I present new methodology for the spectroscopic study at very low temperatures of non-rigid organic moleculesdetected in star formation regions of the interstellar medium. As examples of these studies, recent relevant results are shown. The last works on ethylen glycol (EGLY)and on the C₃ O₆ H₆  isomers are detailed. The firts species, ethylen glycol, was astrophysically identified in 2002. Many medium-sized organic molecules present non-rigidity. Torsional motions and, in general, large amplitude motions, intertransform the different conformers that can stabilize by the formation of intramolecular hydrogen bonds. In the low energy regions of the potential energy surface, these hydrogen bonds can determine the symmetry and they can play important roles on the structure and on the low vibrational energy levels. A variational procedure of reduced dimensionality based on CCSD(T)-F12 calculations is applied to understand the far infrared spectrum of cis-gauche-EGLY [1] where three interacting internal rotations intertransform all the minima. The anisotropy of the surface in the gauche region converts the assignment and classification of the torsional levels into a tricky puzzler. The effects of the isotopic substitutions on the spectra are also analysed. Preliminary results for the C₃ O₆ H₆  species obtained using the new the procedure, are shown.

Boric acid B(OH)₃, as a boron containing molecule, is of interest due to being essential micronutrient for a plant growth. In addition, it is a molecule containing three equivalent internal tops which are hydroxyl groups. As a rigid object, this molecule belongs to C_3H point group, but due to internal rotation B(OH)₃ is actually a non-rigid molecule. Therefore, belongs to D_3H(M) molecular symmetry group. High symmetry of this molecule lets us reduce the volume in 3D(γ₁, γ₂, γ₃) phase space (here γ_i is a torsional coordinate for i-hydroxyl group) in which potential energy and kinematic coefficients have been calculated. Then the entire 3D(γ₁, γ₂, γ₃) phase space was filled by using symmetry operations. All calculations were performed at the MP2/cc-pVQZ level of theory. The calculated 2D PES projections on γ₂, γ₃ plane of the boric acid molecule are shown in Fig. 1. One can observe that for the γ₁=0^° global minimum located near γ₂ = γ₃ = 0^° while for γ₁=180^° global minimum appears near γ₂ = γ₃ = 180^°. The values of the energies of the stationary torsional states were calculated too. Fig. 1 Calculated at MP2/cc-pVQZ level of theory 2D PES for γ₁=0^° on the left and γ₁=180^° on the right.

HOSOH belongs to the interesting group of molecules which is specified by the HXYXH formula, where Y and X are allowed to be identical. Recently we analyzed symmetry properties and the torsional spectra of the HOCH₂OH [1] and HOOOH [2] molecules that belong to that group. Calculation technics of the data preparation, numerical solution of the vibrational Schrödinger equation, using DVR and Fourier methods in the case of very small splitting of the lowest torsional states due to tunneling, was discuss [2]. It was found that tunneling frequencies in the ground state of the HOCH₂OH and HOOOH: of the order of 10⁻⁶ cm⁻¹ and 10⁻¹⁰ cm⁻¹, respectively. As in the case of hydrogen peroxide molecule, it is very interesting how substitution of the oxygen atoms for the sulfur ones in the HOOOH molecule effects the height of the potential barriers and the values of the splitting of the torsional states. For example, possibility to detect the parity violation effect in HSSSH molecule [3]. 2D PES of the HOSOH molecule was calculated at the MP2/cc-pVQZ level of theory. The energies of the stationary torsional states were found by numerical solution of the vibrational Schrödinger equation. It was found that the tunneling frequency in the ground state of the HOSOH molecule is order of 10⁻¹¹ cm⁻¹. Differences in the potential barriers of the HOCH₂OH, HOOOH and HOSOH molecules were discussed. [1] G.A. Pitsevich, A.Ye. Malevich, V.V. Sapeshko, J.Mol.Spectr., 360 (2019) 31-38 [2] G.A. Pitsevich, A.E. Malevich, U.U. Sapeshka, Chem.Phys., 530 (2020) 110633. [3] C. Fabri, L. Horny, M. Quiack, ChemPhysChem 16 (2015) 3584-3589

We presentThis work was done under support of the Volkswagen foundation. The assistance of Science and Technology Center in Ukraine is acknowledged (STCU partner project P686a).he results of a joint analysis of the MIR, FIR, and microwave spectra of the ν₈ vibrational state (C-S stretch) near 710 cm⁻¹, the ν₇ vibrational state (CSH bend) near 802 cm⁻¹, and torsional stack of levels up to fourth excited torsinal state of methyl mercaptan CH₃SH. The analysis employs a new program which was recently developed for fitting several isolated small-amplitude fundamentals embedded in a pure torsional bath in molecules like methyl mercaptan, in which the frame has C_s symmetry and the methyl top has C_3v symmetry. This is the first attempt to perform an analysis of two small amplitude vibrations interacting with torsional bath of states and each other in a molecule with torsional large amplitude motion with this new program. The analysis gave us an opportunity to assign for the first time the pure rotational (microwave) transitions of the ν₇ vibrational state (CSH bend). In our analysis we used data available in the literature [1,2,3] as well as the results of the new measurements from Kharkiv, Köln, and Braunschweig. In the talk the details of this new study will be discussed. [1] L.-H. Xu, R. M. Lees, G. T. Crabbe, et al., J. Chem. Phys. 137, 104313 (2012). [2] R.M. Lees, Li-Hong Xu, B.E. Billinghurst, J. Mol. Spectrosc. 352, 30-38 (2016). [3] R.M. Lees, Li-Hong Xu, B.E. Billinghurst, J. Mol. Spectrosc. 319, 45-56 (2018).

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This work was done under support of the Volkswagen foundation. The assistance of Science and Technology Center in Ukraine is acknowledged (STCU partner project P686a).t

Doubly deuterated methanol CD₂HOH is a non-rigid molecule displaying internal rotation of its asymmetrical CD₂H methyl group. Like the isotopic species of methanol with a symmetrical CH₃ or CD₃ group, it displays a strong rotation-torsion Coriolis coupling. Unlike these species, it also displays a dependence of its generalized inertia tensor on the angle of internal rotation. Its complicated rotation-torsion spectrum was investigated in the microwave,Liu & Quade, J. Mol. Spectrosc. 146 (1991) 252; Su, Liu, & Quade, J. Mol. Spectrosc. 149 (1991) 557; Quade, Liu, Mukhopadhyay, & Su, J. Mol. Spectrosc. 192 (1998) 378; Su & Quade, J. Chem. Phys. 90 (1989) 1396ubmillimeter-wave,^b,c terahertz,^b and FIR^b,c domains. Although more than 3000 transitions have been assigned so far, no global analysis, like the one performed for the similar species CH₂DOH,^d has been carried out. In this talk, new assignments in the terahertz and FIR spectra of CD₂HOH will be reported. Parallel and perpendicular transitions, characterized by a higher K-value than in our previous investigation,^b could be identified up to J=35 for the three lowest lying torsional states. Using the theoretical approach developed for CH₂DOH,^d a global analysis of the available high-resolution data has been attempted in order to check the new assignments and to retrieve spectroscopic parameters such as those describing the hindering potential and the generalized inertia tensor.^e So far, this global analysis has been restricted to transitions with K ≥ 4 as rotation-torsion levels with K < 4 are affected by strong rotation-torsion couplings and cannot be properly modeled. Hopefully this issue will be dealt with by the time of the Symposium.

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Liu & Quade, J. Mol. Spectrosc. 146 (1991) 252; Su, Liu, & Quade, J. Mol. Spectrosc. 149 (1991) 557; Quade, Liu, Mukhopadhyay, & Su, J. Mol. Spectrosc. 192 (1998) 378; Su & Quade, J. Chem. Phys. 90 (1989) 1396s

We study large amplitude motions of methyl group in 9-methylanthracene (9MA) by high-resolution spectroscopy. 9MA molecules are delivered in a supersonic beam and are excited by a single mode Ti:Sapphire laser. The frequency of the laser is controlled with reference to an Er doped fiber optical frequency comb. The figure is the observed rovibronic spectra of S₁(0a₁′) ← S₀(0a₁′) and S₁(1e") ← S₀(1e") transitions. Although the two transitions are overlapped, the rotational lines are well resolved. Now we try to assign the rotational lines and to analyze interactions in the vibronic excited states. r0pt Figure

Quasi-linear molecules display a large amplitude bending mode allowing them to sample their linear configuration. This leads to a strong coupling between the overall rotation and the bending mode and to a singularity in their Hamiltonian. Quasi-linearity has been extensively studied in bent light triatomic molecules like water molecule and the amidogen (NH₂) and methylene (CH₂) radicals as they display a low barrier to linearity ranging from 12 000 cm⁻¹ for NH₂ to less than 2000 cm⁻¹ for CH₂.^a Their rovibrational energy levels can be computed with almost spectroscopic accuracy using variational approaches or, if a higher accuracy is needed, with reduced dimensionality models such as the effective Bending-Rotation approach,^b already applied to treat the anomalous centrifugal distortion of the water molecule^c and the amidogen radical.^d In this talk, the Bending-Rotation approach^b will be tentatively applied to the fitting of high-resolution data pertaining to the vibronic ground state of the methylene radical. The data from previous experimental investigations will be considered in the analysis. Due to the very low barrier to linearity^a of CH₂, its rovibrational energy levels are somewhere in between those of a bent molecule and those of a linear molecule leading to anomalous centrifugal distortion.^e Its rotational energy cannot be computed using the standard approaches developed for semi-rigid molecules. The present analysis will, therefore, provide us with a test for the Bending-Rotation approach.^b In the talk, we will try to see how well the anomalous centrifugal distortion is taken into account and we will try to retrieve the dependence of the spin-rotation coupling on the bending angle.

The molecular jet Fourier-transform microwave spectrum of 4,5-dimethylthiazole has been recorded between 2.0 and 26.5 GHz, revealing torsional splittings arising from two inequivalent methyl internal rotations with relatively low hindering barriers and nitrogen quadrupole hyperfine structures. Two global fits of 97 rotational transitions with 315 torsional and 1009 hyperfine components involving 5 torsional species were performed using the program XIAM H. Hartwig, H. Dreizler, Z. Naturforsch. 51a, 923–932, 1996.nd BELGI-C_s-2Tops-hyperfine, an extended version of the BELGI-C_s-2Tops code M. Tudorie, I. Kleiner, J.T. Hougen, S. Melandri, L.W. Sutikdja, W. Stahl,J. Mol. Spectrosc.269, 211-225, 2011.hich includes the effect of the ¹⁴N quadrupole coupling, giving a root-mean-square deviation of 399.8 kHz and 4.2 kHz, respectively. Compared to the monomethyl substituted thiazole derivatives, the barriers to internal rotation are drastically lower. This is also in contrast to chemical intuition which suggests high barriers due to steric hindrance. Because of the strong interaction between the methyl groups, strong top-top couplings in both the potential energy and kinetic parts of the Hamiltonian were observed.

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H. Hartwig, H. Dreizler, Z. Naturforsch. 51a, 923–932, 1996.a M. Tudorie, I. Kleiner, J.T. Hougen, S. Melandri, L.W. Sutikdja, W. Stahl,J. Mol. Spectrosc.269, 211-225, 2011.w