Fate of the Gas-Phase Reaction Between Oxirane and the CN Radical in Interstellar Conditions

The escalating identification of new complex molecules in the interstellar medium claims for potential formation routes of such species. In this regard, the present work considers the reaction between oxirane and the CN radical as a feasible formation mechanism of species having the C3H3NO molecular formula. Indeed, the compounds of this family are elusive in the interstellar medium and suggestions on which species could be formed at low temperature and low pressure conditions might aid their discovery. The c-C2H4O + CN reaction has been investigated from the thermodynamic and kinetic points of view. The thermodynamic has been studied by means of a double-hybrid density functional and revealed the presence of several mechanisms submerged with respect to the reactants energy, with the potential formation of oxazole and cyanoacetaldehyde. However, the kinetic results suggest that the main reaction pathway is the H-extraction, leading to 2-oxiranyl radical and HCN. The formation of cyanoacetaldehyde + H and of H2CCN + H2CO is also possible with smaller rate constants, while the production of oxazole is negligible due to the presence of a high energy barrier

First Laboratory Detection of N13CO-and Semiexperimental Equilibrium Structure of the NCO-Anion

The cyanate anion (NCO-) is a species of considerable astrophysical relevance. It is widely believed to be embedded in interstellar ices present in young stellar objects but has not yet been detected in the dense gas of the interstellar medium. Here we report highly accurate laboratory measurements of the rotational spectrum of the N13CO-isotopologue at submillimeter wavelengths along with the detection of three additional lines of the parent isotopologue up to 437.4 GHz. With this new data, the rotational spectrum of both isotopologues can be predicted to better 0.25 km s-1in equivalent radial velocity up to 1 THz, more than adequate for an astronomical search in any source. Moreover, a semiexperimental equilibrium structure of the anion is derived by combining the experimental ground-state rotational constants of the two isotopologues with theoretical vibrational corrections, obtained by using the coupled-cluster method with inclusion of single and double excitations and perturbative inclusion of triple excitations (CCSD(T)). The estimated accuracy of the two bond distances is on the order of 5 × 10-4Å: a comparison to the values obtained by geometry optimization with the CCSD(T) method and the use of a composite scheme, including additivity and basis-set extrapolation techniques, reveals that this theoretical procedure is very accurate

Hyperfine-Resolved Near-Infrared Spectra of H2_{2}17^{17}O

Huge efforts have recently been taken in the derivation of accurate compilations of rovibrational energies of water, one of the most important reference systems in spectroscopy. Such precision is desirable for all water isotopologues, although their investigation is challenged by hyperfine effects in their spectra. Frequency-comb locked noise-immune cavity-enhanced optical-heterodyne molecular spectroscopy (NICE-OHMS) allows for achieving high sensitivity, resolution, and accuracy. This technique has been employed to resolve the subtle hyperfine splittings of rovibrational transitions of H$_{2}$$^{17}$Oin the near-infrared region. Simulation and interpretation of the H$_{2}$$^{17}$Osaturation spectra have been supported by coupled-cluster calculations performed with large basis sets and accounting for high-level corrections. Experimental $^{17}$O hyperfine parameters are found in excellent agreement with the corresponding computed values. The need of including small hyperfine effects in the analysis of H$_{2}$$^{17}$O spectra has been demonstrated together with the ability of the computational strategy employed for providing quantitative predictions of the corresponding parameters

DC3_3N observations towards high-mass star-forming regions

We present the study of deuteration of cyanoacetylene (HC$_3$N) 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 DC$_3$N towards 15 high-mass cores. The abundance ratios of DC$_3$N with respect HC$_3$N range in the interval 0.003$-$0.022, lower than those found in low-mas protostars and dark clouds. No significant trend with the evolutionary stage, or with the kinetic temperature of the region, has been found. We compare the level of deuteration of HC$_3$N with those of other molecules towards the same sample, finding weak correlation with species formed only or predominantly in gas phase (N$_2$H$^+$ and HNC, respectively), and no correlation with species formed only or predominantly on dust grains (CH$_3$OH and NH$_3$, respectively). We also present a single-dish map of DC$_3$N towards the protocluster IRAS 05358+3543, which shows that DC$_3$N traces an extended envelope ($\sim$0.37 pc) and peaks towards two cold condensations separated from the positions of the protostars and the dust continuum. The observations presented in this work suggest that deuteration of HC$_3$N 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 HC3_3N: global ro-vibrational analysis and improved line catalogue for astrophysical observations

HC$_3$N 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 HC$_3$N. We have recorded the high-resolution infrared spectrum from 450 to 1350 cm$^{-1}$, a region dominated by the intense $\nu_5$ and $\nu_6$ fundamental bands, located at 660 and 500 cm$^{-1}$, 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$^{-1}$ 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

Gas-phase identification of (Z)-1,2-ethenediol, a key prebiotic intermediate in the formose reaction

Prebiotic sugars are thought to be formed on primitive Earth by the formose reaction. However, their formation is not fully understood and it is plausible that key intermediates could have formed in extraterrestrial environments and subsequently delivered on early Earth by cometary bodies. 1,2-Ethenediol, the enol form of glycolaldehyde, represents a highly reactive intermediate of the formose reaction and is likely detectable in the interstellar medium. Here, we report the identification and first characterization of (Z)-1,2-ethenediol by means of rotational spectroscopy. The title compound has been produced in the gas phase by flash vacuum pyrolysis of bis-exo-5-norbornene-2,3-diol at 750 °C, through a retro-Diels-Alder reaction. The spectral analysis was guided by high-level quantum-chemical calculations, which predicted spectroscopic parameters in very good agreement with the experiment. Our study provides accurate spectral data to be used for searches of (Z)-1,2-ethenediol in the interstellar space

First detection of NHD and ND2_2 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 (NH$_2$D, NHD$_2$, ND$_3$) 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$_2$ 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$_2$, both isotopologues have been searched for in the spectral survey towards the class 0 IRAS 16293-2422, a source in which NH$_3$, 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$_2$. 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$_2$ 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

From the laboratory to the interstellar medium: a strategy to search for exotic molecules in space

The chemistry of the interstellar medium occurs under extreme conditions and can lead to the formation of exotic molecules. These are species that on Earth are unstable and/or highly reactive. Their discovery in space is usually based on the astronomical observation of their rotational fingerprints, which requires an accurate laboratory investigation. This is based on a strategy that starts from the interplay of experiment and theory. State-of-the-art quantum-chemical calculations are used to predict the relevant spectroscopic information required to guide the spectral recording, analysis and assignment. Rotational spectra measurements are then performed in the centimeter-/millimeter-/submillimeter-wave region, thereby exploiting efficient on-the-fly production protocols for exotic molecules. Subsequently, the spectral analysis leads to accurate spectroscopic parameters, which are then used for setting up accurate line catalogs for astronomical searches and detections. This review is based on the strategy developed and the results obtained at the ROT&Comp Lab of the University of Bologna