Methylamine (CH₃NH₂) is the simplest primary amine and a precursor of the simplest amino acid, glycine. Therefore, methylamine is a molecule of particular interest in astrochemistry and prebiotic chemistry. Previous astronomical observations have detected methylamine in multiple star-forming regions, molecular clouds, meteorites, and the atmosphere of Titan. Studying the formation and dissociation mechanisms of methylamine is thus important for the modeling of chemistry in these environments and for laying the groundwork for future astronomical observations. This work focuses on the aminomethyl radical (^·CH₂NH₂), which is one of the expected products arising from the cosmic-ray induced photodissociation of methylamine. A pulsed supersonic expansion of methylamine in argon was coupled with a high-voltage needle discharge source to produce ^·CH₂NH₂ in a vacuum chamber. The rotational spectrum of ^·CH₂NH₂ was collected in the millimeter/submillimeter regime, and the results were compared to theoretical predictions based on the molecular structure of ^·CH₂NH₂. Here we will present the laboratory measurements and the results of the spectral analysis, as well as initial searches for ^·CH₂NH₂ in the interstellar medium.

The ethoxy radical (C₂H₅O) is a reactive intermediate exciting in many important combustion and atmospheric reactions. The near-UV electronic transitions of ethoxy radical were studied by Tan at el. in 1993, where the rotational and spin-rotation splittings were resolved.X. Q. Tan, J. M. Williamson, S. C. Foster and T. A. Miller, J. Phys. Chem. 1993, 97, 9311-9316n the present study, the rotational spectra of C₂H₅O are measured by Fourier-transform Microwave (FTMW) and FTMW-microwave double-resonance spectroscopy in the frequency region of 4-40 GHz. The electric discharge of diluted ethanol is used to generate the ethoxy radical. Four a-type transitions and two b-type transitions including K_a = 0 and K_a = 1 are observed. The 2₀₂-1₀₁ and 1₁₀-1₀₁ transitions are reproduced with the double resonance technique. The rotational and spin-rotation coupling constants agree with Tan et al’s results. However, the hyperfine splittings due to the five protons in the C₂H₅O radical are so complicated that definite assignment has not been obtained yet. We are trying to assign them with the help of the double resonance spectra.

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Footnotes:

X. Q. Tan, J. M. Williamson, S. C. Foster and T. A. Miller, J. Phys. Chem. 1993, 97, 9311-9316I

The family of Criegee intermediates, commonly designated as QOO, where Q is CH₂, CH₃CH, and so on, has been predicted by Rudolf Criegee in 1949 and discovered recently in laboratory studies. These highly unstable and reactive species are important in the atmosphere where they are formed by ozonolysis of alkenes. To investigate its chemistry, CH₂OO has been produced in various laboratory settings by either ozonolysis of CH₂CH₂, photodissociation of CH₂I₂ followed by oxidation, or oxidation of CH₄ under the discharge conditions. At Argonne, we observe CH₂OO resulting from the oxidation of CH₃CH₃ in a continuous-flow SiC microreactor heated to 1700 K. Cold (T_\textrot=7 K) Criegee intermediate is detected in the supersonic molecular beam emerging from the hot microreactor and using the chirped-pulse Fourier transform millimeter-wave spectrometer, which operates in the 60-90 GHz region and is equipped with a fast narrowband digitizer for averaging 10⁷ free induction decay traces in 5 minutes. The branching ratios of CH₂OO to the main oxidation products HO₂ and CH₂O are measured as a function of reactor temperature. We discuss the possible chemical pathways and the thermodynamic conditions within the reactor and outside of it that may lead to the formation and retention of the “fragile” CH₂OO intermediate in this experiment.

Reactions between Criegee intermediates and hydrogen halides are important in atmospheric chemistry, because of their large rate coefficients. Employing a Fourier-transform absorption spectrometer in a step-scan mode or a continuous-scan mode, we recorded infrared spectra of transient species and end products in a flowing mixture of CH₃CHI₂/HCl/N₂/O₂ irradiated at 308 nm. Eight bands near 1383.7, 1357.9, 1323.8, 1271.8, 1146.2, 1098.2, 1017.5 and 931.5 cm⁻¹were experimentally observed and assigned to bands ν₈ to ν₁₅ of the anti-conformer of chloroethyl hydroperoxide (CEHP, CH₃CHClOOH), according to comparison of vibrational wavenumbers and IR intensities predicted with the B3LYP/aug-cc-pVTZ method. We derived a rate coefficient of anti-CH₃CHOO + HCl to be k_HCl = (3.1 ± 0.2)x10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ from the formation of anti-CEHP. At a later reaction period, absorption bands of H₂O and acetyl chloride, CH₃C(O)Cl, at 1819.1 cm⁻¹were observed; these species were produced from the decomposition of anti-CEHP or the secondary reactions of CH₃CHClO + O₂ → CH₃C(O)Cl + HO₂ and OH + HCl → H₂O + Cl according to temporal profiles of CEHP, H₂O, and CH₃C(O)Cl; both O₂ and HCl are major species in the system to participate in the secondary reactions. By adding methanol to deplete anti-CH₃CHOO, we observed only anti-CEHP, indicating that the interconversion from syn-CEHP to anti-CEHP is rapid. The branching ratio of the formation of CH₃C(O)Cl + H₂O to that of CH₃CHClO + OH was estimated to be   0.5 : 0.5. This observation serves as an excellent example that secondary reactions might interfere with the observation of the original products.

Utilizing matrix isolation infrared spectroscopy with a vacuum ultraviolet (VUV) photolysis source (121 nm), a 1:1 complex of the hydroxyl radical (OH) with benzene (C₆H₆) was observed in low temperature argon matrices. Co-deposition experiments with simultaneous VUV photolysis of H₂O and C₆H₆ mixtures were conducted in an argon matrix at 15 K. The OH stretching peak for the OH-C₆H₆ complex was observed at 3502.3 cm⁻¹. Identification of the observed peak of the OH-C₆H₆ complex was established by performing experiments with varying concentrations of the H₂O and C₆H₆ relative to the argon matrix, comparing the co-deposition spectra to the individual monomer spectra of H₂O and C₆H₆ in argon matrices both with and without VUV photolysis, as well as matrix annealing experiments (30 - 35 K). Experiments were also performed using the D₂O isotopomer and the OD stretching peak for the OD-C₆H₆ complex was observed at 2584.1 cm⁻¹. Quantum chemical calculations were performed at the MP2, M06-2X, and ωB97X-D levels of theory with the aug-cc-pVDZ basis set to obtain the optimized geometry and simulated infrared spectrum for the OH-C₆H₆ complex. The observed OH and OD stretching peaks of the OH(D)-C₆H₆ radical complexes observed in argon matrices in the current experiments are in good agreement with the values previously reported in argon matrices using a different production method.A. Mardyukov, E. Sanchez-Garcia, R. Crespo-Otero, and W. Sander, Angew. Chem. Int. Ed. 48, 4804 (2009)^, A.Mardyukov, R.Crespo-Otero, E.Sanchez-Garcia, and W.Sander, Chem.Eur.J.16, 8679 (2010)

l0pt Figure Amino acids, key building blocks of protein, gained enormous attention in interstellar chemistry because they were detected in comets and meteorites; these observations provided strong evidence for the cosmic origin of amino acids on Earth. However, detailed investigations regarding their formation and reactivities with interstellar relevant species under cosmic-like conditions are scarce. We utilized the characteristics of para-hydrogen (p-H₂), which served as a quantum-solid matrix host and a medium for efficient hydrogen-atom reaction, to investigate the reaction between α-alanine [H₂NCH(CH₃)C(O)OH] and H atoms at 3.2 K. To produce H atoms, we performed UV photolysis at 365 nm on a matrix co-deposited with a mixture of H₂NCH(CH₃)C(O)OH/p-H₂ and Cl₂ to produce Cl atoms, followed by infrared irradiation to promote the Cl + H₂ (ν = 1) → H + HCl reaction. Among four different hydrogen-containing moieties of H₂NCH(CH₃)C(O)OH, H abstraction on the -CH moiety to produce C_α-alanyl radical [H₂N•C(CH₃)C(O)OH] from the conformer with the least energy is the most favorable. This radical plays a vital role in the asymmetric synthesis of complex organic molecules. In parallel, possibly H abstraction on both -C(O)OH and CH₃ moieties led to the fragmentation to produce vinylamine (NH₂CH=CH₂) and CO₂ through the second-most favorable channel. Recently, vinylamine has been detected in the interstellar medium.S. Zeng et al. Astrophys. J. Lett. 2021, 921, L27.hese assignments were supported by isotopic substitution experiments and a comparison of experimental results with vibrational wavenumbers of possible products predicted with the B3LYP/aug-cc-pVTZ method.

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Footnotes:

S. Zeng et al. Astrophys. J. Lett. 2021, 921, L27.T

The reactivity of hydrocarbon radicals is strongly influenced by the orbital character and distribution of the unpaired electron. Hyperfine-resolved microwave spectroscopy is an ideal tool for measuring this structure in isolated, gas-phase molecules, providing fundamental insights into their open-shell electronic properties and chemical structure. We present examples of this approach applied to two prototypical reactive species: propargyl (HCCCH₂), the smallest resonance-stabilized π-radical, and phenyl (c-C₆H₅), the simplest aryl σ-radical. Using cavity Fourier transform microwave measurements of isotopically substituted propargyl, combined with highly accurate ab initio rovibrational corrections, we have derived its complete semi-experimental equilibrium structure and unpaired spin distribution, which provide vivid, complementary illustrations of π-electron delocalization. Our parallel work on phenyl has focused on the complete assignment of the complex hyperfine structure associated with its five ¹H nuclear spins. In addition to characterizing the singly occupied, carbon-centered σ-orbital, the precisely determined hyperfine parameters enable us to generate a kHz-accuracy cm-wave catalog. These laboratory data are an essential prerequisite for a sensitive astronomical search for phenyl in radio surveys of narrow-linewidth interstellar objects such as cold, dark molecular clouds, where phenyl is thought to be a critical intermediate in the formation of the first aromatic ring. Our work suggests that other large, weakly polar, open-shell hydrocarbons, including benzyl and indenyl, may be amenable to high-resolution microwave characterization.

Sulfinyl radicals (R-SO) play a critical role in various fields, from biology to atmospheric chemistry, such as aerosol formation in which chlorine atoms was proposed to potentially catalyze the oxidation of sulfur species in Venus' atmosphere. In this study, we performed a Pulsed Laser Photolysis experiment to detect ClSO from Cl₂SO photolysis at 248 nm in a gas flow reactor using time-resolved UV-Vis transient absorption spectroscopy. A strong absorption near 303 nm and a weak one around 385 nm, with a vibrational progression of about 658 cm⁻¹and 227 cm⁻¹, was recorded. Ab initio calculations at the EOMEE-CCSD/ano-pVQZ level revealed that the strong and weak band corresponds to the 1²A" ← X²A" and the 2²A' ← X²A" transitions. Further analysis showed that there might be a conical intersection between the 1²A' and 2²A' state near the ground-state geometry, which poses a challenge for further theoretical work. Furthermore, we constructed a molecular orbital diagram analysis to understand the electronic structure of the sulfinyl functional group. The analysis suggested that sulfinyl radicals tend to form chemical bonds with other radicals. As an example, a fast recombination rate coefficient of Cl + ClSO → Cl₂SO reaction was investigated to be k_Cl+ClSO = (1.48 ± 0.42)×10⁻¹¹ cm³ molecule⁻¹ s⁻¹ at 292 K and 90 Torr utilizing the strong absorption band. These results suggest that the Cl-containing SO_x species might act as radical reservoirs in sulfur oxide-rich environments such as Venus' atmosphere.

Refernce:
1.Wen Chao, Gregory H. Jones, Mitchio Okumura, Carl J. Percival, and Frank A. F. Winiberg*, Spectroscopic and Kinetic Studies of the ClSO Radical from Cl₂SO Photolysis, J. Am. Chem. Soc, 2022, 144, 44, 20323-20331. DOI: 10.1021/jacs.2c07912

Although many non-rigid open-shell molecules can be spectroscopically accounted for without a dedicated approach, a subclass of these requires specific theoretical treatments. Such is the case of the CH₂ and CH₂OH radicals and of the Na₃ trimer for which new approaches were setup to model a large amplitude motion together with the electron spin-rotation coupling.\footnote{Ohashi, Tsuura, Hougen, Ernst, and Rakowsky, {\em J.\ Mol.\ Spec.}~{\bf 184} (1997) 22; Coudert, {\em J.\ Chem.\ Phys.}~{\bf 153} (2020) 144115; and Coudert, Chitarra, Spaniol, Loison, Martin-Drumel, and Pirali, {\em J.\ Chem.\ Phys.}~{\bf 156} (2022) 244301} The acetyl radical CH₃CO is a benchmark molecule for this subclass of molecules as it presents a low barrier torsional motion and fine interaction. This radical was first studied some time ago\footnote{Hirota, Mizoguchi, Ohsima, Katoh, Sumiyoshi, and Endo, {\em Mol.\ Phys.}~{\bf 105} (2007) 455} and a new theoretical model was developed to analyze its spectroscopic data. Unfortunately, only a few low-$J$ transitions could be measured then. An analysis of the new submillimeter wave transitions recorded at the ISMO and PhLAM laboratories for the CH₃CO radical will be presented. The theoretical approach developed parallels that of Hirota {\em et al.}$^c$ It relies on an effective torsion-rotation Hamiltonian\footnote{Nakagawa, Tsunekawa, and Hojima, {\em J.\ Mol.\ Spec.}~{\bf 126} (1987) 329} to which the spin-rotation and hyperfine couplings\footnote{Brown and Sears, {\em J.\ Mol.\ Spec.}~{\bf 75} (1979) 111; and Coudert, Gutl\'e, Huet, Grabow, and Levshakov, {\em J.\ Chem.\ Phys.}~{\bf 143} (2015) 044304} are added. The results of preliminary line frequency analyses will be presented. Values for the torsion-rotation Hamiltonian parameters $A$, $B$, $C$, $D_{ab}$, and $\rho$ were obtained from a data set consisting of 143 rotation-torsion transitions split by spin-rotation and hyperfine couplings. The electron spin-rotation coupling tensor components were also determined.

The simplest symmetric cyclohexadienyl radical produced upon H atom addition to benzene was generated pulse radiolytically and characterized vibrationally using Raman scattering in resonance with its first optical absorption band located at 312 nm. After the initial assignment of Raman bands of the intermediate, we performed corresponding studies on its lower symmetry hydroxyl analog, which we also produced by pulsed irradiation using the addition of OH radical to benzene. To assist in band assignments, parallel experiments were performed with benzene-d6 analogs and theoretical calculations of all intermediates’ molecular geometries and vibrational frequencies using DFT methodology. Resonance Raman spectrum of cyclohexadienyl radical recorded in 500-4800 cm⁻¹consists of 8 bands which can be assigned in terms of strongly enhanced fundamental ν₃ at 1560 cm⁻¹accompanied by two weakly enhanced fundamentals ν₁ and ν₂ at 556 and 1174 cm⁻¹, respectively, and additional weaker overtones and combinations at 1738, 2131, 2744, 3152 and 4715 cm⁻¹. In deuterated analog radical, all of these bands shift to lower wavenumbers. The first three fundamentals shift by 23, 279, and 40 cm⁻¹. Based on these shifts along with DFT computations ν₁- ν₃ fundamental vibrations of cyclohexadienyl radical were assigned to CCC bending, CH bending, and C=C stretching ring modes, or 6a, 9a, and 8a in Wilson notation, respectively. Due to the lower symmetry of the hydroxy-cyclohexadienyl radical, its resonance Raman spectrum exhibits a higher number of bands in the same spectral region. The first eight fundamental bands lay below 1700 cm⁻¹, and the remaining combinations and overtones (6 bands) are above. The strongest fundamental ν₇ at 1560 cm⁻¹was assigned to 8a mode (C=C stretching). Weaker fundamentals located at 554, 961, 1176, and 1420 cm⁻¹were assigned to ring modes 6a, 18a, 9a, and 19a, respectively. Moving the laser excitation wavelength around the absorption peak position allowed us to record the excitation profile of recorded vibrations and observe the change of relative intensity pattern in the observed fundamental bands. Based on this observation, we could discuss the nature of the overlapping excited states contributing to the absorption envelope in these types of intermediates.

Iron-bearing molecules have eluded radio astronomers for decades, in part due to the dearth of laboratory spectroscopic data. To this day, there has been only one definitive detection of an iron-bearing molecule, FeCN. One molecule of much interest is FeH. The rotational spectrum of this radical thus far has only been measured with Laser Magnetic Resonance (LMR), which utilizes the Zeeman effect. Therefore, LMR requires extrapolation to zero field frequencies, which can introduce uncertainties. Here we present the first measurements of the rotational spectrum of FeH and its deuterated analog FeD in their ground electronic state using direct absorption methods in the THz region. The molecule was created in an AC discharge in a mixture of H₂, argon, and Fe(CO)₅, with pressures of 30 milli-torr and 3 milli-torr, respectively. Thus far, five transitions of FeH in the Ω = 1/2, 3/2, and 7/2 ladders (0.7 - 1 THz) and eight transitions of FeD in the Ω = 3/2, 5/2, and 7/2 ladders (0.5 - 1 THz) were recorded. Additional measurements are underway.