1,721,081 research outputs found
A Theoretical Study of the Low-Lying States of the Anionic and Protonated Ionic Forms of Urocanic Acid
No full textA multistate second-order perturbation theory (MS-CASPT2) study of the lowest lying states in the electronic spectra of urocanic acid in vacuo is presented. The anionic trans and cis isomers, as well as the biologically important trans protonated ionic structure, are considered. The vertical and 0-0 excitation spectra were computed for each system at the MS-CASPT2/ANO-L level, describing the lowest lying ππ* and ππ* singlet . and triplet states. In all three systems, a weakly absorbing ππ* singlet state was observed at ∼4.0 eV in the vertical excitation spectrum, suggesting both a novel assignment and an alternative explanation for the previously described wavelength dependent photochemistry of this molecule. The trans anion vertical spectrum otherwise comprises three intense ππ* transitions, at 4.14, 4.59, and 5.00 eV, whereas that of the cis anion shows two, at 4.52 and 5.01 eV. Conversely, the vertical spectrum of the protonated trans system is dominated by a single, intensely absorbing ππ* state, computed at 4.76 eV. The wave functions of the ππ* states all show a multiconfigurational character, which is most pronounced in the protonated ionic structure. The lowest vertical singlet states are of ππ* character (trans anion, 4.13 eV; cis anion, 3.81 eV; trans protonated, 3.19 eV), although the lowest triplet states are due to ππ* transitions (trans anion, 3.40 eV; cis anion, 3.39 eV; trans protonated, 2.56 eV). In each of the anionic systems, the origin of the most intense singlet ππ* transition in the vertical spectrum (trans anion, 4.03 eV; cis anion, 3.81 eV) lies below the corresponding lowest singlet ππ* vertical excitation energy, further suggesting that the latter state may not be directly and efficiently populated. Good agreements with available experimental data are noted throughout. © 2000 American Chemical Society
Ground and excited state CASPT2 geometry optimizations of small organic molecules
No full textA method for computing second-order multiconfigurational perturbation theory (CASPT2) energy gradients numerically has been implemented and applied to a range of elementary organic chromophores, including 1,3 butadiene, acrolein, and two protonated Schiff bases. Geometries of ground and excited states-as well as conical intersections-are compared with the corresponding CASSCF structures, illustrating the effect of including the correction for dynamical electron correlation. It is shown that the differences between the two methods are not readily categorized, but that, while individual changes in bond lengths can be quite large (∼0.01-0.02 Å), the natures and CASPT2 energetics of the structures remain similar. Exceptions to this tend to be systems that have a strong ionic character and that are not well described at the CASSCF level. Basis set effects (double- vs. triple-ζ) were examined for a limited number of examples, and found to be quite dramatic at both levels of theory. © 2003 Wiley Periodicals, Inc
A Theoretical Study of the Low-Lying Excited States of trans- and cis-Urocanic Acid
No full textA multiconfigurational second-order perturbation theory (CASPT2) study of the lowest lying states in the gas-phase electronic spectra of trans- and cw-urocanic acid is presented. Geometries of both isomers have been optimized at the MP2/6-31G(d) and jr-CASSCF/ANO-L(4s3pld, 2s) levels of theory. The geometries are found to differ considerably between the two levels. The vertical and 0-0 excitation spectra were calculated for each isomer. Both singlet and triplet states are described for each, including the lowest lying noπ* excitations and the noπ* excitations. Remarkably, in the trans spectrum, it is found that the noπ* state has a higher vertical excitation energy than the lowest ππ* (5.12 vs 4.93 eV), but a lower band origin (4.10 vs 4.66 eV). Thus, the ππ* and noπ* surfaces cross at a coordinate between that of the ground-state structure and the equilibrium excited state structure. The trans vertical spectrum consists of three intense (nn) electronic transitions at 4.93, 5.40, and 6.00 eV, whereas the cis spectrum is dominated by a single intense transition at 4.15 eV and a weaker one at 5.85 eV. The wave functions of the excitations typically show a multiconfigurational character, with the weighting of doubly excitated configurations exceeding 20% in a number of instances. The lowest lying Rydberg states (π3s) were found at 5.47 and 5.67 eV for the trans and cis isomers, respectively. © 1999 American Chemical Society
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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