Nowadays instrumentation from the microwave to the terahertz domain is efficient enough that acquiring data over a very large spectral region has become routine in many laboratories, even when studying highly reactive species. While the recording of thousands of rotational lines can be achieved in a reasonable amount of time, spectral assignments, however, usually remain significantly much slower, even for species that do not display any particular difficulty regarding to their spectroscopic modelization. Luckily for experimentalists, many tools have been developed over the years to assist this crucial step, from graphical display of series of transitions L. Bonah, O. Zingsheim et al., J. Mol. Spectrosc. 388, 111674 (2022)o semi-automated and fully-automated assignments N. A. Seifert, I. A. Finneran et al., J. Mol. Spectrosc. 312, 13-21 (2015) Regardless, the key point required to initiate spectroscopic assignments, and even identify the molecular carrier of the observed transitions, remains the establishment of a reliable equilibrium structure using high-level quantum chemical calculations. Consequently, most of the laboratory molecular discoveries in the gas phase result from the synergy between experiments and theory. In this talk, I will illustrate how experimental and theoretical state-of-the-art tools can be used in tandem to investigate the rotational structure of molecules. Using the examples of the investigation of the isomers of cyanoethynylbenzene (HCC-C₆H₄-CN) J.-T. Spaniol, K. L. K. Lee, et al., Phys. Chem. Chem. Phys. 25, 6397-6405 (2023)nd of the H₂NCO radical M.-A. Martin-Drumel, A. Coutens, et al., in preparation I will show how such interplay can lead to the gas phase characterization of new molecular species.

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Footnotes:

L. Bonah, O. Zingsheim et al., J. Mol. Spectrosc. 388, 111674 (2022)t N. A. Seifert, I. A. Finneran et al., J. Mol. Spectrosc. 312, 13-21 (2015). J.-T. Spaniol, K. L. K. Lee, et al., Phys. Chem. Chem. Phys. 25, 6397-6405 (2023)a M.-A. Martin-Drumel, A. Coutens, et al., in preparation,

Rotational spectra of the mono- and bi-deuterated forms of (Z)-1,2-ethenediol and of bi-deuterated glycolaldehyde have been recorded in the 80-115 GHz and 240-345 GHz frequency ranges. The samples have been obtained from flash vacuum pyrolysis (at a temperature of 750^°C) of bis-exo-5-norbornene-2,3-diol-d₂, which was purposely synthetized with an isotopic purity better than 97%. The process led to the formation of mono- and bi-deuterated (Z)-1,2-ethenediol and cyclopentadiene (which is the pyrolysis co-product). As already noted for the main isotopic species, (Z)-1,2-ethenediol can isomerize, thus leading to the formation of mono- and bi-deuterated glycolaldehyde (since its mono-substituted variants have already been investigated, they have not been considered in this work). The experimental work has been supported by high-level calculations rooted in the coupled-cluster theory, which provided accurate estimates of the main spectroscopic parameters required to guide the spectral analysis. In detail, the isotopic species studied are: (the other bi-deuterated forms, namely and , have also been searched for, but no clear evidence of their rotational features was found) for glycolaldehyde, and , and for (Z)-1,2-ethenediol. Their spectra have been analyzed using S-reduction of Watson Hamiltonian in the I^r representation, and rotational and quartic centrifugal distortion constants have been obtained to a good accuracy (a few sextic terms have also been determined). A very good agreement between experimental and computed parameters has been noticed.

The first study in machine learning of astrochemical inventories demonstrated that simple machine-learning models can learn and predict entire chemical inventories, specifically of the Taurus molecular cloud (TMC-1). Since publication of the original Taurus molecular cloud (TMC-1) machine learning inventory study, both its astrochemical inventory and the machine learning landscape have grown. Dozens more detections have been made within the molecular cloud; meanwhile machine learning has made its way into the daily chemist’s and astronomer’s toolbox, leading to more techniques specifically equipped for chemical machine learning purposes and large datasets. Much has changed in the last few years, and an update of both the inventory and cheminformatic techniques would be useful at this time. In this talk, we explore the effects of updating the small and diverse molecular inventory of TMC-1 on the machine learning model, propose improvements to the original pipeline, and compare the results of these updates to the original study, while offering a more robust model capable of better predicting column density.

The ethynyl radical cation, CCH⁺ (³Π), is very interesting from an astrochemical point of view, since it is proposed to be a key intermediate in interstellar chemistry. Furthermore, it is fascinating from a fundamental spectroscopic point of view due to its open-shell, linear nature, which gives rise to several vibronic and electronic coupling effects, such as Renner-Teller (RT). The situation gets even more complicated due to the presence of a low lying ³Σ⁻ state, which can mix with the ³Π(A’’) ground state and can in turn lead to strong perturbations in the rovibrational spectrum. In our group we have recorded the broadband vibrational spectra of CCH⁺ and CCD⁺ in the range 350-3500 cm⁻¹, using leak-out spectroscopy,Schmid, P. C. et al. J. Phys. Chem. A, 2022, 126(43), 8111-8117n the cryogenic ion trap instrument FELion interfaced with the FELIX IR free-electron lasersJusko, P. et al., Faraday Discuss. 2019, 217, 172-202. The spectra show an extremely complex splitting pattern of the CCH(D) bending mode. The structure can be related to a combination of RT coupling and a pseudo-Jahn-Teller (PJT) coupling between the ³Π and low lying ³Σ⁻ states. Using a diabatic model the effect of both of these interactions on the vibrational spectrum could be determined ab initio, and it was found that the splitting pattern is largely caused by the PJT coupling. Using additional high-resolution IR data (part 2), we were able to constrain the Σ-Π energy difference and with it assign several features in the broadband IR spectra of CCH⁺ and CCD⁺.

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Footnotes:

Schmid, P. C. et al. J. Phys. Chem. A, 2022, 126(43), 8111-8117i Jusko, P. et al., Faraday Discuss. 2019, 217, 172-202..

The ethynyl radical cation, CCH⁺ (³Π), is very interesting from an astrochemical point of view, since it is proposed to be a key intermediate in interstellar chemistry. However, due to its extremely reactive nature, experimental spectroscopic data, which is needed for its detection in space, is lacking. Here we report on our recent high-resolution infrared spectrum of CCH⁺ in the range 3080-3170 cm⁻¹ by means of leak-out spectroscopySchmid, P. C. et al. J. Phys. Chem. A, 2022, 126(43), 8111-8117sing the cryogenic ion trap apparatus COLTRAPAsvany, O. et al., Appl. Phys. B, 2014, 114, 203-211. This spectral range covers the overtone of the CC stretching mode and the fundamental of the CH stretching mode. For both modes we resolve the P-, Q-, and R-branches (including Λ-doubling) within each of the three fine structure components (Ω=2, 1 and 0) and of several ∆Ω=±1 transitions. The fitted spectroscopic parameters indicate a strong (mode dependent) mixing between the ground ³Π state and the low lying excited ³Σ⁻ state, which is particularly large for the CC stretching mode. This information may be used to help interpret the extremely complex splitting pattern observed in the broadband vibrational spectrum, recently recorded in our group using a cryogenic ion trap interfaced with the FELIX infrared free-electron lasers (see part 1). Furthermore, this data can be directly compared to the data from the James-Webb telescope and may aid detection of this reactive ion.

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Footnotes:

Schmid, P. C. et al. J. Phys. Chem. A, 2022, 126(43), 8111-8117u Asvany, O. et al., Appl. Phys. B, 2014, 114, 203-211..

The high-resolution vibrational and rotational spectra of astrophysically relevant chloromethyl cation, CH₂Cl⁺, have been measured for the first time using a 4K cryogenic ion trap instrument employing leak-out spectroscopy. Rovibrational transitions within the fundamental ν₁ parallel band (symmetric CH-stretching) were measured for the CH₂³⁵Cl⁺ and CH₂³⁷Cl⁺ isotopologues. In addition, the ν₅ perpendicular band (antisymmetric CH-stretching) of the most abundant CH₂³⁵Cl⁺ species was measured also. The spectroscopic searches for the observed bands were guided by the recent report on the low-resolution spectrum of the weakly bound CH₂Cl⁺-Ne complex,S. Thorwirth, K. Steenbakkers, T. Danowski, P. C. Schmid, L. Bonah, O. Asvany, S. Brünken, and S. Schlemmer 2024, Molecules 29, 665nd the rovibrational lines were assigned with the help of results from high-level quantum-chemical calculations. Based on the ground state constants obtained from fits of the infrared data, twelve pure rotational transitions were measured for each isotopologue (CH₂³⁵Cl⁺ and CH₂³⁵Cl⁺) in the 100-250 GHz range using a novel infrared-millimeter-wave double resonance scheme involving leak-out spectroscopy.O. Asvany, S. Thorwirth, P. C. Schmid, T. Salomon, and S. Schlemmer 2023, Phys. Chem. Chem. Phys. 25, 19740he observed rotational transitions exhibit nuclear quadrupole hyperfine structure due to the presence of the ³⁵Cl and ³⁷Cl nuclei (both with I= 3/2). From a combined fit of the infrared and millimeter-wave data, accurate spectroscopic parameters have been derived which will enable radio astronomical searches for in space.

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Footnotes:

S. Thorwirth, K. Steenbakkers, T. Danowski, P. C. Schmid, L. Bonah, O. Asvany, S. Brünken, and S. Schlemmer 2024, Molecules 29, 665a O. Asvany, S. Thorwirth, P. C. Schmid, T. Salomon, and S. Schlemmer 2023, Phys. Chem. Chem. Phys. 25, 19740T

One of the longest standing problems in astronomical spectroscopy is identifying carriers of diffuse interstellar bands (DIBs). The DIBs are a set of absorption features, mostly at optical and near infrared wavelengths. Despite there being close to 600 detected lines, so far only five of them have been conclusively assigned to one species (C60+). This leads us to believe that these lines could be due to cations of existing interstellar molecules. Optical spectra of such cationic species are not widely available. Although, theoretical studies cannot definitely prove or ascertain the presence of a particular molecule in space, they can facilitate the choice of molecules to be studied in the laboratory by identifying potential candidates. Here we present some results from a computational study of possible interstellar cations using equation of motion coupled cluster methods (EOM-CC).

Recent and remarkable experiments by the group of D.M. Neumark at Berkeley have convincingly ended the longstanding debate regarding assignment of the ν₃ vibrational fundamental of the nitrate (NO₃) radical. Via electron photodetachment of the 2ν₃ (e^′) vibrational level of the anion, a feature is clearly observed which can be assigned to the 3₂¹ transition, and which reveals that the ν₃ level of the anion is indeed roughly 1050 cm⁻¹ above the ground vibrational state. Both elementary qualitative reasoning and a simulation of the spectrum lead to the conclusion that this is the correct assignment, and that the nearly coincident 3₂⁰ 1₀¹ feature is not seen in the spectrum. This talk focuses on the simulation of this spectrum, which involves a few challenging aspects. Particular attention will be given to an interference effect that is present in this spectrum whereby both vibronic and Franck-Condon effects contribute to the transition. The impact of this quantum interference effect on the spectrum are discussed.

Orthocarboxylic acids — organic molecules carrying three hydroxyl groups at the same carbon atom — have been distinguished as vital reactive intermediates by the atmospheric science and physical (organic) chemistry communities as fundamental transients in the atmospheric aerosol cycle. Predicted short lifetimes and their trend to dehydrate to a carboxylic acid, free orthocarboxylic acids signify one of the most elusive classes of organic reactive intermediates and even the simplest representative methanetriol (CH(OH)₃) has not previously been detected experimentally. Here, we report the first synthesis of the previously elusive methanetriol molecule in low temperature mixed methanol (CH₃OH) and molecular oxygen (O₂) ices subjected to energetic irradiation. Supported by electronic structure calculations, methanetriol was identified in the gas phase upon sublimation via isomer-selective photoionization reflectron time-of-flight mass spectrometry combined with isotopic substitution studies and detection of photoionization fragments. The first synthesis and detection of methanetriol (CH(OH)₃) reveals its gas-phase stability as supported by a significant barrier hindering unimolecular decomposition. These findings progress our fundamental understanding of the chemistry and chemical bonding of methanetriol and its hydroxyperoxymethane (CH₃OOOH) and hydroxyperoxymethanol (CH₂(OH)OOH) isomers shaping their role in atmospheric secondary aerosol particle formation and global climate of our planet.

Last year at ISMS,^b we presented an analysis of the centimeter, millimeter, and sub-millimeter wave spectra of the acetyl radical which is a benchmark system for non-rigid open-shell molecules displaying a torsional motion. The theoretical modeling of the high-resolution spectrum of the radical relied on a usual rotation-torsion RAM Hamiltonian to which the electron spin-rotation coupling^c was added. Although such an approach successfully reproduced the earlier K_a=0 data, it failed for the new K_a > 0 measurements. An empirical coupling, referred to as the electron spin-torsion coupling, was introduced to achieve a satisfactory analysis. This coupling involves terms in S_x p_γ and S_z p_γ depending on the molecule fixed components S_x and S_z of the electron spin and on the internal rotation angular momentum p_γ. In this paper, a theoretical derivation of the electron spin-torsion coupling will be presented starting from the PAM Hamiltonian of Lin and Swalen and the exact Hamiltonian introduced by Curl for treating fine effects in ²Σ open shell polyatomic molecules.^d The new electron spin-torsion coupling could be theoretically substantiated and an unexpected relation was found between the spectroscopic constants describing the electron spin-torsion coupling and certain components of the electron spin-rotation coupling tensor. In the paper, a line frequency analysis of the centimeter, millimeter, and sub-millimeter wave transitions available now for the radical will be presented in order to determine the spectroscopic parameters describing the electron spin-torsion coupling and to check the validity of this relation.