Rotational transitions belonging to the three lowest torsional states of dimethylether (DME), (CH₃)₂O, have been analyzed using the recently developed model for molecules with two equivalent methyl rotors and C_2v symmetry at equilibrium (PAM_C2v_2tops program) [1]. Significant progress in fitting transitions within the first and second excited torsional states of DME has been achieved and the comprehensive dataset has been reproduced within experimental accuracy by our fit. The dataset comprises not only assignments from the literature and previous measurement campaigns (ground state [2], torsional excited states [3,4]), but also new measurements. These new measurements have been carried out using the Kharkiv spectrometer in the Institute of Radio Astronomy of NASU (Ukraine) from 49 GHz to 180 GHz and from 255 GHz to 400 GHz. Details of the experimental dataset, its analysis and the fit will be discussedThis work was done under support of the Volkswagen foundation. The assistance of Science and Technology Center in Ukraine is acknowledged (STCU partner project P686a). [1] V.V. Ilyushin, J.T. Hougen, J. Mol. Spectrosc. 289, pp.41-49, 2013. [2] C. P. Endres, B. J. Drouin, J. C. Pearson et al. A&A 504, 635–640 (2009). [3] C.P. Endres, H.S.P. Müller, F. Lewen, et al., 65th ISMS, 2010 abstract id. FC01. [4] S.E. Bisshop, P. Schilke, F. Wyrowski, et al., A&A 552, A122, 19 (2013).

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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)..

The known interstellar molecule, methyl formate (HCOOCH₃) has been studied by microwave spectroscopy since 1959 R. F. Curl, J. Chem. Phys. 30, 1529 (1959). The laboratory spectra of methyl formate exhibit many unassigned transitions that are due to rotational transitions in the low-lying vibrational excited states. The first torsional excited state was reported in 2003 H. Odashima, K. Ogata, K. Takagi, & S. Tsunekawa, Molecules 8, 139 (2003)., and up to the second torsional excited state was identified in laboratory and in space. S. Takano, Y. Sakai, S. Kakimoto, M. Sasaki, & K. Kobayashi, Publ. Astron. Soc. Jpn. 64, 89 (2012). K. Kobayashi, K. Takamura, Y. Sakai, S. Tsunekawa, H. Odashima, & N. Ohashi, Astrophys. J. Suppl. Ser. 205, 9 (2013) and references therein. Our laboratory microwave spectra include a series of unknown K_a=0,1 transitions. Based on relative intensities, we estimate that these transitions arise from an excited state about 300 cm^-1 above the ground state. To facilitate identification, we have taken spectra of methyl formate in the 260−370 cm^-1 range at Far−Infrared Beamline of the Canadian Light Source synchrotron. The resolution was instrument−limited to 0.00096 cm^-1. The spectra were very dense, and included the overlapping _12 (C−O−C deformation) and _17 (C−O torsion) modes. Conventional assignment proved very difficult, so we calculated a−type K_a = 0, and 1 rotation−vibration spectra based on the unknown and ground−state microwave data, and gave them arbitrary origins near 300 cm^-1. By correlating them with the far−infrared spectrum, we were able to determine that the new series correspond to _12 near 312 cm^-1.K. Kobayashi, Y. Sakai, M. Fujitake, D. W. Tokaryk, B. E. Billinghurst & N. Ohashi, Can. J. Phys. 98, 551 (2020). We will report on our assignment technique and results, as well as any further developments we have made in extending the assignments of both the microwave and far-infrared spectra.

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R. F. Curl, J. Chem. Phys. 30, 1529 (1959).. H. Odashima, K. Ogata, K. Takagi, & S. Tsunekawa, Molecules 8, 139 (2003).\end S. Takano, Y. Sakai, S. Kakimoto, M. Sasaki, & K. Kobayashi, Publ. Astron. Soc. Jpn. 64, 89 (2012). K. Kobayashi, K. Takamura, Y. Sakai, S. Tsunekawa, H. Odashima, & N. Ohashi, Astrophys. J. Suppl. Ser. 205, 9 (2013) and references therein.\end K. Kobayashi, Y. Sakai, M. Fujitake, D. W. Tokaryk, B. E. Billinghurst & N. Ohashi, Can. J. Phys. 98, 551 (2020).

We present the first study of the millimeter and submillimeter wave spectra of the third and fourth excited torsional states of acetic acid (CH₃COOH). New measurements have been carried out in the 149-183 GHz and 255-420 GHz ranges using the millimeter wave spectrometer in the Institute of Radio Astronomy of NASU (Ukraine). Already published data [1] were combined with the new measurements and fitted using the rho-axis-method torsion–rotation Hamiltonian and RAM36 code [2]. The current fit uses 109 parameters to give an overall weighted root-mean-square deviation of 2.1 for the dataset consisting of 34799 transitions (which due to blending correspond to 21773 measured line frequencies), among which 2663 and 1243 transitions correspond to the third and fourth excited torsional states of CH₃COOH, respectively. The new dataset also contains numerous intertorsional transitions (2407) including those involving energy levels from the third and fourth torsional excited states that are observed due to intensity borrowing via avoided crossing interactions that provide a direct measure of torsional energy spacings associated with torsional potential barrier. In the talk the details of the experimental dataset and analysis will be givenThis work was done under support of the Volkswagen foundation. The assistance of Science and Technology Center in Ukraine is acknowledged (STCU partner project P756). [1] V. V. Ilyushin, C. P. Endres, F. Lewen, S. Schlemmer, B. J. Drouin J. Mol. Spectrosc. 290, pp. 31 - 41, 2013. [2] V. Ilyushin, Z. Kisiel, L. Pszczółkowski, H. Mäder, J. T. Hougen, J. Mol. Spectrosc. 259, pp. 26-38, 2010.

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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 P756)..

Pyruvic acid (C₃H₄O₃, or HOOCOCCH₃) is one of the main reactants in the Krebs Cycle of biological systems where sugar is metabolized and glucose is converted to lactic acid. Its presence in carbonaceous meteorites has already been confirmed, and its abiotic formation in experimental ice analogs implies the potential for exogenous delivery to planets via comets and asteroids. Therefore, it is important to have a complete experimental spectrum within the same wavelengths of astronomical telescopes to facilitate detections of pyruvic acid. Previous work on pyruvic acid reported the spectrum from 160 GHz to 314 GHz. At room temperature, these measurements only cover part of the Boltzmann peak. Additionally, with the spectral coverage now available at telescopes ranging up to and beyond 1 THz, measurements covering a significantly broader region of the molecular spectrum are required. Newly-developed experimental capabilities and sensitivities have since allowed for more sensitive measurement of the rotational transitions in this same wavelength regime and beyond. This work remeasures and extends the spectrum of pyruvic acid from 90 GHz up to 1 THz. Spectral prediction and fitting was conducted using the ERHAM program based on the previous work. Reanalysis using XIAM was then performed due to key advantages and disadvantages in each program. The results of the spectral study and analysis of pyruvic acid from 90 GHz to 1 THz will be presented.

The monitoring of gas phase mononitrotoluenes is crucial for defence, civil security and environmental interests since they are used as taggant for TNT detection. Rotational spectra of the three isomers of nitrotoluene have been recorded at low and room temperatures using a supersonic jet Fourier Transform microwave (MW) spectrometer and a millimeter-wave frequency multiplier chain, respectively. Supported by quantum chemistry calculations, the spectral analysis of pure rotation lines in the vibrational ground state has allowed to characterise the rotational energy, the hyperfine structure due to the ¹⁴N nucleus and the internal rotation splittings arising from the methyl group. An anisotropic internal rotation of coupled -CH₃ and -NO₂ torsional motions was identified for the ortho isomer by quantum chemistry calculations and discussed from the results of its MW analysis. The study of the internal rotation splittings in the spectra of three NT isomers allowed to characterise the internal rotation potentials of the methyl group and to compare them with other mono-substituted toluene derivatives in order to study the isomeric influence on the internal rotation barrier. Roucou et al., CHEMPHYSCHEM, 21(22), 2523-2538, (2020). ^, Roucou et al., CHEMPHYSCHEM, 19, 1056-1067, (2018).

The XIAM code is one of the most frequently used programs to treat the microwave spectra of molecules with up to three methyl internal rotors. XIAM is user-friendly and fast, but often shows difficulties in dealing with low torsional barriers. An example is the case of m-methylanisole where the methyl group attached at the meta-position of the ring undergoes internal rotation with a barrier height of about 56 cm⁻¹ for the cis conformer and 37 cm⁻¹ for the trans conformer.L. Ferres, W. Stahl, H.V.L. Nguyen, J. Chem. Phys. 148, 124304 (2018).he standard deviation obtained with XIAM is much larger than the measurement accuracy for both conformers. Recently, the code has been modified, and two higher order effective parameters connected to the potential term cos(3α) were implemented, which reduced the standard deviations of the fits to almost measurement accuracy.S. Herbers, H.V.L. Nguyen, J. Mol. Spectrosc. 370, 111289 (2020).he XIAM_mod code also succeeded in modeling the microwave spectra of 3-fluorotoluene, 3,4-dimethylfluorotoluene,K.P.R. Nair, S. Herbers, H.V.L. Nguyen, J.-U. Grabow, Spectro. Chim. Acta A 242, 118709 (2020).nd 4-methylacetophenoneS. Herbers, S.M. Fritz, P. Mishra, H.V.L. Nguyen, T.S. Zwier, J. Chem. Phys. 152, 074301 (2020).ith torsional barriers of 17 cm⁻¹, 32 cm⁻¹, and 22 cm⁻¹, respectively, to measurement accuracy.

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L. Ferres, W. Stahl, H.V.L. Nguyen, J. Chem. Phys. 148, 124304 (2018).T S. Herbers, H.V.L. Nguyen, J. Mol. Spectrosc. 370, 111289 (2020).T K.P.R. Nair, S. Herbers, H.V.L. Nguyen, J.-U. Grabow, Spectro. Chim. Acta A 242, 118709 (2020).a S. Herbers, S.M. Fritz, P. Mishra, H.V.L. Nguyen, T.S. Zwier, J. Chem. Phys. 152, 074301 (2020).w

Methyl n-alkyl ketones form a class of molecules with interesting internal dynamics in the gas-phase. They contain two methyl groups undergoing internal rotations. To explore the link between structure and barrier heights in ketones, investigations on a series of saturated methyl n-alkyl ketones were performed, i.e. pentan-2-oneM. Andresen, I. Kleiner, M. Schwell, W. Stahl, H. V. L. Nguyen, J. Phys. Chem. A 2018, 122, 7071-7078exan-2-oneM. Andresen, I. Kleiner, M. Schwell, W. Stahl, H. V. L. Nguyen, ChemPhysChem 2019, 20, 2063-2073 heptan-2-oneM. Andresen, I. Kleiner, M. Schwell, W. Stahl, H. V. L. Nguyen, J. Phys. Chem. A 2020, 124, 1353-1361.nd octan-2-oneM. Andresen, D. Schöngen, I. Kleiner, M. Schwell, W. Stahl, H. V. L. Nguyen, ChemPhysChem 2020, 21, 2206-2216 The molecular jet Fourier-transform microwave spectrum of octan-2-one was recorded between 2.0 and 40.0 GHz, revealing two conformers, one with C₁ and one with C_s symmetry. The barriers to internal rotation of the acetyl methyl group were determined to be 233.340(28) cm⁻¹ and 185.3490(81) cm⁻¹, respectively, confirming the link between conformation and barrier height already established for other methyl alkyl ketones. The study combined high level ab initio calculations with experimentally dervived rotational and torsional parameters using the XIAMH. Hartwig, H. Dreizler, Z. Naturforsch. 1996, 51a, 923-932nd BELGI-C₁ (or BELGI-C_s)I. Kleiner, J. T. Hougen, J. Chem. Phys. 2003, 119, 5505-5509.odes. Results from the various fits will be presented. Finally comparisons to molecules in the literature and structural aspects of the conformers generally observed in methyl n-alkyl ketones will be reviewed.

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M. Andresen, I. Kleiner, M. Schwell, W. Stahl, H. V. L. Nguyen, J. Phys. Chem. A 2018, 122, 7071-7078h M. Andresen, I. Kleiner, M. Schwell, W. Stahl, H. V. L. Nguyen, ChemPhysChem 2019, 20, 2063-2073, M. Andresen, I. Kleiner, M. Schwell, W. Stahl, H. V. L. Nguyen, J. Phys. Chem. A 2020, 124, 1353-1361.a M. Andresen, D. Schöngen, I. Kleiner, M. Schwell, W. Stahl, H. V. L. Nguyen, ChemPhysChem 2020, 21, 2206-2216. H. Hartwig, H. Dreizler, Z. Naturforsch. 1996, 51a, 923-932a I. Kleiner, J. T. Hougen, J. Chem. Phys. 2003, 119, 5505-5509.c

Quadrupole coupling of an iodine nucleus with its coupling constants in the GHz order has always been a challenge in microwave spectroscopy. Methyl internal rotations with very low torsional barriers also often complicate the spectral analysis and modeling.H.V.L. Nguyen and I. Kleiner, Phys. Sci. Rev. DOI: 10.1515/psr-2020-0037, in press.ere, we present the microwave spectrum of a molecule combining the two effects, 3-iodotoluene, observed using a Balle-Flygare Fourier transform microwave spectrometer. The A torsional state including the iodine hyperfine structure has been successfully assigned and fitted utilizing Pickett's SPFIT/SPCAT.H.M. Pickett, J. Mol. Spectrosc. 148, 371 (1991).s expected, the assignment of E torsional state lines has been proven quite challenging due to the extremely low barrier to internal rotation in presence of the iodine hyperfine patterns. Currently, a tentative E state assignment exists and is checked by a separate fit which includes the odd power parameters D_a and D_b (multiplying the P_a and P_b operators) and their higher order parameters in the Hamiltonian. We believe that this is the first microwave report that investigates a molecular species possessing a very low barrier to internal rotation with an iodine embedded in its molecular framework.

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H.V.L. Nguyen and I. Kleiner, Phys. Sci. Rev. DOI: 10.1515/psr-2020-0037, in press.H H.M. Pickett, J. Mol. Spectrosc. 148, 371 (1991).A

Methacrylamide (CH₂=C(CH₃)-CONH₂) is formed by an allyl non-polar frame and an amide polar frame connected by a rotatable bond. Despite the relatively reduced size of the molecule, the presence of four nearby functional groups causes complex internal dynamics. The underlying conformational space has been explored by quantum mechanical modeling and surveyed with millimeter-wave rotational spectroscopy using a Stark-modulated free-jet absorption spectrometer, in the 59.6-104 GHz frequency range. According to the relative orientation of the two unsaturated bonds (defined by the dihedral angle τ, C=C-C=O), two conformers were observed, namely s-trans and s-cis. In the s-cis form, the methylene and carbonyl groups lie on the same side and the overall symmetry is C_s (τ = 0^°), whereas s-trans-methacrylamide consists of two equivalent non-planar minima (τ = ±151^°), which are enantiomers and are separated by a low energy barrier corresponding to a planar skeletal arrangement (τ = 180^°). From relative intensity measurements, s-cis-methacrylamide is estimated to lie 4(2) kJ mol⁻¹ above s-trans-methacrylamide. The rotational spectra are characterized by a complex hyperfine structure which allowed the determination of the methyl internal rotation barrier and the ¹⁴N nuclear quadrupole coupling constants for both conformers. Moreover, the tunneling spliting related to the double minimum potential of s-trans-methacrylamide was determined. A one-dimensional flexible model of the vibro-rotational interaction suggests that the corresponding interconversion barrier is about 2.4 kJ mol⁻¹ and the first torsional quantum state lies 55 cm⁻¹ above the ground state.

The rotational spectra of three planar secondary amines diethylamineH. V. L. Nguyen and W Stahl, J. Chem. Phys. 135, 024310 (2011) ethylmethylamineK. J. Koziol, W. Stahl, H. V. L. Nguyen, J. Chem. Phys 153, 184308 (2020)nd dimethylamineJ. E. Wollrab and V. W. Laurie, J. Chem. Phys. 48, 5058 (1968)ave been investigated. The hydrogen atom of the amino group performs an inversion tunnelling motion, causing splittings of all c-type transitions into doublets. The respective values of the splittings are 1521.54134(40) MHz^b 1980.9176(62) MHz^c and 2646.0 MHz^d. The first microwave spectra of dimethylamine and ethylmethylamine have been recorded in 1967 and 1973R. E. Penn and J. E. Boggs, J. Mol. Spectrosc. 47, 340 (1973) respectively, using Stark modulated spectrometers with a resolution of 25 kHz. The ¹⁴N quadrupole hyperfine structure was reported to be resolved only for some transitions and the effects of internal rotation have not been mentioned. In the present work, the spectra of N-ethylmethylamine and dimethylamine were remeasured using two molecular jet Fourier transform microwave spectrometers operating from 2 to 40 GHz. The proton tunnelling was analysed, taking into account Coriolis interactions which cause splittings in the order of about 0.2 to 1.2 MHz for all b-type transitions. Furthermore, hyperfine splittings due to the quadrupole coupling of the ¹⁴N nucleus were fully resolved. The quadrupole coupling constants χ_aa and χ_bb − χ_cc were determined with high accuracy and calculated well using Bailey's methodW. C. Bailey, Chem. Phys. 252, 57, (2000) Due to the difference in symmetry between ethylmethylamine and dimethylamine, quartets and triplets with splittings in the order of 150 kHz and 200 kHz, respectively, arising from internal rotations of two methyl groups, could be resolved. We observed that large amplitude motions such as internal rotation and/or proton tunnelling influence(s) the hyperfine patterns. Quantum chemical calculations carried out to support the experiment.

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H. V. L. Nguyen and W Stahl, J. Chem. Phys. 135, 024310 (2011), K. J. Koziol, W. Stahl, H. V. L. Nguyen, J. Chem. Phys 153, 184308 (2020)a J. E. Wollrab and V. W. Laurie, J. Chem. Phys. 48, 5058 (1968)h R. E. Penn and J. E. Boggs, J. Mol. Spectrosc. 47, 340 (1973), W. C. Bailey, Chem. Phys. 252, 57, (2000).

Trifluoroacetic acid (TFA, CF₃C(O)OH) is the final degradation product of many hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), and hydrofluoroolifines (HFOs) in the troposphere. While a recent assessment of the TFA impact on the health of humans and the environment shows that its concentration is too small to be a risk, TFA still warrants continued attention, in part because of its very long environmental lifetime.Solomon, K. R., et al., 2016, J. Toxicol. Env. Health B, 19, 289.he ultimate sink of TFA is in surface waters in which it forms salts that are extremely stable and are likely to have half-lives of centuries. From the spectroscopic point of view, TFA is an interesting case of the internal rotation of a heavy C_3v top. The presence of the CF₃ internal rotor leads to relatively strong coupling between internal and overall rotation, ρ = 0.68. However, owing to the high mass of the top, the tunneling probability is low; the A−E splittings are hardly observable in the rotational spectrum, despite relatively low barrier of about 230 cm⁻¹. In the previous studies, the splittings were observed only in the v_t=4 excited torsional state,Stolwijk, W. M. and van Eijck, B. P., 1985, J. Mol. Spec., 113, 196.nd were not observed in the ground state using molecular beam Fourier transform microwave spectroscopy which typical resolution is few kHz.Antolinez, S., et al., 1999, Z. Naturforsch. A, 54, 524.e present new global analysis of the rotational spectrum of TFA using the rho axis method and RAM36 code. The rotational spectrum of TFA was recorded in the range from 50 to 330 GHz. The joint analysis of the v_t= 0, 1, and 2 states resulted in accurate determination of the V₃ and V₆ potential energy terms, 230.36(92) cm⁻¹ and -4.736(82) cm⁻¹ respectively. However, the potential energy terms are strongly correlated with the intrenal rotation constant F that is kept fixed in the fit. Up to now no A−E splittings were observed in the Doppler-limited resolution spectra. The analysis will be extended to v_t=3 and v_t=4 states in which we expect to observe the splittings that will permit to remove the correlation between V and F terms. The latest results will be presented. The present work was funded by the French ANR Labex CaPPA through the PIA contract ANR-11-LABX-0005-01

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Solomon, K. R., et al., 2016, J. Toxicol. Env. Health B, 19, 289.T Stolwijk, W. M. and van Eijck, B. P., 1985, J. Mol. Spec., 113, 196.a Antolinez, S., et al., 1999, Z. Naturforsch. A, 54, 524.W