All molecular system come with their own set of challenges for rotational spectroscopy, theoretically and experimentally: (Multiple) internal interactions might cause complicated energy level schemes and the resulting spectra will be rather difficult to predict theoretically. Experimentally, these spectra are difficult to assess and assign. With today’s broad-band microwave (MW) techniques, finding and identifying such spectral features have lost their major drawback of being very time consuming for many molecules. The unrivalled resolution of advanced fast-passage spectrometers, previously only available for narrow-banded MW techniques, now allows to tackle - at the highest precision – very subtle effects. Historically, (very) low barriers to large amplitude motions leading to (very) large tunnelling splittings often prevented an experimental assessment or if so, their analysis to experimental accuracy. Barriers to large amplitude motions can not only be related to the local atom arrangement but also to the molecular orbital and electron density structure, e.g. chemical information relayed through conjugated π-systems, of the molecule. Theoretically, such systems require calculations at elevated levels, e.g. CCSD(T)/cc-pcVTZ or beyond, but quantitative predictions of the dynamical features often still fall behind those of equilibrium structures. Experimentally, the analysis might require the measurement of tunneling species (in the cm- and mm-wave regions) beyond the torsional ground state to reveal the origin of the observed spectrum and underlying hindering potential. Furthermore - in the realm of barriers to large amplitude motions - details on internal dynamics and the (stereo-chemical) molecular structure encode their nature in the coherent signature of molecular rotation spectra obtained after single- and/or double-resonance excitation. Current examples and new directions together with an outlook will be given.

The rotational constants and quartic centrifugal distortion coefficients of hexafluoro-o-xylene and all singly 13C isotopologues were precisely determined from the 8 to 18GHz gas phase microwave spectrum. A preliminary r0 structure was determined, reproducing the experimental rotational constants with deviations of no more than 15kHz. Interestingly, rather than the C2v symmetry structure expected intuitively, as in o-xylene, calculations with a variety of methods (B3LYP, CAM-B3LYP, ωB97XD, MP2, and CCSD(T)) predict a C2 symmetry structure in which the two CF3 groups rotate in opposite directions by about 16 degrees. Analysis of the interactions between the two CF3 groups using an effective fragment potential (EFP) approach identified two major contributions to their interaction, due to exchange repulsion and electrostatic repulsion, with electrostatic repulsion responsible for the barrier at the C2v geometry. SH, SMF, PM and TSZ gratefully acknowledge support for this work from the Department of Energy Basic Energy Sciences Gas Phase Chemical Physics program under Grant No. DE-FG02-96ER14656. YK and LVS gratefully acknowledge support from the National Science Foundation (NSF CHE-1800505). Present address for TSZ: Combustion Research Facility, Sandia National Laboratory, Livermore, CA 94550.

The rotationally-resolved vibrational spectra of benzaldehyde have been recorded in the far infrared region at room temperature using the Bruker IFS125 Fourier Transform spectrometer at the Canadian Light Source with a resolution of 0.000959 cm⁻¹. The lowest frequency vibrational bands collected correspond to the fundamental and hotbands of the -CHO torsion at 110 cm⁻¹. The assignment and analysis of the dense spectral features in this region will be detailed. In addition, the newly obtained band origin of the -CHO torsional fundamental will be compared to ab initio results in relation to the discrepancy

2-methyl-2-imidazoline, also known as lysidine, is a non-planar heterocyclic molecule containing two ¹⁴N quadrupolar nuclei and one methyl group. The rotational spectra of this molecule were recorded in the frequency range from 7 to 20 GHz using a pulsed molecular jet Fourier transform microwave spectrometerV. Kabourek, P. Cerný, P. Piksa, T. Studecký, P. Kania, S. Urban, Radioengineering, 22, (2013), 1288–1295.nd in selected frequency regions between 90 and 220 GHz using two different millimetre-wave spectrometersP. Kania, L. Stríteská, M. Simecková, S. Urban, J. Mol. Struct., 1–3, (2006), 209–218.Z. Kisiel, A. Kra\'snicki, J. Mol. Spectrosc., 262, (2010), 82–88.. Lysidine displays a very rich millimetre wave spectrum presumably attributable to the presence of two large amplitude motions in the molecule: an internal rotation of the methyl group and a ring-puckering motion. The decimetre-wave spectra are even more complex as the hyperfine structure arising from the nuclear quadrupole coupling interactions of two ¹⁴N nuclei is also resolved. The first analysis of this challenging spectrum guided by quantum chemical calculations is reported in the present contribution.

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V. Kabourek, P. Cerný, P. Piksa, T. Studecký, P. Kania, S. Urban, Radioengineering, 22, (2013), 1288–1295.a P. Kania, L. Stríteská, M. Simecková, S. Urban, J. Mol. Struct., 1–3, (2006), 209–218.

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This talk describes recent updates to the PGOPHER programC. M. Western, J Quant. Spec. Radiat Trans., 186 221 (2017)http://pgopher.chm.bris.ac.uk), including new tools for computer assisted assignment of spectra and simulate spectra involving internal rotation. The new tools for assignment are described in a recent paperC. M. Western and B. E. Billinghurst, Phys. Chem. Chem. Phys., 21 13986 (2019)nd include (I) a method of trying multiple assignments automatically based on the AUTOFIT algorithm of the group of Brooks PateN. A. Seifert, I. A. Finneran, C. Perez, D. P. Zaleski, J. L. Neill, A. L. Steber, R. D. Suenram, A. Lesarri, S. T. Shipman, B. H. Pate, J Mol. Spectrosc. 312, 13, (2015) and (II) a new form of presenting assignments, a nearest lines plot. These latter plots allow possible sets of assignments to be accepted (or rejected) quickly, and also allow the rapid extension of initial assignments to an entire branch or band. Both these tools have been applied to the analysis of high resolution IR spectra, allowing the rapid assignment of  10,000 lines for a band, even in the presence of strong overlapping transitions. These tools are now being supplemented with tools for handling internal rotation in PGOPHER, including a general way of handling the special permutation inversion symmetry that is typically required for such molecules, and calculating levels affected by internal rotation, either by adding empirical terms to a standard asymmetric top Hamiltonian, or a more elaborate approach based on including multiple torsional states. Progress on the development of these tools will be presented, with applications to spectra taken on the far IR beamline of the Canadian light sourceThe Canadian Light Source, is supported by the Canada Foundation for Innovation, Natural Sciences and Engineering Research Council of Canada, the University of Saskatchewan, the Government of Saskatchewan, Western Economic Diversification Canada, the National Research Council Canada, and the Canadian Institutes of Health Research.

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C. M. Western, J Quant. Spec. Radiat Trans., 186 221 (2017)( C. M. Western and B. E. Billinghurst, Phys. Chem. Chem. Phys., 21 13986 (2019)a N. A. Seifert, I. A. Finneran, C. Perez, D. P. Zaleski, J. L. Neill, A. L. Steber, R. D. Suenram, A. Lesarri, S. T. Shipman, B. H. Pate, J Mol. Spectrosc. 312, 13, (2015), The Canadian Light Source, is supported by the Canada Foundation for Innovation, Natural Sciences and Engineering Research Council of Canada, the University of Saskatchewan, the Government of Saskatchewan, Western Economic Diversification Canada, the National Research Council Canada, and the Canadian Institutes of Health Research..

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

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 almost 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 ro-vibrational predissociation spectrum of He-H₃⁺ has been recorded via excitation of the ν₂ vibrational mode of the H₃⁺ sub-unit in a cold 22-pole ion trap. The spectrum for the bare H₃⁺ consists of only a few ro-vibrational lines each for the para and ortho nuclear spin configuration, respectively. Instead, the spectrum of the complex is very rich (several hundred lines) even at the low temperature (4 K) of the trap experiment. Part of this complexity is associated with the (almost) free internal rotation of H₃⁺. The experimental results are compared to theoretical predictions of ro-vibrational spectra on the basis of ab-initio calculations of the He-H₃⁺ potential energy surface. The energy levels result in transitions which agree in many cases with experimental results within a few wavenumbers. In particular the typical band structures of a P- and R-branch associated with an effective diatomic complex seen in the experimental and predicted spectrum help in assigning the rich spectrum. Moreover, an experimental energy term diagram is reconstructed from the observed transitions which can be compared to the rather accurate theoretical predictions. Despite of the floppiness of the complex rotational constants for the effective diatomic complex can be derived and match to the term diagram of a prolate, slightly asymmetric rotor. The influence of the Coriolis interaction resulting from the H₃⁺ internal rotation in a rotating He-H₃⁺ frame shall be discussed.