic acid-mediated cyclizations: generality, scope, and mechanism

The discovery and development of the o-iodoxybenzoic acid (IBX) reaction with certain unsaturated N-aryl amides (anilides) to form heterocycles are described. The application of the method to the synthesis of delta-lactams, cyclic urethanes, hydroxy amines, and amino sugars among other important building blocks and intermediates is detailed. In addition to the generality and scope of this cyclization reaction, this article describes a number of mechanistic investigations suggesting a single electron transfer from the anilide functionality to IBX and implicating a radical-based mechanism for the reaction.     

Concise synthesis of acyl migration-blocked 1,1-difluorinated analogues of lysophosphatidic acid

Lysophosphatidic acid (LPA, 1- or 2-acyl-sn-glycerol 3-phosphate) is an important phospholipid mediator produced by activated platelets and by ovarian cancer cells. Efforts to understand LPA signaling through G-protein-coupled receptors are hampered by the facile acyl migration that results in equilibration to a mixture of the 1- or 2-acyl species under physiological conditions. We describe a new and efficient route to enantiomerically homogeneous lysophospholipid analogues from D-mannitol 1,2:5,6-bis-acetonide to give two 1,1-difluorodeoxy analogues of (2R)-acyl-sn-glycerol 3-phosphate. These compounds are migration-blocked analogues of the labile sn-2 LPA species. The (19)F NMR of diastereotopic fluorines of the difluoromethyl group shows an unexpected solvent dependence.     

Biomimetic dyes as affinity chromatography tools in enzyme purification

Affinity adsorbents based on immobilized triazine dyes offer important advantages circumventing many of the problems associated with biological ligands. The main drawback of dyes is their moderate selectivity for proteins. Rational attempts to tackle this problem are realized through the biomimetic dye concept according to which new dyes, the biomimetic dyes, are designed to mimic natural ligands. Biomimetic dyes are expected to exhibit increased affinity and purifying ability for the targeted proteins. Biocomputing offers a powerful approach to biomimetic ligand design. The successful exploitation of contemporary computational techniques in molecular design requires the knowledge of the three-dimensional structure of the target protein, or at least, the amino acid sequence of the target protein and the three-dimensional structure of a highly homologous protein. From such information one can then design, on a graphics workstation, the model of the protein and also a number of suitable synthetic ligands which mimic natural biological ligands of the protein. There are several examples of enzyme purifications (trypsin, urokinase, kallikrein, alkaline phosphatase, malate dehydrogenase, formate dehydrogenase, oxaloacetate decarboxylase and lactate dehydrogenase) where synthetic biomimetic dyes have been used successfully as affinity chromatography tools.     

Determination of low levels of an antioxidant in polyolefins by large-volume injection temperature-programmed packed capillary liquid chromatography

Sub-ambient column temperatures, promoting strong interactions between the analyte and the stationary phase material, were utilized to focus large volumes of the polyolefin antioxidant Irganox 1076 [benzenepropanoic acid, 3.5-bis(1,1-dimethylethyl)-4-hydroxy-, octadecyl ester] on the column inlet, using pure acetonitrile as sample solvent and mobile phase. Injection volumes up to 100 microl were successfully employed on a 50 cm x 320 microm I.D. capillary column packed with 5 microm Kromasil 100 ODS particles. Irganox 1076 was eluted after completed injection by temperature programming, using a temperature program from 7 to 90 degrees C, in 3 degrees C min(-1). UV detection, using a low-dispersion "U"-shaped flowcell, was performed at 280 nm. The method was applied for the determination of Irganox 1076 that was extracted from low-density polyethylene (0.6 ppm, w/w). Both Soxhlet and microwave-aided solvent extractions were performed, using chloroform and acetonitrile as solvents, respectively. The microwave-aided extraction with acetonitrile was found to give approximately the same yield as the standard Soxhlet reference method. Consequently, small volumes of acetonitrile could be used both as extraction solvent, sample solvent and mobile phase, simplifying the analysis process. The mass limit of detection of the method was found to be 3.3 ng, corresponding to a concentration limit of detection of 33 ng ml(-1), utilizing an injection volume of 100 microl. The within and between day precision of retention times displayed relative standard deviations below 1.2%.     

The pH-dependent role of superoxide in riboflavin-catalyzed photooxidation of 8-oxo-7,8-dihydroguanosine

[reaction: see text]. The riboflavin-catalyzed photooxidation of 2',3',5'-tri-O-acetyl-8-oxo-7,8-dihydroguanosine generates a radical intermediate that is competitively trapped by H(2)O, O2(-)(*), or O(2). The products of H(2)O trapping have been previously described as the spiroiminodihydantoin (pH >or= 7) and iminoallantoin/guanidinohydantoin (pH < 7) nucleosides. Trapping by O2(-)(*) leads to the oxaluric acid (pH or= 8.6) pathways (R' ', R' ' = H or 2,3,5-tri-O-Ac-ribofuranosyl). The pH-dependent role of superoxide was probed using Mn-SOD and compared to guanosine and 8-methoxyguanosine photooxidation.     

Application of parallel processing techniques to improving the efficiency of MM2 molecular mechanics calculations

The application of parallel processing techniques to molecular mechanics calculations is evaluated. Using the standard molecular mechanics package, MM2, four different parallel versions of the program are implemented in a four-processor computing environment. A set of 529 test structures is used to compare the efficiency of the parallel versions of MM2 to a standard serial version of the program. Statistics describing execution times and program execution cycles are gathered and analyzed. The effects of parallel processing overhead and computer system load are explored, and the practical utility of parallel processing in molecular mechanics is estimated. The results of these parallelization experiments indicate that for geometry optimizations requiring significant amounts of computing time an improvement in program execution speed approaching 50% is realizable. © 1993 John Wiley & Sons, Inc.