1,720,983 research outputs found
Switching on molecular iodine elimination through isomerization: The F2C–I–I isomer of difluorodiiodomethane
No full textWe report a UMP2/LANL2MB computational investigation of the ground-state pathways involved in the 350-nm photoinduced formation of molecular iodine from difluorodiiodomethane. Formation of I-2 from the conventional CF2I2 isomer is highly endothermic proceeding through a cyclic-skewed transition state at 62.4 kcal mol (1) above CF2I2. A minimum corresponding to iso- difluorodiiodomethane (F2C-I-I) is calculated at 24.8 kcal mol (1) above CF2I2. The formation of CF2 + I-2 from F2C-I-I is endothermic by only 18.8 kcal mol (1). The results suggest that solvent- induced geminate recombination of CF2I and I photofragments leads to hot F2C-I-I, which dissociates to form I-2 on a picosecond time scale. (c) 2008 Elsevier B. V. All rights reserved
Structure of the Photochemical Reaction Path Populated via Promotion of CF2I2into Its First Excited State
Get PDFThe photochemical reaction path following the promotion of CF2I2 into its lowest-lying excited electronic singlet state has been modeled using ab initio multiconfigurational quantum chemical calculations. It is found that a conical intersection drives the electronically excited CF2I2* species either to the CF2I + I radical pair or back to the starting CF2I2 structure. The structures of the computed relaxation pathways explain the photoproduct selectivity previously observed in the gas phase. Furthermore, the results provide the basis for explaining the condensed-phase photochemistry of CF2I2
Comparative Study of Uracil Excited-State Photophysics in Water and Acetonitrile via RMS-CASPT2-Driven Quantum-Classical Trajectories
No full textWe present a nonadiabatic molecular dynamics study of the ultrafast processes occurring in uracil upon UV light absorption, leading to electronic excitation and subsequent nonradiative decay. Previous studies have indicated that the mechanistic details of this process are drastically different depending on whether the process takes place in the gas phase, acetonitrile, or water. However, such results have been produced using quantum chemical methods that did not incorporate both static and dynamic electron correlation. In order to assess the previously proposed mechanisms, we simulate the photodynamics of uracil in the three environments mentioned above using quantum-classical trajectories and, for solvated uracil, hybrid quantum mechanics/molecular mechanics (QM/MM) models driven by the rotated multistate complete active space second-order perturbation (RMS-CASPT2) method. To do so, we exploit the gradient recently made available in OpenMolcas and compare the results to those obtained using the complete active space self-consistent field (CASSCF) method only accounting for static electron correlation. We show that RMS-CASPT2 produces, in general, a mechanistic picture different from the one obtained at the CASSCF level but confirms the hypothesis advanced on the basis of previous ROKS and TDDFT studies thus highlighting the importance of incorporating dynamic electron correlation in the investigation of ultrafast electronic deactivation processes
Matrix isolation and computational studies of the CF2I radical
No full textWe report experimental and computational studies of the spectroscopy and photochemistry of the CF2I radical, which was generated using pulsed discharge or photolytic techniques and trapped in Ar or Ne matrices held at ∼5 K. Ground state calculations were performed using UM062X and UB3LYP density functionals and the UMP2, UCCSD, and CASPT2 ab initio methods, all with high-quality basis sets. The photochemical reaction path from the lowest-lying electronically excited state was mapped using the CASPT2//CASSCF methodology. A conical intersection was found to drive the excited state species to the photoproduct/reactant wells on the ground state potential energy surface. © 2010 Elsevier B.V. All rights reserved
Roaming isomerization of photoexcited halogenated alkanes in the gas and liquid phases
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Previous issue date: 6Recent experimental and computational gas-phase studies have brought to light a new type of unimolecular decomposition called a “roaming mechanism. It has only been observed in the gas phase, and whether it also occurs in solution is an intriguing question. Using ultrafast transient absorption spectroscopy, we report direct isomerization of CHBr, BBr, and PBr geminal tribromides in solution within the first 100 fs after S-excitation. The gas-phase conditions do not affect the earliest course of similar isomerization of CHBr. High-level ab initio simulations on XBr (X = B, P, and CH) suggest that isomerization is governed by an energetically and dynamically accessible S/S conical intersection and can be best described as a roaming-mediated pathway. Following the initial relaxation from the Franck-Condon point, “wandering” of the central atoms and migration of Br atom starts on a planar region of the S surface, and in the vicinity of the conical intersection (~40 fs) the XBr and Br fragments become separated to 3 AA. After passage through the conical intersection, the partially dissociated bromine atom slips off the XBr bisector plane, and forms the Br–Br bond of the BrXBr–Br isomer (~60 fs). We give examples of similar roaming isomerization in several other di- and polyhalogenated alkanes
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
Watching ultrafast barrierless excited-state isomerization of pseudocyanine in real time
Get PDFThe photoinduced excited-state processes in 1,1'-diethyl-2,2'-cyanine iodine are investigated using femtosecond time-resolved pump-probe spectroscopy. Using a broad range of probe wavelengths, the relaxation of the initially prepared excited-state wavepacket can be followed down to the sink region. The data directly visualize the directed downhill motion along the torsional reaction coordinate and suggest a barrierless excited-state isomerization in the short chain cyanine dye. Additionally, ultrafast ground-state hole and excited-state hole replica broadening is observed. While the narrow excited-state wavepacket broadens during pump-probe overlap, the ground-state hole burning dynamics takes place on a significantly longer time-scale. The experiment reported can be considered as a direct monitoring of the shape and the position of the photoprepared wavepacket on the excited-state potential energy surface
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