1,720,987 research outputs found
CHAPTER 3: Gas-phase Prebiotic Chemistry Driven by Ultraviolet Photolysis of Simple Molecules
No full textThe photolysis of stable simple molecules by means of solar ultraviolet photons is known to be the first step in the chemistry of planetary atmospheres. The transient species produced in this way can react and form more complex molecules that are normally present in trace amounts. Trying to tackle the problem associated with the massive organic system that allegedly preceded the emergence of life on Earth according to the abiotic origin of life theory, we consider here the possibility that prebiotic chemistry on Earth started in its upper atmosphere. A proof-of-concept of this scenario can be provided by investigating the atmospheric chemistry of other planets and moons. In particular, if one searches for good test cases in our Solar System, the atmosphere of Titan stands out for its rich organic chemistry. In this chapter, our comprehension of the chemistry leading to N-rich organic molecules and macromolecules as made possible by resorting to a multidisciplinary approach based on photochemical models and dedicated laboratory experiments will be illustrated. The focus will be on neutral bimolecular reactions leading to nitriles, imines and other organic N-bearing molecules which can react further in the upper atmosphere of Titan up to the formation of macromolecules. Nitriles, imines and N-containing macromolecules are known as possible precursors of amino acids and nucleobases once they come into contact with water
A comparison of the quantum state-specific efficiency of N + N2 reaction computed on different potential energy surfaces
No full textState-to-state exact quantum probabilities of the N + N2 exchange reaction have been calculated on the recently proposed L4 potential energy surface fitted to high level ab initio points using full-dimensional time-independent quantum techniques. Thermal rate coefficient values calculated on L4 were found not to differ from those calculated on a previously proposed potential energy surface. On the contrary, state-specific reaction probabilities calculated on the two surfaces are shown to differ significantly. Arguments for attributing the difference to specific features of the considered potential energy surfaces are provided. © 2009 the Owner Societies
A comparison of the isotope effect for the N + N2 reaction calculated on two potential energy surfaces
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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
A theoretical investigation of the reaction between glycolaldehyde and h+ and Implications for the Organic Chemistry of Star Forming Regions
No full textThe characterization of one of the possible pathways in the reaction between H+ and glycolaldehyde (the channel leading to COH + CH3OH+) has been carried out by performing electronic structure calculations of the stationary points along the minimum energy path. We have employed different theoretical methods verifying that, while geometry optimizations can be performed with a relatively low level of theory, quantitative results for the energies require higher level calculations. The same methodology will be applied to the complete scheme of the title reaction as well as similar processes which are needed to characterize the destruction routes of interstellar complex organic molecules
A theoretical investigation of the reactions of n(2D) with Small Alkynes and Implications for the Prebiotic Chemistry of Titan
No full textThe reactions between atomic nitrogen, in its first electronically excited state (2D), and two different hydrocarbons, methyl acetylene and acetylene, have been analyzed by performing electronic structure calculations of the potential energy surface. For each reaction, H-displacement channels leading to cyclic molecular products have been identified, together with an H2 elimination channel for the reaction N(2D) + acetylene and a methyl (CH3) loss channel for the N(2D) + methylacetylene reaction. Since both reactions have been found to be exothermic and without an entrance barrier, we suggest that they are fast and efficient under the conditions of the upper atmosphere of Titan. These data will be used to perform kinetic calculations and will be compared with detailed experimental results in future work
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