The rotational spectrum of methyl pivalate (t-BuC(O)OCH₃) in the ground vibrational state was observed by molecular beam-Fourier transform microwave spectroscopy. Observed spectral lines for normal species as well as five ¹³C-isotopomers were mainly assigned to the b-type rotational transitions. Some high-K_a lines were found to be split and we have interpreted these splittings in terms of the internal rotation of the methyl group. Some forbidden transitions were also observed for normal species in case where K_a = 2 levels were involved in the internal rotation with E state. The analysis of the observed spectra was carried out by using the XIAM program and thus determined potential barrier V₃ to CH₃ internal rotation was 5.1 kJ mol⁻¹. Since gas electron diffraction study for this molecule shows the large-amplitude motion of the t-Bu group, we are observing the further spectral splittings from the rotational spectra. We are also trying to observe the ¹⁸O-isotopomers.

Diphenyl ether (C₁₂H₁₀O) is a flexible molecule, and it offers two types of competing binding sites to form non-covalently bound complexes, the ether oxygen and the aromatic π system. Its dimer and complexes with water, methanol, tert-butyl alcohol and adamantanol have been investigated with broadband rotational spectroscopy [1,2]. Using this method, we were able to accurately reveal the structures and internal dynamics of these weakly bound molecular clusters isolated in the gas phase. The spectrum of the DPE monomer shows tunneling splitting of each transition due to the concerted internal rotation of both of its phenyl rings around C-O bonds. In the weakly bonded complexes, the tunneling splitting of DPE is quenched due to intermolecular interaction. However, in one of the hydrogen-bond complexes of DPE with one water molecule, the a-type transitions are split. This is due to a concerted motion comprised of the tunneling of the DPE monomer and the internal motion of the water molecule. The analysis of the motions involved in the splitting pattern in DPE and its one water complex will be presented and discussed. [1]M. Fatima et al., Angew. Chem. Int. Edit., 58 (2019), 3108. [2]F. Dietrich et al., Angew. Chem. Int. Edit., 57 (2018), 9534.

Rotational spectroscopy has proved itself to be an invaluable tool for detecting small radicals and organic molecules in the interstellar medium. Unpaired electrons complicate rotational spectra from spin-rotation coupling and the torsions of methyl groups which couple to the molecular rotations. Presently, there exist methods for studying either of these complications but not both simultaneously. Thus preventing the laboratory analysis and the detection of small organic radicals with methyl rotors in space. This work presents a new Hamiltonian for accounting for spin-torsion interactions as well as the established spin-rotation and torsion-rotation terms for C_s molecules. The program utilizes BELGI's two stage diagonalization process to address the torsions with the spin-rotation and spin-torsion terms being added into the second diagonalization stage. Preliminary testing of the program has shown initial agreement with existing programs. This work will provide the means for laboratory analysis of previously unstudied molecules as well as their detection in space.

Methyl tert-butyl ether is a gasoline additive and a water pollutant. Its rotational spectrum was measured from 26.5 - 40 GHz using Chirped-Pulse Fourier Transform Microwave Spectroscopy. Measurements were conducted at low temperature via supersonic expansion and room temperature via static cell. The molecule was previous reported in Suenram et. al 1997 in a range of 9 - 18.6 GHz. This work expands that fit and converts it to the Rho Axis Method utilizing the program RAM36. The improved ground torsional state measurements as well as the room temperature data allowed for tentative assignments of torsionally excited transitions.

In simple cases of methyl group internal rotation, the barrier to internal rotation is equal to the V₃ coefficient in the standard equation of the potential function. In the past few years, methyl internal rotation potentials have been reported with significant V₆ contributions, of which p-toluic acid is just one example.E.G. Schnitzler, et al., J. Phys. Chem. A 121 (2017) 8625or |V₆| << |V₃|, the barrier is still |V₃|. However, if 0 < |V₃/V₆| < 4, there are now two different barriers because there are two different potential minima (or maxima), and none of the barriers is equal to |V₃| or |V₆|. Of course, corresponding effects are also present in systems with two or more internal rotors. However, in these systems additional minima and/or maxima may occur when potential interaction terms become significant, e.g. when V₃₃ and/or V′₃₃ have magnitudes similar to V₃ in a molecule like acetone (molecular symmetry group G₃₆ = [33]C_2v). A number of examples for 1-D and 2-D systems are given and some consequences for the spectroscopy are discussed.

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Footnotes:

E.G. Schnitzler, et al., J. Phys. Chem. A 121 (2017) 8625F

The ground and excited state IR vibronic spectra of jet-cooled methyl anthranilate (MA) in the hydride stretch (2400-3800 cm⁻¹ and mid-IR (1400-1800 cm⁻¹) regions have been recorded and many of the peaks have been assigned. The key exception, and the subject of this talk, is the H-bonded NH stretch on the S₁ excited surface. In contrast to the S₀ surface, where the NH stretches of -NH₂ can be modeled assuming symmetric and asymmetric stretch vibrations, on the S₁ surface only the free NH stretch is observed. Time-dependent density functional electronic structure calculations combined with both normal mode and VPT2 results predict an extremely bright transition between 2900 cm⁻¹ and 3100 cm⁻¹ depending on the level of theory for the hydrogen bonded NH stretch. No corresponding transition is observed experimentally. To explain the discrepancy between the experimental and calculated intensities of the dislocated NH stretch transition in the S₁ excited state a model is proposed based on the adiabatic separation of the NH stretch and other internal coordinates. In this model, the excitation of the NH stretch leads to dramatic structural reorganization, this leading to many Franck-Condon factors that, in turn, lead to substantial shared intensity of the initial bright state over hundreds a wavenumbers, thereby diluting the band sufficiently that it is no longer apparent in the spectra.

Water dimer is probably the most extensively studied hydrogen bonded system. The ground vibrational state of (H₂O)₂ is eight-fold degenerate and it splits into six levels (A₁, E₁, B₁, A₂, E₂, B₂). It has a large a-dipole, which inverses on donor-acceptor interchange tunnelling. The result is that the E states of (H₂O)₂ give spectra of the rigid rotor type whereas the A and B states give rotational tunnelling spectra G. T. Fraser, International Review in Physical Chemistry, 1991, 10, 189–206. The incorporation of argon (Ar) introduces a new dipole in the system (along the trimer a-axis). The (H₂O)₂ (dimer) ‘‘a’’ axis is the ‘‘b’’ axis for the Ar(H₂O)₂ (trimer). In the trimer, the ‘a-dipole’ transitions appear rigid rotor like for all three tunnelling states, whereas the ‘b-dipole’ transitions show tunnelling splitting spectra. Due to the reduced barrier height in Ar(D₂O)₂, the three states namely A₁, E₁, B₁ could be observed previously. The splitting is measured to be only 106 MHzE. Arunan, T. Emilsson, H. S. Gutowsky, The Journal of chemical physics, 2002, 116, 4886-4895n Ar(D₂O)₂ compared to 1 GHz in (D₂O)₂. The tunnelling splitting in Ar(H₂O)₂ could not be observed early as the splitting was expected to be several GHz. Moreover, only the A₁ and E₁ states are allowed for the Ar(H₂O)₂ as the other B₁ state has zero statistical weight. The A₁ state could appear either above or below the E₁ states depending on the K quantum number. With the help of a fourfold periodic potentialJ. D. Lewis, T. B. Malloy Jr, T. H. Chao, J. Laane, Journal of Molecular Structure, 1972, 427-449 we have accurately predicted the fingerprints of b-dipole A₁⁺ ↔ A₁⁻ transitions and observed them using a pulsed nozzle Balle-Flygare Fourier transform microwave spectrometer. Measurement of these transitions enabled us to determine the donor-acceptor tunnelling splitting of 4257.41(4) MHz in Ar(H₂O)₂, compared to about 20 GHz in (H₂O)₂. Also the more detailed structural parameters of the Ar(H₂O)₂ have been evaluated in this work and critically compared with the (H₂O)₂.

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Footnotes:

G. T. Fraser, International Review in Physical Chemistry, 1991, 10, 189–206.. E. Arunan, T. Emilsson, H. S. Gutowsky, The Journal of chemical physics, 2002, 116, 4886-4895i J. D. Lewis, T. B. Malloy Jr, T. H. Chao, J. Laane, Journal of Molecular Structure, 1972, 427-449,

The conformational analysis represents the first step toward a detailed characterization and understanding of the structure-function relationships of molecular systems. In this respect spectroscopic experiments on isolated bio- and organic-molecules allow detection of different binding schemes and three-dimensional (3D) conformations without perturbing effects of the environment. Detection of multiple 3D-geometries concomitantly present in an experimental mixture can be facilitated by “in situ” structural changes induced either by thermal variations, or the interaction with near-IR (NIR) to ultraviolet (UV) light. These sophisticated experiments need to be supported by accurate and reliable computational studies allowing to link the rich experimental data to the desired information on the structure and properties of complex molecular systems. Therefore a reliable computational approach should be able to provide a balanced description of all interactions allowing for extended mapping of the whole conformational space, including the accurate prediction of equilibrium structures, their relative positions on the potential energy surface (PES), free energies corresponding to the specific experimental conditions and the spectroscopic properties. Computations based on the second-order perturbation theory (VPT2) allow accounting for the anharmonicity of both wave function and properties. This results in a correct description of the intensity of non-fundamental transitions and more accurate band-shapes. Moreover, the same anharmonic force fields as employed in the determination of vibrational spectra allow considering vibrational corrections to molecular properties or thermodynamic functions. For flexible molecules, the large amplitude-motion (LAM) free approach, where all LAMs anharmonic constants are excluded, allows overcoming problems due to contaminating the overall VPT2 treatment and higher frequency vibrations. The most reliable structural, spectroscopic and energetic results can be obtained combining various computational methods ranging from density functional theory (DFT) to coupled-cluster (CC), with the latter representing also references for the benchmarking of different DFT methodologies.