1,720,971 research outputs found
Unpolarized and Polarized Raman Spectroscopy of Nylon-6 Polymorphs: A Quantum Chemical Approach
Full text linkExploiting the very recent potentialities of state-of-the-art quantum chemical simulations of crystalline solids, unpolarized Raman spectra of α and γ polymorphs of Nylon-6 obtained through periodic density functional theory calculations are presented for the first time. The computed spectra are compared with the experimental spectra reported in the literature and allow a detailed interpretation to be proposed of the patterns observed, identifying unambiguous Raman marker bands of the different phases. The calculations of single crystal directional intensities gave the further possibility to predict the polarization properties of the Raman spectra of these polymorphs: considering in particular the α phase, polarized Raman spectra have been computed and showed a very good agreement with measurements previously reported for uniaxially oriented samples
Joint Experimental and Computational Investigation of the Structural and Spectroscopic Properties of Poly(vinylidene fluoride) Polymorphs
No full textState-of-the-art density functional theory calculations are here adopted for the investigation of the crystal structure and of the vibrational properties of alpha, beta, gamma and delta phases of poly(vinylidene fluoride) (PVDF), in comparison with IR and Raman measurements. DFT calculations allowed a detailed interpretation of the IR and Raman spectra of alpha and beta phases, giving vibrational assignments useful for qualitative and quantitative characterization of these systems. From a molecular perspective, the computational investigation of the crystal structure and the spectra of PVDF polymorphs helped in clarifying the role of supramolecular dipole-dipole interactions, which indeed modulate the vibrational properties of these systems, indicating also that intermolecular interaction could play a significant role in the modulation of ferroelectric properties. Furthermore, the combined experimental and computational approach allowed us to identify and characterize the thermally and mechanically induced gamma phase, shedding light on the far-IR marker bands of this elusive phase of PVDF
Crystal Structure and Vibrational Spectra of Poly(trimethylene terephthalate) from Periodic Density Functional Theory Calculations
No full textThe crystal structure and the IR
spectrum of crystalline poly(trimethylene
terephthalate), PTT, have been investigated by means of periodic density
functional theory calculations including Grimme’s correction
for dispersion interactions. Both structural and spectroscopic results
have been critically compared to the experimental data taken from
the literature, showing very good agreement between theory and the
experiments. The previous spectral assignments, based only on experimental
investigations, have been revised, and further insights have been
obtained. Furthermore, spectroscopic markers of crystallinity or regularity
(i.e., of the regular conformation of the polymer chain) have been
proposed. In addition to the analysis of the IR spectra, the effect
of computational parameters on the crystal structure determination
(basis sets and parameters for Grimme’s correction) have been
analyzed. This work demonstrates that state-of-the-art computational
methods can provide an unambiguous description of the structural and
vibrational properties of crystalline polymers on the basis of the
peculiar intra- and intermolecular interactions occurring in different
macromolecular materials
Polymorphism of Poly(butylene terephthalate) Investigated by Means of Periodic Density Functional Theory Calculations
No full textThe conformation and solid state
structure of the two α and
β polymorphs of poly(butylene terephthalate) are here studied
by means of state-of-the-art first principles calculations, carried
out both for the crystals and the infinite one-dimensional chain models.
Focusing in details on the debated β form, induced by mechanical
deformation, we verified the setting on of an all-trans conformation,
as also supported by the simulation of the infrared spectra of the
different polymorphs compared to the available experimental spectra.
The transition from the α to the β form is also simulated
by applying increasing strains to the infinite polymer chain: a peculiar
evolution of the intramolecular structure is indeed predicted, showing
a transition from the GTG′ conformation found for the α
form to the TTT conformation of the strained β form
Modeling of Molecular Charge Distribution on the Basis of Experimental Infrared Intensities and First-Principles Calculations: The Case of CH Bonds
No full textDFT calculations are used to predict CH stretching infrared (IR) intensities for about 50 small molecules. B3LYP and PBE1PBE functionals and different basis sets are tested to obtain the best agreement with the experimental absolute IR intensities available. PBE1PBE functional in particular predicts average CH stretching intensities very close to the experimental ones. On the basis of a simple analytical model, it is shown how it is possible to extract atomic charges directly from computed atomic polar tensors (APTs): these IR charges are very close in value to the experimentally derived ones and faithfully reproduce peculiar molecular phenomena. DFT-derived IR charges are also compared with the charges obtained by population schemes such as Mulliken population analysis, natural population analysis (NPA), and with charges obtained by fitting the electrostatic potential according to the Merz−Kollman (MK), CHELP, and CHELPG models. IR charges are found to be similar especially to the charges obtained by these last methods: therefore they can be used as an alternative scheme for the determination of the molecular charge distribution while being strictly connected to experimentally measurable properties. Moreover, the analytical model is further developed to obtain a method for the calculation of the charge fluxes, which take place along the chemical bonds during molecular vibrations
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
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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