1,720,964 research outputs found
Aromatic nitration under tropospheric and combustion conditions. A theoretical mechanistic study
No full textThe oxidized soot surface: Theoretical study of desorption mechanisms involving oxygenated functionalities and comparison with temperature programed desorption experiments
No full textThe desorption mechanism for oxygenated functionalities on soot is investigated by quantum
mechanical calculations on functionalized polycyclic aromatic hydrocarbon PAH models and
compared with recently published temperature programed desorption-mass spectrometry results.
Substituents on PAHs of increasing size up to 46 carbon atoms in the parent PAH are chosen to
reproduce the local features of an oxidized graphenic soot platelet. Initially, the study is carried out
on unimolecular fragmentation extrusion, in some cases processes producing HO, CO, or CO2, in
model ketones, carboxylic acids, lactones, anhydrides, in one aldehyde, one peroxyacid, one
hydroperoxide, one secondary alcohol, and one phenol. Then, a bimolecular process is considered
for one of the carboxylic acids. Furthermore, some cooperative effect which can take place by
involving two vicinal carboxylic groups derived from anhydride hydrolysis is investigated for
other four bifunctionalized models. The comparison between the computed fragmentation
desorption barriers for the assessed mechanisms and the temperature at which maxima occur in
TPD spectra for HO, CO, or CO2 desorption offers a suggestion for the assignment of these
maxima to specific functional groups, i.e., a key to the description of the oxidized surface. Notably,
the computations suggest that 1 the desorption mode from a portion of a graphenic platelet
functionalized by a carboxylic or lactone groups is significantly dependent from the chemical and
geometric local environment. Consequently, we propose that 2 not all carboxylic groups go lost at
the relatively low temperatures generally stated, and 3 lactone groups can be identified as
producing not only CO2 but also CO. © 2006 American Institute of Physics
Diabatic surface methods for the study of the reactivity of organic molecules. 1. Cycloaddition of two ethylenes
No full textIn this paper we discuss the quantitative diabatic surface analysis of the surface associated with the thermal cycloaddition of two ethylene molecules. We show that the main features of a saddle point, such as the index (i.e., the number of negative eigenvalues of the Hessian) and the origin, can be understood by analyzing the behavior of the reactant and product diabatic surfaces computed in a three-dimensional subspace involving two relevant geometrical variables at a time. We show also that the behavior of the constituent diabatic surfaces can be easily rationalized with the simple energy expressions of qualitative MO theory. © 1986, American Chemical Society. All rights reserved
Can a photochemical reaction be concerted? A theoretical study of the photochemical sigmatropic rearrangement of but-1-ene
No full textMC-SCF computations at the 4-31G level using a complete active space (CAS) of four orbitals demonstrate the existence of a concerted photochemical pathway for [1,2] and [1,3] alkyl sigmatropic shifts. The central feature of this concerted path is a conical intersection (e.g., a genuine crossing) between ground and excited state from which a fully efficient return to the ground state is possible. Thus the excited-state surface has no minimum with zero gradient (i.e., a critical point) but only a singularity which corresponds to the lowest energy point of a conical intersection between ground and excited states. Thus there is no bottleneck corresponding to a short-lived intermediate that would correspond to the minimum on the excited-state surface at an avoided crossing. Intrinsic reaction coordinate computations have been performed on the excited-state surface that demonstrate the existence of two "channels" on the excited-state surface that simply continue on the ground-state surface. One of these channels leads to a [1,2] sigmatropic shift, the other to a [ 1,3] sigmatropic shift. The proposed mechanism is consistent with experimental observations where both [1,2]- and [1,3]-shift products are observed, and where the migrating group moves according to a supra process with retention of configuration of the migrating group
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
A quantitative analysis of the factors controlling the inversion barriers in AH3 molecules
No full textVariations 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
Reaction of dialkyl carbonates with alcohols:Defining a scale of the best leaving and entering groups
Full text linkA series of dialkyl and methyl alkyl carbonates has been synthesized and their reactivity investigated. The behavior of preferential leaving and entering groups for the newly synthesized carbonates has been accurately investigated. Both experimental and computational studies agreed that the scale of leaving groups follows the trend: PhCH2O–, MeO– ≥ EtO–, CH3(CH2)2O–, CH3(CH2)7O– > (CH3)2CHO– > (CH3)3CO–. Accordingly, the scale of the entering group has the same trend, with t-butoxide being the worst entering group. A preliminary attempt to rationalize the nucleofugality trends, limited to the (CH3)3CO– and CH3O– groups, has indicated that a likely origin of the observed trends lies in the different entropic contributions and solvation effects
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
- …
