1,721,067 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
Ab Initio Computational Study on Fe2NiP Schreibersite: Bulk and Surface Characterization
No full textPhosphorus is ubiquitous in planet Earth and plays a fundamental role in all living systems. Finding a reasonable prebiotic source of phosphorus is not trivial, as common sources where it is present nowadays are in the form of phosphate minerals, which are rather insoluble and nonreactive materials, making it unavailable for ready incorporation in living organisms. A possible source of phosphorus is from exogenous meteoritic bombardment and, in particular, iron/nickel phosphides. These materials, by simple interaction with water, produce oxygenated phosphorus compounds, which can easily react with organic molecules, thus forming C-O-P bonds. In the present work, periodic ab initio simulations at the PBE level (inclusive of dispersion interactions) have been carried out on metallic Fe2NiP schreibersite, as a relatively abundant component of metallic meteorites, in order to characterize the structural, energetic, and vibrational properties of both bulk and surfaces of this material. The aim is to study the relative stability among different surfaces and also to characterize the nanocrystal morphology of the mineral
Theoretical study of the reaction O(3P) + 1,2-butadiene
No full textThe triplet and singlet potential energy surfaces of the O(3P) + 1,2-butadiene reaction have been investigated by electronic structure calculations at the coupled-cluster (CCSD(T)(aug-cc-pVTZ) level. We focused our attention, in particular, on the different sites of attack of atomic oxygen to 1, 2-butadiene. The results for minima, transition states and reaction channel energetics are compared with the results of previous CCSD(T)(aug-cc-pVTZ)-CBS and CASPT2 calculations to explore the adequacy of simpler computational schemes for discussing the reaction dynamics, in particular the product branching fractions derived from crossed molecular beam experiments
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
Combined crossed molecular beam and theoretical studies of the N(2D) + CH4 reaction and implications for atmospheric models of Titan
No full textThe dynamics of the H-displacement channel in the reaction N(2D) + CH4 has been investigated by the
crossed molecular beam (CMB) technique with mass spectrometric detection and time-of-flight (TOF) analysis
at five different collision energies (from 22.2 up to 65.1 kJ/mol). The CMB results have identified two distinct
isomers as primary reaction products, methanimine and methylnitrene, the yield of which significantly varies
with the total available energy. From the derived center-of-mass product angular and translational energy
distributions the reaction micromechanisms, the product energy partitioning and the relative branching ratios
of the competing reaction channels leading to the two isomers have been obtained. The interpretation of the
scattering results is assisted by new ab initio electronic structure calculations of stationary points and product
energetics for the CH4N ground state doublet potential energy surface. Differently from previous theoretical
studies, both insertion and H-abstraction pathways have been found to be barrierless at all levels of theory
employed in this work. A comparison between experimental results on the two isomer branching ratio and
RRKM estimates, based on the new electronic structure calculations, confirms the highly nonstatistical nature
of the N(2D) + CH4 reaction, with the production of the CH3N isomer dominated by dynamical effects. The
implications for the chemical models of the atmosphere of Titan are discussed
A computational study of the reaction cyanoacetylene and cyano radical leading to 2-butynedinitrile and hydrogen radical
No full textThe present work focuses on the characterization of the reaction between cyanoacetylene and cyano radical by electronic structure calculations of the stationary points along the minimum energy path. One channel, leading to C4N2 (2-Butynedinitrile) + H, was selected due to the importance of its products. Using different ab initio methods, a number of stationary points of the potential energy surface were characterized. The energy values of these minima were compared in order to weight the computational costs in relation to chemical accuracy. The results of this works suggests that B2PLYP (and B2PLYPD3) gave a better description of the saddle point geometry, while B3LYP works better for minima
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
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