The rotational spectrum of 2-methylimidazole has been measured using laser ablation chirped-pulse Fourier transform microwave spectroscopy from 2-18.5 GHz. 2-methylimidazole was laser vaporised then entrained within an argon buffer gas undergoing supersonic expansion allowing for efficient rotational cooling. Carbon-13 and nitrogen-15 isotopologues were measured in natural abundance and substitution coordinates have been determined. The barrier to internal rotation of the methyl group was found to be 122.697(20) cm⁻¹. Nuclear quadropole coupling constants for the two nitrogen nuclei were determined via a rigid rotor fit of the A internal rotor state. Complexes with water and argon were also observed and fit in a similar way.

An approach to connect the su(3) dynamical group- used to describe the bending modes of linear molecules- with configuration space is discussed. The SU(3) group may be seen as a consequence of adding a scalar boson to the SU(2) space of two degenerate harmonic oscillators. The resulting SU(3) group becomes the dynamical group for the bending degrees of freedom of linear molecules, but the connection to configuration space is not obvious. This work aims at providing this connection. Our approach is based on the basis of establishing a mapping between the algebraic and configuration states. An arbitrary operator in configuration space is then expanded in terms of generators of the dynamical algebra. The coefficients are determined through a minimization procedure and given in terms of matrix elements defined in configuration space. As an application we consider the vibrational description of the bending modes of the acetylene molecule, where the force constants are estimated in the framework of the U(3) ×U(3) model.

A new study This work was done under support of the Volkswagen foundation. The assistance of Science and Technology Center in Ukraine is acknowledged (STCU partner project P686).f the dimethylsulfide ((CH₃)₂S) spectrum is reported. The new measurements have been carried out using the Kharkiv spectrometer in the Institute of Radio Astronomy of NASU (Ukraine) and using the Lille spectrometer in the PhLAM laboratory (France). The new millimeter and submillimeter wave measurements cover the frequency range from 49 GHz to 660 GHz. The rotational transitions belonging to the three lowest torsional states of the molecule as well as the new assignments in the FIR torsional band (AILES beamline of the synchrotron SOLEIL) and the microwave data available in the literature A. Jabri, V. Van, H. V. L. Nguyen, H. Mouhib, F. Kwabia Tchana , L. Manceron , W. Stahl, I. Kleiner, A&A 589, A127 (2016).ave been analyzed using recently developed model for the molecules with two equivalent methyl rotors and C_2v symmetry at equilibrium (PAM_C2v_2tops program) Ilyushin V. V., Hougen J. T. J. Mol. Spectrosc. 289 (2013) 41-49. In the talk the details of this new study will be discussed.

---

Footnotes:

This work was done under support of the Volkswagen foundation. The assistance of Science and Technology Center in Ukraine is acknowledged (STCU partner project P686).o A. Jabri, V. Van, H. V. L. Nguyen, H. Mouhib, F. Kwabia Tchana , L. Manceron , W. Stahl, I. Kleiner, A&A 589, A127 (2016).h Ilyushin V. V., Hougen J. T. J. Mol. Spectrosc. 289 (2013) 41-49..

A progress in the analysis of the methyl mercaptan CH₃SH spectrum in its ground, first and second excited torsional states is reported This work was done under support of the Volkswagen foundation. The assistance of Science and Technology Center in Ukraine is acknowledged (STCU partner project P686). The available in the literature data L.-H. Xu, R. M. Lees, G. T. Crabbe, J. A. Myshrall, H. S. P. Müller, C. P. Endres, O. Baum, F. Lewen, S. Schlemmer, K. M. Menten, and B. E. Billinghurst J. Chem. Phys. 137, 104313 (2012).ere reanalyzed using RAM36 code V. Ilyushin, Z. Kisiel, L. Pszczółkowski, H. Mäder, J. T. Hougen // J. Mol. Spectrosc. Vol. 259, pp. 26-38 (2010). ith the main improvement achieved for the root mean square deviation of the microwave data. The updated Hamiltonian model was applied to the further assignments of the methyl mercaptan spectrum using the records obtained in the THz region in the previous study^b . Also a new measurement campaign in subTHz frequency range is planned for the nearest future using a set of spectrometers available in Köln. In the talk the details of this new study will be discussed.

---

Footnotes:

This work was done under support of the Volkswagen foundation. The assistance of Science and Technology Center in Ukraine is acknowledged (STCU partner project P686).. L.-H. Xu, R. M. Lees, G. T. Crabbe, J. A. Myshrall, H. S. P. Müller, C. P. Endres, O. Baum, F. Lewen, S. Schlemmer, K. M. Menten, and B. E. Billinghurst J. Chem. Phys. 137, 104313 (2012).w V. Ilyushin, Z. Kisiel, L. Pszczółkowski, H. Mäder, J. T. Hougen // J. Mol. Spectrosc. Vol. 259, pp. 26-38 (2010). w

Methyl mercaptan (CH₃SH) is a sulfur-containing interstellar molecule, first identified in Sgr B2. R. A. Linke, M. A. Frerking, and P. Thaddeus, Astrophys. J. 234, L139 (1979). Although the abundance of methyl mercaptan was not too high compared with methanol, the identification of CD_3OH towards the IRAS 16293−2422 stimulated our microwave study of CD_3SH. Deuterium fractionation may be possible for the methyl mercaptan molecule. There are many laboratory microwave spectroscopic results on the normal species but the studies on this isotopolog is quite limited. We have observed the spectra of CD_3

Hyperfine splittings in methanol have been revisited in three recent publications. (i) Coudert et al. [JCP 143 (2015) 044304] published an analysis of splittings observed in the low-J range. They calculated 32 spin-rotation, 32 spin-spin, and 16 spin-torsion hyperfine constants using the ACES2 package. Three of these constants were adjusted to fit hyperfine patterns for 12 transitions. (ii) Three present authors and collaborators [JCP 145 (2016) 024307] analyzed medium to high-J experimental Lamb-dip measurements in methanol and presented a theoretical spin-rotation explanation that was based on torsionally mediated spin-rotation hyperfine operators. These contain, in addition to the usual nuclear spin and overall rotational operators, factors in the torsional angle α of the form e^±inα. Such operators have non-zero matrix elements between the two components of a torsion-rotation ^trE state, but have zero matrix elements within a ^trA state. More than 55 hyperfine splittings were successfully fitted using three parameters and the fitted values agree well with ab initio values obtained in (i). (iii) Lankhaar et al. [JCP 145 (2016) 244301] published a reanalysis of the data set from (i), using CFOUR recalculated hyperfine constants based on their rederivation of the relevant expressions. They explain why their choice of fixed and floated parameters leads to numerical values for all parameters that seem to be more physical than those in (i). The results in (ii) raise the question of whether large torsionally-mediated spin-rotation splittings will occur in other methyl-rotor-containing molecules. This abstract presents ab initio calculations of torsionally mediated hyperfine splittings in the E states of acetaldehyde using the same three operators as in (ii) and spin-rotation constants computed by Gaussian09. We explored the first 13 K states for J from 10 to 40 and ν_t = 0, 1, and 2. Our calculations indicate that hyperfine splittings in CH₃CHO are just below current measurement capability. This conclusion is confirmed by available experimental measurements.

Methyl propyl ketone (MPK) or 2-Pentanone is known to be an alarm pheroromone released by the mandibular glands of the bees. It is a highly volatile compound. This molecule was studied by a combination of quantum chemical calculations and microwave spectroscopy in order to get informations about the lowest energy conformers and their structures.The rotational spectrum of 2-pentanone was measured using the molecular beam Fourier transform microwave spectrometer in Aachen operating between 2 and 26.5 GHz. Ab initio calculations determine 4 conformers but only two of them are observed in our jet-beam conditions.The lowest conformer has a C₁ structure and its spectrum shows internal rotation splittings arising from two methyl groups. The internal splittings of 305 transitions for this conformer were analyzed using the XIAM code H. Hartwig, H. Dreizler, Z. Naturforsch. 51a, 923 (1996). It led to the determination of the values for the barrier heights hindering the internal rotation of two methyl groups of 239 cm⁻¹ and 980 cm⁻¹ respectively. The next energy conformer has a C_s structure and the analysis of the internal splittings of 134 transitions using the XIAM code and the BELGI code J. T. Hougen, I. Kleiner and M. Godefroid, J. Mol. Spectrosc., 163, 559-586 (1994).ed to the determination of internal rotation barrier height of 186 cm⁻¹. Comparisons of quantum chemistry and experimental results will be discussed.

---

Footnotes:

H. Hartwig, H. Dreizler, Z. Naturforsch. 51a, 923 (1996).. J. T. Hougen, I. Kleiner and M. Godefroid, J. Mol. Spectrosc., 163, 559-586 (1994).l

In this talk we report on our progress in trying to make the hybrid Hamiltonian competitive with the pure-tunneling Hamiltonian for treating large-amplitude motions in methylamine. A treatment using the pure-tunneling model has the advantages of: (i) requiring relatively little computer time, (ii) working with relatively uncorrelated fitting parameters, and (iii) yielding in the vast majority of cases fits to experimental measurement accuracy. These advantages are all illustrated in the work published this past year on a gigantic v_t = 1 data set for the torsional fundamental band in methyl amineI. Gulaczyk, M. Kreglewski, V.-M. Horneman, J. Mol. Spectrosc., in Press (2017). A treatment using the hybrid model has the advantages of: (i) being able to carry out a global fit involving both v_t = 0 and v_t = 1 energy levels and (ii) working with fitting parameters that have a clearer physical interpretation. Unfortunately, a treatment using the hybrid model has the great disadvantage of requiring a highly correlated set of fitting parameters to achieve reasonable fitting accuracy, which complicates the search for a good set of molecular fitting parameters and a fit to experimental accuracy. At the time of writing this abstract, we have been able to carry out a fit with J up to 15 that includes all available infrared data in the v_t = 1-0 torsional fundamental band, all ground-state microwave data with K up to 10 and J up to 15, and about a hundred microwave lines within the v_t = 1 torsional state, achieving weighted root-mean-square (rms) deviations of about 1.4, 2.8, and 4.2 for these three categories of data. We will give an update of this situation at the meeting.

---

Footnotes:

I. Gulaczyk, M. Kreglewski, V.-M. Horneman, J. Mol. Spectrosc., in Press (2017)..

1,3,5-trisilapentane has been synthesized and studied in the microwave region for the first time using CP-FTMW spectroscopy. The lowest calculated energy structure, C₂ is essentially non-polar with a calculated dipole of 0.063 D. However, slightly higher in energy at 145 cm⁻¹ and 196 cm⁻¹ are the calculated energies for the C₁ and C_2v conformations, respectively. These structures have much larger dipoles calculated at 1.07 D for C₁ and 4.88 D for C_2v. Both of these structures have been confirmed using experiment and the details of such analysis will be discussed. In addition to the structure determination, 1,3,5-trisilapentane has two terminal -SiH₃ groups. The calculated barrier to internal rotation of these groups are calculated to be 327.5 cm⁻¹ for C_2v and 343.2 cm⁻¹ for C₁. This barrier is low enough to exhibit internal rotation splitting in the spectra and treatment of these motions in the analysis will be discussed.

Pure rotational spectra of the title molecules have been recorded using chirped pulse Fourier transform microwave spectroscopy. Under our experimental conditions only one conformer has been observed for each of the four compounds. These conformers have torsional angles of CXCC = 180^o, XCCC = 60^o, CCCC = 180^o, and, for the C₅H₁₁-X-CH₃ species, CCCC_Methyl = 180^o. These angles correspond to anti-gauche-anti conformations for the butyl methyl ether/thioether species, and anti-gauche-anti-anti conformations for the pentyl methyl ether/thioether species. Splittings due to the internal rotation of the X-CH₃ group are observed in both butyl species but are not observed in the pentyl species. The barrier to the X-CH₃ internal rotation has been investigated through spectral analyses and quantum chemical calculations. The differences in the internal rotation barrier between the ethers and thioethers will be discussed and will further be compared to the barriers obtained for similar molecules.