1,721,034 research outputs found
The photoisomerization mechanism of azobenzene: a semiclassical simulation of nonadiabatic dynamics
No full textWe have simulated the photoisomerization dynamics of azobenzene, taking into account internal conversion and geometrical relaxation processes, by means of a semiclassical surface hopping approach. Both n->pi* and p->pi* excitations and both cis-trans and trans-cis conversions have been considered. We show that in all cases the torsion around the N=N double bond is the preferred mechanism. The quantum yields measured are correctly reproduced and the observed differences are explained as a result of the competition between the inertia of the torsional motion and the premature deactivation of the excited state. Recent time-resolved spectroscopic experiments are interpreted in the light of the simulated dynamics
The photoisomerization of a peptidic derivative of azobenzene: a nonadiabatic dynamics simulation of a supramolecular system
No full textThe aim of this work is to investigate the mechanism of photoisomerization of an azobenzenic chromophore in a supramolecular environment, where the primary photochemical act produces important changes in the whole system. We have chosen a derivative of azobenzene, with two cyclopeptides attached in the para positions, linked by hydrogen bonds when the chromophore is in the cis geometry. We have run computational simulations of the cis-trans photoisomerization of such derivative of azobenzene, by means of a surface hopping method. The potential energy surfaces and nonadiabatic couplings are computed ‘‘on the fly” with a hybrid QM/MM strategy, in which the quantum mechanical subsystem is treated semiempirically. The simulations show that the photoisomerization is fast (about 200 fs) and occurs with high quantum yields, as in free azobenzene. However, the two cyclopeptides are not promptly separated, and the breaking of the hydrogen bonds requires longer times (at least several picoseconds), with the intervention of the solvent molecules (water). As a consequence, the resulting trans-azobenzene is severely distorted, and we show how its approach to the equilibrium geometry could be monitored by time-resolved absorption spectroscopy
Ottimizzazione del Progetto di un Accoppiatore Direzionale Verticale con Cristalli Liquidi Ferroelettrici a Superfici Stabilizzate
No full textQM/MM connection atoms for the multistate treatment of organic and biological molecules
No full textWe present an extension of our semiempirical floating occupation MO-CI approach for the determination of ground and excited state potential energy surfaces of interest in
photochemistry. The QM/MM variant of the method, which allows for electrostatic and van der Waals interactions between the QM and MM subsystems, is supplemented with a treatment of covalent interactions based on Antes and Thiel's connection atom approach.
We concentrate on the transferability of the connection atom parameters and on the correct treatment of electrostatic interactions concerning the connection atom itself.
We show the viability of the method with three examples of connection atoms: S in a thioether bridge, C and N in a peptide. The results obtained with the QM/MM treatment compare well
with all-QM results of the same level
Are azobenzenophanes rotation-restricted?
No full textWe simulated the photoisomerization dynamics of an azobenzenophane with a semiclassical surface hopping approach and a semiempirical reparametrized Quantum Mechanics/Molecular Mechanics hamiltonian. Only one of the two azobenzene chromophores in the molecule is taken into account quantum mechanically: the other one is treated by molecular mechanics. Both n -> pi* and pi -> pi* excitations are considered. Our results show that the photoisomerization reaction mainly involves the rotation around the N=N double bond. The excited state relaxation features are in qualitative agreement with experimental time-resolved fluorescence results
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
Simulation of the photodynamics of azobenzene on its rst excited state: comparison of full multiple spawning and surface hopping treatments
No full textWe have run Full Multiple Spawning simulations for the cis -> trans and trans -> cis photoisomerization of azobenzene by n -> pi* excitation. The quantum mechanical description of the nonadiabatic dynamics has been compared with a previous semiclassical treatment based on Surface Hopping. We concentrate on the dynamical effects that determine the photoisomerization quantum yields, namely the rate of radiationless electronic relaxation and the inertia of the motion along the reaction coordinate. The quantal and semiclassical results are in good general agreement, thus confirming our previous analysis of the photodynamics. The Surface Hopping method slightly overestimates the rate of excited state decay, which in this case leads to compute lower quantum yields
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
- …
