452 research outputs found
Importance of the Hydrogen Isocyanide Isomer in Modeling Hydrogen Cyanide Oxidation in Combustion
This article re-examines the chemistry of hydrogen isocyanide (HNC) and its role in combustion nitrogen chemistry
Laboratory Measurement of the Pure Rotational Transitions of the HCNH+ and its Isotopic Species
The pure rotational transitions of the protonated hydrogen cyanide ion,
HCNH+, and its isotopic species, HCND+ and DCND+, were measured in the 107 -
482 GHz region with a source modulated microwave spectrometer. The ions were
generated in the cell with a magnetically confined dc-glow discharge of HCN
and/or DCN. The rotational constant B0 and the centrifugal distortion constant
D0 for each ion were precisely determined by a least-squares fitting to the
observed spectral lines. The observed rotational transition frequencies by
laboratory spectroscopy and the predicted ones are accurate in about 30 to 40
kHz and are useful as rest frequencies for astronomical searches of HCNH+ and
HCND+.Comment: 14 pages in TeX, 1 figures in JPE
The IRAM-30m line survey of the Horsehead PDR: II. First detection of the l-C3H+ hydrocarbon cation
We present the first detection of the l-C3H+ hydrocarbon in the interstellar
medium. The Horsehead WHISPER project, a millimeter unbiased line survey at two
positions, namely the photo-dissociation region (PDR) and the nearby shielded
core, revealed a consistent set of eight unidentified lines toward the PDR
position. Six of them are detected with a signal-to-noise ratio from 6 to 19,
while the two last ones are tentatively detected. Mostly noise appears at the
same frequency toward the dense core, located less than 40" away. We
simultaneously fit 1) the rotational and centrifugal distortion constants of a
linear rotor, and 2) the Gaussian line shapes located at the eight predicted
frequencies. The observed lines can be accurately fitted with a linear rotor
model, implying a 1Sigma ground electronic state. The deduced rotational
constant value is Be= 11244.9512 +/- 0.0015 MHz, close to that of l-C3H. We
thus associate the lines to the l-C3H+ hydrocarbon cation, which enables us to
constrain the chemistry of small hydrocarbons. A rotational diagram is then
used to infer the excitation temperature and the column density. We finally
compare the abundance to the results of the Meudon PDR photochemical model.Comment: 9 pages, 7 PostScript figures. Accepted for publication in Astronomy
\& Astrophysics. Uses aa LaTeX macro
Pressure Dependent Low Temperature Kinetics for CN + CH3CN: Competition between Chemical Reaction and van der Waals Complex Formation
International audienceThe gas phase reaction between the CN radical and acetonitrile CH3CN was investigated experimentally, at low temperatures, with the CRESU apparatus and a slow flow reactor to explore the temperature dependence of its rate coefficient from 354 K down to 23 K. Whereas a standard Arrhenius behavior was found at T > 200 K, indicating the presence of an activation barrier, a dramatic increase in the rate coefficient by a factor of 130 was observed when the temperature was decreased from 168 to 123 K. The reaction was found to be pressure independent at 297 K unlike the experiments carried out at 52 and 132 K. The work was complemented by ab initio transition state theory based master equation calculations using reaction pathways investigated with highly accurate thermochemical protocols. The role of collisional stabilization of a CN⋯CH3CN van der Waals complex and of tunneling induced H atom abstractions were also considered. The experimental pressure dependence at 52 and 132 K is well reproduced by the theoretical calculations provided that an anharmonic state density is considered for the van der Waals complex CH3CN⋯CN and its Lennard-Jones radius is adjusted. Furthermore, these calculations indicate that the experimental observations correspond to the fall-off regime and that tunneling remains small in the low-pressure regime. Hence, the studied reaction is essentially an association process at very low temperature. Implications for the chemistry of interstellar clouds and Titan are discussed
Reaction Networks For Interstellar Chemical Modelling: Improvements and Challenges
We survey the current situation regarding chemical modelling of the synthesis
of molecules in the interstellar medium. The present state of knowledge
concerning the rate coefficients and their uncertainties for the major
gas-phase processes -- ion-neutral reactions, neutral-neutral reactions,
radiative association, and dissociative recombination -- is reviewed. Emphasis
is placed on those reactions that have been identified, by sensitivity
analyses, as 'crucial' in determining the predicted abundances of the species
observed in the interstellar medium. These sensitivity analyses have been
carried out for gas-phase models of three representative, molecule-rich,
astronomical sources: the cold dense molecular clouds TMC-1 and L134N, and the
expanding circumstellar envelope IRC +10216. Our review has led to the proposal
of new values and uncertainties for the rate coefficients of many of the key
reactions. The impact of these new data on the predicted abundances in TMC-1
and L134N is reported. Interstellar dust particles also influence the observed
abundances of molecules in the interstellar medium. Their role is included in
gas-grain, as distinct from gas-phase only, models. We review the methods for
incorporating both accretion onto, and reactions on, the surfaces of grains in
such models, as well as describing some recent experimental efforts to simulate
and examine relevant processes in the laboratory. These efforts include
experiments on the surface-catalysed recombination of hydrogen atoms, on
chemical processing on and in the ices that are known to exist on the surface
of interstellar grains, and on desorption processes, which may enable species
formed on grains to return to the gas-phase.Comment: Accepted for publication in Space Science Review
Comparison of bioinspired algorithms applied to the timetabling problem
The problem of timetabling events is present in various organizations such as schools, hospitals, transportation centers. The purpose of timetabling activities at a university is to ensure that all students attend their required subjects in accordance with the available resources. The set of constraints that must be considered in the design of timetables involves students, teachers and infrastructure. This study shows that acceptable solutions are generated through the application of genetic, memetic and immune system algorithms for the problem of timetabling. The algorithms are applied to real instances of the University of Mumbai in India and their results are comparable with those of a human expert
Importance of the Hydrogen Isocyanide Isomer in Modeling Hydrogen Cyanide Oxidation in Combustion
Review of important reactions for the nitrogen chemistry in the interstellar medium
Predictions of astrochemical models depend strongly on the reaction rate
coefficients used in the simulations. We reviewed a number of key reactions for
the chemistry of nitrogen-bearing species in the dense interstellar medium and
proposed new reaction rate coefficients for those reactions. The details of the
reviews are given in the form of a datasheet associated with each reaction. The
new recommended rate coefficients are given with an uncertainty and a
temperature range of validity and will be included in KIDA
(http://kida.obs.u-bordeaux1.fr).Comment: 39 pages, not published in refereed journal, datasheets are given in
KID
Dissociative recombination of NH: A revisited study
Dissociative recombination of NH is explored in a two-step
theoretical study. In a first step, a diatomic (1D) rough model with frozen NN
bond and frozen angles is adopted, in the framework of the multichannel quantum
defect theory (MQDT). The importance of the indirect mechanism and of the
bending mode is revealed, in spite of the disagreement between our cross
section and the experimental one. In a second step, we use our recently
elaborated 3D approach based on the normal mode approximation combined with
R-matrix theory and MQDT. This approach results in satisfactory agreement with
storage-ring measurements, significantly better at very low energy than the
former calculations.Comment: 9 pages, 5 figures, 1 tabl
Etude des contraintes dynamiques et l'interaction homme-cadre aérien chez les artistes du cirque.
Le but de cette étude et d'analyser le comportement dynamique d'un cadre aérien sujet a des sollicitations induites par les voltigeuses lors d'un numéro. L'étude est divisée en deux parties et pour chaque partie une étude expérimentales et numérique est entreprise. Le calcul par éléments finis permet, entre autres, la simulation très précise et réaliste du comportement mécanique (déplacements, contraintes, déformations...) de structures complexes sans avoir à réaliser de prototype physique. Ainsi, en phase de conception, on peut tirer très avantageusement profit du calcul par éléments finis pour dimensionner, valider ou optimiser des composants ou assemblages complets
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