Anion-pi interactions in cyanuric acids: a combined crystallographic and computational study

Several structures of pi complexes of isocyanuric acid and of several thio derivatives with anions have been computed by using high level ab initio calculations. The nature of the complexes has been studied by means of the method of molecular interaction potential with polarization (MIPp) and Bader's theory of atoms-in-molecules. These molecules form favorable complexes with anions and can be used as binding units for building receptors for the molecular recognition of anions. In several cases, the anion-pi interaction has been demonstrated experimentally by means of X-ray crystallography.     

Excited state properties of mixed phosphine 2-(2'-pyridyl)quinoline complexes of ruthenium(II)

Mixed ligand complexes of the type Ru(pq)(2)(PP)(2+) (pq = 2,2'-pyridylquinoline and PP = one bidentate or two monodentate phosphine ligands) have been prepared from the appropriate phosphine and Ru(pq)(2)Cl(2). The room temperature absorption spectra and low temperature (77 K) emission spectra, emission lifetimes, and quantum yields have been measured for the series of complexes and compared with those of Ru(pq)(3)(2+) and analogous Ru(bpy)(2)(PP)(2+) complexes (bpy = 2,2'-bipyridine) where possible. Emission spectra have been fit using a single mode Franck-Condon analysis. The visible absorption bands and emission bands are assigned to MLCT transitions that are blue shifted relative to Ru(pq)(3)(2+), while the emission lifetimes and quantum yields are increased. The trends in the nonradiative rate constants, k(nr), are described in terms of the energy gap, E(0), and the Huang-Rhys factor, S(M), which were obtained from the spectral fittings, and are correlated with the phosphine ligand structures.     

Asymptotics for Euclidean minimal spanning trees on random points

Summary Asymptotic results for the Euclidean minimal spanning tree onn random vertices inR ^d can be obtained from consideration of a limiting infinite forest whose vertices form a Poisson process in allR ^d. In particular we prove a conjecture of Robert Bland: the sum of thed'th powers of the edge-lengths of the minimal spanning tree of a random sample ofn points from the uniform distribution in the unit cube ofR ^d tends to a constant asn.Whether the limit forest is in fact a single tree is a hard open problem, relating to continuum percolation.Research supported by N.S.F. Grants MCS87-11426 and MCS 90-01710Research supported in part by N.S.F. Grant DMS88-12868, A.F.O.S.R. Grant 89-0301, ARO Grant DAAL03-89-G-0092 and NSA Grant MDA-904-H-2034     

Evaluating Suppliers of Complex Systems: A Multiple Criteria Approach

Organizations are frequently required to make long-term strategic choices regarding the design and manufacture of innovative process equipment and systems which address future needs. An important consideration in such choices is the evaluation of the unique capabilities of available suppliers. Typically, such evaluations must be made relative to multiple attributes or criteria. This paper examines one such supplier evaluation process in the context of a major international manufacturer of telecommunication equipment. The ad hoc method used by the organization to evaluate the multiple factors involved is presented, and an alternative, structured approach to deal with the multiple factors is then discussed.     

REPAIR OF UV-DAMAGED INCOMING PLASMID DNA IN Saccharomyces cerevisiae

A whole-cell transformation assay was used for the repair of UV-damaged plasmid DNA in highly transformable haploid strains of Saccharomyces cerevisiae having different repair capabilities. Six rad alleles were selected from the three epistasis groups: rad 1-1 and rad2-1 from the RAD3 group, rad6-1 and rad18-2 from the RAD6 group, and rad52-1 and rad54-1 from the RAD52 group. Cells carrying single, double and triple rad alleles were transformed to uracil prototrophy by centromeric plasmid DNA (YCp19) modified in vitro with UV (254 nm). Surviving fractions were calculated as the number of transformants at each fluence relative to the number of transformants with unirradiated plasmid DNA. The sensitivity of incoming DNA in single rad mutants shows that most repair is carried out by excision repair and a RAD18-dependent process. In the rad52-1 host, the sensitivity of incoming DNA was intermediate between those found in RAD+ and rad2-1 hosts, suggesting the involvement of a recombinational repair process. Non-epistatic interactions were observed between rad alleles belonging to different epistasis groups. This provides validation for the classification of the three epistasis groups concerning the repair of chromosomal DNA for UV-incoming DNA. In both rad1-1 rad6-1 and rad1-1 rad18-2 rad54-1 hosts, the mean fluence for one lethal event corresponds approximately to one pyrimidine dimer per plasmid molecule, indicating that they are absolute repairless hosts for incoming DNA. A comparison between cell and plasmid survival reveals that there are differences in the repairability of both chromosomal and incoming DNA. The large effect of rad6-1 mutation on cell survival and the small effect on incoming DNA suggest that, in the RAD+ strain, the RAD6 product may be essential for the repair processes which act on chromosomal DNA, but not for those which act on incoming DNA. It is proposed that in yeasts postreplication repair of incoming DNA is limited to supercoiled molecules with 1-2 pyrimidine dimers that can initiate replication.     

FLUORESCENCE PROPERTIES OF PORPHYRIN-GLOBIN FROM HUMAN HEMOGLOBIN

Fluorescence excitation and emission spectra, decays, and quantum yields are reported for the porphyrin-globin of hemoglobin (Hb^desFe) in aqueous solution of pH 8, at 4°C. A very weak fluorescence was observed in the UV (maximum at 334 nm), due to tryptophan and tyrosine residues, in addition to the strong porphyrin emission in the visible (maxima at 624 and 692 nm) reported previously. The absorption and fluorescence properties of the porphyrins of Hb^desFe were compared to those for free porphyrin in organic solvents and in aqueous solution. The close similarity of the fluorescence decays and quantum yields in Hb^desFe and in solution indicate the absence of stronger, specific porphyrin-protein interactions; however, slight spectral shifts point to the existence of water molecules in the Hb^desFe porphyrin environment. The fluorescence study also demonstrates the existence of efficient Trp-porphyrin energy transfer of Förster type. The extent of transfer is in satisfactory agreement with the value expected from crystallographic data for hemoglobin. The results are discussed and compared to previous fluorescence studies of hemoglobin and apohemoglobin. An improved method for the preparation of Hb^desFe is reported.     

Synthesis of the Salmonella type E(1) core trisaccharide as a probe for the generality of 1-(benzenesulfinyl)piperidine/triflic anhydride combination for glycosidic bond formation from thioglycosides

A synthesis of a chromogenic glycoside of the Salmonella anatum group E(1) core trisaccharide is presented in which all three glycosidic bonds, a 1,2-cis-equatorial, a 1,2-trans-axial, and a 1,2-trans-equatorial linkage representing three of the four main classes of glycosidic bond, are formed with thioglycoside donors activated under a single set of conditions by the combination of 1-(benzenesulfinyl)piperidine and trifluoromethanesulfonic anhydride. 2,3-O-Carbonyl- and 2,3-O-isopropylidene-alpha-L-rhamnopyranosyl thioglycosides are found to be highly alpha-selective rhamnosyl donors under these conditions.     

Push-pull effects and emission from ternary complexes of platinum(II), substituted terpyridines, and the strong-field cyanide ion

As part of an effort to develop new lumaphors involving late transition metal ions, this report describes the synthesis and characterization of the first platinum(II) derivatives containing 2,2':6',2'-terpyridine (trpy) and cyanide as co-ligands. According to existing models, including cyanide in the coordination sphere should raise the energies and minimize the influence of short-lived d-d excited states that otherwise compromise the excited-state lifetime. Both [Pt(trpy)(CN)]+ and the 4'-cyano-2,2':6',2'-terpyridine analogue [Pt(CN-T)(CN)]+ are emissive in dichloromethane solution, but the signals are weak. Part of the problem is that the d-pi* charge-transfer excited states also rise in energy, so that the emission actually originates from a (3)pi-pi* state with a relatively low radiative rate constant. However, another member of the series, the 4'-dimethylamino-2,2':6',2'-terpyridine (dma-T) derivative [Pt(dma-T)(CN)]+, proves to be a very promising platform with an emission quantum yield of phi= 0.26 and an excited-state lifetime of tau = 22 micros in room-temperature, deoxygenated dichloromethane solution. In the dma-T complex the electron-rich dimethylamino substituent provides the basis for an emissive, but largely ligand-based, charge-transfer excited state. The orbital parentage is such that the photoluminescence persists in donating solvents like dimethylformamide, which ordinarily quenches d-pi* excited states in complexes of this type.     

First-order semidefinite programming for the direct determination of two-electron reduced density matrices with application to many-electron atoms and molecules

Direct variational calculation of two-electron reduced density matrices (2-RDMs) for many-electron atoms and molecules in nonminimal basis sets has recently been achieved through the use of first-order semidefinite programming [D. A. Mazziotti, Phys. Rev. Lett. (in press)]. With semidefinite programming, the electronic ground-state energy of a molecule is minimized with respect to the 2-RDM subject to N-representability constraints known as positivity conditions. Here we present a detailed account of the first-order algorithm for semidefinite programming and its comparison with the primal-dual interior-point algorithms employed in earlier variational 2-RDM calculations. The first-order semidefinite-programming algorithm, computations show, offers an orders-of-magnitude reduction in floating-point operations and storage in comparison with previous implementations. We also examine the ability of the positivity conditions to treat strong correlation and multireference effects through an analysis of the Hamiltonians for which the conditions are exact. Calculations are performed in nonminimal basis sets for a variety of atoms and molecules and the potential-energy curves for CO and H(2)O.     

The dissociation quotients of formic acid in sodium chloride solutions to 200°C

The dissociation quotients of formic acid were measured potentiometrically from 25 to 200°C in NaCl solutions at ionic strengths of 0.1, 0.3 1.0, 3.0, and 5.0 mol-kg^–1. The experiments were carried out in a concentration cell with hydrogen electrodes. The resulting molal acid dissociation quotients for formic acid, as well as a set of infinite dilution literature values and a calorimetrically-determined enthalpy of reaction, were fitted by an empirical equation involving an extended Debye Hückel term and seven adjustable parameters involving functions of temperature and ionic strength. This regressional analysis yielded the following thermodynamic quantities for 25°C: logK=–3.755±0.002, H^o=–0.09±0.15 kJ-mol^–1, S^o=–72.2±0.5 J-K^–1-mol^–1, and C _p ^o =–147±4 J-K^–1-mol^–1. The isocoulombic form of the equilibrium constant is recommended for extrapolation to higher temperatures.