1,721,019 research outputs found
Towards Electronic Scale Dynamics of Chemical Bonding in Isolated Molecules and Solvated Species
No full textThe chemical bonding in molecules and molecular clusters is determined by the nature of the electron density distribution between the atoms. Over the past century, different theories such as Lewis’s bond, valence-bond theory, molecular orbital theory, and frontier molecular orbital theory, have explained static chemical bond in terms of electron density and their symmetry. Over the past century, “dynamic” nature of the chemical bond has been explored by monitoring the nuclear dynamics in femtosecond and picosecond time regimes. In my thesis work, mostly a computational attempt has been made to explore electronic-scale dynamic nature of chemical bonding in isolated molecules and molecules in solvent environment. As electron density can move over a few angström distance within attosecond time scale, dynamic nature of chemical bonding can be named as attochemistry. To explore attochemistry, vertical ionization scheme has been adopted along with the time-dependent NBO analysis.1
Our discussion of the “dynamic” nature of the chemical bonding must begin with an intriguing comparative study of the potential-energy curves for neutral H2 and cationic H2+, and analogous neutral Li2 and cationic Li2+ species. These curves are well-documented in the literature at different levels of theory. Note that neutral H2 exhibits stronger bonding than neutral Li2 (the potential well of neutral H2 is quite deeper than that of neutral Li2). On the other hand, respective cationic potential energy curves evidence that while the H-H bond becomes weaker following ionization, the same leads to strengthening of the Li-Li bond, revealing a different fate for the adiabatic ions of the respective species. Here, one may ask an important question very relevant to the subject of the present thesis, “Does the vertical ion incorporate the electronic-scale change in the chemical bonding which ultimately dictates the fate of the adiabatic ion?” Characters of the NBOs are determined from the natural hybrid orbitals (NHOs) shows that neutral σH-H is formed by the overlap of two pure 1s orbitals (NBO analysis predicts more than 99% 1s contribution). Similarly, σLi-Li bond also originates from overlap between (almost) pure 2s orbitals (NBO analysis predicts more than 95% 2s contribution). The vertical ionization of the H2 molecule removes an electron from bonding orbital . NBO analysis again predicts more than 99% contribution of the 1s orbital to singly occupied orbital (with α spin) in the vertical H2+ ion. This makes the H-H bond strength weaker upon vertical ionization and as a result, the H-H bond distance of the adiabatic H2+ ion becomes considerably longer than that of the neutral H2 species. Compared with the neutral Li2, the bonding hybrids of the vertical Li2+ ion develops significant sp-hybrid character, resulting in much more directional nature. The wB97xD/6-311+G(d,p) level of theory predicts that bonding NHOs of the vertical Li2+ ion to be of sp0.25 form (with more than 19% p character). In contrast, the corresponding hybridization in neutral Li2 is sp0.05 (with less than 5% p character). This enhanced p-character in NHOs of the vertical Li2+ ion also leads to greater inter-nuclear separation than that of neutral Li2 at the equilibrium geometry. Thus, above comparison of the chemical bonding in Li2 and Li2+ shows that strength of the chemical bond does not merely depend on the number of electrons in the bonding NBO (bond order); rather, it depends on the difference in the bonding hybrids, particularly with regard to the directional p-character. Similarly using NBO fate of a noncovalent bond can be predicted from NBO analysis. For the present discussion, CO:H2O hydrogen bonded complex is taken as an example. The highest E(2) value for the CO:H2O noncovalently bonded complex can be seen (increased after ionization), which also shows the NBOs responsible for the relevant charge transfer. The nC to σOH* charge transfer dominantly contributes to the O-C…..H noncovalent bonding interaction.
NBO analysis suggests that the vertical ionization-induced change of the chemical bonding in the Li2 species, in comparison with the H2 species, occurs at the electronic level without involving any nuclear movement. This electronic change of chemical bonding in the vertical ion dictates the fate (structure and energetics) of the adiabatic ion of the respective species. Using the scheme of the quantum dynamics simulation, one can simulate the temporal evolution of the change of chemical bonding in Li2 species following the vertical ionization scheme. Clearly, time dependent NBO analysis predicts extremely fast periodic expansion and contraction of the electron density in the inter-atomic space following the vertical ionization. This periodic movement represents the initial step of the ionization-induced re-hybridization dynamics of the Li-Li σ-chemical bond. No experiment with attosecond metrology has been performed thus far to unravel this re-hybridization dynamics.
Similarly, a systematic theoretical investigation of the attochemistry of solvated molecules would help one design attosecond experiments under ambient conditions to explore the attochemistry in a liquid environment. With this goal in mind, for the first time, we have explored the attochemistry of molecules surrounded by different non-polar solvent environments. To model solvation effects on the attochemistry of molecules containing gold–chalcogen linkages, we have used an implicit solvent model (Polarizable Continuum Model) under the density functional theory (DFT) formalism for non-polar solvents. We have found that the charge migration time scale in molecules becomes faster in the presence of the solvent environment as compared to that under vacuum. Charge oscillation does not damp quickly in molecules surrounded by the solvent environment as compared to that under vacuum. Furthermore, the direction of the charge migration may change in molecules when they are surrounded by the solvent environment as compared to that under vacuum. Thus, the present work has laid the foundation, for the first time, for thinking of the attochemistry into the realm of liquids.2
In addition to the above-mentioned computational efforts, certain preliminary experimental efforts have also been undertaken. Two important steps towards exploring non-equilibrium solvation effect on attosecond chemical bonding are to prepare a suitable model sample and activate liquid beam inside a vacuum. In this regard, an experimental attempt has been made to carry out laser ablation in liquid to synthesize metal-carbyne3 system which can be a good model system for exploring the attosecond non-equilibrium solvation. Finally liquid beam nozzle has been constructed to activate liquid jet inside the vacuum.
High harmonic generation spectroscopy is a self-probing spectroscopy which can probe the evolving attosecond electronic structure following ionization of a species. Hence, dependency of the HHG spectrum on the direction of recombination of the ejected electron has been explored.4 In the present contribution, taking the ion, atoms, and molecule as examples, we show that V(x) can be constructed from one-dimensional molecular electrostatic potential (MEP) of the respective cation to access theoretical HHG spectra not only of simple atoms but also of multielectron complex molecules. We have shown that the MEP-based formalism not only successfully reproduces the ionization energy of the molecular system but also gives explanation for the orientation-dependent HHG spectral intensity change in terms of the nodal plane of the molecular orbital from where the electron is removed during the HHG process. Further extension of the present one-dimensional model to the two- or three-dimensional potentials is expected to address many interesting and practical issues such as quantum interference and effects of different shapes of orbitals. We intend to explore those effects in the future.
In brief this thesis contributes to our fundamental understanding of electronic scale dynamics of chemical bonding. Our hitherto-taken attempts and the progresses in exploring electronic-scale dynamics of chemical bonding are discussed in the following chapters
Thermal and Femtosecond Laser-Induced CO2-Surface Chemistry on Supported Iron-Oxide Based Nanoparticle Surfaces under UHV
No full textThe thesis has been presented in two parts. The 1st part of the thesis focuses on the
fundamental understanding of room temperature CO2 adsorption on iron-oxide based NP
surfaces. The 2nd part describes the understanding of an interesting catalytic reaction dynamics,
namely of metalized energetic molecules.
The content of the thesis is presented in the following way. In Chapter 2, fabrication,
structural elucidation and CO2 surface chemistry of supported iron-oxide (α-Fe2O3) NPs are
discussed. In Chapter 3, the same on the supported Fe-Pd-oxide(shell)@Pd(core) NP surfaces is
presented where the bimetallization by introducing Pd+2 doping in iron-oxide (α-Fe2O3) NPs,
leads to CO2 adsorption at the room temperature. A periodic density functional theory (DFT)
results for the CO2 adsorption on the model pristine α-Fe2O3(0001) and on the model Pd+2 doped
α-Fe2O3(0001) surfaces are discussed in Chapter 4. In Chapter 5, femtosecond pulse induced
chemistry of CO2 on Pd(core)@Fe-Pd-oxide(shell) NP surfaces is discussed. In Chapter 6 I have
presented conical intersection-mediated nonadiabatic chemical dynamics of a simple analogue nitramine (containing one N-NO2 energetic group) molecule, dimethylnitramine (DMNA) and its
complex with an iron atom (DMNA-Fe). Finally, in Chapter 7, general conclusions and future
directions are presente
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
- …
