Photooxidation of polybutadiene: Quenching of excited triplet states of αβ-unsaturated carbonyls

The quenching of the triplet states of several model αβ-unsaturated ketones and that produced by thermal oxidation of polybutadiene by the triplet quencher cyclooctadiene at low temperatures has been studied by quantum yield and phosphorescence decay-time methods. Both the Perrin and Inokuti–Hirayama relationships were tested and a typical exchange interaction distance for half-quenching of R₀ = 12 ± 1 Å was obtained for both models on all compounds studied.     

Mechanisms of elimination reactions—XXVI: The α'-β mechanism in elimination reactions of sulforium salts

Tracer studies with β-deuterated sulfonium salts have shown that the α'-β, or ylid mechanism can be the major path of elimination in the reaction of sulfonium salts with t-butoxide in t-butyl alcohol, while the E2 reaction is dominant with hydroxide in water or n-butoxide in n-butyl alcohol. The structure of the sulfonium salt also affects the propensity toward α'-β elimination, with 3-pentyl > 3-propyl and eyclopentyl > 3-pentyl > cyclohexyl. The S-methyl protons of the sulfonium salt exchange at a rate much faster than that of the elimination reaction. A strongly basic medium and a syn-periplanar arrangement of the α-C-S and β-C-H bonds seem to be the two most important factors favoring the α'-β mechanism.     

The ORD,CD and UV spectra of sulphides derived from carvone

The ord and cd spectra of aromatic and aliphatic sulphides derived from carvone are compared with the spectra of dicarvone sulphide and related compounds. The spectra are discussed in relation to the structure of the molecules, as evidenced by the nmr spectrum, and correlated with the electronic transitions involved.

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ORD-, CD- und UV-Spektren von Sulfiden des Carvons

Zusammenfassung Die CD- und ORD-Spektren aromatischer und aliphatischer Sulfide von Carvon werden mit Spektren von Dicarvonsulfid und verwandten Verbindungen verglichen. Die Spektren werden in bezug auf Molekülstrukturen, die durch NMR-Spektren bewiesen wurden, diskutiert und mit den betreffenden Elektronenübergängen in Wechselbeziehung gebracht.

     

A Raman spectroscopic and combined analytical approach to the restoration of severely damaged frescoes: the Palomino project

The deterioration of art objects is normally relatively minor, controllable and attributable to environmental changes or bacterial invasion, and until now there has not been any recorded attempt to analyse an artwork that has been deliberately and significantly destroyed. The analytical problems are correspondingly larger but the potential reward from any information that can be forthcoming is thereby proportionately greater. The 17th Century Palomino frescoes on the vaulted ceiling of the Church of Sant Joan del Mercat in Valencia were largely destroyed by insurgents in the Spanish Civil War in 1936. The ensuing gunfire and a series of seven conflagrations inside the church had a devastating effect upon the artwork, and the surviving areas were also rendered unstable with respect to their detachment from the substrate. During the current restoration project being undertaken on these frescoes, an opportunity was provided for the application of several analytical techniques to secure information about the original pigment palette employed, the technology of application used by Palomino and the changes consequent upon the destruction process. Here, we report for the first time the use of analytical Raman spectroscopy, supported by scanning electron microscopy (SEM) and voltammetry of microparticles, for the combined identification of pigments, binders, substrate treatments and pigment alteration in an important, although badly damaged, wall painting for the informing of the ongoing conservation and restoration strategy. Copyright © 2008 John Wiley & Sons, Ltd.     

Highly sensitive detection of dye-labelled DNA using nanostructured gold surfaces

Careful control of surface chemistry results in strong surface enhanced resonance Raman scattering from dye-labelled oligonucleotides assembled on nanostructured gold surfaces, releasing their potential as reliable enhancing surfaces.     

Peroxycarbenium-mediated C-C bond formation: applications to the synthesis of hydroperoxides and peroxides

The Lewis acid-mediated reaction of alkene nucleophiles with peroxyacetals provides an effective route for the synthesis of homologated peroxides and hydroperoxides. In the presence of Lewis acids such as TiCl(4), SnCl(4), and trimethylsilyl triflate, peroxyacetals and peroxyketals undergo reaction with allyltrimethylsilane, silyl enol ethers, and silyl ketene acetals to afford homoallyl peroxides, 3-peroxyketones, and 3-peroxyalkanoates, respectively. Reactions of peroxyacetals are Lewis acid dependent; TiCl(4) promotes formation of ethers while SnCl(4) and trimethylsilyl triflate promote formation of peroxides. Lewis acid-promoted reactions of silylated hydroperoxyacetals furnish silylated hydroperoxides, which can be deprotected to homologated hydroperoxides. Hydroperoxyketals undergo Lewis acid-mediated allylation to furnish 1,2-dioxolanes via attack of hydroperoxide on the intermediate carbocation. Lewis acid-mediated cyclization of unsaturated peroxyacetals furnishes 1,2-dioxanes, 1,2-dioxepanes, and 1,2-dioxacanes through 6-endo/exo, 7-endo/endo, and 8-endo/endo pathways. The corresponding reactions involving 6-endo/endo and 5-endo/exo pathways were unsuccessful.     

The initiation of the free radical polymerization of vinyl monomers by hexakis(arylisocyano) derivatives of chromium (o)

The present paper describes studies carried out to determine the effect of structure on the free radical polymerization of vinyl monomers initiated by means of the hexakis(arylisocyano) derivatives of chromium (o) in which chlorine, methoxy, and methyl groups have been substituted into different positions of the benzene ring. The kinetics of the initiation reactions resembles that studied in detail for the parent compound hexakis(phenylisocyano) chromium (o). The observed changes in the initial rates of the polymerization of methyl methacrylate at 80°C in the presence of carbon tetrachloride can be accounted for by the electronic effects associated with these substituents. However, when steric overcrowding occurs, the initial rates of polymerization are reduced considerably. The results obtained confirm the mechanism of initiation proposed earlier in which the rate determining step is the S_n² displacement of an arylisocyano ligand by a monomer or reactive solvent molecule. An attempt has been made to explain the increased rate of radical formation observed when chlorine is substituted into the benzene ring.     

The initiation of vinyl polymerization by tetrakis (trialkyl-phosphite) nickel (o) derivatives

The study of the polymerization of vinyl monomers initiated by zero-valent nickel phosphite derivatives in the presence of organic halides has been extended to tetrakis (triethylphosphite) nickel (o) and tetrakis (tri 2-chloroethylphosphite) nickel (o). A kinetic study with methyl methacrylate and carbon tetrachloride shows that the polymerization proceeds by a free radical process and that the mechanism of initiation resembles that previously reported for the corresponding triphenylphosphite and carbonyl derivatives. The primary process which becomes rate determining at sufficiently high carbon tetrachloride concentration is the nonreversible S_N1 scission of an alkylphosphite ligand. These alkylphosphite initiators are considerably more active than the corresponding carbonyl derivatives, but are less active than the corresponding triphenyl-phosphite derivative. The difference in activities between the aryl and alkylphosphite derivatives has been accounted for by differences in steric over-crowding of these molecules.     

Neural correlates of semantic and syntactic processes in the comprehension of case marked pronouns: Evidence from German and Dutch

Background  

It is well known that both semantic and syntactic information play a role in pronoun resolution in sentences. However, it is unclear what the relative contribution of these sources of information is for the establishment of a coreferential relationship between the pronoun and the antecedent in combination with a local structural case constraint on the pronoun (i.e. case assignment of a pronoun under preposition governing). In a prepositional phrase in German and Dutch, it is the preposition that assigns case to the pronoun. Furthermore, in these languages different overtly case-marked pronouns are used to refer to male and female persons. Thus, one can manipulate biological/syntactic gender features separately from case marking features.