Double parton scattering inpp collisions at\sqrt s = 63 GeV

In a study ofpp collisions at \(\sqrt s = 63\) GeV with more than 29 GeV total transverse energy emitted into 1.8 units of rapidity in the central region, we have extracted a sample of 4-jet events and compared it with models of the two sources of 4-jet production: double bremsstrahlung and double parton scattering. The data cannot be described by bremsstrahlung alone, and we extract the fraction of 4-jet events attributed to double parton scattering for various definitions of the 4-jet sample. We determine the double parton scattering/2-jet yield ratio, and this leads to a determination of the proton radius. We discuss the implications of our observations for the general understanding of high-ΣE _T events.     

Ligand binding affinity prediction by linear interaction energy methods

A recent method for estimating ligand binding affinities is extended. This method employs averages of interaction potential energy terms from molecular dynamics simulations or other thermal conformational sampling techniques. Incorporation of systematic deviations from electrostatic linear response, derived from free energy perturbation studies, into the absolute binding free energy expression significantly enhances the accuracy of the approach. This type of method may be useful for computational prediction of ligand binding strengths, e.g., in drug design applications.     

Computation of affinity and selectivity: Binding of 2,4-diaminopteridine and 2,4-diaminoquinazoline inhibitors to dihydrofolate reductases

Binding energy calculations for complexes of mutant and wild-type human dihydrofolate reductases with 2,4-diaminopteridine and 2,4-diaminoquinazoline inhibitors are reported. Quantitative insight into binding energetics of these molecules is obtained from calculations based on force field energy evaluation and thermal sampling by molecular dynamics simulations. The calculated affinity of methotrexate for wild-type and mutant enzymes is reasonably well reproduced. Truncation of the methotrexate glutamate tail results in a loss of affinity by several orders of magnitude. No major difference in binding strength is predicted between the pteridines and the quinazolines, while the N-methyl group present in methotrexate appears to confer significantly stronger binding. The recent improvement, which is used here, of our linear interaction energy method for binding affinity prediction, as well as problems with treating charged and flexible ligands are discussed. This approach should be suitable in a drug discovery context for prediction of binding energies of new inhibitors prior to their synthesis, when some information about the binding mode is available.     

Determination of dimethylarsinic acid in seawater in the sub-ppb range by electrothermal atomic absorption spectrometry after preconcentration on an io

Dimethylarsinate (DMA) is preconcentrated on a strong cation-exchange resin. By optimizing the elution parameters, DMA can be separated from other arsenicals and sample components, such as group I and II metals, which can interfere in the final determination. Graphite-furnace atomic absorption spectrometry is used as a sensitive and specific detector for arsenic. The described technique allows DMA to be determined in a sample (20 ml) containing a 10⁵-fold excess of inorganic arsenic with a detection limit of 0.02 ng As ml^-1.     

Single-point titrations: Part 4. Determination of acids and bases with flow injection analysis

A single-point titrimetric system for acids and bases based on the flow injection principle is reported. The sample (30 μl) is introduced into a water stream with a pneumatic injector; this stream reacts with a linear acidic or basic buffer solution in a merging stream, and the peak height is recorded potentiometrically with a glass electrode in a flowthrough cell. The peak maxima are a linear function of the acid or base concentration in the range 0.01–0.1 M. At a sampling rate of 180 samples per hour, the relative standard deviation is less than 1%. The method can be used at sampling rates as high as 720 samples per hour.     

The phosphorescence of benzene obtained byab initio and semi-empirical calculations

Summary Radiative decay and phosphorescence of triplet stare benzene is doubly -orbital and spin- forbidden and is only activated through vibronic coupling among the manifold of triplet states. For this reason the determination of lifetime and transition moments for the decay of triplet benzene has posed a considerable challenge to both theory and experiment. In the present work we have addressed the triplet benzene problem at several levels of theory; by truncated perturbation theory and semiempirical, CNDO/S-CI, calculations; by complete sum-over-state calculations as implemented in recentab initio multiconfiguration quadratic response (MCQR) theory; and by direct MCQR calculations of vibronic phosphorescence. The vibronic coupling is in the two former cases treated by the Herzberg-Teller (H-T) perturbation theory, involving four main mechanisms for the phosphorescent decay of triplet benzene. The results and interpretations given by these approaches as well as their merits and limitations are presented and discussed in some detail. Our calculations indicate that the phosphorescent decay of the³ B _1u state takes place predominantly through vibronic coupling along thee _2g mode. We obtain a phosphorescence that is almost completely out-of-plane polarized, which is in line with more recent measurements by the microwave-induced delayed phosphorescence technique, and could reproduce quite well the intensity ratios for different vibronic bands obtained in that experiment. The final triplet state lifetime is the result of a delicate sum of contributions from several vibronic degenerate and non-degenerate modes. The direct vibronic phosphorescence calculations predict a long lifetime, about one minute — 68 seconds for the best wavefunction — and seem to focus on a doubling of the assumed, albeit not established, best experimental value for the radiative lifetime of triplet benzene; 30 seconds.Dedicated to Inga Fischer-Hjalmars on her 75th birthday     

Valence excitation of linear molecules.I. Excitation and UV spectra of N2, Co,acetylene and HCN

Valence excitation energies of N₂, CO, HCCH and HCN are calculated by use of the new quantum-chemical method HAM/3. The average energy thus obtained is then split to give singlet and triplet energies. For ππ* the Recknagel formulas are used. Theoretical and experimental energies are compared. The V-transitions (ππ*¹σ⁺) in CO, HCCH and HCN have been discussed.     

Synthesis and Aza-[2,3]-Wittig Rearrangements of Vinylaziridines: Scope and Limitations

cis- and trans-2,3-Trisubstituted vinylaziridines have been prepared from cis- and trans-epoxy alcohols, respectively, and used as substrates in the aza-[2,3]-Wittig rearrangement. Five different anion-stabilizing groups have been investigated for their efficiency to promote the rearrangement, and it was found that N-tert-butyl acetyl vinylaziridines were superior in this reaction, affording the corresponding cis-2,6-tetrahydropyridines (>90%) as single isomers when treated with LDA. Similarly, the corresponding (Z)-propenylaziridines gave trans,trans-2,3,6-trisubstituted tetrahydropyridines as the sole products while the (E)-propenylaziridines afforded the cis,cis-2,3,6-derivatives with equally high selectivity. The scope and limitations of the process have been investigated by varying the structure of the substrate, and the mechanism of the rearrangement has been probed to some extent; the mechanistic picture is more complex than assumed previously.     

Derivative potentiometric stripping analysis with a thin film of mercury on a glassy carbon electrode

Derivative potentiometric stripping analysis is based on the pre-concentration of metal analytes in a thin film of mercury on a glassy carbon electrode and subsequent measurement of the electrode potential subject to controlled transport of oxidant to the electrode surface. The mechanism for the stripping process with respect to oxidant concentration, electrode rotation rate and instrument parameters is discussed, together with analytical results and applications.     

Quality Assessments of Recycled Plastics by Spectroscopy and Chromatography

Quality assessments will be important for improved use of recycled polymeric materials. Ongoing preparation of new standards in the area of recycled polymers needs an overview of which properties and thus which polymer characterisation methods that will be important for that purpose. We suggest three polymeric properties as important for this work; these are degree of mixing (composition), degree of degradation and number and amount of low molecular weight compounds (e.g. degradation products, additives, flavour compounds). DSC showed increased degradation as multi modality for LDPE materials obtained from three different sites in a recycling plant. IR demonstrated that the carbonyl index increased during the various steps going from collected material to new product. GC chromatograms obtained for collected film flakes, processed granules and ready-made bags were quite complex with a series of hydrocarbons among other compounds. The recycling process seems, however, to remove some of the low molecular weight compounds found in the incoming dirty material.