1,720,992 research outputs found
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)
The cyclization of parent and cyclic hexa-1,3-dien-5-ynes - A combined theoretical and experimental study
No full textThe thermal cycloisomerization of both parent and benzannelated hexa-1.3-dien-5-yne, as well as of carbocyclic 1,3-dien-5-ynes (ring size 7-14), was investigated by using pure density functional theory (DFT) of Becke, Lee, Yang, and Parr (BLYP) in connection with the 6-31G basis set and the Brueckner doubles coupled-cluster approach [BCCD(T)] with the cc-pVDZ basis set for the parent system. The initial cyclization product is the allenic cyclohexa-1,2,4-triene (isobenzene), while the respective biradical is the transition structure for the enantiomerization of the two allenes. Two consecutive [1,2]-H shifts further transform isobenzene to benzene. For the benzannelated system, the energetics are quite similar and the reaction path is the same with one exception: the intermediate biradical is not a transition state but a minimum which is energetically below isonaphthalene. The cyclization of the carbocyclic 1,3-dien-5-ynes, which follows the same reaction path as the parent system, clearly depends on the ring size. Like the cyclic enediynes, the dienynes were found to cyclize to products with reduced ring strain. This is not possible for the 7- and 8-membered dienynes, as their cyclization products are also highly strained. For 9- to 11-membered carbocycles, all intermediates, transition states, and products lie energetically below the parent system; this indicates a reduced cyclization temperature. All other rings (12- to 14-membered) have higher barriers. Exploratory kinetic experiments on the recently prepared 10- to 14-membered 1,3-dien-5-ynes rings show this tendency, and 10- and 11-membered rings indeed cyclize at lower temperatures
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
)‐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
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
Aromaticity of the Bergman, Myers-Saito, Schmittel, and directly related cyclizations of enediynes
No full textAromaticity criteria (magnetic susceptibility exaltations, nucleus independent chemical shifts (NICS), and aromatic stabilization energy (ASE) evaluations) for enediyne and enyne-allene cyclizations evaluated at (UBS)-BLYP/6-31G all agree that the degrees of cyclic electron delocalization of the benzenoid systems formed by the Bergman (3) and Myers-Saito reactions (5) are comparable to benzene. The reaction enthalpy differences between the parent cyclizations and their benzannelated analogues are not entirely due to disparities in gained ASE during the reactions. The alternative formation of fulvene biradicals is not accompanied by favorable aromatic effects
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