2,213 research outputs found
A Correlated Ab Initio Study of Linear Carbon-Chain Radicals C(sub n)H (n=2-7)
Linear carbon-chain radicals C(sub n) H for n = 2-7 have been studied with correlation consistent valence and core-valence basis sets and the coupled cluster method RCCSD(T). Equilibrium structures, rotational constants, and dipole moments are reported and compared with available experimental data. The ground state of the even-n series changes from 2Sigma(+) to 2Pi as the chain is extended. For C4H, the 2Sigma(+) state was found to lie only 72 cm(exp -1) below the 2Pi state in the estimated complete basis set limit for valence correlation. The C2H(-) and C3H(-) anions have also been characterized
AB Initio Characterization of MgCCH, MgCCH(+), and MgC2, and Pathways to their Formation in the Interstellar Medium
A study of Mg-bearing compounds has been performed in order to determine molecular properties which are critical for planning new astronomical searches and laboratory studies. The primary focus of the work is on MgCCH, MgCCH(+), and the isomers of MgC2. Only MgCCH has been identified in laboratory studies. Additional calculations have been carried out on MgH, MgNC, MgCN, and their cations in an effort to evaluate pathways to the formation of MgCCH and MgCCH(+) in the InterStellar Medium (ISM) or in circumstellar envelopes. Correlated ab initio methods and correlation-consistent basis sets have been employed. Properties including structures, rotational constants, dipole moments, and harmonic frequencies are reported. A transition state between linear MgCC and cyclic MgC2 has been characterized and was found to yield a minimal barrier (approx. 0.5 kcal/mole), indicating easy interconversion to the cyclic form. Direct reactions in the ISM between Mg or Mg(+) and HCCH are precluded by energetic considerations, but a number of ion- molecule or neutral-neutral exchange reactions between CCH and various Mg-containing species offer plausible pathways to MgCCH or MgCCH(+). Weakly bound MgH may react with CCH to form MgCCH, but MgH has not been detected. Both MgNC and MgCN have been observed, but reactions with CCH are slightly endothermic by 1-3 kcal/mole. Although MgH(+), MgNC(+), and MgCN(+) have not been detected, their reactions with CCH to form MgCCH(+) are all exothermic. With only a small barrier separating linear MgCC and cyclic MgC2, the dissociative recombination of MgCCH(+) with an electron is expected to yield cyclic MgC2, and regenerate Mg and CCH. New astronomical searches for MgCCH, MgCCH(+), cyclic MgC2, MgNC(+), and MgCN(+) will provide further insight into organo-magnesium astrochemistry
Astrometric Detection of Double Gravitational Microlensing Events
If a gravitational microlensing event is caused by a widely separated binary
lens and the source approaches both lens components, the source flux is
successively magnified by the individual lenses: double microlensing events. If
events are observed astrometrically, double lensing events are expected to
occur with an increased frequency due to the long range astrometric effect of
the companion. We find that although the trajectory of the source star image
centroid shifts of an astrometric double lensing event has a distorted shape
from both of the elliptical ones induced by the individual single lens
components, event duplication can be readily identified by the characteristic
loop in the trajectory formed during the source's passage close to the
companion. We determine and compare the probabilities of detecting double
lensing events from both photometric and astrometric lensing observations by
deriving analytic expressions for the relations between binary lensing
parameters to become double lensing events. From this determination, we find
that for a given set of the binary separation and the mass ratio the
astrometric probability is roughly an order higher than the photometric
probability. Therefore, we predict that a significant fraction of events that
will be followed up by using future high precision interferometeric instruments
will be identified as double lensing events.Comment: total 6 pages, including 4 figures and no table, ApJ, submitte
Reduction of Activation Energy Barrier of Stone-Wales Transformation in Endohedral Metallofullerenes
We examine effects of encapsulated metal atoms inside a C molecule on
the activation energy barrier to the Stone-Wales transformation using {\it ab
initio} calculations. The encapsulated metal atoms we study are K, Ca and La
which nominally donate one, two and three electrons to the C cage,
respectively. We find that isomerization of the endohedral metallofullerene via
the Stone-Wales transformation can occur more easily than that of the empty
fullerene owing to the charge transfer. When K, Ca and La atoms are
encapsulated inside the fullerene, the activation energy barriers are lowered
by 0.30, 0.55 and 0.80 eV, respectively compared with that of the empty
C (7.16 eV). The lower activation energy barrier of the Stone-Wales
transformation implies the higher probability of isomerization and coalescence
of metallofullerenes, which require a series of Stone-Wales transformations.Comment: 13 pages, 3 figures, 1 tabl
Bibliometric Analysis of Distributed Generation
This paper describes the application of data mining
techniques for eludicating patterns and trends in technological
innovation. Specifically, we focus on the use of bibliometric
methods, viz techniques which focus on trends in the publication
of text documents rather than the content of these documents.
Of particular interest is the relationship between publication
patterns, as characterized by term occurrence frequencies, and
the underlying technological trends and developments which
drive these trends. To focus the discussions and to provide a
concrete example of their applicability, a detailed case study
focussing on research in the area of Distributed Generation (DG)
is also presented; however, the techniques and general approach
devised here will be applicable to a broad range of industries,
situations, and locations. Our results are promising and indicate
that interesting information and conclusions can be derived from
this line of analysis. The results obtained using data extraction
techniques highlight and present the evolution of DG-related
technology focus areas, and their relative importance within this
field
FORMATION OF THE ALMA MOLECULE HOCH2CN AND RELATED SPECIES FROM THE REACTION OF C+ WITH HCN AND HNC IN ICY GRAIN MANTLES
Density functional theory cluster calculations indicate that the intermediate HOCHNC readily forms when \chem{C^+} reacts with HCN embedded in the surface of an icy grain mantle. Subsequent H addition to HOCHNC yields the iscyano compound \chem{HOCH_2NC}. There is enough energy from the H addition for \chem{HOCH_2NC} to isomerize to \chem{HOCH_2CN} (glycolonitrile), an important prebiotic molecule that was recently detected with ALMA observations toward the solar-type protostellar source IRAS 16293-2422 B by Zeng et al. [MNRAS 2019, 484, L43]. It was found that H can also add to HOCHNC to form HOCHNCH without a barrier. The analogous reactions of \chem{C^+} with HNC in ice will also be discussed. Vibrational spectra of the various ice-bound reactants, intermediates, and products will be presented. The calculations were performed with B3LYP using aug-cc-pVDZ sets on C, N, and O and cc-pVDZ sets on H
Astrochemistry lecture and laboratory courses at the University of Illinois: Applied spectroscopy
The Department of Chemistry at the University of Illinois at Urbana-Champaign offers two courses in astrochemistry, one lecture (Chem 450) and one laboratory (Chem 451). Both courses present the opportunity for advanced undergraduate and graduate students to learn about various spectroscopic concepts as they are applied toward an exotic subject, astrochemistry. In the lecture course, each student devotes a substantial fraction of the course work to one of the known astromolecules, building a wiki page for it during the semester, presenting a brief oral description about it in class, and then finally writing a paper about it. The course covers electronic, vibrational, and rotational spectroscopy, along with Einstein coefficients, line widths, and the interpretation of actual astronomical spectra. It also covers relevant reactions and reaction networks. Students learn to use pgopher for modeling rotational spectra. The lab course focuses on the methylidyne radical (CH). It begins with its chemistry and spectroscopy and then moves on to laboratory study of its electronic spectrum as observed in a butane flame and then collected with the university's 12" f/15 Brashear refracting telescope in the campus observatory built in 1896. Students learn to use IGOR to reduce CCD data
Towards better understanding Cybersecurity: Or are "Cyberspace" and "Cyber Space" the same?
Although there are many technology challenges and approaches to attaining cybersecurity, human actions (or inactions) also often pose large risks. There are many reasons, but one problem is whether we all “see the world” the same way. That is, what does “cybersecurity” actually mean – as well as the many related concepts, such as “cyberthreat,” “cybercrime,” etc. Although dictionaries, glossaries, and other sources tell you what words/phrases are supposed to mean (somewhat complicated by the fact that they often contradict each other), they do not tell you how people are actually using them. If we are to have an effective solution, it is important that all the parties understand each other – or, at least, understand that there are different perspectives.
For the purpose of this paper and to demonstrate our methodology, we consider the case of the words, “cyberspace” and “cyber space.” When we started, we assumed that “cyberspace” and “cyber space” were essentially the same word with just a minor variation in punctuation (i.e., the space, or lack thereof, between “cyber” and “space”) and that the choice of the punctuation was a rather random occurrence. With that assumption in mind, we would expect that the usage of these words (as determined by the taxonomies that would be constructed by our algorithms) would be basically the same. As it turned out, they were quite different, both in overall shape and groupings within the taxonomy.
Since the overall field of cybersecurity is so new, understanding the field and how people think about it (as evidenced by their actual usage of terminology, and how usage changes over time) is an important goal. Our approach helps to illuminate these understandings
Exploring Terms and Taxonomies Relating to the Cyber International Relations Research Field: or are "Cyberspace" and "Cyber Space" the same?
This project has at least two facets to it: (1) advancing the algorithms in the sub-field of bibliometrics often referred to as "text mining" whereby hundreds of thousands of documents (such as journal articles) are scanned and relationships amongst words and phrases are established and (2) applying these tools in support of the Explorations in Cyber International Relations (ECIR) research effort. In international relations, it is important that all the parties understand each other. Although dictionaries, glossaries, and other sources tell you what words/phrases are supposed to mean (somewhat complicated by the fact that they often contradict each other), they do not tell you how people are actually using them.
As an example, when we started, we assumed that "cyberspace" and "cyber space" were essentially the same word with just a minor variation in punctuation (i.e., the space, or lack thereof, between "cyber" and "space") and that the choice of the punctuation was a rather random occurrence. With that assumption in mind, we would expect that the taxonomies that would be constructed by our algorithms using "cyberspace" and "cyber space" as seed terms would be basically the same. As it turned out, they were quite different, both in overall shape and groupings within the taxonomy.
Since the overall field of cyber international relations is so new, understanding the field and how people think about (as evidenced by their actual usage of terminology, and how usage changes over time) is an important goal as part of the overall ECIR project
QUANTUM CHEMICAL STUDY OF THE REACTION OF C+ WITH INTERSTELLAR ICE: PREDICTIONS OF VIBRATIONAL AND ELECTRONIC SPECTRA OF REACTION PRODUCTS
The C cation (CII) is the dominant form of carbon in diffuse clouds and an important tracer for star formation in molecular clouds. We studied the low energy deposition of C on ice using density functional theory calculations on water clusters as large as 18 HO. Barrierless reactions occur with water to form two dominant sets of products: HOC + HO and CO + 2HO. In order to provide testable predictions, we have computed both vibrational and electronic spectra for pure ice and processed ice clusters. While vibrational spectroscopy is expected to be able to discern that C has reacted with ice by the addition of HO features not present in pure ice, it does not provided characteristic bands that would discern between HOC and CO. On the other hand, predictions of electronic spectra suggest that low energy absorptions may occur for CO and not HOC, making it possible to distinguish one product from the other
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