To extend the ethane database we recorded a 0.0028 cm⁻¹resolution spectrum of CH₃CH₂D from 650 to 1500 cm⁻¹using a Bruker IFS-125HR at the Jet Propulsion Laboratory. The 98% deuterium-enriched sample was contained in the 0.2038 m absorption cell; one scan was taken with the sample cryogenically cooled to 130 K and another at room temperature. From the cold data, we retrieved line positions and intensities of 8704 individual absorption features from 770 – 880 cm⁻¹using a least squares curve fitting algorithm. From this set of measurements, we assigned 5041 transitions to the ν₁₇ fundamental at 805.3427686(234) cm⁻¹; this band is a c-type vibration, with A and E components arising from internal rotation. The positions were modeled using a 22 term torsional Hamiltonian using SPFIT producing the A and E energy splitting of 5.409(25)x10⁻³ cm⁻¹(162.2(8) MHz) with a standard deviation of 7x10⁻⁴ cm⁻¹(21 MHz). The calculated line intensities at 130 K agree very well with retrieved intensities. To predict line intensities at different temperatures, the partition function value was determined at eight temperatures between 9.8 and 300 K by summing individual energy levels up to J = 99 and K_a = 99 for the six states up through ν₁₇ at 805 cm⁻¹. The resulting prediction of singly-deuterated ethane absorption at 12.5 μm enables its detection in planetary atmospheres, including those of Titan and exoplanets.

The high-resolution infrared spectrum of gaseous ethane-d₁ at 130 K shows transitions that are split into A and E components due to the interaction of overall rotation with the internal rotation of the CH₃ group. An analysis of the spectrum from 680 to 900 cm⁻¹with an expanded version of the program ERHAM P. Groner, J. Chem. Phys. 107 4483 (1997)^, P. Groner, J. Mol. Spectrosc. 278 52 (2012)i

Methanol is the simplest molecule with a three-fold internal rotation and the observation of its ν₈ band served the primary catalyst for the development of internal rotation theory^(a,b). The 75 subsequent years of investigation into the ν₈ band region have yielded a large number assignments, numerous high precision energy levels and a great deal of insight into the coupling of ν_t=3 & 4 with ν₈, ν₇, ν₁₁ and other nearby states^(c). In spite of this progress numerous assignment mysteries persist, the origin of almost half the far infrared laser lines remain unknown and all attempts to model the region quantum mechanically have had very limited success. The C_3V internal rotation Hamiltonian has successfully modeled the ν_t=0,1 & 2 states of methanol and other internal rotors^(d). However, successful modeling of the coupling between torsional bath states and excited small amplitude motion remains problematic and coupling of multiple interacting excited small amplitude vibrations featuring large amplitude motions remains almost completely unexplored. Before such modeling can be attempted, identifying the remaining low lying levels of ν₇ and ν₁₁ is necessary. We present an investigation into the microwave spectrum of ν₇, ν₈ and ν₁₁ along with the underlying torsional bath states in ν_t=3 and ν_t= 4. ^(a) A. Borden, E.F. Barker J. Chem. Phys., 6, 553 (1938). ^(b) J. S. Koehler and D. M. Dennison, Phys. Rev. 57, 1006 (1940). ^(c) R. M. Lees, Li-Hong Xu, J. W. C. Johns, B. P. Winnewisser, and M. Lock, J. Mol. Spectrosc. 243, 168 (2007). ^(d) L.-H. Xu, J. Fisher, R.M. Lees, H.Y. Shi, J.T. Hougen, J.C. Pearson, B.J. Drouin, G.A. Blake, R. Braakman J. Mol. Spectrosc., 251, 305 (2008).

A high resolution (0.0015 cm⁻¹) IR spectrum of propane, C₃H₈, has been recorded with synchrotron radiation at the French light source facility at SOLEIL coupled to a Bruker IFS-125 Fourier transform spectrometer. A preliminary analysis of the ν₂₁ fundamental band (B₁, CH₃ rock) near 921.4 cm⁻¹reveals that the rotational energy levels of 21₁ are split by interactions with the internal rotations of the methyl groups. Conventional analysis of this A-type band yielded band centers at 921.3724(38), 921.3821(33) and 921.3913(44) cm⁻¹for the AA, EE and AE+EA tunneling splitting components, respectively.A. Perrin et al., submitted to J. Mol. Spectrosc.hese torsional splittings most probably are due to anharmonic and/or Coriolis resonance coupling with nearby highly excited states of both internal rotations of the methyl groups. In addition, several vibrational-rotational resonances were observed that affect the torsional components in different ways. The analysis of the B-type band near 870 cm⁻¹(ν₈, sym. C-C stretch) which also contains split rovibrational transitions due to internal rotation is in progress. It is performed by using the effective rotational Hamiltonian method ERHAMP Groner, J. Chem. Phys. 107 (1997) 4483; J. Mol. Spectrosc. 278 (2012) 52.ith a code that allows prediction and least-squares fitting of such vibration-rotation spectra.

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A. Perrin et al., submitted to J. Mol. Spectrosc.T P Groner, J. Chem. Phys. 107 (1997) 4483; J. Mol. Spectrosc. 278 (2012) 52.w

Large amplitude motions in methyl rotor systems have been well studied, especially the coupling between the CH₃ torsion and the CH stretches. The CH₃OO radical is a example of a system where this coupling is relatively small, but its effects still can be observed in the the infrared spectrum taken by the Lee group.K.-H. Hsu, Y.-P. Lee, M. Huang, T. A. Miller, TD08, 68th International Symposium of Molecular Spectroscopy (2013)otational contour simulations based on an asymmetric rotor model show good agreement with the experimental spectrum except for an unexplained broadening of the Q-branch of one of the CH stretch features. The broadening is likely caused by low frequency torsional modes populated at room temperature resulting in sequence band transitions that are slightly shifted from the origin. A reduced dimension model involving the three CH stretches and the CH₃ torsion is applied to CH₃OO to simulate the observed spectrum. The CH stretches are described by a harmonically coupled anharmonic oscillator model in which the parameters depend on the CH₃ torsion angle. Based on these calculations, the observed broadening of the Q-branch can be qualitatively explained by coupling between two CH stretch/CH₃ torsion combination bands which differ by one quantum in torsional excitation. The Ã-~X electronic transitions of halogenated methyl peroxy radicals, CH₂XOO (X-Cl, Br, I), show a complementary structure. At room temperature multiple peaks have been observed in the region of the origin and OO stretch vibronic bands in all three radicals with the spectra for CH₂IO₂ being by far the most complex. This structure may again be the result of hot bands originating from excited torsional levels. Several theoretical models have been investigated to calculate the Franck-Condon factors that govern the structure. A calculation that models the I-C-O-O torsion using curvilinear internal coordinates and molecular geometry and harmonic torsion frequencies predicted by electronic structure calculations shows the best agreement between the CH₂IOO experimental and simulated spectra. The multiple peak structure results from the change in X-C-O-O torsion dihedral between the ~X state and à states. Interestingly, a similar calculation with Cartesian displacement coordinates fails to explain the torsional structure. This study shows the importance of coordinate system choice if a significant displacement in the torsional coordinate occurs upon electronic excitation.

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K.-H. Hsu, Y.-P. Lee, M. Huang, T. A. Miller, TD08, 68th International Symposium of Molecular Spectroscopy (2013)R

The torsion-vibration-rotation analysis of nearly degenerate vibrational states involving both small and large amplitude motion has escaped satisfactory quantum mechanical description. Unfortunately the interstellar medium is filled with many prevalent molecules that feature internal rotation that couples strongly with torsional bath states. Many excited states are observed in emission in hot cores associated with massive star formation and it is likely that absorption in the infrared will be seen by JWST. We present our progress on the analysis of the high resolution pure rotational spectrum of ethyl cyanide, CH₃CH₂CN, which is highly abundant in hot cores with massive star formation and can serve as a sensitive temperature and source size probeA.M. Daly, C. Bermúdez, A. López, B. Tercero, J.C. Pearson, N. Marcelino, J.L. Alonso, J. Cernicharo Astrophys. J., 768 81 (2013) Although the ground state has been assigned to 1.6 THzC.S. Brauer, J.C. Pearson, B.J. Drouin, S. Yu ApJ Suppl. Ser., 184 133 (2009) the two vibrational states ν₁₃ and ν₂₁, the C-C-N bend and torsion, have only been assigned up to 400 GHzD.M. Mehringer, J.C. Pearson, J. Keene, T.G. Phillips Astrophys. J., 608 306 (2004) It is clear that detailed understanding of excited states will help properly model the temperature dependence of the intensity. We will report the progress on the fit up to 1.5 THz for the states ν₁₃ , ν₂₁ , ν₂₀ and ν₁₂, at 206.5 cm⁻¹, 212 cm⁻¹, 375 cm⁻¹and 532 cm⁻¹respectively. In spite of a nearly 1200 cm⁻¹barrier to internal rotation all the vibrational states observed feature A/E splittings inconsistent with such a high barrier suggesting that there is extensive coupling between the torsional bath states and the excited vibrations. The low lying states of ethyl cyanide provide an opportunity to assess all the possible interaction under the C_S group for both A and E symmetry in the high barrier case to serve as a benchmark for developing theory for the analysis of lower barrier cases.

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A.M. Daly, C. Bermúdez, A. López, B. Tercero, J.C. Pearson, N. Marcelino, J.L. Alonso, J. Cernicharo Astrophys. J., 768 81 (2013). C.S. Brauer, J.C. Pearson, B.J. Drouin, S. Yu ApJ Suppl. Ser., 184 133 (2009), D.M. Mehringer, J.C. Pearson, J. Keene, T.G. Phillips Astrophys. J., 608 306 (2004).

The molecular-beam Fourier-transform microwave spectrum of pinacolone (methyl tert-butyl ketone) has been measured in several regions between 2 and 40 GHz. Assignments of a large number of A and E transitions were confirmed by combination differences, but fits of the assigned spectrum using several torsion-rotation computer programs based on different models led to the unexpected conclusion that no existing program correctly captures the internal dynamics of this molecule. A second puzzle arose when it became clear that roughly half of the spectrum remained unassigned even after all predicted transitions were added to the assignment list. Quantum chemical calculations carried out at the MP2/6-311++G(d,p) level indicate that this molecule does not have a plane of symmetry at equilibrium, and that internal rotation of the light methyl group induces a large oscillatory motion of the heavy tert-butyl group from one side of the C_s saddle point to the other. The effect of this non-C_s equilibrium structure was modeled for J = 0 levels by a simple two-top torsional Hamiltonian, where magnitudes of the strong top-top coupling terms were determined directly from the ab initio two-dimensional potential surface. A plot of the resultant torsional levels on the same scale as a one-dimensional potential curve along the zig-zag path connecting the six (unequally spaced) minima bears a striking resemblance to the 1:2:1 splitting pattern of levels in an internal rotation problem with a six-fold barrier. A plot of the six minima closely resembles the potential surface for methylamine. This talk will focus on implications of these resemblances for future work.

The methylisocyanate molecule, CH₃NCO, is of interest as a potential astrophysical species and as a model system for the study of quasisymmetric behavior. The rotational spectrum is made very complex by the presence in CH₃NCO of two large-amplitude motions: an almost free internal rotation and a low barrier skeletal bending motion. This challenging spectrum has, nevertheless, been assigned at 8-38 GHz by Stark spectroscopyJ.Koput, J. Mol. Spectrosc. 115, 131 (1986).nd has been measured at 117-376 GHz with the broadband FASSST technique.Z.Kisiel et al., 65^th OSU Symposium on Molecular Spectroscopy, The Ohio State University, Ohio 2010, RC-13.0.2cm We presently report the results of measuring this spectrum also in supersonic expansion for the transitions below 40 GHz, and at room-temperature in the region between 40 and 120 GHz. In this way we are finally able to confirm the assignment of the ground state and of the internal rotation m=1 state and to analyse the nitrogen hyperfine splitting structure. It is also possible to confidently transfer the Stark-based assignment to the transition sequences measured in the mm-wave region, and to assign high K_a sequences. Various models for fitting this spectrum are explored but, even without more extensive fits, we are now able to present temperature scalable linelists for astrophysical applications.

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J.Koput, J. Mol. Spectrosc. 115, 131 (1986).a Z.Kisiel et al., 65^th OSU Symposium on Molecular Spectroscopy, The Ohio State University, Ohio 2010, RC-13.

A mini-cavity microwave spectrometer was used to record the rotational spectra arising from 2-phenylethyl methyl ether and its weakly bonded argon complex in the frequency range of 10.5 – 22 GHz. Rotational spectra were found for two stable conformations of the monomer: anti-anti and gauche-anti, which are 1.4 kJ mol⁻¹ apart in energy at wB97XD/6-311++G(d,p) level. Doubled rotational transitions, arising from internal motion of the methyl group, were observed for both conformers. The program XIAM was used to fit the rotational constants, centrifugal distortion constants, and barrier to internal rotation to the measured transition frequencies of the A and E internal rotation states. The best global fit values of the rotational constants for the anti-anti conformer are A= 3799.066(3) MHz, B= 577.95180(17) MHz, C= 544.7325(3) MHz and the A state rotational constants of the gauche-anti conformer are A= 2676.1202(7) MHz, B= 760.77250(2) MHz, C= 684.78901(2) MHz. The rotational spectrum of 2-phenylethyl methyl ether – argon complex is consistent with the geometry where argon atom lies above the plane of the benzene moiety of gauche-anti conformer. Tunneling splittings were too small to resolve within experimental accuracy, likely due to an increase in three fold potential barrier when the argon complex is formed. Fitted rotational constants are A= 1061.23373(16) MHz, B= 699.81754(7) MHz, C= 518.33553(7) MHz. The lowest energy solvated ether - water complex with strong intermolecular hydrogen bonding has been identified theoretically. Progress on the assignment of the water complex will also be presented.

Rotational spectra of α-Methylstyrene, cis-β-Methylstyrene, and trans-β-Methylstyrene were examined to investigate their intrinsic tunneling properties. Theoretical calculations at wB97XD/6-311++G(d,p) level predict only one stable conformer for each molecular system. Spectra were recorded in the frequency range of 10.5 - 22.0 GHz using a cavity based Fourier transform microwave spectrometer. A relaxed potential scan for the methyl torsion at wB97XD/6-311++G(d,p) level of theory was used to estimate the associated barrier for the hindered internal rotation. The program XIAM was used to fit the rotational constants, distortion constants and barrier to methyl internal rotation to the measured transition frequencies of the A and E internal rotation states.

Microwave spectra of the neon-acetone van der Waals complex were measured using a cavity-based molecular beam Fourier-transform microwave spectrometer in the region from 5 to 18 GHz. Both ²⁰Ne and ²²Ne containing isotopologues were studied and both c- and weaker a-type rotational transitions were observed. The transitions are split into multiplets due to the internal rotation of two methyl groups in acetone. Electronic structure calculations were done at the MP2 level of theory with the 6-311++g (2d, p) basis set for all atoms and the internal rotation barrier height of the methyl groups was determined to be about 2.8 kJ/mol. The ab initio rotational constants were the basis for our spectroscopic searches, but the multiplet structures and floppiness of the complex made the quantum number assignment very difficult. The assignment was finally achieved with the aid of constructing closed frequency loops and predicting internal rotation splittings using the XIAM code.H. Hartwig and H. Dreizler, Z. Naturforsch. A 51, 923 (1996).nalyses of the spectra yielded rotational and centrifugal distortion constants, as well as internal rotation parameters, which were interpreted in terms of structure and internal dynamics of the complex. H. Hartwig and H. Dreizler, Z. Naturforsch. A 51, 923 (1996).A

Acetyl- and nitrogen containing substances play an important role in chemical, physical, and especially biological systems. This applies in particular for acetamides, which are structurally related to peptide bonds. In this work, N-methyldiacetamide, CH₃N(COCH₃)₂, was investigated by a combination of molecular beam Fourier transform microwave spectroscopy and quantum chemical calculations. In N-methyldiacetamide, at least three large amplitude motions are possible: (1) the internal rotation of the methyl group attached to the nitrogen atom and (2, 3) the internal rotations of both acetyl methyl groups. This leads to a rather complicated torsional fine structure of all rotational transitions with additional quadrupole hyperfine splittings caused by the ¹⁴N nucleus. Quantum chemical calculations were carried out at the MP2/6-311++G(d,p) level of theory to support the spectral assignment. Conformational analysis was performed by calculating a full potential energy surface depending on the orientation of the two acetyl groups. This yielded three stable conformers with a maximum energy difference of 35.2 kJ/mol. The spectrum of the lowest energy conformer was identified in the molecular beam. The quadrupole hyperfine structure as well as the internal rotation of two methyl groups could be assigned. For the N-methyl group and for one of the two acetyl methyl groups, barriers to internal rotation of 147 cm⁻¹ and of 680 cm⁻¹, respectively, were determined. The barrier of the last methyl group seems to be so high that no additional splittings could be resolved. Using the XIAM program, a global fit with a standard deviation on the order of our experimental accuracy could be achieved.

A new hybrid-model fitting program for methylamine-like molecules has been developed, based on an effective Hamiltonian in which the ammonia-like inversion motion is treated using a tunneling formalism, while the internal-rotation motion is treated using an explicit kinetic energy operator and potential energy function. The Hamiltonian in the computer program is set up as a 2x2 partitioned matrix, where each diagonal block consists of a traditional torsion-rotation Hamiltonian (as in the earlier program BELGI), and the two off-diagonal blocks contain all tunneling terms. This hybrid formulation permits the use of the permutation-inversion group G₆ (isomorphic to C_3v) for terms in the two diagonal blocks, but requires G₁₂ for terms in the off-diagonal blocks. Our first application of the new program is to 2-methylmalonaldehyde. Microwave data for this molecule were previously fit (essentially to experimental measurement error) using an all-tunneling Hamiltonian formalism to treat both large-amplitude-motions V.V. Ilyushin, E.A. Alekseev, Yung-Ching Chou, Yen-Chu Hsu, J. T. Hougen, F.J. Lovas, L. Picraux, J. Mol. Spectrosc. 251 (2008) 56-63 For 2-methylmalonaldehyde, the hybrid program achieves a fit of nearly the same quality as that obtained by the all-tunneling program, but fits with the hybrid program eliminate a large discrepancy between internal rotation barriers in the OH and OD isotopologues of 2-methylmalonaldehyde that arose in fits with the all-tunneling program. Other molecules for application of the hybrid program will be mentioned.

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V.V. Ilyushin, E.A. Alekseev, Yung-Ching Chou, Yen-Chu Hsu, J. T. Hougen, F.J. Lovas, L. Picraux, J. Mol. Spectrosc. 251 (2008) 56-63.

The ground state rotational spectrum of itaconic acid (methylenesuccinic acid) and N-acetylethanolamine (AEA) have been collected and analyzed over the frequency range of 7-17.5 GHz. Both molecules displayed an unexpected tunneling splitting pattern caused by a V₂ and V₃ barriers, respectively. AEA’s methyl rotor is directly connected to a carbonyl and is expected to have too high of a barrier to internal motion. Itaconic acid contains no methyl groups or any symmetry, yet a torsional splitting was observed. The origin of this motion as well their barrier heights and lowest energy conformations will be discussed.

In BF₂OH, difluoroboric acid, the OH group is the subject of a large amplitude torsion motion which induces a splitting in the rotational spectrum as well as in the high-resolution infrared spectrum. It is interesting to check whether it is still posible to determine a semiexperimental equilibrium structure for such a molecule. For this goal, the rotation-vibration interactions constants have been experimentally determined by analyzing all the fondamental bands. They have also been computed ab initio using two different levels of theory. The results of the analysis as well as the determination of the structure will be reported.