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 also 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 identified, 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. Phase, indeed, provides pitfalls here and elsewhere. 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 molecular planarity of molecules that contain an alkyl group attached to a system with conjugated double bonds has posed a great challenge for both experiments and theory for a long time. This also holds true for the case of 2-propionylthiophene [1] where a propionyl group is attached at the second position of the planar, aromatic thiophene ring. Quantum chemical calculations performed at different levels of theory yield contradicting results on whether the ethyl group of the propionyl moiety is lying in-plane with the thiophene ring or slightly tilted out-of-plane.
In the microwave spectrum both syn-2PT and anti-2PT were assigned. For the energetically more favourable syn-conformer it was possible to observe several ¹³C-isotopologues. This allowed the determination of a partial experimental heavy atom skeleton structure which revealed a tiny, but non-zero tilt angle of the ethyl group out of the thiophene plane, thereby convincingly confirming the non-planarity of 2-propionylthiophene. The results were further supported by inertial defects of both conformers calculated from the experimental rotational constants.
Aditionally, splittings arising from the internal rotation of the terminal methyl group were analysed, yielding torsional barriers of 806.94(54) cm⁻¹ and 864.5(88) cm⁻¹ for the two observed conformers, respectively.
[1] C. Dindić, W. Stahl, H.V.L. Nguyen, Phys. Chem. Chem. Phys., 2020, 22, 19704-19712.

Methyl tert-butyl ether is a gasoline additive and a water pollutant. Its rotational spectrum was measured from 26.5 - 40 GHz using Chirped-Pulse Fourier Transform Microwave Spectroscopy. Measurements were conducted at low temperature via supersonic expansion with two different backing pressures as well as room temperature via static cell. The two different backing pressures provided spectra at  ∼ 30 K and  ∼ 40 K which allowed meaningful probing of the first excited vibrational state which lies below the first excited torsional state. The room temperature spectrum is highly congested due to low lying vibrational states as well as the torsionally excited states. The molecule was previous reported in Suenram et. al 1997 in a range of 9 - 18.6 GHz which fit 32 lines up to J=3. This work expands that fit to over 400 transitions and converts it to the Rho Axis Method utilizing the program RAM36. The improved ground torsional state measurements as well as the room temperature data allowed for tentative assignments of torsionally excited and vibrationally excited transitions.

Natural essential oils have nowadays a vast range of applications in different areas, from medicine and pharmacology, to food industry and cosmetics. They are composed of several, structurally often similar constituents. Due to the high relevance of essential oils in our everyday life it is of interest to obtain detailed structural and conformational information on their main constituents, to widen our knowledge of their unique properties. Rotational spectroscopy is a powerful technique to address these important questions in an accurate and precise manner. Here we present a comprehensive conformational analysis of menthyl acetate, one of the constituents of peppermint oil (Mentha x piperita). Two conformers of menthyl acetate were observed in the broadband rotational spectrum in the 2-12 GHz frequency range. The structure of the lowest energy conformer was obtained experimentally, based on the assignment of the singly-substituted ¹³C-isotopologues. Additionally, the internal rotation of the methyl top of the acetyl side group was characterized. The experimentally determined low barrier height is in line with previous rotational spectroscopy results for other acetates, for which barrier heights on the order of 1 kJ/mol were reported. A comparison to the other acetates will be discussed in the scope of this talk.

We present the microwave spectra of the fourth molecule containing the four-fold rotor -SF₅, phenylsulfur pentafluoride, c-C₆H₅-SF₄-F (PhSPF). The first three molecules in this series were vinylsulfur pentafluoride (VSPF), propen-1-ylsulfur pentafluoride (PSPF) and buten-1-ylsulfur pentafluoride (BSPF). VSPF exhibited splitting into the A, E, and B torsional states with 10's of MHz between the torsional transitions. PSPF exhibited the torsional splitting with 10's of kHz between transitions. BSPF exhibited no torsional splitting. Likewise, PhSPF shows no torsional splitting in the spectra. This phenomenon is mostly explained by the differences in the values of the four-fold barrier to internal rotation, V₄, in this series of molecules.

The title molecule 2,6-dimethylanisole (26DMA) is one of the six isomers of dimethylanisole systematically studied by microwave spectroscopy. The spectrum of 26DMA was recorded using a pulsed molecular jet Fourier transform spectrometer. The experimental part was supported by quantum chemical calculations carried out at the B3LYP/6-311++G(d,p) level of theory. As calculated and experimentally proven for the three mono-methylanisoles (o-,L. Ferres, H. Mouhib, W. Stahl, and H. V. L. Nguyen, ChemPhysChem 18, 1855-1859, (2017).m-,L. Ferres, W. Stahl, H. V. L. Nguyen, J. Chem. Phys. 148, 124304, (2018).nd p-methylanisoleL. Ferres, W. Stahl, I. Kleiner, and H. V. L. Nguyen, J. Mol. Spectrosc. 343, 44-49, (2018). and three dimethylanisoles (2,3-DMA,L. Ferres, K-N. Truong, W. Stahl, H. V. L. Nguyen, ChemPhysChem 19, 1781-1788, (2018).,4-DMA,L. Ferres, J. Cheung, W. Stahl, H. V. L. Nguyen, J. Phys. Chem. A 123, 3497-3503, (2019).nd 2,4-DMAL. Ferres, W. Stahl, H. V. L. Nguyen, J. Chem. Phys. 151, 104310, (2019)., the barrier to internal rotation of the methoxy methyl rotor surpasses 1000 cm⁻¹, causing unresolvable torsional splittings in the microwave spectrum. With both ortho positions substituted by a methyl group in 26DMA, the methoxy part is highly sterically hindered. It is thus forced to tilt out of the plane spanned by the heavy atoms of the phenyl ring by an angle of 90^°. Many experimental studies have shown that sterical hindrance often increases the barrier to internal rotation. Surprisingly, in the case of 26DMA, the torsional barrier decreases dramatically to about 460 cm⁻¹, leading to observable fine splittings in the microwave spectrum. Thus, 26DMA represents a three-top molecule, featuring two equivalent aryl methyl rotors and one methoxy methyl rotor.

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Footnotes:

L. Ferres, H. Mouhib, W. Stahl, and H. V. L. Nguyen, ChemPhysChem 18, 1855-1859, (2017). L. Ferres, W. Stahl, H. V. L. Nguyen, J. Chem. Phys. 148, 124304, (2018).a L. Ferres, W. Stahl, I. Kleiner, and H. V. L. Nguyen, J. Mol. Spectrosc. 343, 44-49, (2018).) L. Ferres, K-N. Truong, W. Stahl, H. V. L. Nguyen, ChemPhysChem 19, 1781-1788, (2018).3 L. Ferres, J. Cheung, W. Stahl, H. V. L. Nguyen, J. Phys. Chem. A 123, 3497-3503, (2019).a L. Ferres, W. Stahl, H. V. L. Nguyen, J. Chem. Phys. 151, 104310, (2019).)

The Fourier transform microwave spectrum of 4-methylacetophenone recorded from 8 GHz to 18 GHz under jet-cooled conditions has revealed large tunneling splittings arising from a low barrier to internal rotation of the ring methyl group and small splittings from a high torsional barrier of the acetyl methyl group. The large splittings are especially challenging to model, while the small splittings are difficult to analyze due to the resolution limit of 120 kHz. The combination of two methyl groups undergoing internal rotations caused each rotational transition to split into five torsional species, which were resolved and fitted using a modified version of the XIAM code and the newly developed ntop code to a root-mean-square deviation close to measurement accuracy, providing an estimate of the V₃ potential barriers of about 22 cm⁻¹ and 584–588 cm⁻¹  for the ring and the acetyl methyl groups, respectively. The assignment was aided by separately fitting the five torsional species using odd-power order operators. Only one conformer in which all heavy atoms are located on a symmetry plane could be identified in the spectrum, in agreement with results from conformation analysis using quantum chemical calculations.

The microwave spectra of two dimethylfluorobenzene isomers, 2,6- and 3,4-, were measured using two pulsed molecular jet Fourier transform microwave spectrometers operating in the frequency range from 2.0 to 40.0 GHz with the goal of quantifying the steric effects on the barriers to internal rotation of the two methyl groups. Due to the internal rotations of two equivalent methyl groups in 2,6-dimethylfluorobenzeneS. Khemissi, H.V.L. Nguyen, ChemPhysChem 2020, 21, 1682-1687.26DMFB), all rotational transitions split into quartets, while quintets appear for 3,4-dimethylfluorobenzeneJ. Mélan, S. Khemissi, H.V.L. Nguyen, Spectro. Chem. Acta A 2021, 253, 119564.34DMFB) because its two methyl groups are inequivalent. The splittings were analyzed and modeled to deduce a torsional barrier of 236.7922(21) cm⁻¹ for the two methyl groups in 26DMFB and of 456.20(13) cm⁻¹ and 489.78(15) cm⁻¹ for the methyl groups at the meta and para positions, respectively, in 34DMFB. Each torsional species was fitted separately using odd power order parameters to check the correctness of the assignment. For both isomers, a global fit obtained with the program XIAMH. Hartwig, H. Dreizler, Z. Naturforsch. 1996, 51a, 923-932. has achieved a standard deviation close to the measurement accuracy. The experimental results are compared with the predicted values obtained by quantum chemical calculations and those of other toluene derivatives. S. Khemissi, H.V.L. Nguyen, ChemPhysChem 2020, 21, 1682-1687.( J. Mélan, S. Khemissi, H.V.L. Nguyen, Spectro. Chem. Acta A 2021, 253, 119564.( H. Hartwig, H. Dreizler, Z. Naturforsch. 1996, 51a, 923-932.

The rotational spectrum of methyl pivalate (t-BuC(O)OCH₃) in the ground vibrational state was observed by molecular beam-Fourier transform microwave spectroscopy. Observed spectral lines for normal species as well as five ¹³C-isotopomers were mainly assigned to the b-type rotational transitions. Some high-K_a lines were found to be split and we have interpreted these splittings in terms of the internal rotation of the methyl group. Some forbidden transitions were also observed for normal species in case where K_a = 2 levels were involved in the internal rotation with E state. The analysis of the observed spectra was carried out by using the XIAM program and thus determined potential barrier V₃ to CH₃ internal rotation was 5.1 kJ mol⁻¹. Since gas electron diffraction study for this molecule shows the large-amplitude motion of the t-Bu group, we are observing the further spectral splittings from the rotational spectra. We are also trying to observe the ¹⁸O-isotopomers.

Silicon chemistry is an extensive research topic due to its importance for industry and technology, and because of its natural abundance on earth. Silicon is a versatile element, its applications range from pure silicon to compounds. To understand the physical and chemical properties of silicon-containing molecules it is essential to characterize the gas-phase structures. The rotational spectrum and structural analysis of sec-butoxytrimethylsilane (sBT-Si) will be presented. The spectrum of sBT-Si has been recorded using the Hamburg COMPACT spectrometer, which is a chirped-pulse Fourier transform microwave spectrometer, in the 2 – 8 GHz frequency range. Quantum-chemical calculations have been carried out to study the conformational flexibility of sBT-Si, and the measured rotational spectrum was examined for the lowest energy conformers. The spectrum is complicated due to the fact that three of the five methyl groups in sBT-Si have a rotational barrier lower than 7 kJ/mol, leading to internal rotation splitting. Despite this, the lowest energy conformer is prominent in the spectrum and has been successfully assigned. Weak signals of the conformer second lowest in energy have also been assigned. In this talk, the conformational flexibility and internal motion of this molecule will be discussed and compared to related molecules.