r0pt Figure Previous work on 1H,2H-hexafluorocyclopentene and 1H-heptafluorocyclopenteneRing puckering potentials of three fluorinated cyclopentenes: C5F8, C5HF7, and C5H2F6 E. A. Arsenault, B. E. Long, Wallace C. Pringle, Yoon Jeong Choi, S. A. Cooke, Esther J Ocola, Jaan Laane, Talk MG05, 70th International Symposium on Molecular Spectroscopy, 2015as been improved upon and broadened. A test set of small pseudo-four membered rings with known experimental barriers to planarity has been used to explore suitable quantum mechanical approaches to this type of molecular problem. The best method has then been applied to 1H,2H-hexafluorocyclopentene and 1H-heptafluorocyclopentene. The existing data set on these fluorinated ring compounds has been extended by recording low resolution far infrared spectra which allowed an experimental determination of the barrier to planarity for these compounds. It is found that the barrier to planarity decreases in the order C₅H₂F₆ > C₅HF₇ > C₅F₈. Results will be presented together with an attempted rationale. Ring puckering potentials of three fluorinated cyclopentenes: C5F8, C5HF7, and C5H2F6 E. A. Arsenault, B. E. Long, Wallace C. Pringle, Yoon Jeong Choi, S. A. Cooke, Esther J Ocola, Jaan Laane, Talk MG05, 70th International Symposium on Molecular Spectroscopy, 2015h

The vapor-phase and Raman spectra of 3-cyclopenten-1-ol (3CPOL) and 3-cyclopenten-1-amine (3CPAM) have been recorded and analyzed. The spectra demonstrate the existence of six conformers for each molecule, two pairs of which are mirror images of each other and with equivalent vibrational modes. The conformational minima are achieved for selected values of the ring-puckering coordinate and the internal rotation coordinate of either the -OH or -NH2 group. Theoretical CCSD/cc-pVTZ computations for both molecules were carried out and their two-dimensional potential energy surfaces (PEFs) were calculated. The PEFs confirm the existence of all conformers and provide the relative energies for these. The structures with the π-type intramolecular hydrogen bonding are at the lowest calculated energies and 301 to 411 cm⁻¹lower than the other conformations for 3CPOL, and 165 to 197 cm⁻¹lower for 3CPAM.

The H₅⁺ molecular ion has been studied since the 1960s due to its role as the product of H₂ + H₃⁺ collisions, which is of astrochemical relevance as an intermediate in the interstellar production of HD. Since the mid-1980s, much experimental and theoretical effort has gone into both obtaining and simulating the IR absorption spectrum of H₅⁺. Multiple assignments for the observed peaks have been proposed, from those resulting from very simple harmonic treatments to those arising from full-dimensional multi-configuration time-dependent Hatree (MCTDH) calculations. This full-dimensional MCTDH approach, while in excellent agreement with experimental energies and intensities, poses a challenge to assignment as it does not directly provide eigenfunctions of the vibrational Hamiltonian. Motivated by newly recorded spectra of H₅⁺ and D₅⁺ between 4500 and 7500 cm⁻¹, we have developed an alternate approach, based on a four-dimensional Hamiltonian that is adiabatically separated into a pair of coupled two-dimensional Hamiltonians whose eigenfunctions may be directly calculated by a discrete variable representation. This approach has the benefit of providing good agreement with experiment, while dramatically simplifying the assignment of the transitions. After assignment, it became clear that the progressions of peaks observed in the IR spectrum of H₅⁺ arise from the effects of proton tunneling between two configurations of the system that look like H₂ ·H₃⁺ and H₃⁺ ·H₂ respectively. This tunneling behavior is further demonstrated by the analysis of a one-dimensional model Hamiltonian with non-adiabatic couplings. This treatment has also been applied to the deuterated isotoplogues D₅⁺, H₂D⁺H₂, and D₂H⁺D₂ with each system displaying the same tunneling behavior, but with differences in the length of the progression supported.

Microwave spectra of benzene-water and related complexesS. Suzuki, P. G. Green, R. E. Bumgarner, S. Dasgupta, W. A. Goddard, G. A. Blake, Science 257 (1992) 942-945; H. S. Gutowsky, T. Emilsson, E. Arunan, J. Chem. Phys. 99 (1993) 4883-4893; E. Arunan, T. Emilsson, H. S. Gutowsky, J. Chem. Phys. 101 (1994) 861-868; B. R. Prasad, M. S. Krishnan, E. Arunan, J. Mol. Spectrosc. 232 (2005) 308-314; T. Emilsson, H. S. Gutowsky, G. de Oliveira, C. E. Dykstra, J. Chem. Phys. 112 (2000) 1287-1294; L. Evangelisti, K. Brendel, H. Mäder, W. Caminati, S. Melandri, Angew. Chem. Int. Ed., 56 (2017) 13699-13703; W. Caminati, A. Maris, A. Dell'Erba, P. G. Favero, Angew. Chem. Int. Ed. 45 (2006) 6711-6714; D. M. Bittner, D. P. Zaleski, S. L. Stephens, N. R. Walker, A. C. Legon, ChemPhysChem 16 (2015) 2630-2634.ave been fit in the past using a free internal rotor model, which was first proposed for the CF₃H−NH₃ complexG. T. Fraser, F. J. Lovas, R. D. Suenram, D. D. Nelson, W. Klemperer, J. Chem. Phys. 84 (1986) 5983-5988. Not all the lines observed experimentally could be fit using excited state torsional quantum number as an additional quantum state for the observed microwave transitions. Some of the lines give an RMS deviation of several hundred kHz if included in the fit. A detailed study of the literature revealed that a very similar spectral pattern was observed for the vibrationally excited state (V₆=1 or V₈=1) of the CF₃NC molecule, consisting of a relatively compact central group of lines flanked by two equidistant single linesD. Christen, K. Ramme, Z. Naturforsch. A 39 (1984) 865-870. We propose a HamiltonianK. M. T. Yamada, M. Bester, M. Tanimoto, G. Winnewisser, J. Mol. Spectrosc. 126 (1987) 118-128; H. W. Kroto, Molecular Rotation Spectra, Dover Publications, Inc., Mineola, New York, 2003. including Coriolis interactions for an axially symmetric top molecule to fit the excited state spectra of benzene-water and related molecular complexes published so far by including all the omitted lines. We show that the new fit leads to RMS deviations within experimental accuracy. Details will be presented in the talk.

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S. Suzuki, P. G. Green, R. E. Bumgarner, S. Dasgupta, W. A. Goddard, G. A. Blake, Science 257 (1992) 942-945; H. S. Gutowsky, T. Emilsson, E. Arunan, J. Chem. Phys. 99 (1993) 4883-4893; E. Arunan, T. Emilsson, H. S. Gutowsky, J. Chem. Phys. 101 (1994) 861-868; B. R. Prasad, M. S. Krishnan, E. Arunan, J. Mol. Spectrosc. 232 (2005) 308-314; T. Emilsson, H. S. Gutowsky, G. de Oliveira, C. E. Dykstra, J. Chem. Phys. 112 (2000) 1287-1294; L. Evangelisti, K. Brendel, H. Mäder, W. Caminati, S. Melandri, Angew. Chem. Int. Ed., 56 (2017) 13699-13703; W. Caminati, A. Maris, A. Dell'Erba, P. G. Favero, Angew. Chem. Int. Ed. 45 (2006) 6711-6714; D. M. Bittner, D. P. Zaleski, S. L. Stephens, N. R. Walker, A. C. Legon, ChemPhysChem 16 (2015) 2630-2634.h G. T. Fraser, F. J. Lovas, R. D. Suenram, D. D. Nelson, W. Klemperer, J. Chem. Phys. 84 (1986) 5983-5988.. D. Christen, K. Ramme, Z. Naturforsch. A 39 (1984) 865-870.. K. M. T. Yamada, M. Bester, M. Tanimoto, G. Winnewisser, J. Mol. Spectrosc. 126 (1987) 118-128; H. W. Kroto, Molecular Rotation Spectra, Dover Publications, Inc., Mineola, New York, 2003.,

Acetaldehyde is of great interest because of its two large amplitude motions, CH₃ internal rotation and pyramidal deformation (mainly C-H wagging), and there have been extensive spectroscopic studies so far L. H. Xu, J. T. Hougen, R. M. Lees, and M. A. Mekhtiev, J. Mol. Spectrosc. 214, 175 (2002).I. Kleiner, J. T. Hougen, J.-U. Grabow, S. P. Belov, M. Yu. Tretyakov, and J. Cosleou,J. Mol. Spectrosc., 179, 41 (1996). Y.-C. Chou, C.-L. Huang, I.-C. Chen, C.-K. Ni, and A. H. Kung, J. Chem. Phys., 116, 1003 (2002).Y.-C. Chou, J. T. Hougen, and I.-C. Chen, J. Chem. Phys., 120, 2255 (2004). M. Baba, I. Hanazaki, and U. Nagashima, J. Chem. Phys., 82, 3938 (1985).M. Baba, U. Nagashima, and I. Hanazaki, J. Chem. Phys., 83, 3514 (1985).. In order to accurately investigate the vibrational and rotational levels in the S₁(n,π*) state, rotationally-resolved high-resolution fluorescence excitation spectra were measured by crossing a cw single-mode UV laser beam perpendicular to a collimated molecular beam. 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 acetaldehyde molecule is pyramidal in the S₁(n,π*) state, and interactions between the vibrational modes are significant. Our final goal is to perform a global fit of observed rotational transitions and to accurately analyze each interaction.

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L. H. Xu, J. T. Hougen, R. M. Lees, and M. A. Mekhtiev, J. Mol. Spectrosc. 214, 175 (2002).

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Y.-C. Chou, C.-L. Huang, I.-C. Chen, C.-K. Ni, and A. H. Kung, J. Chem. Phys., 116, 1003 (2002).

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M. Baba, I. Hanazaki, and U. Nagashima, J. Chem. Phys., 82, 3938 (1985).

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The microwave spectrum of 2,6-dimethylfluorobenzene, one of the six isomers in the dimethylfluorobenzene family, was measured using a pulsed molecular jet Fourier transform microwave spectrometer in the frequency range from 2 to 40 GHz. Quantum chemical calculations were performed at the B3LYP/6-311++G(d,p) and MP2/6-31G(d,p) levels of theory to obtain optimized molecular geometries. The latter level yielded values of the rotational constants which were in almost exact agreement with the experimental values, and had eased tremendously the spectral assignment. Due to the internal rotation of the two equivalent methyl groups, all rotational transitions split into quartets with separations of up to several hundreds of MHz. The splittings were analyzed and modeled using the XIAMH. Hartwig, H. Dreizler, Z. Naturforsch. 51a, 923, (1996).nd the ntop codeL. Ferres, W. Stahl, H.V.L. Nguyen, J. Chem. Phys. 151, 104310, (2019).o measurement accuracy. The deduced V₃ potential value of 206.4 cm⁻¹ is in reasonable agreement with the values predicted by quantum chemistry. H. Hartwig, H. Dreizler, Z. Naturforsch. 51a, 923, (1996).a L. Ferres, W. Stahl, H.V.L. Nguyen, J. Chem. Phys. 151, 104310, (2019).t

2-methylmalonaldehyde (2-MMA) has two large amplitude strongly coupled large amplitude vibrations (LAVs), internal rotation of the methyl group and proton tunneling between two equivalent oxygen atoms, which results in a multiple splitting of the ground vibrational state. Since the symmetry group of 2-MMA is G₁₂ isomorphic to a point group C_6v, each vibrational level of LAV is split into nondegenerate (A₁, A₂, B₁, B₂) and degenerate (E₁, E₂) substates. In the present paper a four-dimensional model for 2-MMA is discussed where the out-of-plane vibrations are treated together with LAVs. In the standard approach the out-of-plane vibrations are handled as non-degenerate. In the non-rigid picture both out-of plane vibrations can be treated as a degenerate mode strongly coupled to LAVs and are also split into nondegenerate and degenerate substates. This effect is shown numerically using a simple harmonic model for out-of-plane degenerate mode. The results show that the out-of-plane vibrations are split into several components and their vibrational excitation should create a system of several transitions belonging to different species of the G₁₂ symmetry group.

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 113 parameters to give an overall weighted root-mean-square deviation of 1.94 for the dataset consisting of 39272 transitions (which due to blending correspond to 24479 measured line frequencies), among which 3458 and 1591 transitions correspond to the third and fourth excited torsional states of CH₃COOH, respectively. The new dataset also contains numerous intertorsional transitions (2425) 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. [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..