1,720,970 research outputs found
Wavelength dependent photoextrusion and tandem photoextrusion reactions of ninhydrin bis-acetals for the synthesis of 8-ring lactones, benzocyclobutenes and orthoanhydrides
No full textNinhydrin bis-acetals give access to 8-ring lactones, benzocyclobutenes and spirocyclic orthoanhydrides through photoextrusion and tandem photoextrusion reactions. Syntheses of fimbricalyxlactone B, isoshihunine and numerous heterocycles demonstrate the value of the methods, while TA-spectroscopy and TD-DFT studies provide mechanistic insights on their wavelength dependence
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
Understanding Ultrafast Molecular Structural Dynamics in Liquids : Ultrafast Raman Loss and Transient Absorption Study
No full textThe present thesis deals with the detailed ultrafast experimental studies on the ex-cited state vibrational dynamics of model chemical systems in condensed phase. The molecular systems are studied by means of Ultrafast Raman Loss Spectroscopy (URLS) and femtosecond transient absorption (TA) spectroscopy. The primary objective of the current thesis is to look at structural dynamics immediately after Franck-Condon excitation, influenced by surrounding solvent molecules.
The thesis is divided into six chapters and a brief summary of each chapter is given below.
In Chapter 1, I have included the concept of femtosecond time-resolved spectroscopy and nuclear dynamics. Some important aspects of spontaneous and stimulated Raman scattering are discussed. Subsequently, I have described the theoretical framework and methodologies that are important to understand non-linear spectroscopy. Important aspects of time-resolved stimulated Raman pro-cess are described here. A brief outline of the current thesis is also presented at the end of this chapter.
In Chapter 2, I have described in detail the instrumentation and data process-ing methods. A detailed explanation of spectroscopic techniques like transient ab-sorption and femtosecond stimulated Raman spectroscopy and their experimen-tal aspects are discussed. The optimization of the setup, detailed data analysis processes are described. Finally, I have described step-by-step data processing method using DaFitUma software which is developed in our lab.
In Chapter 3, I have studied the excited state isomerization dynamics of tetrap-henylethylene in various solvents using femtosecond TA and URLS spectroscopy. Tetraphenylethylene, a model molecular motor, known to exhibit low-frequency torsional dynamics. TA measurements in the S1 state reveal a step-wise struc-tural rearrangement from Franck-Condon (FC) state ! relaxed state ! twisted state on photoexcitation. Furthermore, oscillation in TA kinetics was observed with-in 1 ps. Fourier transformation of this oscillation leads to a 130 cm 1 low-frequency mode which is responsible for the excited state wavepacket dynamics during the course of FC state ! relaxed state structural rearrangement. Two vi-brational marker band Cet=Cet stretching ( 1512 cm 1) and Cph=Cph stretching ( 1584 cm 1) modes appear immediately after photoexcitation in URLS spectra. From the peak frequency shift, we confirm that the initial structural relaxation is dominated by phenyl torsion rather than central Cet=Cet twist.
In Chapter 4, the structural dynamics and solvent induced internal conversion of 2-(20-Hydroxyphenyl)benzothiazole (HBT), an extensively studied excited state intramolecular proton transfer (ESIPT) system, are investigated. We have obtained snapshots of vibrational spectra of the photoexcited HBT, by URLS, providing de-tailed structural change at an early time. From the URLS peak amplitude analysis we have observed 400 fs amplitude decay in acetonitrile. This is assigned to a rapid structural change induced by polar acetonitrile due to the torsional mo-tion. The ultrafast amplitude decay is associated with coherent excitation of tor-sional mode ( 40 cm 1). Oscillatory peak position in the high-frequency Raman modes indicates a coupling between low-frequency torsional and high-frequency vibrations. Activation of torsional motion in polar solvents provides an additional route for non-radiative decay, responsible for a short lifetime ( 12 ps) and low fluorescence quantum yield. For hexane, the URLS spectral evolution is assigned to vibrational cooling (2-4 ps) followed by population relaxation (115 ps).
Chapter 5, deals with the ultrafast intersystem crossing (ISC) dynamics of thioxanthone (TX) in various solvents. It has striking solvent dependent photo-physical properties. For example, the fluorescence quantum yield ( f ) and life-time ( f ) of the primarily populated singlet (1 *) state strongly depends on the solvent properties. We have studied TX in three different solvents: acetonitrile (ACN; aprotic), methanol (MeOH; polar protic) and in cyclohexane (CX; nonpo-lar). Following electronic excitation to the S1 (1 ) state, the molecule relaxes to the lowest T1 (3 ) state in 50 ps, 4 ps in ACN and in CX respectively. In the case of TX in methanol, two stages of ISC with time constants of 4 ps and > 2ns were obtained. These two stages are assigned to direct ( 4 ps) ISC to the T2 state (1 ! 3n ) followed by slow internal conversion ( 2 ns) to the T1 state (3 ). Analysis of peak position dynamics further allows us to obtain vibrational relaxation times during ISC process. The present study sheds light on a complete understanding of the structural changes accompanying the ISC process.
Finally, in Chapter 6, we have briefly summarized the main results of the the-sis. This chapter also includes our comments on some of the future extensions of the present work
Femtosecond Time-Resolved Raman Spectroscopy Reveals Structural Evidence for meta Effect in Stilbenols
No full textThe met a effect in substituted aromatics plays a crucial role in their excited-state photophysical properties. Meta-substituted hydroxyarenes such as naphthols, stilbenols, and chromophoric constituents of green fluorescent proteins show unusual photoacidity and enhanced fluorescence lifetime and quantum yield when compared to their para-derivatives. Variation in the excited state features of the meta-derivatives when compared to the para-derivatives in stilbenols has been attributed to the enhanced torsional barrier for interconversion between the planar and the twisted perpendicular forms. Herein, we employed femtosecond time-resolved Raman spectroscopy to provide the direct structural evidence for the enhanced torsional barrier in meta-stilbenol. The Raman band profiles of the olefinic C=C stretch related to the torsional motion are found to decay with time constants of similar to 750 and similar to 13 ps in meta-stilbenol and para-stilbenol respectively, unraveling the structural evidence for the observed enhanced photoacidity originating from enhanced rates of excited-state proton transfer. Further, time-resolved fluorescence measurements are performed to elucidate the relaxation pathways of the excited states of the stilbenols
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
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