The Cologne Database for Molecular Spectroscopy, CDMS: a useful tool for astronomers and spectroscopists

Formation of S-Bearing Complex Organic Molecules in Interstellar Clouds via Ice Reactions with C2H2, HS, and Atomic H

The chemical network governing interstellar sulfur has been the topic of unrelenting discussion for the past few decades due to the conspicuous discrepancy between its expected and observed abundances in different interstellar environments. More recently, the astronomical detections of CH3CH2SH and CH2CS highlighted the importance of interstellar formation routes for sulfur-bearing organic molecules with two carbon atoms. In this work, we perform a laboratory investigation of the solid-state chemistry resulting from the interaction between C2H2 molecules and SH radicals-both thought to be present in interstellar icy mantles-at 10 K. Reflection absorption infrared spectroscopy and quadrupole mass spectrometry combined with temperature-programmed desorption experiments are employed as analytical techniques. We confirm that SH radicals can kick-start a sulfur reaction network under interstellar cloud conditions and identify at least six sulfurated products: CH3CH2SH, CH2CHSH, HSCH2CH2SH, H2S2, and tentatively CH3CHS and CH2CS. Complementarily, we utilize computational calculations to pinpoint the reaction routes that play a role in the chemical network behind our experimental results. The main sulfur-bearing organic molecule formed under our experimental conditions is CH3CH2SH, and its formation yield increases with the ratios of H to other reactants. It serves as a sink to the sulfur budget within the network, being formed at the expense of the other unsaturated products. The astrophysical implications of the chemical network proposed here are discussed.

Electron-driven molecular processes for cyanopolyacetylenes HC2n+1N (n = 3, 4, and 5)

Linear carbon series cyanopolyacetylenes (HC2n+1N) (n = 3, 4, and 5) are astromolecules found in the atmosphere of Titan and interstellar media such as TMC-1 (Taurus molecular cloud-1). All these compounds are also detected in IRC + 10 216. In the present work, we comprehensively investigate electron interaction with important cyanopolyacetylene compounds, viz. HC7N (cyano-tri-acetylene), HC9N (cyano-tetra-acetylene), and HC11N (cyano-penta-acetylene). The study covers incident electron energies ranging from the ionization threshold to 5 keV. Various electron-driven molecular processes are quantified in terms of total cross-sections. The quantum spherical complex optical potential (SCOP) is used to determine elastic (Qel) and inelastic (Qinel) cross-sections. Ionization is the most important inelastic effect that opens various chemical pathways for the generation of different molecular species; we computed the ionization cross-section (Qion) and discrete electronic excitation cross-section (ΣQexc) using the complex scattering potential-ionization contribution (CSP-ic) method. The cyanopolyacetylene compounds are difficult to handle experimentally owing to the health risks involved. Therefore, there are no prior experimental data available for these molecules; only Qion have been reported theoretically. Thus, the present work is the maiden report on computing Qel, Qinel, ΣQexc, and QT. In order to provide an alternative approach and further validation of the present work, we employed our recently developed two-parameter semi-empirical method (2p-SEM) to compute Qel and QT. Additionally, we predict the polarizability of the HC11N molecule, which has not been reported in the existing literature. This prediction is based on a correlation study of polarizabilities of molecules with Qion values from the same series of molecules.

High-Temperature Line List of Phosphaethyne (HCP)

Phosphaethyne (HCP) has been detected in circumstellar envelopes; its spectroscopic line list is helpful for modeling the relevant atmospheric opacity. We present the first comprehensive line list for HCP(X1Σ+) using robust first-principles methods. The analytical potential energy surface and dipole moment surface were constructed based on 26478 ab initio points from coupled-cluster calculations, along with the considerations of core-valence electron correlation and scalar relativistic effects. The variational nuclear motion program TROVE was used to obtain the ro-vibrational energy levels, Einstein A coefficients, and so on. The J-dependent Coriolis-decoupled Hamiltonian was adopted in the variational calculations with k ≤ 20, and the linear molecule treatment was applied to consider the l-type doubling of the bending vibration. The line list contains almost 0.45 billion transitions between 1.21 million levels with rotational excitation up to J = 200. It covers the wavenumber range of 0-9000 cm-1 (wavelengths above 1.11 μm) and is suitable for temperatures up to 3000 K. The millimeter wave spectra agree well with the experiments, and the Fermi resonance between 2v2 and v3 bands has been reproduced.

Tracing Prebiotic Molecules: Rotational Spectroscopy of Deuterated Glycolaldehyde and (Z)-1,2-Ethenediol

Glycolaldehyde, an important prebiotic molecule, along with its monodeuterated species and its higher energy tautomer, (Z)-1,2-ethenediol, has been detected in the interstellar medium. Although the elemental D/H ratio in the universe is only ∼1.6 × 10-5, the deuterium relative abundance in interstellar molecules might be by far larger than this. As such, it provides a remarkable and almost unique diagnostic tool. In particular, it might help elucidate the reaction mechanisms that lead to the formation of the so-called complex organic molecules. It is therefore crucial to extend the census of the interstellar deuterated molecules. To this aim, in this work, we present for the first time a spectroscopic investigation of the rotational spectra of the CHDOD-CHO bideuterated variant of glycolaldehyde and of mono- and bideuterated species of (Z)-1,2-ethenediol (CHOD═CHOD, CHOD═CHOH, and CHOH═CHOD rotamers). For each species, more than a hundred transitions have been assigned. Their analysis led to the accurate determination of all rotational constants as well as quartic and sextic centrifugal distortion terms, thus providing spectroscopic line catalogs suitable for supporting astronomical searches. In addition, the rotational constants of the bideuterated glycolaldehyde isotopologue studied in this work allowed us to improve the semiexperimental equilibrium structure determination for this molecule.

High-temperature spectra of the PNO molecule based on robust first-principles methods

The PNO molecule is an important species found in the interstellar medium, and its spectroscopic information is helpful for its detection. We present the first line list of PNO (X1Σ+) using robust first-principles methods. The analytical potential energy surface and the dipole moment surface were constructed based on 11 942 ab initio points. The variational nuclear motion calculation was implemented in TROVE to obtain the rovibrational energy levels, Einstein A coefficients and other parameters. The J-dependent Coriolis-decoupled Hamiltonian was adopted with k ≤ 15, and the l-type doubling was considered for the bending vibration of the linear molecule. The line list contained almost 5.87 billion transitions between 3.61 million levels with rotational excitation up to J = 200 and was used to generate the PNO spectrum below 3000 K in the wavenumber range from 0 to 6000 cm-1. The millimetre wave spectrum agrees well with available experimental benchmarks. The Fermi resonance effects in the PNO spectrum are universal and complex, resulting in significant intensity increment of the related weak transition. This line list may be helpful for the spectroscopic characterization and possible astronomical detection of PNO, especially in high-temperature environments.

High-temperature rotation-vibration spectrum of iminosilylene (HNSi)

Iminosilylene (HNSi) has been observed in the laboratory and is expected to occur in the envelopes of carbon-rich stars. However, the lack of spectroscopic information for HNSi has hampered its further astrophysical detection. Using robust ab initio methods, we present the first and comprehensive molecular line list for HNSi (X 1Σ+). The new line list contains almost 3.36 billion transitions between 1.57 million levels with rotational excitation up to J = 160. It is suitable for temperatures up to 3000 K and covers the wavenumber range of 0-9000 cm-1 (wavelengths λ > 1.11 μm). This new line list can be helpful for the future spectroscopic characterization and molecular detection of HNSi in the laboratory and interstellar space.

The Cyanoketenyl Anion [NC3O]. Angew Chem Int Ed Engl 2024;63:e202403766. [PMID: 38470943 DOI: 10.1002/anie.202403766] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Cumulenes and heterocumulenes with three or more cumulative multiple bonds are usually reactive species that serve as valuable building blocks for more complex molecules but tend to isomerize or cyclize and therefore are difficult to isolate. Using a mild ligand exchange reaction at the carbon in α-metalated ylides, we have now succeeded in the synthesis and gram-scale isolation of the elusive cyanoketenyl anion [NC3O]-. Despite its assumed cumulene-like structure and the delocalization of the negative charge across the whole 5-atom molecule, it features a bent geometry with a nucleophilic central carbon atom. Computational studies reveal an ambiguous bonding situation in the anion, which can be illustrated only by a combination of different resonance structures. Nonetheless, the anion features remarkable stability, thus allowing the storage of its potassium-crown ether salt and its application as a highly functional synthetic building block. The cyanoketenyl anion readily reacts with a series of small molecules to form more complex organic compounds, including industrially valuable compounds such as cyanoacetate. This work demonstrated that reactive species can be generated by novel synthesis methods and open up atom-economic pathways to complex compounds from small abundant molecules.

Isomerism of CH 2 SO : Accurate structural, energetic, and spectroscopic characterization

A recent work [Ye et al. Mon. Not. R. Astron. Soc. 2023, 525, 1158] on the gas-phase formation of t-HC(O)SH, already detected in the interstellar medium, pointed out that the trans form of HC(S)OH is a potential candidate for astronomical observations. Prompted by these results, theCH 2 SO family of isomers has been investigated from an energetic point of view using a double-hybrid density functional in combination with a partially augmented triple-zeta basis set. This preliminary study showed that the most stable species of the family are the cis and trans forms of HC(O)SH and HC(S)OH. For their structural and spectroscopic characterization, a composite scheme based on coupled cluster (CC) calculations that incorporates up to the quadruple excitations and accounts for the extrapolation to the complete basis set limit and core correlation effects has been employed. This approach opens to the prediction of rotational constants with an accuracy of 0.1%. A hybrid scheme, based on harmonic frequencies computed using the CC singles, doubles and a perturbative treatment of triples method (CCSD(T)) in conjunction with a quadruple-zeta basis set, allowed us to obtain fundamental vibrational frequencies with a mean absolute error of about 1%.

High-temperature molecular line list of hydroboron monoxide (HBO)

Hydroboron monoxide (HBO) is expected to occur in envelopes of the asymptotic giant branch (AGB), but a lack of spectroscopic data is hampering its possible detection. Using the state-of-the-art ab initio method, we present the first, comprehensive molecular line list for HBO which is suitable for temperatures up to T = 3000 K. This new line list covers the wavenumber range of 0-9000 cm-1 (wavelengths of λ ≥ 1.11 μm), and it contains almost 75 million transitions between 435 631 energy levels with rotational excitation up to J = 120. The new line list of HBO can facilitate its future molecular detection in the laboratory and interstellar space.

A Vision for the Future of Astrochemistry in the Interstellar Medium by 2050

By 2050, many, but not nearly all, unattributed astronomical spectral features will be conclusively linked to molecular carriers (as opposed to nearly none today in the visible and IR); amino acids will have been observed remotely beyond our solar system; the largest observatories ever constructed on the surface of the Earth or launched beyond it will be operational; high-throughput computation either from brute force or machine learning will provide unprecedented amounts of reference spectral and chemical reaction data; and the chemical fingerprints of the universe delivered by those of us who call ourselves astrochemists will provide astrophysicists with unprecedented resolution for determining how the stars evolve, planets form, and molecules that lead to life originate. Astrochemistry is a relatively young field, but with the entire universe as its playground, the discipline promises to persist as long as telescopic observations are made that require reference data and complementary chemical modeling. While the recent commissionings of the James Webb Space Telescope and Atacama Large Millimeter Array are ushering in the second "golden age" of astrochemistry (with the first being the radio telescopic boom period of the 1970s), this current period of discovery should facilitate unprecedented advances within the next 25 years. Astrochemistry forces the asking of hard questions beyond the physical conditions of our "pale blue dot", and such questions require creative solutions that are influential beyond astrophysics. By 2050, more creative solutions will have been provided, but even more will be needed to answer the continuing question of our astrochemical ignorance.

Low temperature reaction kinetics inside an extended Laval nozzle: REMPI characterization and detection by broadband rotational spectroscopy

Chirped-Pulse Fourier-Transform millimeter wave (CP-FTmmW) spectroscopy is a powerful method that enables detection of quantum state specific reactants and products in mixtures. We have successfully coupled this technique with a pulsed uniform Laval flow system to study photodissociation and reactions at low temperature, which we refer to as CPUF ("Chirped-Pulse/Uniform flow"). Detection by CPUF requires monitoring the free induction decay (FID) of the rotational coherence. However, the high collision frequency in high-density uniform supersonic flows can interfere with the FID and attenuate the signal. One way to overcome this is to sample the flow, but this can cause interference from shocks in the sampling region. This led us to develop an extended Laval nozzle that creates a uniform flow within the nozzle itself, after which the gas undergoes a shock-free secondary expansion to cold, low pressure conditions ideal for CP-FTmmW detection. Impact pressure measurements, commonly used to characterize Laval flows, cannot be used to monitor the flow within the nozzle. Therefore, we implemented a REMPI (resonance-enhanced multiphoton ionization) detection scheme that allows the interrogation of the conditions of the flow directly inside the extended nozzle, confirming the fluid dynamics simulations of the flow environment. We describe the development of the new 20 K extended flow, along with its characterization using REMPI and computational fluid dynamics. Finally, we demonstrate its application to the first low temperature measurement of the reaction kinetics of HCO with O2 and obtain a rate coefficient at 20 K of 6.66 ± 0.47 × 10-11 cm3 molec-1 s-1.

Streamers feeding the SVS13-A protobinary system: astrochemistry reveals accretion shocks?

We report Atacama Large Millimeter/submillimeter Array (ALMA) high-angular resolution (∼50 au) observations of the binary system SVS13-A. More specifically, we analyse deuterated water (HDO) and sulfur dioxide (SO2) emission. The molecular emission is associated with both the components of the binary system, VLA4A and VLA4B. The spatial distribution is compared to that of formamide (NH2CHO), previously analysed in the system. Deuterated water shows an additional emitting component spatially coincident with the dust-accretion streamer, at a distance ≥120 au from the protostars, and at blue-shifted velocities (>3 km s-1 from the systemic velocities). We investigate the origin of the molecular emission in the streamer, in light of thermal sublimation temperatures calculated using updated binding energy (BE) distributions. We propose that the observed emission is produced by an accretion shock at the interface between the accretion streamer and the disk of VLA4A. Thermal desorption is not completely excluded in case the source is actively experiencing an accretion burst.

New interstellar laboratories in the molecular ring

Much of what is known about chemistry in star-forming regions comes from observations of nearby (d < 500 pc) low-mass protostars. For chemistry in high-mass star-forming regions, several more distant (d ∼ 2-8 kpc), exceptionally bright molecular sources have also been the subjects of repeated observations but with concomitantly poorer linear spatial resolution. Facilities such as ALMA and JWST, however, now provide the means for observing distant sources at dramatically higher spatial resolution and sensitivity. We used the modest resolving power of the Atacama Compact Array, a dedicated subset of ALMA antennas, to carry out a pilot survey of 11 giant molecular clouds selected from the Bolocam Galactic Plane Survey [Battisti & Heyer, Astrophys. J., 2014, 780, 173] within the so-called molecular ring between about 4 and 8 kpc from the galactic center. Within our observed sample, molecular emission regions-most of which correspond to at least one (candidate) young stellar object-exhibit a range of chemical complexity and diversity. Furthermore, nine target giant molecular clouds contain well-fit methanol emission, giving us a first look at the spatial chemical variability within the objects at relatively high (compared to past observations) resolutions of ∼5''. This work lays the foundation for future high angular resolution studies of gas-phase chemistry with the full ALMA.

Hunting for interstellar molecules: rotational spectra of reactive species

Interstellar molecules are often highly reactive species, which are unstable under terrestrial conditions, such as radicals, ions and unsaturated carbon chains. Their detection in space is usually based on the astronomical observation of their rotational fingerprints. However, laboratory investigations have to face the issue of efficiently producing these molecules and preserving them during rotational spectroscopy measurements. A general approach for producing and investigating unstable/reactive species is presented by means of selected case-study molecules. The overall strategy starts from quantum-chemical calculations that aim at obtaining accurate predictions of the missing spectroscopic information required to guide spectral analysis and assignment. Rotational spectra of these species are then recorded by exploiting the approach mentioned above, and their subsequent analysis leads to accurate spectroscopic parameters. These are then used for setting up accurate line catalogs for astronomical searches.

Ab Initio Rovibrational Spectroscopy of the Acetylide Anion

In this work the rovibrational spectrum of the acetylide anion HCC- is investigated using high-level electronic structure methods and variational rovibrational calculations. Using a composite approach the potential energy surface and dipole surface is constructed from explicitly correlated coupled-cluster accounting for corrections due to core-valence correlation, scalar relativistic effects and higher-order excitation effects. Previous approaches for approximating the latter are critically evaluated. Employing the composite potential, accurate spectroscopic parameters determined from variational calculations are presented. In comparison to the few available reference data the present results show excellent agreement with ground state rotational constants within 0.005% of the experimental value. Intensities determined from the variational calculations suggest the bending fundamental transition ν2 around 510 cm-1 to be the best target for detection. The rather weak CD stretching fundamental ν1 in deuterated isotopologues show a second-order resonance with the (0,20,1) state and the consequences are discussed in some detail. The spectroscopic parameters and band intensities provided for a number of vibrational bands in isotopologues of the acetylide anion should facilitate future spectroscopic investigations.

Peptide Bonds in the Interstellar Medium: Facile Catalytic Formation from Nitriles on Water-Ice Grains

A recent suggestion that acetamide, CH₃C(O)NH₂, could be readily formed on water-ice grains by the acid induced addition of water across the C≡N bond has now been shown to be credible. Computational modeling of the reaction between R-CN (R = H, CH₃) and a cluster of 32 molecules of water and one H₃O⁺ proceeds catalytically to form first a hydroxy imine R-C(OH)═NH and second an amide R-C(O)NH₂. Quantum mechanical tunneling, computed from small-curvature estimates, plays a key role in the rates of these reactions. This work represents the first reasonable effort to show, in general, how amides can be formed from nitriles and water, which are abundant substrates, reacting on a water-ice cluster containing catalytic amounts of hydrons in the interstellar medium with consequential implications toward the origins of life.

Millimeter-Wave and High-Resolution Infrared Spectroscopy of 2-Furonitrile─A Highly Polar Substituted Furan

The rotational spectrum of 2-furonitrile (2-cyanofuran) has been obtained from 140 to 750 GHz, capturing its most intense rotational transitions at ambient temperature. 2-Furonitrile is one of two isomeric cyano-substituted furan derivatives, both of which possess a substantial dipole moment due to the cyano group. The large dipole of 2-furonitrile allowed over 10 000 rotational transitions of its ground vibrational state to be observed and least-squares fit to partial octic, A- and S-reduced Hamiltonians with low statistical uncertainty (σ_fit = 40 kHz). The high-resolution infrared spectrum, obtained at the Canadian Light Source, allowed for accurate and precise determination of the band origins of its three lowest-energy fundamental modes (ν₂₄, ν₁₇, and ν₂₃). Similar to other cyanoarenes, the first two fundamental modes (ν_24, A″, and ν₁₇, A', for 2-furonitrile) form an a- and b-axis Coriolis-coupled dyad. More than 7000 transitions from each of these fundamental states were fit to an octic A-reduced Hamiltonian (σ_fit = 48 kHz), and the combined spectroscopic analysis determines fundamental energies of 160.1645522 (26) cm^-1 and 171.9436561 (25) cm^-1 for ν₂₄ and ν₁₇, respectively. The least-squares fitting of this Coriolis-coupled dyad required 11 coupling terms, G_a, G_a^J, G_a^K, G_a^JJ, G_a^KK, F_bc, F_bc^J, F_bc^K, G_b, G_b^J, and F_ac^K. Using both the rotational and high-resolution infrared spectra, a preliminary least-squares fit was obtained for ν₂₃, providing its band origin of 456.7912716 (57) cm^-1. The transition frequencies and spectroscopic constants provided in this work, when combined with theoretical or experimental nuclear quadrupole coupling constants, will provide the foundation for future radioastronomical searches for 2-furonitrile across the frequency range of currently available radiotelescopes.

Performance of a chirped-pulse Fourier transform millimeter wave spectrometer in the range of 75-110 GHz

We present a home-built chirped-pulse Fourier transform millimeter wave (CP-FTMMW) spectrometer. The setup is devoted to the sensitive recording of high-resolution molecular spectroscopy in the W band between 75 and 110 GHz. We describe the experimental setup in detail, including a characterization of the chirp excitation source, the optical beam path, and the receiver. The receiver is a further development of our 100 GHz emission spectrometer. The spectrometer is equipped with a pulsed jet expansion and a DC discharge. Spectra of methyl cyanide as well as hydrogen cyanide (HCN) and hydrogen isocyanide (HNC) products from the DC discharge of this molecule are recorded to characterize the performance of the CP-FTMMW instrument. The formation of the HCN isomer is favored by a factor of 63 with respect to HNC. Hot/cold calibration measurements enable a direct comparison of the signal and noise levels of the CP-FTMMW spectra to those of the emission spectrometer. For the CP-FTMMW instrument, we find many orders of magnitude of signal enhancement and a much stronger noise reduction due to the coherent detection scheme.

Electric Dipole Moments from Stark Effect in Supersonic Expansion: n-Propanol, n-Butanol, and n-Butyl Cyanide

The orientation and magnitude of the molecular electric dipole moment are key properties relevant to topics ranging from the nature of intermolecular interactions to the quantitative analysis of complex gas-phase mixtures, such as chemistry in astrophysical environments. Stark effect measurements on rotational spectra have been the method of choice for isolated molecules but have become less common with the practical disappearance of Stark modulation spectrometers. Their role has been taken over by supersonic expansion measurements within a Fabry-Perot resonator cavity, which introduces specific technical problems that need to be overcome. Several of the adopted solutions are described and compared. Presently, we report precise electric dipole moment determinations for the two most stable conformers of the selected molecules of confirmed or potential astrophysical relevance: n-propanol, n-butanol, and n-butyl cyanide. All dipole moment components have been precisely determined at supersonic expansion conditions by employing specially designed Stark electrodes and a computer program for fitting the measured Stark shifts, inclusive of cases with resolved nuclear quadrupole hyperfine structure. The experimental values are compared with suitable quantum chemistry computations. It is found that, among the tested levels of computation, vibrationally averaged dipole moments are the closest to the observation and the molecular values are, as in the lighter molecules in the series, largely determined by the hydroxyl or nitrile groups.

Convergence of series expansions in rovibrational configuration interaction (RVCI) calculations

Rotational and rovibrational spectra are a key in astrophysical studies, atmospheric science, pollution monitoring, and other fields of active research. The ab initio calculation of such spectra is fairly sensitive with respect to a multitude of parameters and all of them must be carefully monitored in order to yield reliable results. Besides the most obvious ones, i.e., the quality of the multidimensional potential energy surface and the vibrational wavefunctions, it is the representation of the μ-tensor within the Watson Hamiltonian, which has a significant impact on the desired line lists or simulated spectra. Within this work, we studied the dependence of high-resolution rovibrational spectra with respect to the truncation order of the μ-tensor within the rotational contribution and the Coriolis coupling operator of the Watson operator. Moreover, the dependence of the infrared intensities of the rovibrational transitions on an n-mode expansion of the dipole moment surface has been investigated as well. Benchmark calculations are provided for thioformaldehyde, which has already served as a test molecule in other studies and whose rovibrational spectrum was found to be fairly sensitive. All calculations rely on rovibrational configuration interaction theory and the discussed high-order terms of the μ-tensor are a newly implemented feature, whose theoretical basics are briefly discussed.

Rotational Spectra of Unsaturated Carbon Chains Produced by Pyrolysis: The Case of Propadienone, Cyanovinylacetylene, and Allenylacetylene

Several interstellar molecules are highly reactive unsaturated carbon chains, which are unstable under terrestrial conditions. Laboratory studies in support of their detection in space thus face the issue of how to produce these species and how to correctly model their rotational energy levels. In this work, we introduce a general approach for producing and investigating unsaturated carbon chains by means of selected test cases. We report a comprehensive theoretical/experimental spectroscopic characterization of three species, namely, propadienone, cyanovinylacetylene, and allenylacetylene, all of them being produced by means of flash vacuum pyrolysis of a suitable precursor. For each species, quantum-chemical calculations have been carried out with the aim of obtaining accurate predictions of the missing spectroscopic information required to guide spectral analysis and assignment. Rotational spectra of the title molecules have been investigated up to 400 GHz by using a frequency-modulation millimeter-/submillimeter-wave spectrometer, thus significantly extending spectral predictions over a wide range of frequency and quantum numbers. A comparison between our results and those available in the literature points out the clear need of the reported laboratory measurements at higher frequencies for setting up accurate line catalogs for astronomical searches.

New Carbenes and Cyclic Allenes Energetically Comparable to Experimentally Known 1-Azulenylcarbene

1-Azulenylcarbene (18; 0 kJ mol^-1) is experimentally known as the key reactive intermediate for the rearrangement reactions of aryl carbenes in the laboratory. Here, using coupled-cluster methods up to the fc-CCSD(T)/cc-pVTZ//fc-CCSD(T)/cc-pVDZ level, thirteen new carbenes and one new cyclic allene are theoretically identified within the C₁₁H₈ elemental composition that either energetically lie below or very close to 18. While the cyclic allene, bicyclo[5.4.0]undeca-2,3,5,7,9,11-hexene (1; -166 kJ mol^-1), is the experimentally known lowest energy isomer, three other cyclic allenes, bicyclo[5.4.0]undeca-1,2,4,6,8,10-hexene (2; -100 kJ mol^-1), bicyclo[5.4.0]undeca-1,3,4,6,8,10-hexene (3; -97 kJ mol^-1), and bicyclo[6.3.0]undeca-1,2,4,6,8,10-hexene (13; -42 kJ mol^-1), demand new experimental studies. In total, thirty-one isomers are studied in this work (within -166 to +15 kJ mol^-1 from 18) and all are found to be polar (μ ≠ 0). Among these, 1H-benzo[7]annulen-1-ylidene (17; -4 kJ mol^-1; μ = 5.24 D), bicyclo[5.4.0]undeca-2,4,6,8,11-pentaene-10-ylidene (24; 13 kJ mol^-1; μ = 7.59 D), 5-methylene-naphthalen-1-ylidene (26; 15 kJ mol^-1; μ = 5.32 D), 6-methylene-naphthalen-2-ylidene (27; -43 kJ mol^-1; μ = 6.60 D), and 8-methylene-naphthalen-2-ylidene (28; -39 kJ mol^-1; μ = 5.55 D) are competitively polar compared to 18 (μ = 5.39 D). Therefore, these carbene molecules are potential targets for rotational spectroscopists and radioastronomers. Considering the importance of naphthyl and azulenylcarbenes in reactive intermediate chemistry, mechanisms of different rearrangement reactions and plausible formation pathways of some of these new carbenes are studied in this work using density functional theory.

Theoretical Investigation of Rotationally Inelastic Collisions of OH(X2Π) with Hydrogen Atoms

State-to-state cross sections and rate coefficients for transitions between rotational/fine- structure levels of OH(X2Π) induced by collisions with atomic hydrogen are reported in this work. The scattering calculations take into account the full open-shell character of the OH + H system and include the four potential energy surfaces (1A′, 1A′′, 3A′, 3A′′) that correlate with the OH(X2Π) + H(2S) asymptote. Three of these surfaces are repulsive, while the deep H2O well is present on one surface (1A′). The OH + H potential energy curves calculated by Alexander et al. [J. Chem. Phys. 121, 5221 (2004)] are employed in this work. Time independent quantum scattering calculations were performed using the quantum statistical method of Manolopoulos and co-workers [Chem. Phys. Lett. 343, 356 (2001)] because of the presence of the deep H2O well. The computed cross sections include contributions from direct scattering, as well formation and decay of a transient collision complex since the transient HO-H complex is expected to decay nonreactively. Rate coefficients for OH-H inelastic collisions are of interest for astrophysical applications.

Multichannel Radical-Radical Reaction Dynamics of NO + Propargyl Probed by Broadband Rotational Spectroscopy

Chirped-pulse rotational spectroscopy in a quasi-uniform flow has been used to investigate the reaction dynamics of a multichannel radical-radical reaction of relevance to planetary atmospheres and combustion. In this work, the NO + propargyl (C₃H₃) reaction was found to yield six product channels containing eight detected species. These products and their branching fractions (%), are as follows: HCN (50), HCNO (18), CH₂CN (12), CH₃CN (7.4), HC₃N (6.2), HNC (2.3), CH₂CO (1.3), HCO (1.8). The results are discussed in light of previous unimolecular photodissociation studies of isoxazole and prior potential energy surface calculations of the NO + C₃H₃ system. The results also show that the product branching is strongly influenced by the excess energy of the reactant radicals. The implications of the title reaction to the planetary atmospheres, particularly to Titan, are discussed.

Identification of interstellar amino acetonitrile in the hot molecular core G10.47＋0.03: Possible glycine survey candidate for the future

Amino acids are the essential keys that contribute to the study of the formation of life. The simplest amino acid, glycine (NH2CH2COOH), has been searched for a long time in the interstellar medium, but all surveys of glycine have failed. Since the detection of glycine in the interstellar medium was extremely difficult, we aimed to search for the precursor of glycine. After detailed searches of the individual prebiotic molecular species, we successfully identified the emission lines of possible glycine precursor molecule amino acetonitrile (NH2CH2CN) towards the hot molecular core G10.47 + 0.03 using the Atacama Large Millimeter/Submillimeter Array. The estimated statistical column density of amino acetonitrile was (9.10 ± 0.7) × 10¹⁵ cm^-2 with rotational temperature (T_rot) 122 ± 8.8 K. The estimated fractional abundance of amino acetonitrile was 7.01 × 10^-8. We found that the estimated fractional abundance of NH2CH2CN fairly agrees with the theoretical value predicted by the three-phase warm-up model from Garrod (2013).

Terahertz continuous wave spectroscopy: a portable advanced method for atmospheric gas sensing

Motivated by the increasing demand to monitor the air-quality, our study proved the feasibility of a new compact and portable experimental approach based on Terahertz (THz) continuous wave high resolution spectroscopy, to detect the presence of the air's contaminants as greenhouse gases (GHG) and volatile organic compounds (VOCs). In this specific work, we first characterized, determining their molar absorption coefficient in the spectral region (0.06-1.2) THz, the pure optical response of the vapor of five VOCs: methanol, ethanol, isopropanol, 1-butanol and 2-butanol. In particular, 1-butanol and 2-butanol are characterized for the first time in literature at THz frequencies. Then we studied the optical response of their mixtures achieved with ambient air and ethanol. The results show that it is possible for a differentiation of single components by describing their spectral absorption in terms of the linear combination of pure compounds absorption. This proof of concept for this apparatus study and set-up paves the way to the use of THz Continuous wave high resolution spectroscopy for the environmental tracking of air pollutants.

HeH+ Collisions with H2: Rotationally Inelastic Cross Sections and Rate Coefficients from Quantum Dynamics at Interstellar Temperatures

We report for the first time an accurate ab initio potential energy surface for the HeH⁺–H₂ system in four dimensions (4D) treating both diatomic species as rigid rotors. The computed ab initio potential energy point values are fitted using an artificial neural network method and used in quantum close coupling calculations for different initial states of both rotors, in their ground electronic states, over a range of collision energies. The state-to-state cross section results are used to compute the rate coefficients over a range of temperatures relevant to interstellar conditions. By comparing the four dimensional quantum results with those obtained by a reduced-dimensions approach that treats the H₂ molecule as an averaged, nonrotating target, it is shown that the reduced dimensionality results are in good accord with the four dimensional results as long as the HeH⁺ molecule is not initially rotationally excited. By further comparing the present rate coefficients with those for HeH⁺–H and for HeH⁺–He, we demonstrate that H₂ molecules are the most effective collision partners in inducing rotational excitation in HeH⁺ cation at interstellar temperatures. The rotationally inelastic rates involving o-H₂ and p-H₂ excitations are also obtained and they turn out to be, as in previous systems, orders of magnitude smaller than those involving the cation. The results for the H₂ molecular partner clearly indicate its large energy-transfer efficiency to the HeH⁺ system, thereby confirming its expected importance within the kinetics networks involving HeH⁺ in interstellar environments.

Synthesis, Purification, and Rotational Spectroscopy of 1-Cyanocyclobutene (C5H5N)

The rotational spectrum of 1-cyanocyclobutene from 130 to 360 GHz has been observed, assigned, and least-squares fit for the ground state and the two lowest-energy vibrationally excited states. Synthesis by UV photochemical dimerization of acrylonitrile and subsequent base-catalyzed dehydrocyanation affords a highly pure sample, yielding several thousand observable rotational transitions for this small organic nitrile. Over 2500 a-type, R-branch transitions of the ground state have been least-squares fit to low error with partial-octic A- and S-reduced Hamiltonians, providing precise determinations of the corresponding spectroscopic constants. In both reductions, computed spectroscopic constants are in close agreement with their experimentally determined counterparts. Two vibrationally excited states (ν₂₇ and ν₁₇) form a Coriolis-coupled dyad, displaying many a-type and b-type local resonances and related nominal interstate transitions. Somewhat unexpectedly, despite the very small permanent b-axis dipole moment, a number of b-type transitions could be observed for the ν₁₇ state; this is explained in terms of state mixing by the Coriolis perturbations. Over 2200 transitions for each of these states have been least-squares fit to a low-error, two-state, partial-octic, A-reduced Hamiltonian with nine Coriolis-coupling terms (G_a , G_a^J, G_a^K, G_a^JJ, F_bc , F_bc^K, G_b , G_b^J, and F_ac). The availability of so many observed rotational transitions, including resonant transitions and nominal interstate transitions, enables a very accurate and precise determination of the energy difference (ΔE_27,17 = 14.0588093 (43) cm^-1) between ν₂₇ and ν₁₇. The spectroscopic constants presented herein provide a starting point for future astronomical searches for 1-cyanocyclobutene.

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.

The QUIJOTE line survey of TMC-1

We present the recent results obtained with the QUIJOTE line survey of the cold dark core TMC-1. The observations have been carried out with the YEBES 40m radio telescope (see Figure 1) in the Q-band (31-50 GHz). A new set of receivers have been installed in the telescope within the frame of the ERC synergy Nanocosmos project that allows to cover the whole 31-50 GHz band in dual polarization [1]. The spectral resolution is 38.15 kHz. The sensitivity achieved so far varies between 0.12 and 0.25 mK, and allows to search for new molecules in a line by line (no stacking) detection procedure. These new data have permitted to detect the many relatively small protonated species of abundant molecules and eight sulfur-bearing species. The most exciting result is the detection of hydrocarbon chains and cycles with low permanent dipole moment, such as CH2CHCCH, CH2CCHCCH, the propargyl radical (C3H3), cyclopentadiene, indene (the first PAH detected in space), ortho-benzyne and two ethynyl derivatives of cyclopentadiene (c-C5H5CCH) ([2–7]). We have found that the gas-phase chemistry of hydrocarbons in TMC-1 has to be revisited in depth.

Rotational action spectroscopy of trapped molecular ions

Rotational action spectroscopy is an experimental method in which rotational spectra of molecules, typically in the microwave to sub-mm-wave domain of the electromagnetic spectrum (∼1-1000 GHz), are recorded by action spectroscopy. Action spectroscopy means that the spectrum is recorded not by detecting the absorption of light by the molecules, but by the action of the light on the molecules, e.g., photon-induced dissociation of a chemical bond, a photon-triggered reaction, or photodetachment of an electron. Typically, such experiments are performed on molecular ions, which can be well controlled and mass-selected by guiding and storage techniques. Though coming with many advantages, the application of action schemes to rotational spectroscopy was hampered for a long time by the small energy content of a corresponding photon. Therefore, the first rotational action spectroscopic methods emerged only about one decade ago. Today, there exists a toolbox full of different rotational action spectroscopic schemes which are summarized in this review.

Integration of theory, simulation, artificial intelligence and virtual reality: a four-pillar approach for reconciling accuracy and interpretability in computational spectroscopy

The established pillars of computational spectroscopy are theory and computer based simulations. Recently, artificial intelligence and virtual reality are becoming the third and fourth pillars of an integrated strategy for the investigation of complex phenomena. The main goal of the present contribution is the description of some new perspectives for computational spectroscopy, in the framework of a strategy in which computational methodologies at the state of the art, high-performance computing, artificial intelligence and virtual reality tools are integrated with the aim of improving research throughput and achieving goals otherwise not possible. Some of the key tools (e.g., continuous molecular perception model and virtual multifrequency spectrometer) and theoretical developments (e.g., non-periodic boundaries, joint variational-perturbative models) are shortly sketched and their application illustrated by means of representative case studies taken from recent work by the authors. Some of the results presented are already well beyond the state of the art in the field of computational spectroscopy, thereby also providing a proof of concept for other research fields.

Full spectroscopic model and trihybrid experimental-perturbative-variational line list for CN

Accurate line lists are important for the description of the spectroscopic nature of small molecules. While a line list for CN (an important molecule for chemistry and astrophysics) exists, no underlying energy spectroscopic model has been published, which is required to consider the sensitivity of transitions to a variation of the proton-to-electron mass ratio. Here we have developed a Duo energy spectroscopic model as well as a novel hybrid style line list for CN and its isotopologues, combining energy levels that are derived experimentally (Marvel), using the traditional/perturbative approach (Mollist), and the variational approach (from a Duo spectroscopic model using standard ExoMol methodology). The final Trihybrid ExoMol-style line list for ¹²C¹⁴N consists of 28 004 energy levels (6864 experimental, 1574 perturbative, the rest variational) and 2285 103 transitions up to 60 000 cm^-1 between the three lowest electronic states (X ²Σ⁺, A ²Π, and B ²Σ⁺). The spectroscopic model created is used to evaluate CN as a molecular probe to constrain the variation of the proton-to-electron mass ratio; no overly promising sensitive transitions for extragalactic study were identified.

Synthesis, Purification, and Rotational Spectroscopy of (Cyanomethylene)Cyclopropane-An Isomer of Pyridine

The gas-phase rotational spectrum of (cyanomethylene)cyclopropane, (CH₂)₂C═CHCN, generated by a Wittig reaction between the hemiketal of cyclopropanone and (cyanomethylene)triphenylphosphorane, is presented for the first time. This small, highly polar nitrile is a cyclopropyl-containing structural isomer of pyridine. The rotational spectra of the ground state and two vibrationally excited states were observed, analyzed, and least-squares fit from 130 to 360 GHz. Over 3900 R-, P-, and Q-branch, ground-state rotational transitions were fit to low-error, partial octic, A- and S-reduced Hamiltonians, providing precise determinations of the spectroscopic constants. The two lowest-energy vibrationally excited states, ν₁₇ and ν₂₇, form a Coriolis-coupled dyad displaying small a- and b-type resonances. Transitions for these two states were measured and least-squares fit to a two-state, partial octic, A-reduced Hamiltonian in the I^r representation with nine Coriolis-coupling terms (G_a, G_a^J, G_a^K, G_a^JJ, F_bc, F_bc^J, F_bc^K, G_b, and G_b^J). The observation of many resonant transitions and nine nominal interstate transitions enabled a very accurate and precise energy difference between ν₁₇ and ν₂₇ to be determined: ΔE_17,27 = 29.8975453 (33) cm^-1. The spectroscopic constants presented herein provide the foundation for future astronomical searches for (cyanomethylene)cyclopropane.

Rotational Spectra of Three Cyanobutadiene Isomers (C5H5N) of Relevance to Astrochemistry and Other Harsh Reaction Environments

Three cyanobutadiene isomers have been synthesized and their rotational spectra analyzed in the 130-375 GHz frequency range. These species, which are close analogues of known interstellar molecules and are isomers of the heterocyclic aromatic molecule pyridine (C₅H₅N), offer the opportunity of revealing important insights concerning the chemistry in astronomical environments. The s-trans conformers of E-1-cyano-1,3-butadiene and Z-1-cyano-1,3-butadiene are observed, while both the anti-clinal and syn-periplanar conformers of 4-cyano-1,2-butadiene are evident in the rotational spectra. Over 1000 transitions for s-trans-Z-1-cyano-1,3-butadiene and for syn-periplanar-4-cyano-1,2-butadiene are fit to an octic, distorted-rotor Hamiltonian with low uncertainty (<50 kHz). Although neither s-trans-E-1-cyano-1,3-butadiene nor anti-clinal-4-cyano-1,2-butadiene can be fully treated with a distorted-rotor Hamiltonian in this frequency range, we provide herein minimally perturbed, single-state least-squares fits of over 1000 transitions for each species, yielding sets of spectroscopic constants that are expected to enable accurate prediction of high-intensity transitions at frequencies up to 370 GHz for both isomers. The assigned transitions and spectroscopic constants for these cyanobutadienes have already enabled the identification of two isomers in harsh reaction environments and should be sufficient to enable their identification in astronomical environments by radio astronomy.

The sulphur saga in TMC-1: Discovery of HCSCN and HCSCCH

We report the detection, for the first time in space, of cyano thioformaldehyde (HCSCN) and propynethial (HCSCCH) towards the starless core TMC-1. Cyano thioformaldehyde presents a series of prominent a- and b-type lines, which are the strongest previously unassigned features in our Q-band line survey of TMC-1. Remarkably, HCSCN is four times more abundant than cyano formaldehyde (HCOCN). On the other hand, HCSCCH is five times less abundant than propynal (HCOCCH). Surprisingly, we find an abundance ratio HCSCCH/HCSCN of ∼ 0.25, in contrast with most other ethynyl-cyanide pairs of molecules for which the CCH-bearing species is more abundant than the CN-bearing one. We discuss the formation of these molecules in terms of neutral-neutral reactions of S atoms with CH2CCH and CH2CN radicals as well as of CCH and CN radicals with H2CS. The calculated abundances for the sulphur-bearing species are, however, significantly below the observed values, which points to an underestimation of the abundance of atomic sulphur in the model or to missing formation reactions, such as ion-neutral reactions.

Detection of deuterated methylcyanoacetylene, CH2DC3N, in TMC-1

We report the first detection in space of the single deuterated isotopologue of methylcyanoacetylene, CH2DC3N. A total of fifteen rotational transitions, with J = 8-12 and Ka = 0 and 1, were identified for this species in TMC-1 in the 31.0-50.4 GHz range using the Yebes 40m radio telescope. The observed frequencies were used to derive for the first time the spectroscopic parameters of this deuterated isotopologue. We derive a column density of (8.0 ± 0.4) × 1010 cm-2. The abundance ratio between CH3C3N and CH2DC3N is ∼22. We also theoretically computed the principal spectroscopic constants of 13C isotopologues of CH3C3N and CH3C4H and those of the deuterated isotopologues of CH3C4H for which we could expect a similar degree of deuteration enhancement. However, we have not detected either CH2DC4H nor CH3C4D nor any 13C isotopologue. The different observed deuterium ratios in TMC-1 are reasonably accounted for by a gas phase chemical model where the low temperature conditions favor deuteron transfer through reactions with H2D+.

Cumulene carbenes in TMC-1: Astronomical discovery of l-H2C5 ★

We report the first detection in space of the cumulene carbon chain l-H2C5. A total of eleven rotational transitions, with Jup = 7-10 and Ka = 0 and 1, were detected in TMC-1 in the 31.0-50.4 GHz range using the Yebes 40m radio telescope. We derive a column density of (1.8±0.5)×1010 cm-2. In addition, we report observations of other cumulene carbenes detected previously in TMC-1, to compare their abundances with the newly detected cumulene carbene chain. We find that l-H2C5 is ~4.0 times less abundant than the larger cumulene carbene l-H2C6, while it is ~300 and ~500 times less abundant than the shorter chains l-H2C3 and l-H2C4. We discuss the most likely gas-phase chemical routes to these cumulenes in TMC-1 and stress that chemical kinetics studies able to distinguish between different isomers are needed to shed light on the chemistry of C n H2 isomers with n > 3.

Pure hydrocarbon cycles in TMC-1: Discovery of ethynyl cyclopropenylidene, cyclopentadiene and indene

We report the detection for the first time in space of three new pure hydrocarbon cycles in TMC-1: c-C3HCCH (ethynyl cyclopropenylidene), c-C5H6 (cyclopentadiene) and c-C9H8 (indene). We derive a column density of 3.1 × 1011 cm-2 for the former cycle and similar values, in the range (1-2) × 1013 cm-2, for the two latter molecules. This means that cyclopentadiene and indene, in spite of their large size, are exceptionally abundant, only a factor of five less abundant than the ubiquitous cyclic hydrocarbon c-C3H2. The high abundance found for these two hydrocarbon cycles, together with the high abundance previously found for the propargyl radical (CH2CCH) and other hydrocarbons like vinyl and allenyl acetylene (Agúndez et al. 2021; Cernicharo et al. 2021a,b), start to allow us to quantify the abundant content of hydrocarbon rings in cold dark clouds and to identify the intermediate species that are probably behind the in situ bottom-up synthesis of aromatic cycles in these environments. While c-C3HCCH is most likely formed through the reaction between the radical CCH and c-C3H2, the high observed abundances of cyclopentadiene and indene are difficult to explain through currently proposed chemical mechanisms. Further studies are needed to identify how are five- and six-membered rings formed under the cold conditions of clouds like TMC-1.

First detection of doubly deuterated methyl acetylene (CHD2CCH and CH2DCCD)★

We report the first detection in space of the two doubly deuterated isotopologues of methyl acetylene. The species CHD2CCH and CH2DCCD were identified in the dense core L483 through nine and eight, respectively, rotational lines in the 72-116 GHz range using the IRAM 30m telescope. The astronomical frequencies observed here were combined with laboratory frequencies from the literature measured in the 29-47 GHz range to derive more accurate spectroscopic parameters for the two isotopologues. We derive beam-averaged column densities of (2.7 ± 0.5) × 1012 cm-2 for CHD2CCH and (2.2 ± 0.4) × 1012 cm-2 for CH2DCCD, which translate to abundance ratios CH3CCH/CHD2CCH = 34 ± 10 and CH3CCH/CH2DCCD = 42 ± 13. The doubly deuterated isotopologues of methyl acetylene are only a few times less abundant than the singly deuterated ones, concretely around 2.4 times less abundant than CH3CCD. The abundances of the different deuterated isotopologues with respect to CH3CCH are reasonably accounted for by a gas-phase chemical model in which deuteration occurs from the precursor ions C3H6D+ and C3H5D+, when the ortho-to-para ratio of molecular hydrogen is sufficiently low. This points to gas-phase chemical reactions, rather than grain-surface processes, as responsible for the formation and deuterium fractionation of CH3CCH in L483. The abundance ratios CH2DCCH/CH3CCD = 3.0 ± 0.9 and CHD2CCH/CH2DCCD = 1.25 ± 0.37 observed in L483 are consistent with the statistically expected values of three and one, respectively, with the slight overabundance of CHD2CCH compared to CH2DCCD being well explained by the chemical model.

O-bearing complex organic molecules at the cyanopolyyne peak of TMC-1: detection of C2H3CHO, C2H3OH, HCOOCH3, and CH3OCH3

We report the detection of the oxygen-bearing complex organic molecules propenal (C2H3CHO), vinyl alcohol (C2H3OH), methyl formate (HCOOCH3), and dimethyl ether (CH3OCH3) toward the cyanopolyyne peak of the starless core TMC-1. These molecules are detected through several emission lines in a deep Q-band line survey of TMC-1 carried out with the Yebes 40m telescope. These observations reveal that the cyanopolyyne peak of TMC-1, which is the prototype of cold dark cloud rich in carbon chains, contains also O-bearing complex organic molecules like HCOOCH3 and CH3OCH3, which have been previously seen in a handful of cold interstellar clouds. In addition, this is the first secure detection of C2H3OH in space and the first time that C2H3CHO and C2H3OH are detected in a cold environment, adding new pieces in the puzzle of complex organic molecules in cold sources. We derive column densities of (2.2 ± 0.3) × 1011 cm™2, (2.5 ± 0.5) × 1012 cm-2, (1.1 ± 0.2) × 1012 cm-2, and (2.5 ± 0.7) × 1012 cm-2 for C2H3CHO, C2H3OH, HCOOCH3, and CH3OCH3, respectively. Interestingly, C2H3OH has an abundance similar to that of its well known isomer acetaldehyde (CH3CHO), with C2H3OH/CH3CHO ~ 1 at the cyanopolyyne peak. We discuss potential formation routes to these molecules and recognize that further experimental, theoretical, and astronomical studies are needed to elucidate the true mechanism of formation of these O-bearing complex organic molecules in cold interstellar sources.

SiO, 29SiO, and 30SiO emission from 67 oxygen-rich stars. A survey of 61 maser lines from 7 to 1 mm

Circumstellar environments of oxygen-rich stars are among the strongest SiO maser emitters. Physical processes such as collisions, infrared pumping and overlaps favors the inversion of level population and produce maser emission at different vibrational states. Despite numerous observational and theoretical efforts, we still do not have an unified picture including all the physical processes involved in the SiO maser emission. The aim of this work is to provide homogeneous data in a large sample of oxygen-rich stars. We present a survey of 67 oxygen-rich stars from 7 to 1 mm, in their rotational transitions from J = 1 → 0 to J = 5 → 4, for vibrational numbers v from 0 to 6 in the three main SiO isotopologues. We have used one of the 34 m NASA antennas at Robledo and the IRAM 30 m radio telescope. The first tentative detection of a v = 6 line is reported, as well as the detection of new maser lines. The highest vibrational levels seem confined to small volumes, presumably close to the stars. The J = 1 → 0, v = 2 line flux is greater than the corresponding v = 1 in almost half of the sample, which may confirm a predicted dependence on the pulsation cycle. This database is potentially useful in models which should consider most of the physical agents, time dependency, and mass-loss rates. As by-product, we report detections of 27 thermal rotational lines from other molecules, including isotopologues of SiS, H2S, SO, SO2, and NaCl.

TMC-1, the starless core sulfur factory: Discovery of NCS, HCCS, H2CCS, H2CCCS, and C4S and detection of C5S

We report the detection of the sulfur-bearing species NCS, HCCS, H2CCS, H2CCCS, and C4S for the first time in space. These molecules were found towards TMC-1 through the observation of several lines for each species. We also report the detection of C5S for the first time in a cold cloud through the observation of five lines in the 31-50 GHz range. The derived column densities are N(NCS) = (7.8±0.6)×1011 cm-2, N(HCCS) = (6.8±0.6)×1011 cm-2, N(H2CCS) = (7.8±0.8)×1011 cm-2, N(H2CCCS) = (3.7±0.4)×1011 cm-2, N(C4S) = (3.8±0.4)×1010 cm-2, and N(C5S) = (5.0±1.0)×1010 cm-2. The observed abundance ratio between C3S and C4S is 340, that is to say a factor of approximately one hundred larger than the corresponding value for CCS and C3S. The observational results are compared with a state-of-the-art chemical model, which is only partially successful in reproducing the observed abundances. These detections underline the need to improve chemical networks dealing with S-bearing species.

Discovery of CH2CHCCH and detection of HCCN, HC4N, CH3CH2CN, and, tentatively, CH3CH2CCH in TMC-1

We present the discovery in TMC-1 of vinyl acetylene, CH2CHCCH, and the detection, for the first time in a cold dark cloud, of HCCN, HC4N, and CH3CH2CN. A tentative detection of CH3CH2CCH is also reported. The column density of vinyl acetylene is (1.2±0.2)×1013 cm-2, which makes it one of the most abundant closed-shell hydrocarbons detected in TMC-1. Its abundance is only three times lower than that of propylene, CH3CHCH2. The column densities derived for HCCN and HC4N are (4.4±0.4)×1011 cm-2 and (3.7±0.4)×1011 cm-2, respectively. Hence, the HCCN/HC4N abundance ratio is 1.2±0.3. For ethyl cyanide we derive a column density of (1.1 ±0.3)×1011 cm-2. These results are compared with a state-of-the-art chemical model of TMC-1, which is able to account for the observed abundances of these molecules through gas-phase chemical routes.