Six-fold-symmetry internal rotation in toluenes: the low barrier challenge of 2,6-and 3,5-difluorotoluene

Pure six-fold symmetry (V6) internal rotation poses significant challenges to experimental and theoretical determination, as the very low torsional barriers result in huge tunneling splittings difficult to identify and to model. Here we resolved the methyl group internal rotation dynamics of 2,6- and 3,5-difluorotoluene using a newly developed computer code especially adapted to V6 problems. The jet-cooled rotational spectra of the title molecules in the 5–25 GHz region revealed internal rotation tunneling doublings of up to 3.6 GHz, which translated in methyl group potential barriers of V6 = 0.14872(24) and 0.0856(10) kJ mol−1, respectively, in the vibrational ground-state. Additional information on Stark effects and carbon isotopic species in natural abundance provided structural data and the electric dipole moments for both molecules. Ab initio calculations at the MP2 level do not reproduce the tiny torsional barriers, calling for experiments on other systems and additional theoretical models.DFGMINECO/CTQ2012-39132-C02-0

Sensitivity to a possible variation of the proton-to-electron mass ratio torsion-wagging-rotation transitions in methylamine CH3NH2

9 x 10(-6) is deduced

THE ROTATIONAL SPECTRUM OF 13^{13}CH3_3NH2_2 UP TO 1 THz

This work is supported by the Programme National de Physico-Chimie du Milieu Interstellaire (PCMI-CNRS) and by the contract ANR-08-BLAN-0054.Author Institution: Laboratoire PhLAM, CNRS UMR 8523, Universite de Lille 1, 59655 Villeneuve d'Ascq Cedex, France; Institute of Radio Astronomy of NASU, Chervonopraporna 4, 61002 Kharkov, UkraineMethylamine (CH$_3$NH$_2$) is a molecule of astrophysical importance detected in interstellar medium for the first time in 1974~{\bf 191} (1974) L135.}. Also it has been discovered in the atmosphere of Jupiter~{\bf 4} (1977) 203. }. It is suggested that methylamine can be a precursor of the simplest amino acid glycine. In this context we present a new study of rotational spectrum of the ground vibrational state of $^{13}$C isotopologue of methylamine in the frequency range up to 1 THz. The spectrum of $^{13}$CH$_3$NH$_2$ was recorded and analyzed for the first time. All the spectra were obtained using the Lille spectrometer based on the solid state sources. The analysis of the rotational spectrum of methylamine is complicated by two large-amplitude motions: CH$_3$ torsion and NH$_2$ wagging. The Hamiltonian used in the present study is based on the group-theoretical high-barrier tunneling formalism developed by Ohashi and Hougen~{\bf 121} (1987) 474.}. This model proved to be efficient in the previous studies of the parent species of methylamine~{\bf 229} (2005) 170.} since it allowed fitting within experimental accuracy all the rotational transitions of the ground vibrational state with $J \leq 30$. In view of extended frequency range of the present study the fitting program will be modified in order to take into account the rotational transitions with $J > 30$. For the parent isotopic species, measurements and analysis using the same approach are in progress. The latest results will be discussed

MOLECULES WITH A SIX-FOLD BARRIER: MICROWAVE SPECTRUM OF TOLUENE

G.~O.~S{\oAuthor Institution: Institute of Radio Astronomy of NASU, Chervonopraporna 4, 61002 Kharkov, Ukraine; Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warszawa, Poland; Institut fur Physikalische Chemie, Christian-Albrechts Universitat zu Kiel, Olshausenstrasse 40, D-24098, Kiel, Germany; Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8441Recent progress in analysis and fitting of transitions in the toluene microwave spectrum characterized by $J$$\leq$30, $K$$_a$$\leq$12, and the free rotor quantum number $|$$m$$|$$\leq$3 will be presented. The analysis was carried out using a new program which is specifically designed for six-fold barrier molecules, and allows the user to select almost any symmetry-allowed torsion-rotation term for inclusion in the fitting Hamiltonian. The program is based on the theoretical framework developed by S{\o}rensen and Pedersenrensen, T.~Pedersen, {\it Studies in Physical and Theoretical Chemistry}, Elsevier, {\bf 23}, 219--236 (1983).} in their application of the Longuet-Higgins permutation-inversion group G$_{12}$ to the microwave spectrum of CH$_3$NO$_2$. In the analysis we have used the published and rather extensive unpublished data from previous studies as well as recent new measurements of the toluene microwave spectrum in the 8 -- 18 GHz range. A rather successful fit of over 350 toluene microwave transitions in the abovementioned quantum number range has been achieved. The fit is close to the estimated experimental accuracy and includes more than thirty intertorsional $m$=~+3 $\Leftrightarrow$ $m$=~-3 transitions

SPECTROSCOPY OF THE GROUND, FIRST AND SECOND EXCITED TORSIONAL STATES OF ACETALDEHYDE FROM 0.05 TO 1.6 THz.

Author Institution: Institute of Radio Astronomy of NASU, Chervonopraporna 4, 61002 Kharkov, Ukraine; Laboratoire de Physique des Lasers, Atomes et Molecules, UMR 8523 CNRS-Universite Lille 1, Batiment P5, F-59655 Villeneuve d'Ascq Cedex, France; Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109-8099, USAA new global study of the acetaldehyde (CH${_3}$CHO) spectrum is reported. The new measurements cover the frequency range from 49 GHz to 1.6 THz and have been carried out using three different spectrometers in IRA NASU (Ukraine), PhLAM Lille (France), and JPL (USA). The rotational transitions belonging to the three lowest torsional states, as well as previously published data on the FIR torsional bands, of the molecule have been analyzed using the rho-axis-method. The dataset consisting of more than 19700 line frequencies and including rotational transitions with {\it J} up to 66 and {\it K${_a}$} up to 22 was fit using a model consisting of 117 parameters and weighted root-mean-square deviation of 0.71 has been achieved. Details of this new study and problems encountered in analysis of the second torsional state will be discussed

MICROWAVE STUDY OF A HYDROGEN-TRANSFER-TRIGGERED METHYL-GROUP INTERNAL ROTATION IN 5-METHYLTROPOLONE

Author Institution: Institute of Radio Astronomy of Nasu, Chervonopraporna 4, 61002 Kharkov, Ukraine; DEPARTMENT OF CHEMISTRY AND BIOCHEMISTRY, COLLEGE OF CHARLESTON, CHARLESTON, SC 29424, USA; DEPARTMENT OF NATURAL SCIENCE, TAIPEI MUNICIPAL UNIVERSITY OF EDUCATION, TAIPEI 10048, TAIWAN; SUN CHEMICAL, CINCINNATI, OH 45232, USA; OPTICAL TECHNOLOGY DIVISION, NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, GAITHERSBURG, MD 20899-8441, USA; DEPARTMENT OF CHEMISTRY AND BIOCHEMISTRY, COLLEGE OF CHARLESTON, CHARLESTON, SC 29424, USAWe present here the first experimental and theoretical study of the microwave spectrum of 5-methyltropolone, which can be visualized as a 7-membered ?aromatic? carbon ring with a five-membered hydrogen-bonded cyclic structure at the top and a methyl group at the bottom. The molecule exhibits two large-amplitude motions, an intramolecular hydrogen transfer and a methyl torsion. The former motion is particularly interesting because transfer of the hydrogen atom from the hydroxyl to the carbonyl group induces a tautomerization in the molecule, which then triggers a 60^irc} internal rotation of the methyl group. Measurements were carried out by Fourier-transform microwave spectroscopy in the 8 to 24 GHz frequency range. Theoretical analysis was carried out using a tunneling-rotational Hamiltonian based on a G$_{12}^m$ extended-group-theory formalism. Our global fit of 1015 transitions to 20 molecular parameters gave a root-mean-square deviation of 1.5 kHz. The tunneling splitting of the two $J = 0$ levels arising from a hypothetical pure hydrogen transfer motion is calculated to be 1310 MHz. The tunneling splitting of the two $J = 0$ levels arising from a hypothetical pure methyl-top internal rotation motion is calculated to be 885 MHz. Some theoretical difficulties in interpreting the low-order tunneling parameters in this and the related molecule 2-methylmalonaldehyde will be discussed

Deuterated methyl mercaptan (CH3SD): Laboratory rotational spectroscopy and search toward IRAS 16293-2422 B

Methyl mercaptan (also known as methanethiol), CH3SH, has been found in the warm and dense parts of high-as well as low-mass star-forming regions. The aim of the present study is to obtain accurate spectroscopic parameters of the S-deuterated methyl mercaptan CH3SD to facilitate astronomical observations by radio telescope arrays at (sub) millimeter wavelengths. We have measured the rotational spectrum associated with the large-amplitude internal rotation of the methyl group of methyl mercaptan using an isotopically enriched sample in the 150-510 GHz frequency range using the Koln millimeter wave spectrometer. The analysis of the spectra has been performed up to the second excited torsional state. We present modeling results of these data with the RAM36 program. CH3SD was searched for, but not detected, in data from the Atacama Large Millimeter/submillimeter Array (ALMA) Protostellar Interferometric Line Survey (PILS) of the deeply embedded protostar IRAS 16293-2422. The derived upper limit corresponds to a degree of deuteration of at most similar to 18%

Rotational spectroscopy of isotopic species of methyl mercaptan at millimeter and submillimeter wavelengths: CH

Methyl mercaptan (CH3SH) is an important sulfur-bearing species in the interstellar medium, terrestrial environment, and potentially in planetary atmospheres. The aim of the present study is to provide accurate spectroscopic parameters for the most abundant minor isotopolog CH334SH to support radio astronomical observations at millimeter and submillimeter wavelengths. The rotational spectrum of CH334SH, which is complicated by the large-amplitude internal rotation of the CH3 group versus the 34SH frame, was investigated in the 49−510 GHz and 1.1−1.5 THz frequency ranges in natural isotopic abundance. The analysis of the spectrum was performed up to the second excited torsional state, and the obtained data were modeled with the RAM36 program. A fit within experimental accuracy was obtained with a RAM Hamiltonian model that uses 72 parameters. Predictions based on this fit are used to search for CH334SH with the Atacama Large Millimeter/submillimeter Array (ALMA) toward the hot molecular core Sgr B2(N2), but blends with emission lines of other species prevent its firm identification in this source