1,720,974 research outputs found
High Resolution Free Jet Millimeter Wave Absorption Spectroscopy: a bridge to Astrochemistry
No full textConventional absorption spectroscopy is still the workhorse in high-resolution rotational laboratory spectroscopy.1 The data obtained from these kind of instruments are relevant for astronomical searches of complex molecules that represent excellent probes of the physical and chemical environments and history of the sources where they are detected.2 Nowadays, observations performed by the Atacama Large Millimeter Array (ALMA) open up new opportunities to reveal the chemical complexity of solar systems analogues. At the same time the huge amount of data collected and the extremely rich surveys represent a challenge for the astrochemistry community. The chance to detect molecules with an increasing large number of atoms, goes hand in hand with the complexity of their conformational equilibria, often associated with large amplitude motions, that need to be analysed in laboratory before taking on an astronomical search. For this reason a strong interplay between the laboratory spectroscopists and the observational astronomers is increasingly required to be able to unravel the spectra, which are rather difficult to predict theoretically, mainly in the sub-mm wave region.
In this talk laboratory results on diols and thiols of potential astronomical interest, obtained using the only Free Jet Absorption Millimeter Wave (FJAMMW) spectrometer working at the University of Bologna,3 will be presented. The rotational spectra (59.6 - 74.4 GHz, corresponding to 5.03 - 4.03 mm) reveal the presence of six conformers for 1,2-butanediol (C4H10O2) and four conformers for 1,3-propanedithiol (C3H8S2), proving the complexity of the conformational landscapes of these kind of compounds.
Moreover, taking advantage of the existing public ALMA data, some considerations on the rich molecular line spectrum of the Class 0 protostellar binary IRAS 16293-2422 will be discussed. References
[1] S. Brünken, S. Schlemmer, arXiv:1605.07456, 2016
[2] E. Herbst, E. F. van Dishoeck, Annu. Rev. Astron. Astrophys. 47, 427, 2009
[3] C. Calabrese, A. Maris, L. Evangelisti, L. B. Favero, S. Melandri, W. Caminati, J. Phys. Chem. A. 117, 13712, 201
High Resolution Millimeter Wave Absorption Spectroscopy: from the laboratory data to the astronomical surveys
No full textConventional absorption spectroscopy is still the workhorse in high-resolution rotational laboratory spectroscopy.1 The data obtained from these kind of instruments are relevant for astronomical searches of complex molecules that represent excellent probes of the physical and chemical environments and history of the sources where they are detected.2 Nowadays, observations performed by the Atacama Large Millimeter Array (ALMA) open up new opportunities to reveal the chemical complexity of solar systems analogues. At the same time the huge amount of data collected and the extremely rich surveys represent a challenge for the astrochemistry community. To reach this goal, the spectroscopic know-how is fundamental in recognizing typical pattern lines due to multiple internal interactions and motions that cause complicated energy level schemes, since the resulting spectra will be rather difficult to predict theoretically, mainly in the sub-mm wave region. For this reason a strong interplay between laboratory spectroscopists and observational astronomers is increasingly required to be able to unravel the experimental data.
In this talk the features and the potential of the high resolution rotational spectroscopy technique will be pointed out, providing some results on different kind of molecules characterized by complex conformational landscapes. In particular, molecular spectra of potential astronomical interest will be presented, obtained using the free jet absorption millimeter wave and the free space cell absorption sub-mm wave spectrometers working at University of Bologna.3
References
[1] S. Brünken, S. Schlemmer, arXiv:1605.07456, 2016
[2] E. Herbst, E. F. van Dishoeck, Annu. Rev. Astron. Astrophys. 47, 427, 2009
[3] C. Calabrese, A. Maris, L. Evangelisti, L. B. Favero, S. Melandri, W. Caminati, J. Phys. Chem. A. 117, 13712, 201
Gas phase spectroscopic recognition of complex organic molecules in space
No full textIt is commonly believed that accumulation of organic molecules of increasing complexity is an important step in the series of events that led from elementary particles to the emergence of life. For this reason a lot of resources have been and are currently devoted to the discovery and quantification of molecular species in extra-terrestrial environments.
Through the recognition of the rotational spectral lines, radioastronomy represents one of the most important techniques for the unequivocal detection of complex organic molecules (COMs) in circumstellar envelopes and interstellar medium (ISM): of the among 130 compounds that have been detected about 90 species are neutral molecules, that typically are characterized by functional groups common in organic chemistry, such as amines, alcohols, aldehydes, ketones, etc.
COMs of greater size are expected to be detected exploiting the increasing sensitivity and resolution of new radiotelescopes, therefore the challenge is nowadays twice: (i) on the one hand the spectral analysis of the huge amount of data contained in the ongoing astronomical observations, (ii) on the other hand the laboratory observation and assignment of the rotational spectra of COMs, which are often complicated due to flexibility typical of these molecules.
Considering these tasks, we will here present first the rotational spectra of some COMs recorded in laboratory, highlighting the relationships between the spectral features and the conformational landscapes, then an example of spectral survey achieved by data reduction of the freely available Atacama Large Millimeter/submillimeter interferometer Array (ALMA) data on Class 0 protostellar binary IRAS 16293-2422, evidencing the issues related to the molecular recognition in dense spectra
On the torsional flexibility of the dihydrolipoic acid's pharmacophore: 1,3-propanedithiol
No full textThe conformational space of the antioxidant dihydrolipoic acid has been explored through the investigation of its pharmacophore 1,3-propanedithiol. Five of the possible 25 non equivalent isomers (namely: gGGg’, gGGg, g’AGg, gAGg and g’AGg’) were observed in the 59.6-74.4 GHz frequency region by free-jet absorption rotational spectroscopy and for three of them also the 34S mono-substituted isotopologues were detected in natural abundance. Theoretical simulations show that the balance of steric and electronic intramolecular interactions arises on a shallow conformational potential energy surface and suggest that in polar solvents the dithiol chain’s flexibility is greater than in the isolated phase
Conformational preferences of molecules, biomolecules and molecular complexes: the importance of the hydrogen bond and other non-bonding interactions
No full textRotational spectroscopy of non-covalently bound complexes of medium size organic molecules
Full text linkThe conformational space of non-covalently bound complexes of medium size organic molecules is shaped by competing interactions occurring within the molecules or with the partners. It usually presents a high number of low energy conformations very close in energy with shallow potential energy barriers through which the molecular system can tunnel.
The conformational preferences of non-covalently bound complexes can be studied to a very high degree of accuracy by free jet rotational spectroscopy1 and from the detailed structural and dynamical data that can be obtained, the site and geometry of the interaction and information on the binding energy can be inferred without ambiguity.
The questions usually addressed are: which is the preferred binding site, which type of interactions are established, and whether any conformational change takes place in the monomers upon complexation. Answers to these questions allow insight into the molecular interaction process at the molecular level, bridging the gap between gas-phase and bulk properties.
Chosen examples of published and unpublished results of complexes of medium-size organic molecules with different partners formed in a supersonic expansion and characterized by rotational spectroscopy will be discussed. The partner molecules are held together by hydrogen bonds, weak hydrogen bonds and lone-pair--hole interactions.
It will be shown how non-bonding interactions compete to shape the conformational space of the complexes, the structural changes brought to the conformers of the monomers by complexation and how these interactions can be drastically changed through atomic or functional group substitution
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
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
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