1,720,982 research outputs found
Exocyclic push-pull conjugated compounds. Part 1. Theoretical study of the effect of ring size on the structure, electronic properties and rotational barriers of cyclic analogoues of 1,1-diamino-2,2-dicyanoethylene
No full textAn MO theoretical ab initio study was performed on 2-exo-methylene push-pull derivatives having as donor groups nitrogen atoms, components of a heterocyclic ring, and as acceptors CN and COOEt groups. Five-, six- and seven-membered ring derivatives were considered. Calculations were also performed on a number of push-pull ethylenes whose experimental properties were reported in the chemical literature in order to test the soundness of the conclusions from theoretical approaches. The physical properties calculated for the latter molecules were compared with known experimental values, in order to check the predictive ability of the theoretical approaches employed. Geometrical features and torsional barriers in solution are satisfactorily reproduced. Results were obtained with different basis sets, second-order Moller-Plesset theory, in order to perform comparisons at different theoretical levels with a view to carrying out calculations in larger molecular systems. The widest range of comparisons between calculated values for the different molecules were carried out at the HF/6-31G*//HF/6-31G* level. The origin of the torsional barrier for isomer interconversion as a function of the electronic properties of these molecules is discussed, in particular by examining the polarized character of the exo double bond. The role of the lone pairs of the nitrogen atoms in the push-pull mechanism is investigated, also in competition with an unsaturated bond within the ring. The different conjugation patterns that can be exploited within these molecules is examined within the donor-acceptor model and Natural Bond Orbital (NBO) theory. Empirical correlations are proposed in order to estimate dipole moments and absorption wavelengths for the family of push-pull olefins. (C) 2000 Elsevier Science B.V. All rights reserved
Exocyclic push-pull conjugated compounds. Part 4. rotational barriers in poorly polarized push-pull ethylenes
No full textThe rotational barrier for substituted ethylenes was calculated with MO ab initio methods, both in the Hartree-Fock (HF) single determinant scheme and with multiconfigurational self-consistent field (MCSCF) approaches. The HF model affords reliable results only when applied to molecules substituted with strong electron donor groups, assigning a push-pull character to the molecule. The MCSCF approach was employed for calculating rotational barriers in poorly polarized ethylenes not directly amenable to the HF methods; however, this method was found hard to handle for low symmetry molecules with substituents interacting with the double bond. A method is proposed based on the interpolation of the energy of the perpendicular conformation from a Fourier truncated function constructed with HF molecular energies calculated for frozen conformations twisted up to 60 degrees, The application of HF theory for studying internal rotation in substituted ethylenes with poorly polarized character is discussed and an upper limit of 35-40 kcal/mol can be set up for having reliable barriers from the calculated energy of the rotational transition state at this level of theory. (C) 2001 Elsevier Science B.V. All rights reserved
Exocyclic push-pull conjugated compounds. Part 2. The effect of donor and acceptor substituents on the rotational barrier of push-pull ethylenes
No full textThe energy of the rotational barriers and electronic structure of the transition state in substituted ethylenes are discussed in the light of the results obtained from different theoretical MO ab initio approaches. The 6-31G* basis set at Hartree-Fock (HF) level and with second-order Moller-Plesset perturbation theory (MP2) was employed, critical points were localized through Full geometry relaxation and characterized by vibrational analysis. A multiconfigurational approach (MCSCF) with different active spaces was also employed. For alkenes the correct rotational transition state is obtained only from the MCSCF approach, whereas for push-pull olefins the HF approach with correlation corrections at MP2 level provides correct answers for the internal rotation around C(sp(2))-C(sp(2)) bonds. The choice is more critical when only acceptor or donor groups are present, especially when change of hybridization occurs at the atoms at the edges of the C-C bond in the critical points. (C) 2000 Published by Elsevier Science B.V. All rights reserved
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
Exocyclic push-pull conjugated compounds. Part 3. An experimental NMR and theoretical MO ab initio study of the structure, the electronic properties and barriers to rotation about the exocyclic partial double bond in 2-exo-methylene-and 2-cyanoimino-quinazolines and-benzodiazepines
No full textThe structure of a number of 2-exo-methylene substituted quinazolines and benzodiazepines, respectively, 1, 3a,b, I (X = -CN, -COOEt) and their 2-cyanoimino substituted analogues 2, 3c,d (X = -CN, -SO2C6H4-Me(p) was completely assigned by the whole arsenal of 1D and 2D NMR spectroscopic methods, The E/Z isomerism at the exo-cyclic double bond was determined by both NMR spectroscopy and confirmed by ab initio quantum chemical calculations; the Z isomer is the preferred one, its amount proved dependent on steric hindrance. Due to the push-pull effect in this part of the molecules the restricted rotation about the partial C-2,C-11 and C-2,N-11 double bonds, could also be studied and the barrier to rotation measured by dynamic NMR spectroscopy. The free energies of activation of this dynamic process proved very similar along the compounds studied but being dependent on the polarity of the solvent. Quantum chemical calculations at the ab initio level were employed to prove the stereochemistry at the exo-cyclic partial double bonds of 1-4, to calculate thr barriers to rotation but also to discuss in detail both the ground and the transition state of the latter dynamic process in order to better understand electronic, inter- and intramolecular effects on the barrier to rotation which could be determined experimentally. in the cyanoimino substituted compounds 2, 3c,d, the MO ab initio calculations evidence the isomer interconversion to be better described by the internal rotation process than by the lateral shift mechanism. (C) 2000 Elsevier Science Publishers B.V. All rights reserved
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
NMR spectroscopic and theoretical structural analysis of 5-benzyl substituted hydantoins in solution
No full textThe vicinal H,H and H,C coupling constants along the Ph-CH2AB-CRHM-C-X(=O)R' spin system of 5-benzyl substituted hydantoins together with the results of accompanying force field (TRIPOS) and quantum chemical ab initio calculations (GAUSSIAN-94 with atomic basis set of 3-21G type) were employed to report on the rotamer population about the hydantoin C(5)-benzyl substituent bond. The conformation having the phenyl ring folded over the hydantoin ring system proved to be preferred. (C) 1999 Elsevier Science B.V. All rights reserved
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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