1,722,166 research outputs found
Spectroscopic detection, reactivity and acid base behaviour of ring-dimethoxylated phenylethanoic acid radical cations and radical zwitterions in aqueous solution.
No full textA product and time-resolved kinetic study of the one-electron oxidation of ring-dimethoxylated phenylethanoic acids has been carried out at different pH values. Oxidation leads to the formation of aromatic radical cations or radical zwitterions depending on pH, and pK(a) values for the corresponding acid-base equilibria have been measured. The radical cations undergo decarboxylation with first-order rate constants (k(dec)) ranging from <10(2) to 5.6 x 10(4) s(-1) depending on radical cation stability. A significant increase in k(dec) (between 10 and 40 times) is observed on going from the radical cations to the corresponding radical zwitterions. The results are discussed in terms of the ease of intramolecular side chain to ring electron transfer required for decarboxylation, in both the radical cations and radical zwitterions
Reactivity and acid-base behavior of ring-methoxylated arylalkanoic acid radical cations and radical zwitterions in aqueous solution. Influence of structural effects and pH on the benzylic C-H deprotonation pathway
No full text[GRAPHICS] A product and time-resolved kinetic study of the one-electron oxidation of ring-methoxylated phenylpropanoic and phenylbutanoic acids (Ar(CH2)(n)CO2H, n = 2, 3) has been carried out at different pH values. Oxidation leads to the formation of aromatic radical cations (Ar center dot+(CH2)(n)CO2H) or radical zwitterions (Ar center dot+(CH2)(n)CO2-) depending on pH, and pK(a) values for the corresponding acid-base equilibria have been measured. In the radical cation, the acidity of the carboxylic proton decreases by increasing the number of methoxy ring substituents and by increasing the distance between the carboxylic group and the aromatic ring. At pH 1.7 or 6.7, the radical cations or radical zwitterions undergo benzylic C-H deprotonation as the exclusive side-chain fragmentation pathway, as clearly shown by product analysis results. At pH 1.7, the first-order deprotonation rate constants measured for the ring-methoxylated arylalkanoic acid radical cations are similar to those measured previously in acidic aqueous solution for the alpha-C-H deprotonation of structurally related ring-methoxylated alkylaromatic radical cations. In basic solution, the second-order rate constants for reaction of the radical zwitterions with -OH (k(-OH)) have been obtained. These values are similar to those obtained previously for the -OH-induced alpha-C-H deprotonation of structurally related ring-methoxylated alkylaromatic radical cations, indicating that under these conditions the radical zwitterions undergo benzylic C-H deprotonation. Very interestingly, with 3,4-dimethoxyphenylethanoic acid radical zwitterion, that was previously observed to undergo exclusive decarboxylation up to pH 10, competition between decarboxylation and benzylic C-H deprotonation is observed above pH 11
One-electron oxidation of 2-(4-methoxyphenyl)-2-methylpropanoic and 1-(4-methoxyphenyl)cyclopropanecarboxylic acids in aqueous solution. The involvement of radical cations and the influence of structural effects and pH on the side-chain fragmentation reactivity
No full textA product and time-resolved kinetic study on the one-electron oxidation of 2-(4-methoxyphenyl)-2methylpropanoic acid (2), 1-(4-methoxyphenyl)cyclopropanecarboxylic acid (3), and of the corresponding methyl esters (substrates 4 and 5, respectively) has been carried out in aqueous solution. With 2, no direct evidence for the formation of an intermediate radical cation 2(center dot+) but only of the decarboxylated 4-methoxycumyl radical has been obtained, indicating either that 2(center dot+) is not formed or that its decarboxylation is too fast to allow detection under the experimental conditions employed (k > 1 x 10(7) s(-1)). With 3, oxidation leads to the formation of the corresponding radical cation 3(center dot+) or radical zwitterion -3(center dot+) depending on pH. At pH 1.0 and 6.7, 3(center dot+) and -3(center dot+) have been observed to undergo decarboxylation as the exclusive side-chain fragmentation pathway with rate constants k = 4.6 x 10(3) and 2.3 x 10(4) s(-1), respectively. With methyl esters 4 and 5, direct evidence for the formation of the corresponding radical cations 4(center dot+) and 5(center dot+) has been obtained. Both radical cations have been observed to display a very low reactivity and an upper limit for their decay rate constants has been determined as k < 10(3) s(-1). Comparison between the one-electron oxidation reactions of 2 and 3 shows that the replacement of the C(CH3)(2) moiety with a cyclopropyl group determines a decrease in decarboxylation rate constant of more than 3 orders of magnitude. This large difference in reactivity has been qualitatively explained in terms of three main contributions: substrate oxidation potential, stability of the carbon-centered radical formed after decarboxylation, and stereoelectronic effects. In basic solution, -3(center dot+) and 5(center dot+) have been observed to react with -OH in a process that is assigned to the -OH-induced ring-opening of the cyclopropane ring, and the corresponding second-order rate constants (k-(OH)) have been obtained. With -3(center dot+), competition between decarboxylation and -OH-induced cyclopropane ring-opening is observed at pH >= 10, with the latter process that becomes the major fragmentation pathway around pH 12
The Role of structural effects on the side chain fragmentation pathways of ring methoxylated arylalkanoic acid radical cations
Full text linkInvestigation on the effect of horizontal and vertical deflectors on the near-wake of a square-back car model
No full textThe near-wake flow field downstream of a square-back car model is analyzed to investigate on the combined effect of horizontal and vertical deflectors. Aerodynamic coefficients and near-wake velocity fields are presented for a range of deflectors size and slant angles, for Reynolds number Re ≈ 250,000 based on free stream velocity and model height. Mean velocity fields and turbulence quantities are shown to shed light onto the interactions taking place within the wake. The underlying rationale is to couple a decrease of the wake extent along the vertical direction, achieved by reducing the size of the top separated region, to a simultaneous reduction of the fluctuations in the horizontal plane induced by the lateral longitudinal vortices. Vertical deflectors height is reported to play a key role in this scenario markedly affecting the resulting drag and lift coefficients. Analysis of velocity fields points out the existence of an optimal vertical spoiler size, in the presented set-up equal to 2/3 of model height, which features the overall best performance compared to other tested configurations. The beneficial effect of combined spoilers is found to be more relevant when horizontal and vertical deflectors feature a moderate (20°) slant angle with respect to free stream direction
Experimental investigations on secondary structures in a fully developed turbulent jet
No full textThe effect of streamwise vorticity on entrainment in jet flows has been the subject of various works and is related to many applications involving effective fluid mixing. In this work the near-field of a fully developed turbulent round jet was investigated at different Reynolds numbers ranging from ≈ 3000 to ≈ 30000 by means of time-resolved particle image velocimetry. Acquisitions were carried out on transverse planes at different downstream positions, in order to visualize the evolution of vortical structures and to assess their features. Results from average velocity fields and rms show a strong impact of crosswise vortical structures on radial velocity distributions as downstream distance increases. Velocity correlation functions analysis exhibits a decrease of crosswise structures size as the Reynolds number increases, reaching an asymptotic behaviour which may be also observed in TKE distributions, which feature high levels in the outer radial area. Entrainment rates calculations supported by spectral analysis confirm the decisive role of streamwise vorticity in the control of the entrainment process as well as its dependence on Reynolds number. Entrainment rates approaching an asymptotic state versus Reynolds number suggests a connection between the effectiveness of this process and crosswise vortical structures size
Effect of windshield configuration and pilot position on motorcycle performances
No full textThe effect of three windshield shapes on the aerodynamic performance of a racing-type motorcycle is investigated experimentally via global wind-tunnel force balance measurement and detailed local wake analysis by Particle Image Velocimetry. Results are compared to two driver's stances as well as to a baseline case. Results are dependent mainly on the windshield length as coupled to the driver positioning. Full-size windshield is characterized by a quite uniform wake structure and reduced level of velocity fluctuations and turbulence and is associated to improved stability, in spite of a relative increase of drag. When the additional safety and driver's protection from weather conditions are considered, the use of a full-size windshield appears an effective solution for commercial motorcycles
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Introduction
No full textThis book illustrates how pragmatics transcends the boundaries of linguistics. This volume covers Gricean pragmatics as well as topics including: conversation and collective belief, the norm of assertion, speech acts, what a context is, the distinction between semantics and pragmatics and implicature and explicature, pragmatics and epistemology, the pragmatics of belief, quotation, negation, implicature and argumentation theory, Habermas’ Universal Pragmatics, Dascal’s theory of the dialectical self, theories and theoretical discussions on the nature of pragmatics from a philosophical point of view.
Conversational implicatures are generally meaning augmentations on top of explicatures, whilst explicatures figure prominently in what is said. Discussions in this work reveal their characteristics and tensions within current theories relating to explicatures and implicatures. Authors show that explicatures and implicatures are calculable and not (directly) tied to conventional meaning
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