1,721,013 research outputs found
Mechanism of Carbon−Halogen Bond Reductive Cleavage in Activated Alkyl Halide Initiators Relevant to Living Radical Polymerization: Theoretical and Experimental Study
No full textThe mechanism of reductive cleavage of model alkyl halides (methyl 2-bromoisobutyrate, methyl 2-bromopropionate, and 1-bromo-1-chloroethane), used as initiators in living radical polymerization (LRP), has been investigated in acetonitrile using both experimental and computational methods. Both theoretical and experimental investigations have revealed that dissociative electron transfer to these alkyl halides proceeds exclusively via a concerted rather than stepwise manner. The reductive cleavage of all three alkyl halides requires a substantial activation barrier stemming mainly from the breaking C-X bond. The activation step during single electron transfer LRP (SET-LRP) was originally proposed to proceed via formation and decomposition of RX•- through an outer sphere electron transfer (OSET) process (Guliashvili, T.; Percec, V. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 1607). These radical anion intermediates were proposed to decompose via heterolytic rather than homolytic C-X bond dissociation. Here it is presented that injection of one electron into RX produces only a weakly associated charge-induced donor-acceptor type radical anion complex without any significant covalent σ type bond character between carbon-centered radical and associated anion leaving group.Therefore, neither homolytic nor heterolytic bond dissociation applies to the reductive cleavage of C-X in these alkyl halides inasmuch as a true radical anion does not form in the process. In addition, the whole mechanism of SET-LRP has to be revisited since it is based on presumed OSET involving intermediate RX•-, which is shown here to be nonexistent
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
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
[[alternative]]Theoretical Studies of Isomerization Reactions of 2-Pentoxy Radical and Its Derivatives in the Troposphere
No full text[[abstract]]Abstract
This thesis deals with the calculation of isomerization reactions of 2-pentoxy radical and its derivatives by density function theory. All of the local minimum structures are optimized with 6-31G and 6-311++G** basis set at the levels of HF and B3LYP. There are five sections rendered here.
Section 1: We study the intramolecular hydrogen transfer from five different positions of carbon of the 2-pentoxy. There are five paths of hydrogen transfer discussed, which include the transition structure of 3-, 4-, 5-, and 6-member ring. The results indicate that the rate constant of 6-member ring calculated at the B3LYP/6-311++G** level is very close to the experimental value. The rate constant is 1.25×105(s-1). The order of the energy barriers among five possible pathways are 3-, 4- >5- >6- member ring. The energy barrier of 6-member ring is 8.63 kcal/mol. The ring strain of the transition state surely dominates the energy barrier of the intramolecular hydrogen transfer.
Section 2: We study the double bond isomerization reactions at these series reactions. The ring strain of the transition state is not the only influence factor on the barrier of the double bond reaction. The second factor is the stability of each product. The pent-1-ene-4-oxy radical undergoes isomerization by 1,3-hydrogen shift via a lower energy barrier than that of 2-pentoxy radical. The energy barrier is 20.89 kcal/mol. The energy barrier of pent-2-ene-4-oxy radical isomerization via a six-member ring transition state, 6.9 kcal/mol, is also lower than the analogous process of 2-pentoxy radical.
Section 3: We study substitution effect of isomerization of 2-pentoxy radical. In this section, we only discuss isomerization proceeding through a 6-member ring. The lowest barrier of these reactions is 5-methyl-2-hexoxy radical. The energy barrier is 3.77 kcal/mol. We find that the relative energy of the product is an important factor relating to the barrier-height of these isomerization reactions.
Section 4: We study isomerization reactions of R(NH)?and RS?proceeding through 4-,5-, and 6-member ring. The reactions of R(NH)? are similar to the reactions of RO?, but the reactions of RS? are quite different. The barrier of the reactions of RS? via 5-member ring is lower than that via 6-member ring.
Section 5: We summarize two important factors on the barriers of isomerization reactions of alkoxy radicals and its derivatives. One is ring strain of the transition structure, and the other is the relative energy of the product.
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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