1,721,081 research outputs found
Theoretical study of the SN2 reaction of Cl-(H2O)+CH3Cl using our own N-layered integral molecular orbital and molecular mechanics polarizable continuum model method (ONIOM-PCM)
No full textAn ab Initio Study of the Photochemical Decomposition of 3,3-Dimethyldiazirine
No full textPhotochemical decomposition of 3,3-dimethyldiazirine (DMD) has been computationally investigated by using high-level ab initio calculations in conjunction with the 6-31G* and cc-pvdz basis sets. The geometries of minima and transition states, as well as conical intersection points in the seam of crossing of two surfaces, have been optimized with the complete active space self-consistent field (CAS-SCF) method, and their energies, recalculated with second-order multireference perturbation (CAS/MP2) theory. The reaction path starting at the excited n-π* state of DMD is predicted to occur via a nonadiabatic mechanism, giving carbene and molecular dinitrogen (both in their singlet ground states) as the main products; the computed barrier height (1.0 kcal mol-1) agrees well with the experimental estimate of the activation energy in the singlet excited state (0.0-1.5 kcal mol-1). Ground state of dimethylcarbene is the only species where a 1,2-hydrogen shift takes place, being the only source of propene. The calculated potential energy barrier height for dimethylcarbene to propene isomerization (2.6 kcal mol-1) agrees well with the observed activation energy (2.56 kcal mol-1). No evidence for rearrangement in the first singlet excited state of DMD has been found; such a process would lead to a higher activation energy than the observed one. Consequently, 1,2-hydrogen migration concurrent with N2 extrusion in the excited state has been ruled out
Ultrafast Rationless Deactivation of Orgnic Dyes: Evidence for a Two-state Two-mode Pathway in Polymethine Cyanine
No full textCASSCF quantum chemical calculations (including dynamics) have been used to investigate the ultrafast photoisomerization of three symmetric cyanine dye models of different chain lengths. For the 'model' trimethine cyanine, the photochemical isomerization path can be divided into two phases: initial barrierless skeletal stretching coupled with torsional motion and the decay process that takes place in the region of the twisted intramolecular charge- transfer (TICT) minimum state with an adjacent conical intersection. The path is consistent with both biexponential decay of fluorescence without rise time at short wavelengths and the rise time followed by monoexponential decay at long wavelengths observed in time-resolved experiments. For penta- and heptamethine cyanines, the photoisomerization about different C-C bonds is shown to be an activated process, where the torsional reaction path terminates, again, at a TICT state and the decay takes place at a twisted S1/S0 conical intersection. In agreement with the experimental results, the activation energies increase with the length of the polymethine chain. In contrast to the differenCeS in the potential energy surface between short and long cyanines, we demonstrate that the excited state evolution of these systems can be understood in terms of the same two-state two-mode model of the reaction coordinate previously reported for the (isoelectronic) retinal protonated Schiff base models
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
Potential-Energy Surface for Ultrafast Photochemistry. Static and Dynamic Aspects
No full textThe first singlet excited states (S-1) which control the ultrafast (i.e. subpicosecond) photochemistry of 2-cis-penta-2,4-dieniminium cation (2-cis-C5H6NH2+), all-trans-hexa-1,3,5-triene (all-trans-HT) and cyclohexa-1,3-diene (CHD) have been investigated using ab initio MCSCF and multireference MP2 theories. The structure of the corresponding potential energy surfaces (PESs) has been characterized by computing novel unconstrained and symmetry-constrained minimum-energy paths (MEP) starting from Franck-Condon and S-2/S-1 conical intersection points on S-1. Furthermore, analytical frequency computations have been used to produce quantitative information on the surface curvature.
We show that the S-1 energy surface is characterized by two domains, region I and region II. Region I controls the initial acceleration of the excited state molecule. In contrast, region II is a low-lying region of S-1 and controls the evolution towards fully efficient decay to the ground state. The energy surface structure indicates that the double-bond isomerization of 2-cis-C5H6NH2+ and all-trans-MT and the ring-opening of CMD are prototypes of three classes of barrierless reactions characterized by a different excited state dynamics. In 2-cis-C5H6NK2+ and, more loosely, in all-trans-HT the initial relaxation results in the production of a totally symmetric S-1 transient. The following triggering of the S-1 -->S-0 decay requires energy redistribution along a symmetry-breaking (torsional) mode leading to an S-2/S-1 conical intersection (CI). In contrast, the shape of region I of CHD indicates that an almost direct (i.e. impulsive) motion towards an asymmetric S-1/S-0 CI occurs upon initial relaxation. Previously reported and novel semi-classical trajectory computations and the available experimental evidence seem to support these conclusions
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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