Methacrolein and methyl vinyl ketone are the two major oxidation products of isoprene emitted in the troposphere. New spectroscopic information is provided with the aim to allow unambiguous identification of these molecules, characterized by a large amplitude motion associated with the methyl top. State-of-the-art millimeter-wave spectroscopy experiments coupled to quantum chemical calculations have been performed. Comprehensive sets of molecular parameters have been obtained. The torsion-rotation-vibration effects will be discussed in detail. From the atmospheric application point of view the results provide precise ground state molecular constants essential as a foundation (by using the Ground State Combination Differences method) for the analysis of high resolution spectrum, recorded from 600 to 1600 cm⁻¹. The infrared range can be then refitted using appropriate Hamiltonian parameters. The present work is funded by the French ANR through the PIA under contract ANR-11-LABX-0005-01 (Labex CaPPA), by the Regional Council Nord-Pas de Calais and by the European Funds for Regional Economic Development (FEDER).

The Fourier transform spectra of the CSH-bending and CH₃-rocking infrared bands of CH₃SH have been investigated at 0.001 cm⁻¹ resolution employing synchrotron radiation at the Canadian Light Source in Saskatoon. The relative band strengths and structures are remarkably different from those for the analogous CH₃OH relative, with the CSH bend being very weak and both the in-plane and out-of-plane CH₃ rocks being strong with comparable intensities. The CSH bend, centered at 801.5 cm⁻¹, has parallel a-type character with no detectable b-type component. The out-of-plane CH₃ rock at 957.0 cm⁻¹ is a purely c-type perpendicular band, whereas the in-plane rock around 1074 cm⁻¹ is of mixed a/b character. The K-reduced v_t = 0 sub-state origins for the CSH bend follow the normal oscillatory torsional pattern as a function of K with an amplitude of 0.362 cm⁻¹, as compared to 0.653 cm⁻¹ for the ground state and 0.801 cm⁻¹ for the C-S stretching mode. The torsional energy curves for the out-of-plane rock are also well-behaved but are inverted, with an amplitude of 1.33 cm⁻¹. In contrast, the sub-state origins for the in-plane rock do not display a clear oscillatory structure but are scattered over a range of about 2 cm⁻¹, with indications of some significant perturbations. The assignments for the three bands all extend up to about K = 10 and are well-determined from GSCD relations, particularly for the a/b in-plane rock for which ∆K = 0, +1 and −1 transitions are all observed.

The adiabatic separation in methyl mercaptan of the high-frequency asymmetric CH stretch vibrations from the low-frequency torsional (γ) and CSH bend (ρ) coordinates yields a set of 7 vibrational conical intersections (CIs). The three CIs in the staggered conformation at ρ = 79^° are close to the global minimum energy geometries (ρ_e = 83.3^°), accounting for the observed near-degeneracy of the two asymmetric CH stretch vibrations. The vibrational frequencies were computed at the CCSD(T)/aug-cc-pVTZ level. A new high-order Exe Jahn-Teller model, which involves a spherical harmonic expansion in ρ and γ, fits the calculated electronic and vibrational energies over the whole range of γ and for ρ between 0^° and 100^° to within a standard deviation of 0.2 cm⁻¹. The pattern of the CIs contrasts with that in methanol where the CIs occur only in the eclipsed conformation near the top of the torsional barrier. An examination of three alternative diabatization schemes for the two molecules points to rather different nuclear dynamics. In CH₃SH crossings between the upper and lower adiabatic surfaces are predicted to occur predominantly with motion along the CSH bending coordinate; whereas in CH₃OH, such crossings are predicted to occur predominantly with torsional motion.

The spectroscopic study of molecules with large amplitude vibrations have led to reconsider the concept of molecular structure. Sometimes identifying definite bond lengths and angles is not enough to reproduce the experimental data so one must have information on the large amplitude molecular vibration potential energy function and dynamics. 3,3-dimethyloxetane (DMO) has non-planar ring equilibrium configuration and a double minimum potential function for ring-puckering with a barrier of 47 cm⁻¹. J. A. Duckett, T. L. Smithson, and H. Wieser, J. Mol. Spectrosc. 1978, 69 , 159; J. Mol. Struct. 1979, 56, 157^, J. C. López, A. G. Lesarri, R. M. Villamañán and J. L. Alonso, J. Mol. Spectrosc. 1990, 141, 231he observation of endocyclic ^13C and ^18O monosubstituted isotopologues R. Sánchez, S. Blanco, A. Lesarri, J. C. López and J. L. Alonso, Phys. Chem. Chem. Phys. 2005, 7, 1157llow to conclude that the ring is puckered. However an interesting feature was observed for the ^13C substitutions at the methyl carbon atoms. While two different axial and equatorial ^13C−methyl groups spectra are predicted from a rigid non−planar ring DMO model, only one species was found. The observed rotational transitions appear at a frequency close to the average of the frequencies predicted for each isotopologue. The observed lines have the same intensity as that found for the ^13C_ isotopomer and double that that found for the ^13C_ isotopomer.^c J. C. Lpez, A. G. Lesarri, R. M. Villamañán and J. L. Alonso, J. Mol. Spectrosc. 1990, 141, 231T R. Snchez, S. Blanco, A. Lesarri, J. C. Lpez and J. L. Alonso, Phys. Chem. Chem. Phys. 2005, 7, 1157a

Secondary organic aerosols (SOA) have a significant effect on climate change. They are mainly produced in the atmosphere by oxidation of gaseous precursors. Fu et al.T.- M. Fu et al., J. Geophys. Res., 113, (2008).ave suggested trans-methylglyoxal (MG) as a possible precursor of SOA in the cloud for its presence in large quantities in the atmosphere. The characterization of SOAs precursors by laboratory spectroscopy allows providing elements for the understanding of the process of formation of these aerosols. For this purpose, we completed the existing pure rotational spectrum of MG in the 12-40 GHz rangeC.E. Dyltick-Brenzinger and A. Bauder, Chem. Phys. 30, 147 (1978).y new records in a supersonic jet in the 4-20 GHz range (FTMW) and at room temperature in the 150-500 GHz range (mm/submm-wave spectrometer). The analysis was made with the support of quantum chemistry calculations (MP2/CBS and B98/CBS using the Gaussian 09 software). The adjustment of the spectroscopic parameters, taking into account the internal rotation related to the presence of a methyl group, was performed using the RAM36 code. The spectra have been reproduced at the experimental precision up to maximal values of J and K_a equal to 85 and 35, respectively. The data obtained for the isolated molecule, both experimentally and theoretically, will allow the study of its hydrated complexes and, by comparison, will give access to (micro-) hydration properties. For this purpose, two stable complexes predicted by theoretical calculations will be studied.

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Footnotes:

T.- M. Fu et al., J. Geophys. Res., 113, (2008).h C.E. Dyltick-Brenzinger and A. Bauder, Chem. Phys. 30, 147 (1978).b

A partially rotationally resolved infrared spectrum of CH₃OO· in the CH stretch region has been reportedK.-H. Hsu, Y.-H. Huang, Y.-P. Lee, M. Huang, T. A. Miller and A. B. McCoy J. Phys. Chem. A, in press, DOI: 10.1021/acs.jpca.5b12334 The rotational contour of the ν₂ CH stretch band in the experimental spectrum can be simulated with an asymmetric rotor model. The simulation shows good agreement with the experimental spectrum except that the broadening of the Q-branch in the experimental spectrum remains unexplained. This broadening is likely due to the sequence band transitions from the torsionally excited levels populated at room temperature to combination levels involving the CH stretch and the same number of torsional quanta. A four dimension model involving three CH stretches and the CH₃ torsion is applied to the CH₃OO· radical to obtain the frequencies and intensities of the vibrational transitions in the CH stretch region. Based on these calculations, the torsional sequence bands are calculated to be slightly shifted from the origin band, because of the couplings between the CH stretches and CH₃ torsion, thereby causing the apparent broadening observed for the ν₂ fundamental. Due to the accidental degeneracy of two different CH stretch and CH₃ torsion combination levels which differ by one quantum in the torsional excitation, the frequencies of the torsional sequence bands will be very sensitive to details of the potential, which makes the shifts difficult to precisely predict with electronic structure calculations. Complementary analyses are now underway for the other two CH stretch vibrational bands, ν₁ and ν₉. K.-H. Hsu, Y.-H. Huang, Y.-P. Lee, M. Huang, T. A. Miller and A. B. McCoy J. Phys. Chem. A, in press, DOI: 10.1021/acs.jpca.5b12334.

A variational procedure of reduced dimensionality based on CCSD(T)-F12 calculations is applied to understand the far infrared spectrum of Ethylene-Glycol. This molecule can be classified in the double molecular symmetry group G8 and displays nine stable conformers, gauche and trans. In the gauche region, the effect of the potential energy surface anisotropy due to the formation of intramolecular hydrogen bonds is relevant. For the primary conformer, the ground vibrational state rotational constants are computed at 6.3 MHz, 7.2 MHz and 3.5 MHz from the experimental parameters. Ethylene glycol displays very low torsional energy levels whose classification is not straightforward. Given the anisotropy, tunneling splittings are significant and unpredictable. The ground vibrational state splits into 16 sublevels separated approximately 142 cm⁻¹. Transitions corresponding to the three internal rotation modes allow assign previous observed Q branches. Band patterns, calculated between 362.3 cm⁻¹ and 375.2 cm⁻¹, between 504 cm⁻¹ and 517 cm⁻¹ and between 223.3 cm⁻¹ and 224.1 cm⁻¹, that correspond to the tunnelling components of the v21 fundamental (ν₂₁ = OH-torsional mode), are assigned to the prominent experimental Q branches.

l0pt Figure Hydrogen is one of the most abundant interstellar species. Observation of rotational and vibrational spectra of H₂ containing complexes is of great importance because they are possible candidates for radio-astronomical detection. CO, HCN, HCCH are as isoelectronic molecules of N₂, each with a strong triple bond. It had been a big challenge to predict reliable theoretical rovibrational spectra of complexes including such species because the higher order electron correlation energy plays a non-negligible role in improving the accuracy. However, recent works on CO−H₂J. Chem. Phys., 139, 164315 (2013); Science, 336, 1147 (2012).ave shown that it is possible to reproduce the experimental spectra quantitatively. In this work, we calculate a five-dimension potential energy surface (PES) of HCN−H₂ complex which explicitly include the intramolecular asymmetric stretching vibrational mode(C−H,Q₃) coordinate at CCSD(T)/aug-cc-pVQZ+3s3p2d1f1g level, corrected with electron correlation energy from the triple and quadruple excitation. Vibrational average over intramolecular vibration mode is made with HCN monomer at ground and the first excited vibrational states respectively, and the averaged data are fitted to two four-dimension Morse/Long-Range (MLR) potential energy functions. Based on the MLR PESs, for the first time, we calculated the spectra of HCN−paraH₂ and HCN−orthoH₂. The results for HCN−orthoH₂ are in good agreement with the published experimental dataJ. Chem. Phys., 115, 5137 (2001).ith root-mean-square-difference (RMSD) only 0.01cm⁻¹, which validates the accuracy of the PESs.

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Footnotes:

J. Chem. Phys., 139, 164315 (2013); Science, 336, 1147 (2012).h J. Chem. Phys., 115, 5137 (2001).w

r0pt Figure The rotational spectra of H-C-C-C-N ••• H-C-C-C-N, cyanoacetylene dimer,were recorded using Balle-Flygare type Fourier transform microwave (FTMW) spectrometers at Wesleyan and Arizona. The low-J transitions were measured down to 1.3 GHz at very high resolution, FWHM  1 kHz using a large-cavity spectrometer. The spectral hyperfine structure due to the ¹⁴N nuclear quadrupole coupling interactions is well-resolved below 4 GHz using a low frequency spectrometer at the University of Arizona. The experimental spectroscopic constants were fitted as: B₀ = 339.292331(8) MHz, D_J = 32.15(8) Hz, H = -0.0015(2) Hz, eqQ(¹⁴N₁) = -3.990(1) MHz, and eqQ(¹⁴N₂) = -4.171(1) MHz. The vibrationally averaged dimer configuration is
H-C-C-C-N₁ ••• H-C-C-C-N₂. Using a simple linear model, the vibrational ground state and the equilibrium hydrogen bond lengths are determined to be: r₀(N ••• H) = 2.2489(3) Å and r_e(N ••• H) = 2.2315 Å. The equilibrium center-of-mass distance between the two HCCCN subunits is = 7.0366 Å. Using the rigid precession model, the vibrational ground state center-of-mass distance and the pivot angles which HCCCN subunits make with the a-axis of H-C-C-C-N₁ ••• H-C-C-C-N₂ are = 7.0603 Å, θ₁ = 13^°, and θ₂ = 8.7^°, respectively. The calculated hydrogen bond energy of H-C-C-C-N ••• H-C-C-C-N is 1466 cm⁻¹ using the MP2/aug-cc-PVTZ method in present work.

Only a few weakly-bound complexes containing the O₂ molecule have been characterized by high-resolution spectroscopy, notably N₂O-O₂ [1] and HF-O₂ [2]. This neglect is no doubt due in part to the complications added by the oxygen unpaired electron spin. Here we report an extensive infrared spectrum of CO-O₂, as observed in the CO fundamental band region ( ∼ 2150 cm⁻¹) using a tunable quantum cascade laser to probe a pulsed supersonic jet expansion. The derived energy level pattern consists of ‘stacks’ characterized by K, the projection of the total angular momentum on the intermolecular axis. Five such stacks are observed in the ground vibrational state, and ten in the excited state, v(CO) = 1. They are divided into two groups, with no observed transitions between groups, and we believe these groups correlate with the two lowest rotational states of O₂, namely (N, J) = (1, 0) and (1, 2). In many ways, the spectrum and energy levels are similar to those of CO-N₂ [3], and we use the same approach for analysis, simply fitting each stack with its own origin, B-value, and distortion constants. The rotational constant of the lowest stack in the ground state (with K = 0) implies an effective intermolecular separation of 3.82 Å, but this should be interpreted with caution since it ignores possible effects of electron spin. [1] H.-B. Qian, D. Seccombe, and B.J. Howard, J. Chem. Phys. 107, 7658 (1997). [2] W.M. Fawzy, C.M. Lovejoy, D.J. Nesbitt, and J.T. Hougen, J. Chem. Phys. 117, 693 (2002); S. Wu, G. Sedo, E.M. Grumstrup, and K.R. Leopold, J. Chem. Phys. 127, 204315 (2007). [3] M. Rezaei, K.H. Michaelian, N. Moazzen-Ahmadi, and A.R.W. McKellar, J. Phys. Chem. A 117, 13752 (2013), and references therein.