1,721,053 research outputs found
Rotational Spectroscopy Pinpoints the Tetrahydrate as the Onset of Water Self-Aggregation in Sevoflurane Hydration
No full textCharacterizing the interactions between water and volatile anesthetics at a molecular level is crucial for understanding their mechanisms of action. We employed broadband molecular rotational spectroscopy (CP-FTMW) and extensive isotopic substitution experiments to generate and characterize the stepwise addition of up to four water molecules to the volatile anesthetic sevoflurane, a flexible molecule with multiple binding sites. The substantial amount of isotopic data enabled the conclusive derivation of accurate structural information. The observed structures contain the most stable conformer of the previously identified monomer, with water clusters favorably interacting with the molecule to form an open chain with up to three water molecules. Notably, two isomers were detected for the tetrahydrate, which exhibit a cyclic structure with either a clockwise or anticlockwise orientation, resembling that of the pure water tetramer. The four-water marks a transition where water–water interactions dominate over direct sevoflurane–water interactions driving the assembly of the water network
Rotational Signatures of Dispersive Stacking in the Formation of Aromatic Dimers
No full textThe aggregation of aromatic species is dictated by inter‐ and intramolecular forces. Not only is characterizing these forces in aromatic growth important for understanding grain formation in the interstellar medium, but it is also imperative to comprehend biological functions. We report a combined rotational spectroscopic and quantum‐chemical study on three homo‐dimers, comprising of diphenyl ether, dibenzofuran, and fluorene, to analyze the influence of structural flexibility and the presence of heteroatoms on dimer formation. The structural information obtained shows clear similarities between the dimers, despite their qualitatively different molecular interactions. All dimers are dominated by dispersion interactions, but the dibenzofuran dimer is also influenced by repulsion between the free electron pairs of the oxygen atoms and the π‐clouds. This study lays the groundwork for understanding the first steps of molecular aggregation in systems with aromatic residues
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
Headspace Analysis Of Volatile Compounds Using Segemented Chirped-pulse Fourier Transform Mm-wave Spectroscopy
Full text linkA chirped-pulse Fourier transform mm-wave spectrometer has been tested in analytical chemistry applications of headspace analysis of volatile species. A solid-state mm-wave light source (260-290 GHz) provides 30-50 mW of power. This power is sufficient to achieve optimal excitation of individual transitions of molecules with dipole moments larger than about 0.1 D. The chirped-pulse spectrometer has near 100\% measurement duty cycle using a high-speed digitizer (4 GS/s) with signal accumulation in an FPGA. The combination of the ability to perform optimal pulse excitation and near 100\% measurement duty cycle gives a spectrometer that is fully optimized for trace detection. The performance of the instrument is tested using an EPA sample (EPA VOC Mix 6 – Supelco) that contains a set of molecules that are fast eluting on gas chromatographs and, as a result, present analysis challenges to mass spectrometry. The ability to directly analyze the VOC mixture is tested by acquiring the full bandwidth (260-290 GHz) spectrum in a “high dynamic range” measurement mode that minimizes spurious spectrometer responses. The high-resolution of molecular rotational spectroscopy makes it easy to analyze this mixture without the need for chemical separation. The sensitivity of the instrument for individual molecule detection, where a single transition is polarized by the excitation pulse, is also tested. Detection limits in water will be reported. In the case of chloromethane, the detection limit (0.1 microgram/L), matches the sensitivity reported in the EPA measurement protocol (EPA Method 524) for GC/MS.Made available in DSpace on 2014-09-17T16:56:22Z (GMT). No. of bitstreams: 3
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Previous issue date: 2014-06-1
Two Instruments In One: A New Cp-ftmw Experimental Setup To Measure Multiple Frequency Bands Simultaneously
Full text linkThe introduction by Pate and coworkers\footnote{Brown, G. G.; Dian, B. C.; Douglass, K. O.; Geyer, S. M.; Shipman, S. T.; Pate, B. H. A Broadband Fourier Transform Microwave Spectrometer Based on Chirped Pulse Excitation. \textit{Rev. Sci. Instrum.} \textbf{2008}, 79 (5), 053103. https://doi.org/10.1063/1.2919120.} of broadband capabilities in rotational spectrometers revolutionized the field and widened the scope of applications of rotational spectroscopy.\footnote{Park, G. B.; Field, R. W. Perspective: The First Ten Years of Broadband Chirped Pulse Fourier Transform Microwave Spectroscopy. \textit{J. Chem. Phys.} \textbf{2016}, 144 (20), 200901. https://doi.org/10.1063/1.4952762.} Since its introduction, many different experimental setups have been reported depending on the end application. In most cases, the bandwidth is restricted to that of commercially available electronic components (mainly high-power amplifiers). The most common setups operate, thus, in limited bands of 2-8, 8-18, 18-26GHz, etc., and the collection in different bands entails the recording of the spectrum several times with the consequent increase in time and sample consumption. Here we present a new experimental approach that allows for the collection of two of those otherwise separate bands in a single measurement, which reduces the time and sample consumption by half. We use two dual-polarization broadband horn antennae to simultaneously polarize the molecular sample in the 2-8 GHz (horizontal polarization) and 8-12 GHz (vertical polarization). The molecular emission is then combined and directly collected on a fast oscilloscope as in a traditional broadband experiment. We will show the performance of this setup compared to separated measurements, which allows for a faster collection of the whole frequency range without compromising the sensitivity. This approach only requires minimal modifications to the current spectrometers.Made available in DSpace on 2021-09-24T21:09:53Z (GMT). No. of bitstreams: 2
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Previous issue date: 2021-06-22Made available in DSpace on 2022-01-21T16:09:56Z (GMT). No. of bitstreams: 4
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Previous issue date: 2021-06-2
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
The electrical discharge products of multi-component mixtures probed by broadband millimeter-wave rotational spectroscopy
Full text linkMade available in DSpace on 2019-07-15T22:17:14Z (GMT). No. of bitstreams: 2
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Previous issue date: 2019-06-18Made available in DSpace on 2020-01-25T19:29:21Z (GMT). No. of bitstreams: 4
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Previous issue date: 2019-06-18Electrical discharge sources have been used extensively for the generation of molecular ions, radicals, and long chain unsaturated hydrocarbons, which have consequently been discovered to be present in the interstellar medium (ISM) [1]. We have used a discharge source to produce a number of medium-sized, astronomically relevant, neutral organic molecules from multi-component mixtures. The discharge products are characterised with our segmented chirped-pulse Fourier transform millimeter-wave spectrometer, which has been previously applied in our room-temperature studies of cyanides and alcohols [2]. We will concentrate here on the discharge products of mixtures of molecules already detected in the ISM, such as aldehydes in mixture with the nitrogen-containing molecules acetonitrile and ammonia. Any newly-observed species in our spectra can be assigned with rest frequencies and rotational parameters already suitable for interstellar searches with millimeter-wave radio telescopes, such as the Atacama Large Millimeter/submillimeter Array. Results from our experiments will allow us to better consider reaction pathways towards complex organic molecules in extra-terrestrial environments.
References:
[1] M. C. McCarthy et al., Astrophys. J. Suppl. Ser. 129 (2000), 611-623.
[2] B.E. Arenas et al., Phys. Chem. Chem. Phys. 19 (2017), 1751-1756
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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