Imidazoles are a class of aromatic compounds with significant biochemical properties. Some imidazoles are found to be key compounds in cancer growth while functionalizing imidazole in order to synthesize new drugs is a field of increasing interest among medicinal chemists. The ground state rotational spectra of 1-, 2-, 4- and 5-methylimidazole were recorded in the 7.0-18.5 GHz frequency range by Chirped Pulse Fourier Transform Microwave Spectroscopy. Solid samples of 2-, 4-, and 5-methylimidazole were transferred into the gas phase through laser ablation in the presence of argon gas undergoing supersonic expansion. Liquid 1-methylimidazole was placed in a bespoke reservoir which allows direct introduction of the molecule into the flow of argon. Ground state rotational constants, A₀, B₀, C₀, centrifugal distortion constants, D_J, D_JK, D_K, d₁, d₂, and nuclear quadrupole coupling constants, x_aa(N), x_bb(N)-x_cc(N), were determined for all structural isomers of methylimidazole. Detection of 13C and 15N isotopologues in their natural abundance allowed for determination of atomic coordinates of heavy atoms. V₃ barriers to internal rotation of the methyl group were determined and provide an insight into the flexibility of the structural isomers.

Many molecular symmetry (MS) groups for molecules with periodic large-amplitude motions are semidirect products H ∧F where the invariant subgroup H is a pure permutation group and F is a point group.C.M. Woodman, Mol. Phys. (1970) 19, 753-780roup H is often a direct product of cyclic permutation groups. The point group F is formed by the equivalent rotations and, for starred PI operators, the E^* operator. In a modified notation for such MS groups based on the semidirect product decomposition,P. Groner, J. Mol. Spectrosc. (2018) 343, 34-43uch as G₈ = [4]C_s or G₃₆ = [33]C_2v, the numbers within square brackets refer to the cyclic operators in H and the point group symbol refers to F. This has many advantages over just G₈ or G₃₆ because (1) it contains the number of periodic internal motions and their periodicities; (2) the symmetry of rotational, vibrational and transition dipole operators are the same as those of the point group; (3) this makes it easy to correlate the symmetry labels to those of possible equilibrium structures and vibronic levels; (4) and to derive qualitative splitting patterns. Each of these advantages is illustrated with examples. The notation can be extended to molecules with coaxial internal rotors on a linear frame. For such molecules with inequivalent rotors, two different valid notations are possible. If the rotors are symmetrically equivalent, the semidirect product corresponds to the extended PI (or EMS) group.

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C.M. Woodman, Mol. Phys. (1970) 19, 753-780G P. Groner, J. Mol. Spectrosc. (2018) 343, 34-43s

We presentThis 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 work in Köln was supported by the Deutsche Forschungsgemeinschaft (DFG) via SFB 956, project B3, and the Gerätezentrum Cologne Terahertz Center.he results of our analysis of the FIR and microwave spectra of the ν₈ vibrational state (C-S stretch) of methyl mercaptan CH₃SH near 710 cm⁻¹. The analysis employs a new program which was recently developed for fitting several isolated small-amplitude fundamentals embedded in a pure torsional bath in molecules like methyl mercaptan, in which the frame has C_s symmetry and the methyl top has C_3v symmetry. Our study involves the energy levels that belong to the ν₈ vibrational state itself as well as to v_t = 0, 1, 2, 3, 4 torsional vibrational states of methyl mercaptan. In our analysis we used data available in the literature [1,2,3] as well as the results of the new measurements from Kharkiv, Köln, and Braunschweig. In the talk the details of this new study will be discussed. [1] L.-H. Xu, R. M. Lees, G. T. Crabbe, et al., J. Chem. Phys. 137, 104313 (2012). [2] R.M. Lees, Li-Hong Xu, B.E. Billinghurst, J. Mol. Spectrosc. 352, 30-38 (2016). [3] R.M. Lees, Li-Hong Xu, B.E. Billinghurst, J. Mol. Spectrosc. 319, 45-56 (2018).

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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 (STCU partner project P686). The work in Köln was supported by the Deutsche Forschungsgemeinschaft (DFG) via SFB 956, project B3, and the Gerätezentrum Cologne Terahertz Center.t

We presentThis talk is dedicated to the memory of Dr. Jon T. Hougen (who recently passed away) in recognition of his essential contribution to this project.he results of achieved progress in our analysis of the far-IR and microwave spectra of the ν₁₀ vibrational state of acetaldehydeThis work was done under support of the Volkswagen foundation. The assistance of STCU is acknowledged (partner project P686). The analysis of the gs - ν₁₀ band near 509 cm⁻¹involves the energy levels that belong to the v_t =0, 1 torsional states of ν₁₀ vibrational mode as well as v_t =0, 1, 2, 3, 4 torsional states of the ground vibrational state. The intervibrational interactions between ν₁₀ vibrational state and torsional bath is taken explicitly into account with the help of a new program which was recently developed for fitting several isolated small-amplitude fundamentals embedded in a pure torsional bath in molecules with C_s frame and C_3v top. Obtained results provide significant progress in comparison with the previous fitting attempts for gs - ν₁₀ band of acetaldehyde near 509 cm⁻¹. In the talk the details of the results with emphasis on the remaining fitting problems for the hot torsional bandsThe authors are indebted to Dr. A.R.W. McKellar for providing raw spectrum data on torsional bands of acetaldehyde from previous study.f acetaldehyde will be discussed.

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This talk is dedicated to the memory of Dr. Jon T. Hougen (who recently passed away) in recognition of his essential contribution to this project.t This work was done under support of the Volkswagen foundation. The assistance of STCU is acknowledged (partner project P686).. The authors are indebted to Dr. A.R.W. McKellar for providing raw spectrum data on torsional bands of acetaldehyde from previous study.o

The infrared Fourier transform spectrum of the lower vibrational modes of CD₃SH has been recorded in the 400-1200 cm⁻¹ region using synchrotron radiation at the FIR beamline of the Canadian Light Source in Saskatoon. Torsion-rotation assignments have been made for a relatively strong parallel band centered at 644 cm⁻¹ and a weaker perpendicular band centered at 727 cm⁻¹. Comparison with the spectra for the normal CH₃SH species as well as the analogous CD₃OH and CH₃OH methanol molecules would suggest an obvious association of the 644 cm⁻¹ band with the C-S stretching mode, with the 727 cm⁻¹ mode likely to be the out-of-plane methyl rock. However, a previous vibrational normal mode analysis [Byler and Gerasimowicz, J. Mol. Struct. 112 (1984) 207-219] showed strong coupling between the C-S stretch and CSH bending modes. They assign the 644 cm⁻¹ band to the latter, and attribute the C-S stretch instead to a feature at 688 cm⁻¹ that we find no clear evidence for in our spectrum. For normal CH₃SH, the CSH bend is very weak and lies between the strong C-S stretch and CH₃-rocking bands. A Gaussian quantum chemistry calculation of the vibrational frequencies and transition moments was carried out, and indeed there is a mode predicted to lie in between our two observed bands with almost vanishing intensity and a reduced mass and effective force constant corresponding closely to those calculated for the C-S stretch of normal CH₃SH. This apparent dramatic extinction of the normally very strong C-S stretching band is quite remarkable!

We observed the rotationally resolved fluorescence excitation spectrum of the S₁ ← S₀ transition of jet-cooled 9-methylanthracene. The potential energy curve for CH₃ internal rotation is six-fold symmetric. The barrier to rotation (V₆) is  ∼  120 cm⁻¹ [2], which is considerably larger than that of toluene (  ∼  5 cm⁻¹ ) [1]. The V₆ value is the energy difference between two isomers staggered and eclipsed. The main contribution to V₆ is expected to be hyperconjugation, which is the π-like interaction between the aromatic π orbital and methyl sp³ orbital. The final goal of this work is to quantitatively elucidate the vibrational and rotational energy levels for CH₃ internal rotation. It is essential to observe the high-resolution and high-precision electronic spectra and theoretically search the best effective Hamiltonian to reproduce the experimental results. We are now developing a new laser control system with optical frequency comb locked to GPS, and an ab initio method which provides the reliable potential energy curve.
[1] M. Baba, et al., J. Phys. Chem. A 113, 2369 (2009)
[2] D. R. Borst and D. W. Pratt, J. Chem. Phys. 113, 3658 (2000)

The vibrational specificity and isotopic dependence of hindered proton-transfer dynamics have been explored in the lowest-lying singlet excited state, ùB₂ (π^*π), of 6-hydroxy-2-formylfulvene (HFF) and its monodeuterated isotopolog (HFF-d). Both systems have been probed under bulk-gas conditions by employing polarization-resolved degenerate four-wave mixing (DFWM) spectroscopy, where judicious selection of incident and detected polarization geometries served to alleviate spectral complexity and to allow for the quantitative extraction of rotation-tunneling information. The observed > 1000-fold decrease in tunneling rate that accompanies the π^*← π electron promotion (transitioning from ultrafast ground-state dynamicsZ. N. Vealey, L. Foguel and P. H. Vaccaro, J. Phys. Chem. Lett. 9, 4949 (2018)o near-complete quenching of analogous excited-state behavior) makes HFF a compelling model system for investigating the nuanced nature of low-barrier hydrogen bonding and its ability to regulate attendant hydron-migration events. A thorough analysis of low-energy vibronic bands in the ùB₂ manifold will be presented, with the dependence of unimolecular reactivity on heavy atom motion and isotopic modification being discussed in the context of structural predictions emerging from high-level quantum-chemical calculations.

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Z. N. Vealey, L. Foguel and P. H. Vaccaro, J. Phys. Chem. Lett. 9, 4949 (2018)t