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Structural data from microwave spectra of monomers and oligomers of the methyl methacrylate system compared to theoretical predictions allows for accurate predictions of the structure and physical properties of higher oligomers or even polymers. The idea behind this project is to start with small building blocks and to successively increase the size of the oligomers in order to obtain more and more accurate predictions. Following the previous analysis of the monomer, this contribution focuses on the dimer of methyl methacrylate. In the dimer phase of methyl methacrylate, which was subjected to a thermal self-polymerization process, the linear methyl methacrylate dimer was identified by means of rotational spectroscopy. The analysis was performed using the coaxially oriented beam-resonator arrangement Fourier-transform microwave (COBRA-FTMW) spectrometer. The dimer comprises three methyl rotors. Coupling of the methyl internal rotation to the overall rotation causes a complicated, challenging splitting of the rotational spectrum. The fact that only the two methoxymethyl groups contributed resolvable ( > 5kHz) splittings simplified the spectrum a little and a fit of molecular parameters to the experimental data was achieved with experimental accuracy utilizing the program XIAM.

The monitoring of gas phase mononitrotoluenes is crucial for defence, civil security and environmental interests since they are used as taggant for TNT detection and in the manufacturing of industrial compounds such as dyestuffs. We have succeeded to measure and analyse at high resolution the room temperature rotationally resolved millimeter-wave spectrum of 3-nitrotoluene (3-NT). Experimental and theoretical difficulties have been overcome, in particular, those related to the low vapour pressure of 3-NT and to the internal rotation of a CH₃ in almost free rotation regime (V₃=6.7659(24) cm⁻¹). Rotational spectra have been recorded in the microwave and millimeter-wave ranges using a supersonic jet Fourier Transform microwave spectrometer (T_rot < 10 K) M. Tudorie, et al., J. Chem. Phys. 134, (2011), 074314nd a millimeter-wave frequency multiplication chain (T=293 K) G. Mouret, et al., IEEE Sens. J. 13, (2013), 133-138. respectively. Spectral analysis of pure rotation lines in the vibrational ground state and in the first torsional excited state supported by quantum chemistry calculations permits to characterise the rotational energy of the molecule, the hyperfine structure due to the ¹⁴N nucleus and the internal rotation of the methyl group.A. Roucou et al., ChemPhysChem (2018)html:<hr /><h3>Footnotes: M. Tudorie, et al., J. Chem. Phys. 134, (2011), 074314a G. Mouret, et al., IEEE Sens. J. 13, (2013), 133-138., A. Roucou et al., ChemPhysChem (2018)

To protect atmosphere, hydrofluorocarbons (HFCs) are the current substituents of the dangerous CFCs and other ODS (ozone depleting substances).Burkholder J.B. et al., Chem.Rev., 2015, 115, 3704lthough HFCs are not ODS, they are potent greenhouse gases and, thus, they would be harmful to climate. Consequently, there is a keen interest on monitoring their reaction and the decomposition products in order to measure their effects. Fluoral (trifluoroacetaldehyde, ) is one of the stable decomposition products of several families of ODS substituents. Monitoring it in the atmosphere is hampered by the few spectroscopic data available in the literature. The rotational spectrum of fluoral from 8-40 GHz was measured previously,Woods R.C., 1967, J. Chem. Phys., 46, 4789owever the performed analysis of the spectrum was rather limited due to difficulties in theoretical description. These difficulties reside mainly in the hindered internal rotation of the group. Compared to acetaldehyde, in fluoral, the group represents the major part of the molecular mass. Therefore, there is a strong coupling between the overall molecular rotation and the internal rotation of the top ρ = 0.92. As such, previously used principal axis method, where the axes remain unaffected by the large amplitude motion, is not fully suitable for the analysis. We present the analysis of new high resolution microwave and millimeter wave spectra of fluoral in the ranges 6-26 and 50-305 GHz, respectively, employing rho-axis method implemented in RAM36 program. The rotational distortional and internal rotational parameters that reproduce the spectral at experimental accuracy were determined for the ground state and several lowest excited torsional states.

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Footnotes:

Burkholder J.B. et al., Chem.Rev., 2015, 115, 3704A Woods R.C., 1967, J. Chem. Phys., 46, 4789h

The microwave spectrum of o-fluorotuene has been investigated at -12 ^°C from 8.7-26.5 GHz with waveguide chirped-pulse Fourier transform microwave spectroscopy (CP-FTMW). This molecule has a measured V₃ barrier of 238.3 cm⁻¹. The low barrier height resulted in some challenges when fitting the excited states in XIAM. This work improves on our previous fit by extending into the 18-26.5 GHz frequency range and by switching to use the RAM36 fitting software instead. Based on newly collected data and our previous assignments of excited torsional states, a global fit of the ground and first two excited states has been conducted in RAM36. Additionally, scans were taken at 3 ^°C, 19 ^°C, and 35 ^°C to increase the population of the excited torsional modes. Details of the fit including improvements from previous work will be discussed in the talk.

The microwave spectrum of isoprene has been investigated in a cryogenic (4-7 K) He buffer gas cell (12-26.5 GHz) and in a room temperature waveguide (8.7-26.5 GHz). Rotation about the single C-C bond converts the lower-energy trans rotamer to the higher-energy gauche-conformer via an unstable, planar, cis transition state. As in butadiene, it is counterintuitive that gauche is more stable than cis because it has long been believed that planarity is required for π electron delocalization, a factor that often imparts greater stability. r0pt Figure In standard jet experiments, observation of the higher-energy gauche conformer has proven challenging. However in the buffer gas cell, collisions with cold He result in rapid but gentle conformational cooling, allowing for straightforward observation of the gauche-isoprene rotamer. The rotational spectrum of gauche is complex owing to the combined effects of rotational line splitting (0⁺ / 0⁻ from gauche-gauche inversion) and A/E splitting of the methyl rotor. On the basis of new theoretical calculations, steric hindrance of methyl rotation is predicted to be lower in the gauche than in the trans conformer (V₃ barrier approximately 653 cm⁻¹ or 7.8 kJ/mol). In addition to the on-going spectroscopic analysis, efforts are now underway to better characterize the energetics of C-C bond rotation in isoprene, with the goal of ultimately understanding the factors that result in the greater stability of the gauche relative to the cis isoprene rotamer.

Hyperfine splittings in the ground state of CH₃OH have recently been studied by several groups [JCP(2015)143\textunderscore044304, (2016)145\textunderscore024307, (2016)145\textunderscore244301]. In our work [JCP(2016)145\textunderscore024307], we treated splittings in the Lamb-dip sub-mm-wave transitions between some torsion-rotation states of E symmetry. These doublets increase nearly linearly with J, and we attributed them to the effect of torsionally mediated spin-rotation interaction of the methyl protons. Hyperfine doublets of this type have so far been observed only in methanol. The focus of this talk is on hyperfine doublet, "triplet" and quartet splittings observed in the first excited E torsional state of CH₃OH from three laboratories. Four series of lines dominate the available data. Measurements are: (i) from Kharkov/Lille, K = 6 ← 7, Q branch, quartets, with 7 ≤ J ≤ 15; (ii) from NNOV, K = 3 ← 2, Q branch, with 3 ≤ J ≤ 18, where the series starts as quartets, changes to doublets at J = 7, and then finally to singlets at J = 17; (iii) from NNOV, K = -2 ← -3, P branch, doublets, with 8 ≤ J ≤ 12; (iv) from Kharkov/Lille, K = 8 ← 7, Q branch, with 8 ≤ J ≤ 24, where the series starts as triplets and becomes doublets at J = 15. We have ignored the central features of the triplets, since we believe they might be due to unusual double-N crossover resonances; and (v) a few measurements that don't form branches. We have empirically modeled these hyperfine splittings with spin-torsion, spin-rotational and spin-spin terms for the two I = \frac12 spin systems arising from the OH and CH₃ protons, respectively. Work is in progress to understand the deeper physical meaning of these fitting parameters and compare them with ab initio calculations.

A few years ago, the authors wrote a hybrid program to fit rotational levels in molecules with one CH₃ internal-rotation large-amplitude motion, one NH₂ inversion large-amplitude motion, and symmetry described by the G₁₂ PI group. This program was applied with success to the MW spectrum of 2-methylmalonaldehyde, but the rather small data set for this molecule did not provide a stringent test of the model. More challenging is the application of the hybrid program to CH₃NH₂, since this molecule has a much larger data set, containing both MW and IR transitions, as well as having a more extensive v_t, J, and K quantum number coverage. In our ISMS talk this year we will first give an overview of our best least-squares fit to date: The data set contains slightly more than 2500 MW and 11000 IR transitions with J ≤ 32 and K ≤ 14, which are fit to a weighted standard deviation of 1.64 using 71 parameters. Next, we present an assessment of this fit’s strong points (e.g., significantly less parameters, ability to predict spectra in higher torsional states) and weak points (e.g., somewhat larger standard deviation, greater parameter correlation) when compared to the best all-tunneling-model fit in the literature. Based on this assessment, we believe that our fit, as well as the predictive abilities of the program, are sufficiently good that we can now begin considering collaborations with measurement and assignment campaigns of v_t = 1 MW data and v_t = 2, 3 IR data already underway in other laboratories. Finally, we will present a slightly modified ordering scheme for the operators in this hybrid program, and describe the need for devising a contact transformation treatment to specify determinable parameters in the hybrid Hamiltonian, in order to reduce parameter-correlation problems during the trial-and-error fitting process. A knowledge of determinable parameters would be particularly useful here, since there is little previous experience to guide the choice of “a good set” of higher-order constants to float when carrying out large fits having over 10000 lines, J_max = 40, K_max = 15, v_t = 0 and 1 torsional states, A₁, A₂, B₁, B₂, E₁, E₂ symmetry species, and nearly 100 parameters.

A new program is described for fitting several isolated small-amplitude fundamentals embedded in a pure torsional bath in molecules like acetaldehyde, in which the frame has C_s symmetry and the methyl top has C_3v symmetry. The program is based on the Longuet-Higgins group theoretical ideas and uses the Rho-axis method. In the talk the basic ideas, the structure of the program as well as the strategy for checking the program will be discussed. Also we present the first results 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 application of the new program to the spectrum of the ν₁₀ vibrational state of acetaldehyde, CH₃CHO, near 509 cm⁻¹. The analysis of the 509 cm⁻¹band is accompanied by the analysis of microwave spectrum of the ν₁₀ vibrational state and v_t = 3,4 torsional states of acetaldehyde.

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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

Spectra of molecules with a 3-fold internal rotor become much more interesting in the presence of another large-amplitude motion (LAM) that leads to tunneling between equivalent asymmetric forms which may also tunnel to a different conformer. An effective rotational Hamiltonian has been derived for such a system of which ethanol, CH₃CH₂OH, is a typical example J.C. Pearson et al., J. Mol. Spectrosc. 251 (2008) 394 For isolated vibrational states of molecules with two symmetric rotors with sufficiently "high" barriers, the ERHAM codeP. Groner, J. Mol. Spectrosc. 278 (2012) 52–67orks well. Modifications were explored to find out whether ERHAM can be coaxed to treat ethanol-type systems, using "ancient" unpublished microwave data from vibrational ground and excited states of ethylphospine, CH₃CH₂PH₂, as test data. For gauche ethylphosphine, the splitting between the a-type Coriolis-coupled ground states is 5.215(6) MHz whereas it is 229.9(2) MHz in the ν₂₄ state (PH₂ torsion). The tunneling energy coefficients ϵ₀₁ for the methyl internal rotation are -0.63(2) MHz and 2.93(5) MHz (sign undeterminable), respectively. These results look promising; however, up to now, sets of assigned frequencies had to be omitted from fits to experimental uncertainty of 25 kHz: (a) for the ground state, all c-type transitions J_4,J−3 - J_3,J−3 (41 < J < 48) for systematic large deviations (reason unknown); (b) for the ν₂₄ state, half of the quartets of the J_3,J−2 - J_2,J−2 series (28 < J < 32) because of interactions with a state of the trans conformer) and some of the K_a = 1, 2 low-J transitions (incorrect assignments or unknown reasons). Analyses of data for the ν₂₃ (CH₃ torsion) and ν₂₂ (CCP deformation) states are in progress.

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J.C. Pearson et al., J. Mol. Spectrosc. 251 (2008) 394. P. Groner, J. Mol. Spectrosc. 278 (2012) 52–67w

r0pt Figure The rotational spectra of vinylsulfur pentafluoride, and three isotopologues (S-34 and both C-13's) have been recorded in the frequency region of 6 GHz to 20 GHz. Measurements were made using both cavity and chirped pulse Fourier transform microwave spectrometers. The four-fold barrier to the internal rotation of the –SF₅ group against the vinyl group has been approximated from the spectral data which is possible due to the observation of easily resolved pure rotational transitions in each of the A, B, and doubly degenerate E torsional substates. All transitions were successfully fit simultaneously using the ERHAM code. We note that this work, we believe, represents the first use of pure rotational spectroscopy to characterize a four-fold barrier internal rotation problem. Rotational constants, structure, and the internal rotation barrier height will be presented and compared to results from quantum chemical calculations.

The rotational spectra of the two title molecules and several isotopologues have been recorded in the frequency region of 6 GHz to 20 GHz using Fourier transform microwave spectroscopy. r0pt Figure For propen-1-ylsulfur pentafluoride, triplets of rotational transitions were observed appropriate for the A, B, and doubly degenerate E torsional substates arising from the four-fold barrier to internal rotation of the –SF₅ group against the propen-1-yl frame. However, the observed splittings, which are on the order of tens of kHz, were considerably smaller in magnitude than those analogous splittings observed in the spectra of the vinylsulfur pentafluoride, which were on the order of thousands of kHz. For the buten-1-ylsulfur pentafluoride, for which two conformers have been observed, at the resolution of the chirped pulse FTMW spectrometer used, splittings were not observable and the observed spectrum could be fit using the Hamiltonian of a semi-rigid rotor. Further experiments using a cavity FTMW spectrometer are underway. Constants from the spectral analyses, together with the results of quantum chemical calculations have allowed for the alkene-SF₅ barrier to internal rotation to be examined as a function of alkene chain length and the results will be presented.

Vitamin B6, like the rest of vitamins, is an important compound involved in numerous biological functions. Concretely, takes part in brain and nervous system health, in the metabolism of carbohydrates to produce energy and in the process of removing unwanted chemicals from our blood, among others. Vitamine B6 is found in a variety of forms, and here we present the rotational study of the pyridoxine form. Pyridoxine has been brought into gas-phase by means of laser ablation and probed by broadband LA-CP-FTMW microwave spectroscopy in the range 2-8 GHz. The presence of a methyl group in the structure offer us a nice internal rotation problem reflected in the spectrum, that together with the hyperfine structure due to the ¹⁴N atom, makes this study very challenging. The high resolution of LA-MB-FTMWC. Bermúdez, S. Mata, C. Cabezas and J. L. Alonso, Angew. Chemie - Int. Ed., 2014, 53, 11015–11018.pectroscopy has been crucial to overcome this problematic.

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Footnotes:

C. Bermúdez, S. Mata, C. Cabezas and J. L. Alonso, Angew. Chemie - Int. Ed., 2014, 53, 11015–11018.s