1,720,970 research outputs found
H-bonding additives act like Lewis acid catalysts
No full text[GRAPHICS] A combination of NMR, IR, and ab initio techniques reveals the striking structural similarities of an exemplary H-bonded complex of an N-acyloxazolidinone with an N,N'-disubstituted electron-poor thiourea and the corresponding Lewis acid complex. Although the H-bond association constant is lower than for the Lewis acid adduct, Diels-Alder reactions are accelerated and stereochemically altered in a fashion similar to weak Lewis acids
Metal-free, noncovalent catalysis of Diels-Alder reactions by neutral hydrogen bond donors in organic solvents and in water
No full textWe examined the catalytic activity of substituted thioureas in a series of Diets-Alder reactions and 1,3-dipolar cycloadditions. The kinetic data reveal that the observed accelerations in the relative rates are more dependent on the thiourea substituents than on the reactants or solvent. Although the catalytic effectiveness is the strongest in noncoordinating, nonpolar solvents, such as cyclohexane, it is also present in highly coordinating polar solvents, such as water. In 1,3-dipolar cycloadditions, the thiourea catalysts demonstrate only very moderate selectivites for reactions with inverse electron demand. Our experiments emphasize that both hydrophobic and polar interactions can co-exist, making these catalysts active, even in highly coordinating solvents. This class of catalysts increases the reaction rates and endo-selectivities of Diets-Alder reactions, in a similar manner to weak Lewis acids, without concomitant product inhibition
Can fulvenes form from enediynes? A systematic high-level computational study on parent and benzannelated enediyne and enyne-allene cyclizations
No full textApart from the well-known Bergman, Myers-Saito, and Schmittel ring closure reactions of parent enediyne (4) and enyne-allene (3), novel cyclization modes were identified using density functional (DFT) and coupled-cluster methods. The geometries obtained with several DFT functionals are quite similar; for consistency's sake, we employed BLYP/6-31G geometries; Brueckner double energy single points [BCCD(T)/cc-pVDZ] on these geometries were used to determine the relative energies. The C-1-C-5 cyclization of 4 leading to fulvene biradical 8 is 40 kcal mol(-1) endothermic, and the product ties 31 kcal mol(-1) above 1,4-didehydrobenzene 7 because of the lack of aromatic stabilization. The heat of formation (Delta H-f(o)) of 8 is predicted to be 172.0 +/- 1.0 kcal mol(-1). Yet another ring closure of 4 leading to dimethylenecyclobutene biradical 12 is 69 kcal mol(-1) endothermic and is hardly of preparative interest. A new cyclization of 3 should lead to the seven-membered ring biradical 13, which is located 33 kcal mol(-1) above 3 and 24 kcal mol(-1) above the Schmittel product 6. As the transition structure for both cyclizations differ by 11 kcal mol(-1), 13 may form under suitable conditions. All other possible modes of cyclization of 4 did not lead to stable products. Benzannelation has a significant effect on the endothermicities of the Bergman and Myers-Saito cyclizations, which are 8-9 kcal mol(-1) above the parent reactions due to reduced aromatization energy in the naphthalene derivatives. The endothermicities of the other cyclization pathways are largely unaffected by benzannelation
Is SH4, the simplest 10-S-4 sulfurane, observable?
No full textThe kinetic stability of SH4 was investigated theoretically with the coupled cluster ansatz. The two possible modes of decomposition into SH2 and H-2 through either a C-2v or a C-1 transition structure (TS) were investigated using intrinsic reaction coordinate (IRC) computations; orbital interactions along the reaction paths were analyzed. The two dissociation modes are due to differences in the electron delocalization in the TSs. While the C-2v TS is bonded rather covalently by a three center-four electron (3c-4e) interaction which is lost in a strictly synchronous way (two electrons occupy the same orbital at a time along the reaction coordinate), the bonding orbital in the C-1 TS is merely occupied by a single electron. Surprisingly, this highly polarized TS has a lower barrier. Computations at the CCSD(T)/cc-pVQZ level of theory show that the zero-point corrected enthalpy (Delta H(0)double dagger) of the C-1 TS is 16 kcal mol(-1) above the C-4v symmetric ground state; the barrier along the C-2v path is 40 kcal mol(-1). The overall exothermicity for the dissociation into SH2 and H-2 was estimated to be Delta H-0 = -76 kcal mol(-1). The fundamental IR absorptions of SH4 (obtained by scaling the computed harmonic vibrational frequencies taken from the CCSD(T)/cc-pVQZ level of theory) are 1432 and 2037 cm(-1)
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
)‐1,5‐hexadiyne‐3‐enes: a systematic computational study
No full textThe effects of several substituents (-BH2, -BF2, -AlH2, -CH3, -C6H5, -CN, -COCH3, -CF3, -SiH3, -NH2, -NH3+, -NO2, -PH2, -OH, -OH2+, -SH, -F, -Cl, -Br) on the Bergman cyclization of (Z)-1,5-hexadiyne-3-ene (enediyne, 3) were investigated at the Becke-Lee-Yang Parr (BLYP) density functional (DFT) level employing a 6-31G basis set. Some of the substituents, (-NH3+, -NO2, -OH, -OH2+, -F, -Cl, -Br) are able to lower the barrier (up to a minimum of 16.9 kcal mol(-1) for difluoro-enediyne 7rr) and the reaction enthalpy (the cyclization is predicted to be exergonic for -OH2+ and -F) compared to the parent system giving rise to substituted 1,4-dehydrobenzenes at physiological temperatures. (C) 2001 John Wiley & Sons, Inc
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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
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