Millimeter/submillimeter spectroscopy is a powerful analytical technique for identifying the components of complex gas mixtures. In this work, millimeter/submillimeter spectroscopy has been utilized to detect and quantify organic species sublimated from a UV photolyzed methanol ice. Eleven species were uniquely identified by their structure-specific pure rotational transitions, and the integrated intensities of their rotational transitions were used to determine their rotational temperatures and gas densities by a rotation diagram analysis. This talk will outline the experimental design and the latest results with SubLIME. The abundance ratios of detected organic species will also be compared to those detected in star-forming regions and cometary comae.

Detected through the aromatic infrared bands (3-20 μm), polycyclic aromatic hydrocarbons (PAHs) are considered to lock up a large fraction of the carbon in the interstellar medium (ISM) and to significantly impact its physics and chemistry. Despite their importance, much is unknown about their formation and reactivity under the harsh conditions of the ISM. We are looking at the reactivity of these molecules in the laboratory by means of an electrical discharge nozzle coupled with chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy in the 2-12 GHz frequency range. Under such energetic conditions, PAHs are expected to undergo fragmentation processes and/or a recombination chemistry. The formed species are then unambiguously identified via their unique microwave signature. Herein, we present our results obtained from discharge experiments on the PAHs naphthalene (C₁₀H₈) and phenanthrene (C₁₄H₁₀), both pure and in mixture with acetonitrile (CH₃CN), a simple nitrogen-containing interstellar molecule. An interesting chemistry has been observed for the investigated PAHs, which will be compared and discussed.

Using a Fourier transform infrared spectrophotometer and a low temperature cryostat, we have obtained the IR spectra of formaldehyde dissolved in liquid krypton between 125 K and 137 K. Monomeric formaldehyde in gas phase was prepared by thermal decomposition of para-formaldehyde. Peak positions (ν), wavenumber shifts(∆ν), and full widths at half maximum (∆ν_1/2) are reported. Influence of the solvent on fundamental vibrational frequencies are studied using the polarizable continuum model (PCM). When used in combination with calculated anharmonic frequencies, the PCM model shows qualitative agreement with frequency shifts in liquid Kr. The importance of formaldehyde IR characterization around 100 K will be discussed in reference to its detection in the interstellar medium B. Zuckerman et al., Observations of Interstellar Formaldehyde, Astrophys. J., 1970, 160: 485-506. the IR detection of the ν₁ vibration in the protostar W33AE. Roueff et al., Infrared Detection of Gas Phase Formaldehyde Towards the High Mass Protostar W33A, A&A, 2006, 447: 963-969 and although the Cassini infrared spectrometer (CIRS) was not able to detect methanol, formaldehyde, and acetonitrile in Titan's atmosphere, upper limits on their abundances and formation conditions have been proposed. C.A. Nixon et al., Upper Limits for Undetected Trace Species in the Stratosphere of Titan. Faraday Discuss. 2010, 147: 65-81.^, B.N. Tran et al., Titan's Atmospheric Chemistry: Photolysis of Gas Mixtures Containing Hydrogen Cyanide and Carbon Monoxide at 185 and 254 nm, Icarus, 2008, 193: 224232

More than 200 molecules have so far been detected in the interstellar medium (ISM), of which close to one third are complex organic molecules containing six or more atoms. Over the last decades, laboratory experiments simulating the conditions in cold molecular clouds have demonstrated that these COMs can form from interaction of ionizing radiation with simple ices deposited on interstellar dust particles. These experiments have unveiled multiple pathways towards the formation of acetaldehyde (CH₃CHO) in such ices, explaining its detection in many interstellar and circumstellar environments including tentative detections in interstellar ices. By condensing acetaldehyde and carbon monoxide at 5 K and irradiating the ice with 5 keV electrons, we simulate secondary electrons generated in the track of galactic cosmic rays interacting with ices around cosmic dust particles. Combined infrared and photoionization reflectron time-of-flight mass spectrometry studies were employed to unambiguously identify pyruvic acid (CH₃COCOOH) and its enol, 2-hydroxyacrylic acid (CH₂(COH)COOH), as reaction product from the irradiation by a barrierless radical-radical reaction of the acetyl (CH₃CO) and hydroxycarbonyl (HOCO) radicals. These results present an abiotic pathway towards the formation of this prebiotic molecule in the interstellar medium. Pyruvic acid constitutes a key starting material for the Krebs cycle, which supplies living organisms with energy. Furthermore, it is a precursor for more complex biomolecules such as lactic acid or alanine. In a second experiment, condensing pure methanol and mixed methanol and carbon monoxide ices, 1,2-ethenediol (HOCH=CHOH) was detected after irradiation with 5 keV electrons at 5 K. Three distinct pathways to its formation could be inferred from isotopic substitutions. 1,2-ethenediol, the enol of glycolaldehyde, is the reactive key intermediate in the formose cycle to form complex sugars from formaldehyde but has hitherto not been isolated. The detection of 1,2-ethendiol in such simple ices therefore suggests a facile reaction network to enable complex sugar formation.

All isomers of C₂H₄O₂, i.e. glycolaldehyde (HCOCH₂OH), acetic acid (CH₃COOH) and methyl formate (HCOOCH₃), have been observed abundantly around the Galactic center, in dark clouds, and hot cores of the interstellar medium (ISM), as well as in some comets of the Solar System. However, their exact gas-grain formation and destruction pathway is still unclear. According to El-Abd et al. (2019), the observed column densities of methyl formate and acetic acid are well-correlated, and are likely simply tracking the relative total gas mass in star forming regions. Methyl formate and glycolaldehyde, however, display a stark dichotomy in their relative column densities. The latter finding suggests that different formation/destruction routes are at play for the three isomers. To date, there is a strong laboratory evidence for an efficient production of glycolaldehyde, methyl formate, and acetic acid in the ISM (Gerakines et al. 1996; Bennett and Kaiser 2007; Modica et al. 2012). However, so far models and laboratory studies cannot fully reproduce the observed mutually exclusive presence of specific isomers in certain star formation regions. Understanding the formation of the C₂H₄O₂ isomers is an important step to verify the formation of yet more complex molecules that are necessary for life. In this talk, I will present our latest results obtained at the ion accelerator ATOMKI facility in Debrecen (Hungary) using the novel ultrahigh vacuum ICA end station. Results will be discussed in light of upcoming JWST mission.

Nitrogen-containing heterocycles pose an intriguing astrochemical mystery. 45 different varieties have been detected on meteorites with non-terrestrial isotopic abundances, but none have been detected in space, despite numerous search attempts. It is unclear if these species are more likely to form through low-temperature gas-phase chemistry, photoprocessing of icy grains, aqueous chemistry in a meteorite parent body, or a combination of processes. Further exploration of this question requires a broader catalog of potential N-heterocycle precursors, particularly for low temperature gas-phase chemistry. Here, results of ab initio molecular dynamics simulations of gas-phase N-heterocycle forming reactions will be discussed. These simulations were performed with an ab initio nanoreactor, a computational tool developed for chemical reaction discovery. Multiple novel gas-phase N-heterocycle formation pathways have been revealed, and many reactants are similar to known interstellar molecules. These simulations also reinforce previous experimental and theoretical studies which demonstrated that smaller N-heterocycles are potential precursors to larger ones. While these simulations cannot provide direct insight into interstellar chemistry, they have revealed nonintuitive N-heterocycle precursors that warrant further experimental and astronomical study.

The T₂ time of HCN and HNC in He was measured at low temperatures using chirped pulse Fourier transform millimeter wave spectroscopy, to compare to scattering calculations and to investigate the difference of interaction between the two structural isomers. These molecules are commonly observed in the interstellar medium (ISM), where their abundance ratio can be used to elucidate conditions within an interstellar environment. Within these environments, they are expected to collide with He and H₂, which could lead to non-thermal excitation. Previous ab initio calculations of the collisions with He and H2 with HCN and HNC have shown that the rates for HNC with these colliders are much greater, despite their similarities in structure. We have performed CPUF (Chirped pulse in Uniform Flow) experiments to measure the difference in T₂ of HCN and HNC at cold temperatures in He. The J=1-0 transitions of HCN and HNC were observed in under CRESU (Reaction Kinetics in Uniform Supersonic Flow) conditions using cold flows of He. Pulsed laser photolysis of vinyl cyanide was used to produce HCN and HNC under comparable conditions. The T₂ time was fit directly from the free induction decays of these experiments at different temperatures, which is directly comparable to the linewidth in the frequency domain. This was then compared to pressure broadening cross sections produced from close coupling scattering calculations performed on ab initio potential energy surfaces. We find that there is a difference in interaction of HCN and HNC with He at low temperatures and will discuss the impacts this may have for observing these species in the ISM.

It remains an open question as to whether polycyclic aromatic hydrocarbons (PAHs) can be efficiently formed in the ISM by bottom-up mechanisms involving growth from small aromatic precursors like benzene. However, the lack of dipole moment renders benzene invisible in the radio regime making estimation of the abundance of benzene in the ISM difficult. The recent detection of benzonitrile in the Taurus Molecular Cloud (TMC)-1 has caused excitement in the astrochemical community as it is the first aromatic molecule detected in the interstellar medium (ISM) using radio astronomy. Benzonitrile is thought to form via the neutral-neutral reaction between the CN radical and benzene, and therefore may serve as a chemical proxy to determine the abundance of benzene. The abundances of aromatic species in ISM environments are not well understood, in part due to a lack of experimental kinetics data. Both rate constants and product-branching ratios for the reactions of aromatic molecules must be measured at low temperature in order to input these reactions into astrochemical models and accurately predict abundances. Benzene and toluene are two of the aromatic species detected in the atmosphere of Titan and their reactions with the CN radical have been studied down to 105 K by Trevitt et al. Here, we have extended this study down to 15 K to approach dense cloud conditions and have measured the rate constants of the reactions of benzene and toluene with the CN radical using the well-established CRESU technique (Cinétique de Réaction en Ecoulement Supersonique Uniforme, or Reaction Kinetics in Uniform Supersonic Flow) combined with the Pulsed Laser Photolysis-Laser-Induced Fluorescence method. I will also discuss our recent progress in combining chirped-pulse micro/mm-wave spectroscopy with the CRESU method and how we plan to employ this technique to measure product branching ratios for reactions of the CN radical with aromatics at low temperatures.

O(¹D) insertion reactions with methylamine are a predicted pathway to form aminomethanolB. M. Hays, S. L. Widicus Weaver, J. Phys. Chem. A, 117, 32, 7142-7148 (2013)^,M. E. Wolf, P. R. Hoobler, J. M. Turney, H. F. Schaefer III, Phys. Chem. Chem. Phys., 2019,21, 24194−24205 a possible glycine precursor in the interstellar medium. In a preliminary study, O(^1D) insertion into methane and ethylene have shown to successfully result in the formation and spectral detection of methanol and vinyl alcohol, respectivelyB. M. Hays, N. Wehres, B. Alligood DePrince, A. A.M. Roy, J. C. Laas, S. L. Widicus Weaver, Chem. Phys. Lett., 630, 18 (2015) Despite the detection of these insertion products, the complex mixture resulting under these conditions has not been well characterized. Likewise, O(^1 M. E. Wolf, P. R. Hoobler, J. M. Turney, H. F. Schaefer III, Phys. Chem. Chem. Phys., 2019,21, 24194-24205,B. M. Hays, N. Wehres, B. Alligood DePrince, A. A.M. Roy, J. C. Laas, S. L. Widicus Weaver, Chem. Phys. Lett., 630, 18 (2015).

Origin of organic molecules on Earth is discussed to be interstellar clouds, because comets carried molecules produced in clouds. Methyl isocyanate CH₃NCO is presumably one of the delivered organic molecules at the primitive earth. However, observed abundance ratios of [CH₃NCO]/[HNCO] in molecular clouds [1] are clearly lower than that in the comet 67P [2]. Recently a theoretical study has suggested that this abundance anomaly can be explained by a model of dust-surface formation of CH₃NCO in molecular clouds [3]. If that is the case, high-temperature CH₃NCO desorbed from dust surface makes a compact distribution in a central region of a star-forming core. To examine this temperature and distribution condition, the spatial and spectral data of CH₃NCO toward the (N1) core of the star-forming region Sagittarius B2 observed by ALMA were investigated in the 94–113 GHz region for 24 rotational lines. Considering intensity maps and velocity structures, a compact distribution relating to a bipolar molecular flow and an accretion disk already suggested [4] was imaged by CH₃NCO. Rotational temperatures are 20–60 K and column densities are 10¹⁵ cm⁻². These results suggest hot and dense conditions of this molecule in the central region of the core. Hence, CH₃NCO detected in the present data is likely to be produced on dust surface. The model explaining the abundance anomaly is observationally supported. [1] Halfen et al., 2015, ApJL, 812, L5. [2] Goesmann et al., 2015, Sci., 349, 6247. [3] Majumdar et al., 2018, MNRAS, 473, L59. [4] Lis et al. 1993, ApJ, 402, 238.

C_nH molecules are the simplest linear carbon chains in space. They are crucial for not only probes of young clouds but also benchmarks of calculations of chemical reaction network. However, their abundances occasionally show anomaly. For example, observed column densities of C₄H in various sources are one order of magnitude higher than theoretically estimated values. Herbst & Osamura suggested that these excesses of C₄H come from the theoretically determined dipole moment of C₄H.^a The dipole moment in the electronic ground state of ²Σ⁺ was calculated to be 0.87 D by the RCCSD(T)/aug-cc-pVDZ level of ab initio theory.^b However, the mixing of wavefunctions between the ground state and the low-lying electronic excited state of ²Π having the large dipole moment of 4.4 D occurs, giving a higher dipole moment to the ground state. By using a higher dipole moment, a smaller column density is derived via observed line intensities. In the present study, we re-calculated the dipole moment of C₄H by quantum chemical calculations including the mixing.^c The calculations were carried out by the multi-reference configuration interaction level of ab initio theory using the cc-pVQZ basis set. The new dipole moment was derived to be 2.10 D, which is about 2.4 times larger than the value of 0.87 D used so far. Reported lines of C₄H were analyzed to revise column densities by using the new dipole moment. Revised values are about a factor of 6 smaller than those in the previous works. Using the revised column density of C₄H, abundances of the C_2nH (n = 1–4) series show exponential smooth decreases with carbon-chain length in various sources.

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Footnotes:

^aHerbst & Osamura, 2008, ApJ, 679, 1670. ^bWoon, 1995, Chem. Phys. Lett. 244, 45. ^cOyama et al., 2020, ApJ, 890, 39.

Early ab initio calculationsSaebø, Radom, and Schaefer, J. Chem. Phys. 78 (1983) 845; Marenich and Boggs, J. Chem. Phys. 119 (2003) 3098; and Ibid., J. Chem. Phys. 119 (2003) 10105evealed that the hydroxymethyl radical () is a non-rigid species exhibiting a complicated potential energy surface. It displays 4 C₁ non-superimposable, energetically equivalent, equilibrium configurations and 4 C_s maxima, approximately 300 and 1500 cm⁻¹ above the equilibrium configurations. The large amplitude motion of the radical can be pictured as an internal rotation of the group with respect to the group. The axis of internal rotation is the bond and the two-fold symmetry hindering potential is characterized by a barrier height of 1500 cm⁻¹. Aided by new ab initio results, the torsional levels associated with this torsional motion were computed. The energy levels display a tunneling splitting which, for the ground torsional level, was found to be smaller than 0.1 cm⁻¹. As the large amplitude motion of the hydroxymethyl radical seems to be well described by the high barrier approximation, the tunneling IAM water dimer formalismHougen, J. Mol. Spectrosc. 114 (1985) 395; and Coudert and Hougen, J. Mol. Spectrosc. 130 (1988) 86as used to derive a fitting approach aimed at accounting for its rotation-tunneling energy. The effects of the fine spin-rotation and hyperfine spin-spin couplings were also included since there is an unpaired electron. In the paper, the analysis with the IAM approach^b of already available^c,d and newly measured sub-millimeter wave spectroscopic data will be reported. The fitted value of the tunneling splitting will be compared to that retrieved from the torsional energy level calculation. The rotational dependence of the tunneling splitting will be discussed.

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Footnotes:

Saebø, Radom, and Schaefer, J. Chem. Phys. 78 (1983) 845; Marenich and Boggs, J. Chem. Phys. 119 (2003) 3098; and Ibid., J. Chem. Phys. 119 (2003) 10105r Hougen, J. Mol. Spectrosc. 114 (1985) 395; and Coudert and Hougen, J. Mol. Spectrosc. 130 (1988) 86w

From an astrochemical point of view ketenimine (CH₂CNH) is a complex organic molecule (COM) and therefore likely to be a building block for biologically relevant molecules. Since it has been detected in the star-forming region Sagittarius B2(N), it is of high relevance in this field. Although experimental data are available for certain bands, for some energy ranges such as above 1200 cm⁻¹ reliable data virtually do not exist. In addition, high-level ab initio calculations are neither reported for ketenimine nor for one of its deuterated isotopologues. In this paper, we provide for the first time data from accurate quantum chemical calculations and a thorough analysis of the full rovibrational spectrum. Based on high-level potential energy surfaces obtained from explicitly correlated coupled-cluster calculations including up to 4-mode coupling terms, the (ro)vibrational spectrum of ketenimine has been studied in detail by variational calculations relying on rovibrational configuration interaction (RVCI) theory. Strong Fermi resonances were found for all isotopologues. Rovibrational infrared intensities have been obtained from dipole moment surfaces determined from the distinguishable cluster approximation. A comparison of the spectra of the CH₂CNH molecule with experimental data validates our results, but also reveals new insight about the system, which shows very strong Coriolis coupling effects.