One of the current “frontiers” in the spectroscopy of molecules with large amplitude motion is the extension of the analysis of excited vibrational states to move beyond the isolated state model to a more global treatment including several coupled vibrational modes. Before that, of course, one needs to explore the isolated states themselves, and there are still states even for small molecules like methanol, methyl mercaptan and methylamine whose structures have not yet been characterized in detail. In this task, the use of Loomis-Wood plots and Excel spreadsheets has proved to be very helpful, in conjunction with solid bases of ground-state energy term values provided by the pioneering work of the microwave and THz community. The Excel difference table approach relies on systematic trends in the J-dependence of the spectral sub-bands, and can be a sensitive pointer to perturbations arising from level-crossing resonances between interacting modes. The vibrational modes can also exhibit anharmonic resonances with the ground-state torsional manifold as it rises up through the vibrational regions. Isotopic substitution can then serve as a useful tool for tuning the resonances with two degrees of freedom, one along the energy axis by altering the relative energies of the coupled states, and a second along the K-axis by changing the value of the ρ parameter which governs the periodic K-dependence of the torsional energies. Examples will be presented of this “isotopic tuning” for isotopologues of methanol, along with discussion of features of the Excel spreadsheet approach. We will also illustrate how isotopic shifts may give insight into sub-band vibrational assignments from comparison of the torsion-vibration manifolds of the lower modes of normal CH₃OH and its O-18 relative.

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⁻¹and 584–588 cm⁻¹for 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.

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.

1,2-Benzenediol, commonly known as catechol, is emitted directly from biomass burning as well as produced in the atmosphere through the gas-phase reaction of benzene and phenol with OH radicals.Z. Finewax, J. A. de Gouw, P. J. Ziemann, Environ. Sci. Technol. 52, 1981-1989 (2018).atechol displays appreciable gas-phase reactivity and its monitoring in the atmosphere via rovibrational spectroscopy has a strong interest. From a molecular point of view, catechol is interesting because its two vicinal hydroxyl groups can act interchangeably as both hydrogen donors and acceptors in internal hydrogen bonding.W. Caminati, S. Di Bernardo, J. Mol. Struct., 240, 263-274 (1990).e performed a rotationally resolved analysis of the low-frequency out of plane bending modes of the intramolecular H-bond of catechol. Using synchrotron-based FT-Far-IR spectroscopy at the AILES beamline of SOLEILJ.B. Brubach, L. Manceron, M. Rouzieres, O. Pirali, D. Balcon, F. K. Tchana, V. Boudon, M. Tudorie, T. Huet, A. Cuisset, P. Roy, AIP Conf. Proc. 1214, 81–84 (2009).nd high level of theory anharmonic quantum chemistry calculations, we have fully resolved and analyzed the rovibrational structures of the -OH acceptor and -OH donor torsional bands. Numerous hot bands involving the lowest vibrational energy modes are observed and an attempt of assignment is performed. Finally, using a versatile millimeter-wave spectrometer,G. Mouret, M. Guinet, A. Cuisset, L. Croizé, S. Eliet, R. Bocquet, F. Hindle, IEEE Sens. J. 13, 133-138 (2013).he room temperature Doppler limited rotational spectrum of catechol has been measured in the 70-220 GHz frequency range. Pure rotational lines belonging to the ground and the four lowest energy vibrationally excited states have been assigned and a global fit gathering the far-IR and millimeter-wave data provides the rotational and centrifugal distorsion constants of the different far-IR modes involving the intramolecular hydrogen bond.

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Z. Finewax, J. A. de Gouw, P. J. Ziemann, Environ. Sci. Technol. 52, 1981-1989 (2018).C W. Caminati, S. Di Bernardo, J. Mol. Struct., 240, 263-274 (1990).W J.B. Brubach, L. Manceron, M. Rouzieres, O. Pirali, D. Balcon, F. K. Tchana, V. Boudon, M. Tudorie, T. Huet, A. Cuisset, P. Roy, AIP Conf. Proc. 1214, 81–84 (2009).a G. Mouret, M. Guinet, A. Cuisset, L. Croizé, S. Eliet, R. Bocquet, F. Hindle, IEEE Sens. J. 13, 133-138 (2013).t

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 and 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 (20012). 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, Submitted to Can. J. Phys. 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 (20012). 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, Submitted to Can. J. Phys.

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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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).)

Ammonia is an important “weed” molecule in interstellar medium, planetary and exoplanetary atmosphere studies. In last ten years, new experimental IR analysis have been reported in the extended region between 5000 cm⁻¹(or 2 μm) and 10,000 cm⁻¹(or 1 μm). But reliable line list is still missing for the 6000 cm⁻¹(or 1.63 μm) region. Combining the JPL experimental measurements with the line positions predicted on our Ames-Pre3 potential energy surface and the 296K intensity predicted by the UCL-C2018 line list, we have been able to successfully assign more than 1300 transitions in that range. The transitions belong to following bands: ν₂+ν₃+ν₄ (0111), ν₁+ν₂+ν₄ (1101), 3ν₂+ν₃ (0310), ν₁+3ν₂ (1300), 6ν₂ (0600s),and a "hot" band 2ν₂+ν₃+ν₄ (0211) - ν₂ (0100). The combination difference for the determined experimental energy levels are about 1E-3 cm⁻¹, close to the resolution of lab measurements. Our Ames-Pre3 predictions for most J=0-10 transitions are found to be accurate within ±  0.05 cm⁻¹, better than the C2018 line positions. Newly determined band origins and rovibrational levels will be presented along with band-by-band simulations comparing to the observed spectra. More complete Effective Hamiltonian model analysis is the target due for future work.

With recent advances in ultrashort laser technology, many studies using intense nonresonant laser fields have been conducted to control vibrational or rotational wave packets. In particular, control of large-amplitude low-frequency vibration, e.g., torsional motion, is important because such vibration may cause a substantial change in molecular structure. For instance, torsional motion of biphenyl has deserved much attention since chirality and physical property of the molecule depend on its torsional angle. In this study, we coherently excite torsional vibration of substituted biphenyl derivatives by the interaction with ultrashort laser fields and the resultant vibrational excitation is monitored by recording resonant two-photon ionization (R2PI) spectrum. We further adopt double-pulse excitation to control vibrational state distribution via wave-packet interference. Adiabatically cooled molecular sample of 2-fluorobiphenyl is irradiated by the fundamental output from a fs Ti:Sapphire laser. This pump pulse induces vibrational excitation through impulsive Raman process. With an appropriate delay after the pump-pulse irradiation, the S₁−S₀ excitation spectrum of the molecules is recorded via R2PI with the doubled output of a nanosecond dye laser ( ∼ 280 nm). A progression with almost constant spacings appears in the R2PI spectrum without the pump pulse. It has been assigned to that of the torsional mode from vibrational ground state, i.e., v = 0 (v being the quantum number of the torsional mode in the electronic ground state).H. S. Im and E. R. Bernstein, J. Chem. Phys. 88, 7337 (1988).hen the pump pulse is introduced, the intensity of each band is reduced and new bands appear. These bands are assigned to the progression from v = 1. These observations indicate that impulsive Raman excitation of torsional vibration is realized. We also conduct a double-pump pulse experiment, where a pair of pulses are implemented for excitation. In this experiment, we succeeded in controlling the state distribution of torsional vibration by adjusting the time delay between the two pump pulses.

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H. S. Im and E. R. Bernstein, J. Chem. Phys. 88, 7337 (1988).W

r0pt Figure Dimethyl ether is one of the most abundant molecule in star-forming regions. Like other complex organic molecules, its formation process is not yet clearly established. The study of deuteration may provide crucial hints. The mono-deuterated species (CH₂DOCH₃) was studied in 2012. This analysis led to a detection in ISM and results have been published by Richard et al.C. Richard et al., A&A, 552, A117, 2013 The spectra of the doubly deuterated species were recorded in Lille from 150 to 1500 GHz. We used the ERHAM code to treat the torsion of the methyl-group. So far, the analysis of both conformer symmetric and asymmetric are still in progress and we will present the first spectroscopic results, and their ISM search as well. This project has received financial support from the CNRS through the MITI interdisciplinary programs.

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C. Richard et al., A&A, 552, A117, 2013.