1,720,986 research outputs found
Vibrational Signature of Water Molecules in Asymmetric Hydrogen Bonding Environments
No full textThe O-H stretching vibrational modes of water molecules are sensitive to their local environments. Here, we applied effective normal mode analysis to isolate contributions of each of the two hydrogen atoms to the vibrational modes nu(1) and nu(3) of water molecules in the liquid phase. We demonstrate that the decoupling of the two contributions f(d) and the frequency splitting of the vibrational modes Delta omega(13) are inextricably related to the symmetry of the hydrogen bonding environment. We show that ambient liquid water modeled at the density functional level of theory exhibits the characteristics of an asymmetric environment with an average decoupling of 0.82 and a splitting of 137 inverse centimeters Such large value of decoupling and splitting would account for the inhomogeneous broadening as observed in the vibrational spectra of liquid water. The computational protocols and the results of this work will facilitate the interpretation of experimental Raman and infrared spectra of interfacial water molecules at hydrophobic, membrane, and protein surface
Variable-Metric Localization of Occupied and Virtual Orbitals
No full textThe key idea of the variable-metric
approach to orbital localization
is to allow nonorthogonality between orbitals while, at the same time,
preventing them from becoming linearly dependent. The variable-metric
localization has been shown to improve the locality of occupied nonorthogonal
orbitals relative to their orthogonal counterparts. In this work,
numerous localization algorithms are designed and tested to exploit
the conceptual simplicity of the variable-metric approach with the
goal of creating a straightforward and reliable localization procedure
for virtual orbitals. The implemented algorithms include the steepest
descent, conjugate gradient (CG), limited-memory Broyden–Fletcher–Goldfarb–Shanno
(L-BFGS), and hybrid procedures as well as trust-region (TR) methods
based on the CG and Cauchy-point subproblem solvers. Comparative analysis
shows that the CG-based TR algorithm is the best overall method to
obtain nonorthogonal localized molecular orbitals (NLMOs), occupied
or virtual. The L-BFGS and CG algorithms can also be used to obtain
NLMOs reliably but often at higher computational cost. Extensive tests
demonstrate that the implemented methods allow us to obtain well-localized
Boys–Foster (i.e., maximally localized Wannier functions) and
Pipek–Mezey, orthogonal and nonorthogonal, and occupied and
virtual orbitals for a variety of gas-phase molecules and periodic
materials. The tests also show that virtual NLMOs, which have not
been described before, are, on average, 13% (Boys–Foster) and
18% (Pipek–Mezey) more localized than their orthogonal counterparts
Unraveling the Origins of Strong and Reversible Chemisorption of Carbon Dioxide in a Green MetalOrganic Framework
No full textCyclodextrin-derived
metal–organic frameworks (MOFs) are
remarkable not only because of their ability to absorb carbon dioxide
strongly and reversibly but also because they can be readily obtained
from inexpensive, renewable, and environmentally benign components.
Despite the wealth of data on the carbon dioxide intake by CD-MOF-2,
a representative of these MOFs, the nature and structural characteristics
of its diverse adsorption sites, capable of binding CO2 in the irreversible, reversible, and weak regimes, remain unclear.
A comprehensive analysis of the results of the density functional
theory modeling performed in this work in conjunction with experimental
data shows that the hydroxyl counterions in CD-MOF-2 pull the protons
away from the cyclodextrin alcohol groups, increasing their nucleophilic
strength and turning them into strongly binding alkoxide chemisorption
sites. At the same time, the diverse hydrogen bonding environments
of the alkoxide sites reduce their nucleophilic character to a different
extent, tuning their CO2 binding to become irreversible,
reversible, or weak. By linking the acid–base proton equilibrium
and hydrogen bondingtwo chemical concepts widely used for
liquidsto the strength of the CO2 binding in CD-MOF-2,
this work suggests new strategies for advancing design of tunable
solid materials for CO2 capture or detection
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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