A number of internal rotation codes can deal with the combination of one or two internal rotor(s) with one ¹⁴N quadrupole nucleus, but not many treats the internal rotations with two ¹⁴N nuclei. Here we present the extended version of our internal rotor program (BELGI-C_s), called BELGI-2N using the Rho-Axis Method (RAM)J. T. Hougen, I. Kleiner, and M. Godefroid, J. Mol. Spectrosc. 163, 559 (1994)lobal approach to deal with the compounds containing one methyl top and two weakly coupling ¹⁴N nuclei. For molecules containing a ¹⁴N nucleus with a nuclear spin I equal to 1, all rotational transitions of the rigid rotor split into several hyperfine components. The quadrupole moment is relatively small and can be treated using a first order perturbation approximation. To test our new code, we applied it to the microwave data previously recorded for N-, 2-, 4- and 5-methylimidazole, using a chirped-pulse Fourier transform microwave spectroscopy in the 7.0–18.5 GHz frequency rangeE. Gougoula, C. Medcraft, J. Heitkämper, and N. R. Walker, J. Chem. Phys. 151, 144301 (2019) Compared to this study, we were able to perform global fits with root-mean-square deviations within the experimental accuracy and to increase the number of assigned lines with the high predictive power of the fits. J. T. Hougen, I. Kleiner, and M. Godefroid, J. Mol. Spectrosc. 163, 559 (1994)g E. Gougoula, C. Medcraft, J. Heitkämper, and N. R. Walker, J. Chem. Phys. 151, 144301 (2019).

Producing spectroscopic data in the infrared range for a number of VOCs (volatile organic compounds)to be used for detecting and measuring their abundance with remote sensing is highly needed to control air pollution and air quality. Toluene is one of the important VOCs and abundant hydrocarbon in the Earth’s atmosphere and it is a major anthropogenic pollutant emitted by various sources. The ultimate goal of our study is twofold: (i) develop an offspring of the RAM36 code V. V. Ilyushin, Z. Kisiel, L. Pszczólkowski, H. Mäder, J. T. Hougen, J. Mol. Spectrosc. 259, 26 (2010)apable of simultaneous fitting torsion-rotation spectra in several vibrational states of a molecule with C_3v internal rotor and C_2v frame, and (ii) build an infrared database for toluene in the spectral range 600-800 cm⁻¹ for remote sensing purposes. For the second goal new data were recorded using the JET-AILES experiment at the SOLEIL synchrotron around the band ν₃₆ at 729 cm⁻¹. In order to account for possible hot bands in the infrared spectrum we also performed a search for the transitions belonging to the lowest vibration state of toluene ν₃₈ at 204 cm⁻¹. The details of the new code and the first assignments of the torsion-rotation spectrum for the lowest vibrational state of toluene in the microwave range will be presented.We acknowledge support by the Dim Qi2 program.html:<hr /><h3>Footnotes: V. V. Ilyushin, Z. Kisiel, L. Pszczólkowski, H. Mäder, J. T. Hougen, J. Mol. Spectrosc. 259, 26 (2010)c We acknowledge support by the Dim Qi2 program.

r0pt Figure Perfluorophenol (C₆F₅OH) is analogous to hexafluorobenzene (C₆F₆), with one fluorine atom replaced by a hydroxyl group. This substitution has a threefold effect: it provides a permanent dipole moment, the molecule remains very close to the oblate symmetric top limit, and intramolecular hydrogen bonds between the H and adjacent fluorine atoms are formed. Hydrogen can tunnel through the barrier to internal rotation of the OH group, lifting the degeneracy of the torsional states. These factors result in the unusual case of a polar, tunneling, near-symmetric top. To probe these effects, the 1 K rotational spectrum of perfluorophenol between 7.5 and 17.5 GHz has been measured using chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy. The asymmetry parameter (κ) was experimentally determined to be 0.944, in agreement with the near-oblate geometries predicted by MP2/6-311++G(d,p) calculations. Tunneling splitting was observed for both a-type and b-type transitions due to the C-O bond axis lying between the a and b inertial axes; a consequence of the near-symmetry of perfluorophenol. The energy difference between the split 0⁺ and 0⁻ tunnelling levels was established to be 24.850 MHz. Preliminary wB97XD/6-311++G** calculations estimate a barrier to internal rotation (V₂) of approximately 1211 cm⁻¹. The observed energy splitting is much lower than might be expected of this barrier height when compared to other phenolic derivatives, the exact reason for which is still to be understood.

The spectroscopic characterization of explosives taggants, like nitrotoluenes (NT) used for the TNT detection, is a research subject of growing interest. Recently, the spectroscopic studies of the three NT isomers in the microwave and millimeter-wave ranges were reported^1,2. We present the gas-phase rotational spectroscopic study of weakly volatile dinitrotoluenes (DNT) isomers. The pure rotational spectrum of, 2,4-DNT and 2,6-DNT was recorded in microwave range (2-20 GHz) using a Fabry-Perot Fourier-transform microwave (FP-FTMW) technique coupled to a pulsed supersonic jet. The spectral analysis was supported by quantum chemical calculations carried out at the B98/cc-pvtz and MP2/cc-pvtz levels of theory. The spectra of DNT were complicated by the presence of two ¹⁴N nucleus giving rise to congested hyperfine structures. The methyl group internal rotation barriers were calculated at the B98/cc-pvtz level of theory to be V₃=563 cm⁻¹and V₃= 696 cm⁻¹for 2,4- and 2,6-DNT, respectively. Although no splitting due to internal rotation was observed for 2,6-DNT, several splittings were observed for 2,4-DNT and their analysis in under progress. The semi-rigid and the nuclear quadrupole couplings descriptions obtained from the spectral analysis are presented. An anisotropic internal rotation of the coupled -CH₃ and -NO₂ torsional motions, as already mentioned for 2-NT¹, will be discussed for 2,4-DNT. ¹Roucou et al., CHEMPHYSCHEM, 21, 2523-2538, (2020). ²Roucou et al., CHEMPHYSCHEM, 19, 1056-1067, (2018). Acknowledgment: This work received financial support from the French Agence Nationale de la Recherche via funding of the project Millimeter-wave Explosive Taggant vapors Investigations using Spectral taxonomy (METIS) under contract number ANR-20-ASTR-0016-03.

The rotational spectra of two conformers of valine methyl ester (ValOMe) have been measured and assigned using a cavity based Fourier-transform microwave spectrometer in the range of 9-18 GHz as a part of a project investigating the structures of a series of amino acid methyl esters. We modeled 15 possible conformers of ValOMe using the ωB97XD/6-311++G(d,p)) level of theory. 59 rotational transitions assigned to conformer I were fit to Watson's A-reduced Hamiltonian: A = 2552.01(1) MHz, B = 1041.821(2) MHz, and C = 938.549(2) MHz. ¹⁴N nuclear quadrupole hyperfine splittings were resolved, and the 137 hyperfine components were fit to χ_aa = -4.20(2) MHz and χ_bb−χ_cc = 1.26(1) MHz. The spectrum of conformer I also reveals tunneling splittings from the ester methyl rotor. The XIAMH. Hartwig and H. Dreizler, Z. Naturforsch. 51a, (1996) 923.rogram was used to fit the barrier to the internal rotation of the methyl rotor. The best fit V₃ barrier was found to be 387.8(8) cm⁻¹. 20 rotational transitions were assigned for conformer II and the fitted rotational constants are A = 2544.405(9) MHz, B = 1092.337(2) MHz, and C = 896.301(1) MHZ. The transitions were split by nuclear quadrupole coupling and tunneling, and complete assignment of these components is ongoing.

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H. Hartwig and H. Dreizler, Z. Naturforsch. 51a, (1996) 923.p

Acetoin (CH₃COCH(OH)CH₃) is a common additive to e-cigarette fluids. Though not toxic itself, it decomposes into diacetyl (CH₃COCOCH₃), which is known to cause lung damage. Diacetyl may be important in interstellar chemistry because it has been observed as a VUV desorption product from an interstellar ice analog experiment studying acetaldehyde-based ices. We reported on an attempt to study this molecule at this conference in 2021. Given its extremely small dipole moment and multiple methyl rotors, the study of diacetyl is challenging. Acetoin, conversely, has a strong dipole moment of 2.55 D, which allows its spectrum to be easily observed. The microwave spectrum of acetoin has been collected by Gou and coworkers as reported in the Microwave Newsletter. We extended measurements of rotational lines from 70 to 115 GHz and from 140 to 800 GHz. These data were collected using a long-path length direct absorption flow cell spectrometer. The spectral analysis is underway. The results of this spectrum will enable astronomical observations for both acetoin and, by proxy, diacetyl. Here we will report on the millimeter/submillimeter spectrum of acetoin and our progress towards its analysis and comparison to observations.

While recording the cm-wave spectrum of the title compoundG.S. Grubbs, P. Groner, Stewart E. Novick, S.A. Cooke, "Methyl group internal rotation and the choice of Hamiltonian for the rotational spectrum of 1,1-difluoroacetone", Journal of Molecular Spectroscopy, Volume 280, 2012, Pages 21-26e discovered that the Watson A reduction r0pt Figure in the I^r representation resulted in a poorly fitting Hamiltonian to the observed ground state transitions. However, the Watson S reduction in the I^r representation gave satisfactory results as did both reductions in the III^r representation. The prior work used only measurements between 6 GHz and 16 GHz and only quartic centrifugal distortion (CD) constants were needed in the fits. In order to further explore the reduction/representation-dependence of the spectroscopic fits quartic and sextic CD constants have been obtained from quantum chemical calculations. Furthermore higher frequency measurements have been recorded which i) provide greater certainty in the experimental CD constants, and ii) now require up to decadic CD constants in the Hamiltonian. Further insights into the failure of the A-I^r approach will be presented. In the course of performing this work the methyl group barrier to internal rotation has been improved and will also be discussed.

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G.S. Grubbs, P. Groner, Stewart E. Novick, S.A. Cooke, "Methyl group internal rotation and the choice of Hamiltonian for the rotational spectrum of 1,1-difluoroacetone", Journal of Molecular Spectroscopy, Volume 280, 2012, Pages 21-26w

The infrared spectra of cyclopropylamine (c-C₃H₅NH₂) in the region of 35-600 cm⁻¹ have been measured at 298K with a resolution of 0.00096 cm⁻¹ using the far-infrared beamline at the Canadian Light Source synchrotron. We report here the results of the rovibrational analysis of the ν₂₇ (253.87 cm⁻¹) –NH₂ torsional fundamental, as well as the pure rotational analysis of transitions associated with the ground state and the first excited state of the –NH₂ torsional mode between 35 and 60 cm⁻¹. The ongoing assignment and analysis of hot bands and overtones involving higher torsional states will also be discussed.

We present a line position analysis of a large body of data pertaining to H₂¹⁸O and involving all 5 vibrational states up to the First Triad, namely, the lowest lying states (000), (010), (020), (100), and (001). The data set contains infrared lines retrieved in this work, from FTS and from high-temperature emission spectra, and already published high-resolution measurements including microwave and THz transitions, and kHz accuracy transitions.^b The analysis, carried out with the Bending-Rotation fitting Hamiltonian,^c allows us to reproduce more than 11700 data with a unitless standard deviation of 1.6 up to J=20 and K_a=16. The highly accurate THz transitions^b are reproduced with an RMS of 0.2 MHz and the kHz accuracy transitions^b with an RMS better than 0.3 MHz. A line intensity analysis of absorption transitions involving the same vibrational states will also be presented. FIR line intensities measured in this work using FTS were fitted in addition to previously measured line intensities. 3890 line intensities are accounted for with a unitless standard deviation of 1.4. The absorption line list calculated using these results will be compared to that recently obtained from theoretical calculations.^d With the present set of spectroscopic parameters, discrepancies up to 0.09 cm⁻¹ are noted for the line positions.

Acetaldehyde is one of a prototype molecule to study large amplitude motion. In the ground state, the energy level structure were well understood by considering the methyl torsional motion (ν₁₅ mode). I. Kleiner, J. T. Hougen, J. -U. Grabow, M. Mckhtiev, and J. Cosleou, J. Mol. Spectrosc., 179, 41 (1996).n the other hand, in the S₁ state, it is necessary to consider the aldehyde-hydrogen inversion mortion (ν₁₄ mode) M. Baba, I. Hanazaki, U. Nagashima, J. Chem. Phys., 82, 3938 (1985).M. Nobleand K. C. Lee, J. Chem. Phys., 81, 1632 (1984). in addition to the methyl torsion. Rotationally-resolved spectrum of the S₁ ← S₀ transition were observed by using a pulsed amplified CW laser, and obtained effective rotational constants. Y.-C. Chou, C.-L. Huang, I-C. Chen, C.-K. Ni, A. H. Kung, J. Chem Phys., 115, 5089 (2001).H. Liu, E. C. Lim, A. Nino, C. Munoz-Caro, R. H. Judge, D. C. Moule, J. Mol. Spectrosc., 190, 78 (1998). In this work, rotationally-resolved high-resolution fluorescence excitation spectra of the S₁ ← S₀ transition of acetaldehyde have been observed. Sub-Doppler excitation spectra were measured by crossing a single-mode UV laser beam perpendicular to a collimated molecular beam. The typical linewidth of observed spectra was about 40 MHz. The absolute wavenumber was calibrated with accuracy 0.0002 cm⁻¹ by measurement of the Doppler-free saturation spectrum of iodine molecule and fringe pattern of the stabilized etalon. The observed spectra around 30118 cm⁻¹ and 30375 cm⁻¹ correspond to 14⁰⁻₀ 15²₀ and 14⁰⁺₀ 15⁴₀ band, respectively. We are trying to analyze the rotational structure including the interaction with the large amplitude motions and then determine the parameters of the S₁ state.

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Footnotes:

I. Kleiner, J. T. Hougen, J. -U. Grabow, M. Mckhtiev, and J. Cosleou, J. Mol. Spectrosc., 179, 41 (1996).O M. Baba, I. Hanazaki, U. Nagashima, J. Chem. Phys., 82, 3938 (1985).

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Y.-C. Chou, C.-L. Huang, I-C. Chen, C.-K. Ni, A. H. Kung, J. Chem Phys., 115, 5089 (2001).

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Propanol occurs in two isomers, as a primary alcohol normal-propanol (CH₃CH₂CH₂OH) and as a secondary alcohol iso-propanol (CH₃CH(OH)CH₃). Moreover, normal-Propanol occurs in five different conformers: Ga, Gg, Gg', Aa, and Ag. Rotational spectra of all three conformers of the G family are well described [1], hence, an astronomical search of their rotational fingerprints is possible, in contrast to Aa and Ag. Rotational spectra of normal-propanol were recorded in the frequency region of 18 to 500 GHz. Double-modulation double-resonance (DM-DR) measurements were performed additionally, in particular to unambiguously assign weak transitions of the Aa and to verify assignments of the Ag conformer. An extended quantum mechanical model for Aa was derived, based on Ref. [2]. Furthermore, the existence of two tunneling states, Ag⁺ and Ag⁻, has been proven by unambiguously assigned transitions, but a quantum mechanical model description for Ag could not be given yet. The astronomical detection of all five conformers is now possible, but the quantum mechanical description of the A family should still be improved in the future. [1] Kisiel, Z., Dorosh, O., Maeda, A., et al., Phys. Chem. Chem. Phys. 12 (2010) 8329. [2] Dreizier, H. & Scappini, F. Z., Naturforsch. A 36 (1981), 1187.