1,721,003 research outputs found
Cagelike clusters as models for the isolated hydroxyls of silica: ab-initio B3-LYP calculationsof the interaction with ammonia
No full textCage-like clusters as models for the hydroxys of silica: ab-initio calculations of 1H and 29Si NMR chemical shifts
No full textVibrational modes of isolated hydroxyls of silica computed ab-initio in a cluster approach
No full textAb Initio Calculation of the Crystalline Structure and IR Spectrum of Polymers: Nylon 6 Polymorphs
No full textState-of-the-art computational methods in
solid-state chemistry were applied to predict the structural
and spectroscopic properties of the α and γ crystalline
polymorphs of nylon 6. Density functional theory calculations
augmented with an empirical dispersion correction (DFT-D)
were used for the optimization of the two different crystal
structures and of the isolated chains, characterized by a
different regular conformation and described as one-dimen-
sional infinite chains. The structural parameters of both
crystalline polymorphs were correctly predicted, and new
insight into the interplay of conformational effects, hydrogen
bonding, and van der Waals interactions in affecting the
properties of the crystal structures of polyamides was obtained.
The calculated infrared spectra were compared to experimental data; based on computed vibrational eigenvectors, assignment of
the infrared absorptions of the two nylon 6 polymorphs was carried out and critically analyzed in light of previous investigations.
On the basis of a comparison of the computed and experimental IR spectra, a set of marker bands was identified and proposed as
a tool for detecting and quantifying the presence of a given polymorph in a real sample: several marker bands employed in the
past were confirmed, whereas some of the previous assignments are criticized. In addition, some new marker bands are proposed.
The results obtained demonstrate that accurate computational techniques are now affordable for polymers characterization,
opening the way to several applications of ab initio modeling to the study of many families of polymeric materials
Spectroscopic and thermodynamic study of the H-bonding of olefins onto the isolated hydroxyl of amorphous silica
No full textIntermolecular Interaction Energies in Molecular Crystals: Comparison and Agreement of Localized Møller–Plesset 2, Dispersion-Corrected Density Functional, and Classical Empirical Two-Body Calculations
No full textA comparative analysis of the intermolecular energy for a data set including 60 molecular crystals with a large variety of functional groups has been carried out using three different computational approaches: (i) a method based on a physically meaningful empirical partition of the interaction energy (PIXEL), (ii) density functional methods with a posteriori empirical correction for the dispersion interactions (DFT-D), and (iii) a full periodic ab initio quantum mechanical method based on Møller-Plesset perturbation theory for the electron correlation using localized crystal orbitals (LMP2). Due to the large computational cost, LMP2 calculations have been restricted to a subset of seven molecular crystal comprising benzene, formic acid, formamide, succinic anhydride, urea, oxalic acid, and nitroguanidine, and the results compared with PIXEL and DFT-D data as well as with the experimental data show excellent agreement among all adopted methods. This shows that both DFT-D and PIXEL approaches are robust predictive tools for studying molecular crystals. A detailed analysis shows a very similar dispersion contribution of the two methods across the 60 considered molecular crystals. The study also confirms that pure DFT shows serious deficiencies in properly handling molecular crystals in which the dispersive contribution is large. Due to the negligible requested computational resources, PIXEL is the method of choice in screening of a large number of molecular crystals, an essential step to predict crystal polymorphism or to study crystal growth processes. DFT-D can then be used to refine the ranking emerged from PIXEL calculations due to its general applicability and robustness in properly handling short-range interactions
Adducts of alkali-metal ions with the C-C triple bond: an experimental and ab-initio study
No full text
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
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