53 research outputs found
DCN observations towards high-mass star-forming regions
We present the study of deuteration of cyanoacetylene (HCN) towards a
sample of 28 high-mass star-forming cores divided into different evolutionary
stages, from starless to evolved protostellar cores. We report for the first
time the detection of DCN towards 15 high-mass cores. The abundance ratios
of DCN with respect HCN range in the interval 0.0030.022, lower than
those found in low-mas protostars and dark clouds. No significant trend with
the evolutionary stage, or with the kinetic temperature of the region, has been
found. We compare the level of deuteration of HCN with those of other
molecules towards the same sample, finding weak correlation with species formed
only or predominantly in gas phase (NH and HNC, respectively), and no
correlation with species formed only or predominantly on dust grains (CHOH
and NH, respectively). We also present a single-dish map of DCN towards
the protocluster IRAS 05358+3543, which shows that DCN traces an extended
envelope (0.37 pc) and peaks towards two cold condensations separated
from the positions of the protostars and the dust continuum. The observations
presented in this work suggest that deuteration of HCN is produced in the
gas of the cold outer parts of massive star-forming clumps, giving us an
estimate of the deuteration factor prior to the formation of denser gas.Comment: Accepted in Monthly Notices of the Royal Astronomical Society -- 11
pages, 7 Figures, 2 Tables. Version with some typos correcte
Rotational and high-resolution infrared spectrum of HCN: global ro-vibrational analysis and improved line catalogue for astrophysical observations
HCN is an ubiquitous molecule in interstellar environments, from external
galaxies, to Galactic interstellar clouds, star forming regions, and planetary
atmospheres. Observations of its rotational and vibrational transitions provide
important information on the physical and chemical structure of the above
environments. We present the most complete global analysis of the spectroscopic
data of HCN. We have recorded the high-resolution infrared spectrum from
450 to 1350 cm, a region dominated by the intense and
fundamental bands, located at 660 and 500 cm, respectively, and their
associated hot bands. Pure rotational transitions in the ground and
vibrationally excited states have been recorded in the millimetre and
sub-millimetre regions in order to extend the frequency range so far considered
in previous investigations. All the transitions from the literature and from
this work involving energy levels lower than 1000 cm have been fitted
together to an effective Hamiltonian. Because of the presence of various
anharmonic resonances, the Hamiltonian includes a number of interaction
constants, in addition to the conventional rotational and vibrational l-type
resonance terms. The data set contains about 3400 ro-vibrational lines of 13
bands and some 1500 pure rotational lines belonging to 12 vibrational states.
More than 120 spectroscopic constants have been determined directly from the
fit, without any assumption deduced from theoretical calculations or
comparisons with similar molecules. An extensive list of highly accurate rest
frequencies has been produced to assist astronomical searches and data
interpretation. These improved data, have enabled a refined analysis of the
ALMA observations towards Sgr B2(N2).Comment: 35 pages, 14 figures, accepted for pubblication in ApJ Supplemen
Computational molecular spectroscopy
Spectroscopic techniques can probe molecular systems non-invasively and investigate their structure, properties and dynamics in different environments and physico-chemical conditions. Different spectroscopic techniques (spanning different ranges of the electromagnetic field) and their combination can lead to a more comprehensive picture of investigated systems. However, the growing sophistication of these experimental techniques makes it increasingly complex to interpret spectroscopic results without the help of computational chemistry. Computational molecular spectroscopy, born as a branch of quantum chemistry to provide predictions of spectroscopic properties and features, emerged as an independent and highly specialized field but has progressively evolved to become a general tool also employed by experimentally oriented researchers. In this Primer, we focus on the computational characterization of medium-sized molecular systems by means of different spectroscopic techniques. We first provide essential information about the characteristics, accuracy and limitations of the available computational approaches, and select examples to illustrate common trends and outcomes of general validity that can be used for modelling spectroscopic phenomena. We emphasize the need for estimating error bars and limitations, coupling accuracy with interpretability, touch upon data deposition and reproducibility issues, and discuss the results in terms of widely recognized chemical concepts
First detection of NHD and ND in the interstellar medium
Deuterium fractionation processes in the interstellar medium (ISM) have been
shown to be highly efficient in the family of nitrogen hydrides. To date,
observations were limited to ammonia (NHD, NHD, ND) and imidogen
radical (ND) isotopologues. We want to explore the high frequency windows
offered by the \emph{Herschel Space Observatory} to search for deuterated forms
of amidogen radical NH and to compare the observations against the
predictions of our comprehensive gas-grain chemical model. Making use of the
new molecular spectroscopy data recently obtained at high frequencies for NHD
and ND, both isotopologues have been searched for in the spectral survey
towards the class 0 IRAS 16293-2422, a source in which NH, NH and their
deuterated variants have been previously detected. We used the observations
carried out with HIFI (Heterodyne Instrument for the Far Infrared) in the
framework of the key program "Chemical Herschel surveys of star forming
regions" (CHESS). We report the first detection of interstellar NHD and ND.
Both species are observed in absorption against the continuum of the protostar.
From the analysis of their hyperfine structure, accurate excitation temperature
and column density values have been determined. The latter were combined with
the column density of the parent species NH to derive the deuterium
fractionation in amidogen. The amidogen D/H ratio measured in the low-mass
protostar IRAS 16293-2422 is comparable to the one derived for the related
species imidogen and much higher than that observed for ammonia. Additional
observations of these species will give more insights into the mechanism of
ammonia formation and deuteration in the ISM. We finally indicate the current
possibilities to further explore these species at submillimeter wavelengths.Comment: 11 pages, 5 figures, 7 tables. Accepted for publication in A&
An improved rovibrational linelist of formaldehyde, H₂¹²C¹⁶O
Published high-resolution rotation-vibration transitions of H₂¹²C¹⁶O the principal isotopologue of methanal, are analyzed using the MARVEL (Measured Active Rotation-Vibration Energy Levels) procedure. The literature results are augmented by new, high-accuracy measurements of pure rotational transitions within the ground, ν_{3}, ν_{4}, and ν_{6} vibrational states. Of the 16 596 non-redundant transitions processed, which come from 43 sources including the present work, 16 403 could be validated, providing 5029 empirical energy levels of H₂¹²C¹⁶O with statistically well-defined uncertainties. All the empirical rotational-vibrational energy levels determined are used to improve the accuracy of ExoMol’s AYTY line list for hot formaldehyde. The complete list of collated experimental transitions, the empirical energy levels determined, as well as the extended and improved line list are provided as Supplementary Material
A Journey from Thermally Tunable Synthesis to Spectroscopy of Phenylmethanimine in Gas Phase and Solution
Phenylmethanimine is an aromatic imine with a twofold relevance in chemistry: organic synthesis and astrochemistry. To tackle both aspects, a multidisciplinary strategy has been exploited and a new, easily accessible synthetic approach to generate stable imine-intermediates in the gas phase and in solution has been introduced. The combination of this formation pathway, based on the thermal decomposition of hydrobenzamide, with a state-of-the-art computational characterization of phenylmethanimine laid the foundation for its first laboratory observation by means of rotational electric resonance spectroscopy. Both E and Z isomers have been accurately characterized, thus providing a reliable basis to guide future astronomical observations. A further characterization has been carried out by nuclear magnetic resonance spectroscopy, showing the feasibility of this synthetic approach in solution. The temperature dependence as well as possible mechanisms of the thermolysis process have been examined. © 2020 The Authors. Published by Wiley-VCH Gmb
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