Electrochemical studies of ion pairs and triplets formed by alkali metal cations with 1,2-naphthosemiquinone and 1,2-naphthoquinone dianion

The properties of ion associates formed by alkali metal cations with 1,2-naphthosemiquinone radical anions and 1,2-naphthoquinone dianions have been polarographically studied in dimethylsulfoxide solutions. It was established that these radicals and dianions coexist in equilibria with corresponding ion pairs and triple ions. The values of association constants were calculated on the basis of the slightly modified method of De Ford and Hume. Some structural aspects of ion association in the systems investigated are discussed.     

Interpretation of13C NMR spectra of 7-substituted 9,11-dideoxy-PGF 1 analogues and their synthons

A full interpretation of¹³C NMR spectra of the methyl esters of 7-hydroxy-and 7-oxo-9,11-dideoxy-PGF ₁ analogues and their synthons is reported.

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Interpretation von¹³C-NMR-Spektren von 7-substituierten 9,11-Dideoxy-PGF₁-Analogen und deren Synthonen

Zusammenfassung Es wird eine vollständige Zuordnung der¹³C-NMR-Resonanzen von den Methylestern der 7-Hydroxy- und 7-Oxo-9,11-dideoxy-PGF ₁-Analogen und ihrer Synthone beschrieben.

     

Multifunctionality in Epoxy Resins

Abstract

Epoxy resin will continue to be in the forefront of many thermoset applications due to its versatile properties. However, with advancement in manufacturing, changing societal outlook for the chemical industries and emerging technologies that disrupt conventional approaches to thermoset fabrication, there is a need for a multifunctional epoxy resin that is able to adapt to newer and robust requirements. Epoxy resins that behave both like a thermoplastic and a thermoset resin with better properties are now the norm in research and development. In this paper, we viewed multifunctionality in epoxy resins in terms of other desirable properties such as its toughness and flexibility, rapid curing potential, self-healing ability, reprocessability and recyclability, high temperature stability and conductivity, which other authors failed to recognize. These aspects, when considered in the synthesis and formulation of epoxy resins will be a radical advance for thermosetting polymers, with a lot of applications. Therefore, we present an overview of the recent finding as to pave the way for varied approaches towards multifunctional epoxy resins.     

Rates of the halide ion cleavage from halo-9,10-diphenylanthracene anion radicals in DMF

The rate constants k for the cleavage of the carbon-halogen bond in anion radicals of 1- and 2-chloro-9,10-diphenylanthracene, 9,10-dichloroanthracene and 2-bromo-9,10-diphenylanthracene have been determined and compared with the literature data for other anthracenyl halides. There is no significant correlation between log k and the formal potentials of the anion radical formation. However, linear relationships of the experimental RT In k vs. the relative thermodynamic contribution to the activation barrier as well as the relative intrinsic activation energy (calculated on the basis of the recent Savéant model and using the ionic component of the bond enthalpy estimated from the absolute electronegativity and hardness) have been found.     

Comparison of spectroscopic and electrochemical studies of disproportionation equilibria of 1,3,5-triphenylverdazyl radical inDMF containing carboxylic acids

Summary Stoichiometry and equilibrium constants for the disproportionation of the title radicals in N,N-dimethylformamide containing salicilic, chloroacetic, and phenylacetic acids were determined on the basis of absorption spectra. On the other hand, only apparent equilibrium constants depending on concentrations of an acid and a radical could be obtained from electrochemical measurements at a mercury electrode. Significant differences in reaction stoichiometry and in order of magnitudes of disproportionation constants found under spectroscopic and electrochemical conditions were discussed in terms of an influence of the electric field in the double layer on the distribution of different associates formed by verdazyl species with acids.

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Vergleich spektroskopischer und elektrochemischer Untersuchungen von Disproportionierungsgleichgewichten von 1,3,5-Triphenylverdazylradikalen inDMF/Carbonsäure-Mischungen

Zusammenfassung Die Stöchiometrie und die Gleichgewichtskonstanten für die Disproportionierung der Titelradikale wurden in N,N-Dimethylformamid mit einem Gehalt an Salizyl-, Chloressig- bzw. Phenylessigsäure auf Basis der Absorptionsspektren bestimmt. Andererseits wurden lediglich effektive Gleichgewichtskonstanten — in Abhängigkeit der Konzentration von einer Säure und einem Radikal — aus elektrochemischen Messungen an der Quecksilberelektrode erhalten. Signifikante Unterschiede der Reaktionsstöchiometrie und die Größenordnungen in den Disproportionierungskonstanten, die bei den spektroskopischen bzw. elektrochemischen Bedingungen gefunden wurden, werden im Hinblick auf den Einfluß des elektrischen Feldes in der Doppelschicht auf die Verteilung verschiedener Assoziate von Verdazylspezies mit Säuren diskutiert.

     

Random mappings with exchangeable in‐degrees

In this article we introduce a new random mapping model, , which maps the set {1,2,…,n} into itself. The random mapping is constructed using a collection of exchangeable random variables which satisfy . In the random digraph, , which represents the mapping , the in‐degree sequence for the vertices is given by the variables , and, in some sense, can be viewed as an analogue of the general independent degree models from random graph theory. We show that the distribution of the number of cyclic points, the number of components, and the size of a typical component can be expressed in terms of expectations of various functions of . We also consider two special examples of which correspond to random mappings with preferential and anti‐preferential attachment, respectively, and determine, for these examples, exact and asymptotic distributions for the statistics mentioned above. © 2007 Wiley Periodicals, Inc. Random Struct. Alg., 2008     

Effects of species and sites on metal concentrations in byssal threads of two mytilids

Concentrations of 11 metals (Al, Cu, Fe, Mg, Mn, Mo, Ni, Pb, Sr, V and Zn) were determined by inductively coupled plasma quadruple mass spectrometer measurements in byssus of two mytilid species Mytilus galloprovincialis and Xenostrobus securis from three sites situated along the coastline of Ría de Vigo (NW Spain). The species coexist only at one of those sites, thus it was possible to compare the impact of both factors, site and species, on metal concentrations. Principal component and regression analyses showed a dominant effect of mussel species and not the site. In all samples, the highest concentration was observed for iron in accordance with the literature data for M. galloprovincialis. One order of magnitude higher concentrations of iron detected in the byssus of X. securis than that of M. galloprovincialis are discussed considering a different microstructure of byssus threads as shown by transmission electron microscopy images.     

Fluorescent probe based on proton transfer in 4′-(diethylamino)-3-hydroxyflavone: Spectral and luminescent characteristics upon selective excitation

The spectral characteristics of acetonitrile solutions of 4′-(diethylamino)-3-hydroxyflavone dye with dual fluorescence are studied under selective excitation. This dye is a structural analog of 3-hydroxyflavone and exhibits excited-state proton transfer, which, in contrast to 3-hydroxyflavone, has a thermodynamic rather than a kinetic character. The fluorescence spectra at different excitation photon energies and the excitation spectra of different fluorescence bands are studied. It is found that the intensity ratio of the normal and tautomeric fluorescence bands lying near 507 and 570 nm, respectively, depends on the excitation wavelength, namely, this ratio is 1.45 and almost does not change in the region of the main absorption band (370–420 nm), while, in the region of the second singlet band (near 280 nm), it decreases to 1.15. This can be explained by an increase in the probability of proton transfer with formation of a tautomeric form in the case of excitation into the second band. Another interesting feature is the existence of a latent third emission band peaked at 535 nm, which was found and reliably recorded upon excitation at wavelengths of 470–500 nm. Addition of water quenches this emission, which indicates that it belongs to the anionic form of the dye.     

Spectral properties of molecular charge-transfer probe QMOM

The spectral characteristics of solutions of a dye with dual fluorescence, 1-methyl-2-(4-methoxy)phenyl-3-hydroxy-4(1H)-quinolone, in acetonitrile are studied upon selective excitation. This dye is a structural analogue of 3-hydroxyflavone and also exhibits excited-state proton transfer, which, as well as in the case of 3-hydroxyflavone, has a kinetic nature. The fluorescence spectra are studied upon excitation by photons of various energies, and the excitation spectra are recorded at wavelengths of different fluorescence bands. It is found that the intensity ratio of the emission of the normal and tautomeric forms (at wavelength of 415 and 518 nm, respectively) is almost the same (0.23–0.25) for excitation in the regions of the main and the second absorption bands. At the same time, in the case of excitation between these bands, this ratio decreases to 0.19. The second interesting feature is the existence of a third latent emission band peaked at about 480 nm, which is reliably detected upon excitation at wavelengths in the region of 400–450 nm. This study shows that this emission belongs to the anionic form of the dye. This form is also responsible for a decrease in the intensity ratio of the emission of the two main forms in the case of excitation between the first and second absorption bands.     

Direct observation of proton transfer from the excited <Emphasis Type="Italic">S</Emphasis><Subscript>2</Subscript> state in the 3-hydroxyflavone molecule

The spectra of dual fluorescence of 3-hydroxyflavone molecules excited by 44-ps pulses in the region of the S ₁ and S ₂ absorption bands are measured with a picosecond resolution. The dynamics of the spectra directly demonstrates the time development of the proton transfer from the carboxyl to the carbonyl group of the molecule. Upon excitation into the main absorption band, the transfer process occurs for about 210 ps. The excitation into the region of the S ₂ band results in a faster (～170 ps) process, and the relative contribution made to the total spectrum by the long-wavelength band, which belongs to the proton-transfer state, is higher in this case for all the time ranges of luminescence recording. The data obtained directly point to an additional channel of proton transfer via the S ₂ state. The probability of this process is estimated to be 0.84 × 10¹² s^?1.