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Hydrocarbon activation with cerium(IV) ammonium nitrate: Free radical versus oxidative pathways
No full textThe reactivity of propellane C-C bonds towards cerium(IV) ammonium nitrate (CAN) was studied utilizing photochemical initiation in acetonitrile. Synthetic as well as computational (B3LYP/6-311+G and MP2/6-31G ) data strongly suggest that the activation of the C-C bonds in cyclopropane and cyclobutane derivatives involves NO3 radical attack on the hydrocarbon ring and does not proceed through single electron transfer. The product structures are not consistent with the intermediate formation of propellane radical cations. These propellane systems could be generated independently by oxidation with charged electrophiles, by anodic oxidation, and through photo-oxidation with 1,2,4,5-tetracyanobenzene; the observed chemical behavior of radical cations is clearly different
Halogenation of cubane under phase-transfer conditions: Single and double C-H-bond substitution with conservation of the cage structure
No full textThe first highly selective C-H chlorination, bromination. and iodination of cubane (1) utilizing polyhalomethanes as halogen sources under phase-transfer (PT) conditions is described, Isomeric dihalocubanes with all possible combinations of chlorine, bromine, and iodine in ortho, meta, and para positions were also prepared by this method; m-dihalo products form preferentially. Ab initio and density functional theory (DFT) computations were used to rationalize the pronounced differences in the reactions of 1 with halogen (Hal(.)) vs carbon-centered trihalomethyl (Hal(3)C(.)) radicals (Hal = Cl, Br). For Hal(3)C radicals the C-H abstraction pathway is less unfavorable (DeltaG(298)(++) = 21.6 kcal/mol for Cl3C. and 19.4 kcal/mol for Br3C. at B3LYP/6-311+G // B3LYP/6-31G ) than the fragmentation of the cubane skeleton via S(H)2-attack on one of the carbon atoms of 1 (DeltaG(298)(++) = 33.8 and 35.1 kcal/mol, respectively). In stark contrast, the reaction of 1 with halogen atoms preferentially follows the fragmentation pathway (DeltaG(298)(++) = 2.1 and 7.5 kcal/mol) and C-H abstraction is more unfavorable (DeltaG(298)(++) = 4.6 and 12.0 kcal/mol). Our computational results nicely agree with the behavior of 1 under PT halogenation conditions (where Hal(3)C(.) is involved in the activation step) and under free-radical photohalogenation with Hair (Della, E. W., et al. J.Am. Chern. Sec. 1992, 114, 10730). The incorporation of a second halogen atom preferentially in the meta position of halocubanes demonstrates the control of the regioselectivity by molecular orbital symmetry
Hydrocarbon activation with cerium(IV) ammonium nitrate: Free radical versus oxidative pathways
No full textThe reactivity of propellane C-C bonds towards cerium(IV) ammonium nitrate (CAN) was studied utilizing photochemical initiation in acetonitrile. Synthetic as well as computational (B3LYP/6-311+G and MP2/6-31G ) data strongly suggest that the activation of the C-C bonds in cyclopropane and cyclobutane derivatives involves NO3 radical attack on the hydrocarbon ring and does not proceed through single electron transfer. The product structures are not consistent with the intermediate formation of propellane radical cations. These propellane systems could be generated independently by oxidation with charged electrophiles, by anodic oxidation, and through photo-oxidation with 1,2,4,5-tetracyanobenzene; the observed chemical behavior of radical cations is clearly different
The rearrangement of the cubane radical cation in solution
No full textThe rearrangement of the cubane radical cation (1(.+)) was examined both experimentally (anodic as well as (photo)chemical oxidation of cubane 1 in acetonitrile) and computationally at coupled cluster, DFT, and MP2 [BCCD(T)/cc-pVDZ//B3LYP/6-31G + ZPVE as well as BCCD(T)/cc-pVDZ//MP2/6-31G + ZPVE] levels of theory. The interconversion of the twelve C-2v degenerate structures of 1(.+) is associated with a sizable activation energy of 1.6 kcal mol(-1). The barriers for the isomerization of 1(.+) to the cuneane radical cation (2(.+)) and for the C-C bond fragmentation to the secocubane- 4,7-diyl radical cation (10(.+)) are virtually identical (DeltaH(0)(double dagger) = 7.8 and 7.9 kcal mol(-1), respectively). The low-barrier rearrangement of 10(.+) to the more stable syn-tricyclooctadiene radical cation 3(.+) favors the fragmentation pathway that terminates with the cyclooctatetraene radical cation 6(.+). Experimental single-electron transfer (SET) oxidation of cubane in acetonitrile with photoexcited 1,2,4,5-tetracyanobenzene, in combination with back electron transfer to the transient radical cation, also shows that 1(.+) preferentially follows a multistep rearrangement to 6(.+) through 10(.+) and 3(.+) rather than through 2(.+). This was confirmed by the oxidation of syn-tricyclooctadiene (3), which, like 1, also forms 6 in the SET oxidation/rearrangement/electron-recapture process. In contrast, cuneane (2) is oxidized exclusively to semibullvalene (9) under analogous conditions. The rearrangement of 1(.+) to 6(.+) via 3(.+), which was recently observed spectroscopically upon ionization in a hydrocarbon glass matrix, is also favored in solution
ChemInform Abstract: Electrophilic and Oxidative Activation of the Central C-C Bond in [3.3.n]Propellanes: A Theoretical Study.
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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
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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