86,830 research outputs found
The C(3P) + NH3 reaction in interstellar chemistry: II. Low temperature rate constants and modeling of NH, NH2 and NH3 abundances in dense interstellar clouds
A continuous supersonic flow reactor has been used to measure rate constants
for the C + NH3 reaction over the temperature range 50 to 296 K. C atoms were
created by the pulsed laser photolysis of CBr4. The kinetics of the title
reaction were followed directly by vacuum ultra-violet laser induced
fluorescence (VUV LIF) of C loss and through H formation. The experiments show
unambiguously that the reaction is rapid at 296 K, becoming faster at lower
temperatures, reaching a value of 1.8 10-10 cm3 molecule-1 s-1 at 50 K. As this
reaction is not currently included in astrochemical networks, its influence on
interstellar nitrogen hydride abundances is tested through a dense cloud model
including gas-grain interactions. In particular, the effect of the
ortho-to-para ratio of H2 which plays a crucial role in interstellar NH3
synthesis is examined
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Low-temperature formation of polycyclic aromatic hydrocarbons in Titan’s atmosphere
The detection of benzene in Titan’s atmosphere led to the emergence of polycyclic aromatic hydrocarbons (PAHs) as potential nucleation agents triggering the growth of Titan’s orange-brownish haze layers. However, the fundamental mechanisms leading to the formation of PAHs in Titan’s low-temperature atmosphere have remained elusive. We provide persuasive evidence through laboratory experiments and computations that prototype PAHs like anthracene and phenanthrene (C14H10) are synthesized via barrierless reactions involving naphthyl radicals (C10H7•) with vinylacetylene (CH2=CH–C≡CH) in low-temperature environments. These elementary reactions are rapid, have no entrance barriers, and synthesize anthracene and phenanthrene via van der Waals complexes and submerged barriers. This facile route to anthracene and phenanthrene—potential building blocks to complex PAHs and aerosols in Titan—signifies a critical shift in the perception that PAHs can only be formed under high-temperature conditions, providing a detailed understanding of the chemistry of Titan’s atmosphere by untangling elementary reactions on the most fundamental level
Computational study of boron nitride nanotube synthesis: how catalyst morphology stabilizes the boron nitride bond
In an attempt to understand why catalytic methods for the growth of boron
nitride nanotubes work much worse than for their carbon counterparts, we use
first-principles calculations to study the energetics of elemental reactions
forming N2, B2 and BN molecules on an iron catalyst. We observe that in the
case of these small molecules, the catalytic activity is hindered by the
formation of B2 on the iron surface. We also observe that the local morphology
of a step edge present in our nanoparticle model stabilizes the boron nitride
molecule with respect to B2 due to the ability of the step edge to offer sites
with different coordination simultaneously for nitrogen and boron. Our results
emphasize the importance of atomic steps for a high yield chemical vapor
deposition growth of BN nanotubes and may outline new directions for improving
the efficiency of the method.Comment: submitted to physical review
Influence of geometry on the emitting properties of 2,3-naphthalimides
The luminescence properties of 2,3-naphthalimides have been studied using picosecond and nanosecond spectroscopies.
In acetonitrile solution N-phenyl-2,3-naphthalimid(e3 ) is found to emit dual fluorescence with emission maxima at 385 and 490 nm, respectively. The short-wavelength emission corresponds to the known fluorescence of the naphthalimides and is demonstrated for 3 to originate from a molecular conformation in which the phenyl substituent and the naphthalimide skeleton are orthogonal to each other. The long-wavelength emission is assumed to originate from a singlet excited state formed by a ca. 90° rotation of the phenyl group so that the two moieties are coplanar. Only a small dipole moment change is found between this excited state and the ground state. Only short-wavelength emission is observed with a lifetime in the nanosecond range as in the case of 1 and 2 when phenyl rotation is blocked with a bulky ortho tert-butyl group (compound 4). Increasing the viscosity of a glycerol/ethanol medium enhances both the efficiency and the lifetime of the short-wavelength emission
of 3. It appears that at 77 K the emission originates directly from the Franck-Condon state. At room temperature, the other two emitting species are shown to arise from the Franck-Condon state by competitive intramolecular geometrical relaxation processes. Structures 5 and 6 are tentatively put forward to explain the formation of naphthazepinedione 8 by a 2 \pi + 2 \pi photochemical cycloaddition reaction
Isotopic fractionation of carbon, deuterium and nitrogen : a full chemical study
Context. The increased sensitivity and high spectral resolution of millimeter
telescopes allow the detection of an increasing number of isotopically
substituted molecules in the interstellar medium. The 14N/ 15N ratio is
difficult to measure directly for carbon containing molecules. Aims. We want to
check the underlying hypothesis that the 13C/ 12C ratio of nitriles and
isonitriles is equal to the elemental value via a chemical time dependent gas
phase chemical model. Methods. We have built a chemical network containing D,
13C and 15N molecular species after a careful check of the possible
fractionation reactions at work in the gas phase. Results. Model results
obtained for 2 different physical conditions corresponding respectively to a
moderately dense cloud in an early evolutionary stage and a dense depleted
pre-stellar core tend to show that ammonia and its singly deuterated form are
somewhat enriched in 15N, in agreement with observations. The 14N/ 15N ratio in
N2H+ is found to be close to the elemental value, in contrast to previous
models which obtain a significant enrichment, as we found that the
fractionation reaction between 15N and N2H+ has a barrier in the entrance
channel. The large values of the N2H+/15NNH+ and N2H+/ N15NH+ ratios derived in
L1544 cannot be reproduced in our model. Finally we find that nitriles and
isonitriles are in fact significantly depleted in 13C, questioning previous
interpretations of observed C15N, HC15N and H15NC abundances from 13C
containing isotopologues.Comment: 21 pages, 9 figures in the text, 3 Figures in the appendices. 7
tables in the text, 4 tables in the appendices. Accepted for publication by
Astronomy Astrophysic
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