Direct infrared spectra predictions for van der Waals (vdW) complexes rely on accurate intra-molecular vibrationally excited inter-molecular potential. Due to computational cost increasing with number of freedom, constructing an effective reduced-dimension potential energy surface, which only includes direct relevant intra- molecular modes, is the most feasible way and widely used in the recent potential studies. However, because of strong intra-molecular vibrational coupling, some indirect relevant modes are also play important roles in simulating infrared spectra of vdW complexes. The questions are how many intra-molecular modes are needed, and which modes are most important in determining the effective potential and direct infrared spectra simulations. Here, we explore these issues using a simple, flexible and efficient vibration-averaged approach, and apply the method to vdW complex C₂H₂−H₂. With initial examination of the intra-molecular vibrational coupling, an effective seven-dimensional ab initio potential energy surface(PES) for C₂H₂−H₂, which explicitly takes into account the Q₁,Q₂ symmetric-stretch and Q₃ asymmetric-stretch normal modes of the C₂H₂ monomer, has been generated. Analytic four-dimensional PESs are obtained by least-squares fitting vibrationally averaged interaction energies for ν₃(C₂H₂)=0 and 1 to the Morse/long-range(MLR) potential function form. We provide the first prediction of the infrared spectra and band origin shifts for C₂H₂−H₂ dimer. We particularly examine the dependence of the symmetric-stretch normal mode on asymmetric-stretch frequency shift for the complex.

Ammonia and trimethylamine (TMA) were used to probe preference of hydrogen over halogen bonding in molecular complexes containing CHF₂I via chirped pulse Fourier transform microwave spectroscopy. The halogen bonded complex of TMA is ≈ 2 kJ/mol more energetically favourable (extrapolation to CCSD(T)/CBS level) than the hydrogen bonded complex. The reverse is true for the ammonia complex where the hydrogen bonded complex is ≈ 3kJ/mol more favourable. Although the spectra of both complexes were perturbed by large amplitude motions around the intermolecular bond effective fits of the lower rotational energy levels appear to confirm that TMA prefers to bind to the iodine whilst ammonia prefers the hydrogen.

The rotational spectrum of the pyridine…water complex has been observed in the 2-18 GHz region using chirped-pulse and cavity Fourier transform microwave spectroscopy. The water is hydrogen bonded to the nitrogen, as expected, but the hydrogen bond is bent, with the oxygen tilted toward either of the ortho hydrogens of the pyridine. This gives rise to a pair of equivalent configurations and the possibility of a tunneling motion involving an in-plane rocking of the water. DFT calculations support this view. Experimentally, a pair of states with severely perturbed rotational structure has been identified and the spectra assigned. Analysis of the perturbations in the a-type (pure rotation) spectra has enabled an accurate determination of the tunneling splitting, which has been confirmed by direct observation of b-type (rotation-tunneling) transitions. A simultaneous fit of the a- and b- type transitions gives the most accurate value of the tunneling splitting. Results for the H₂O, D₂O, and D-bound HOD complexes will be presented. The tunneling splittings are as follows: H₂O-pyridine (10402.9 MHz), HOD-pyridine (12513.4 MHz, determined only from perturbation analysis), and D₂O-pyridine (13582.3 MHz). Curiously, the tunneling splitting increases with increased deuteration. Additional small splittings have been observed in some transitions, indicating the possibility of further internal dynamics. This system offers an interesting test case for theoretical treatments of large amplitude motion.

Methyl thioformate CH₃SC(O)H, is a monosulfur derivative of methyl formate, a relatively abundant component of the interstellar medium (ISM)E. Chruchwell, G. Winnewisser, A&A, 45, 229 (1975) Methyl thioformate being the thermodynamically most stable isomer with a C₂H₄OS formula, it can be reasonably proposed for detection in the ISM. Theoretical investigations on this molecule have been done recently by Senent et al.M. L. Senent, C. Puzzarini, M. Hochlaf, R. Domínguez-Gómez and M. Carvajal, J. Chem. Phys., 141, 104303 (2014) Previous experimental studies on this molecule have been performed by Jones et al.G. I. L. Jones, D. G. Lister and N. L. Owen, J. Mol. Spectrosc., 60, 348 (1976)nd Caminati et al.W. Caminati, B. P. V. Eijck and D. G. Lister, J. Mol. Spectrosc., 90, 15 (1981)nd its microwave spectrum was recorded between 10 and 41 GHz.
In this study, S-methyl thioformate has been synthesized by reaction of methyl mercaptan with formic-acetic anhydride. The millimeter wave spectrum was then recorded for the first time from 150 to 660 GHz with the Lille’s spectrometer based on solid-state sources O. Zakharenko, R. A. Motiyenko, L. Margulès, T. R. Huet., J. Mol. Spectrosc., 317, 41 (2015) Around 2300 lines were assigned up to J = 70 and K = 15 and a fit for the ground torsional state ν_t=0 performed with the BELGI-C_s codeJ. T. Hougen, I. Kleiner, and M. Godefroid, J. Mol. Spectrosc. 163, 559 (1994)ill be presented and discussed. Our aim is to provide a line list for an astrophysical research.

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E. Chruchwell, G. Winnewisser, A&A, 45, 229 (1975). M. L. Senent, C. Puzzarini, M. Hochlaf, R. Domínguez-Gómez and M. Carvajal, J. Chem. Phys., 141, 104303 (2014). G. I. L. Jones, D. G. Lister and N. L. Owen, J. Mol. Spectrosc., 60, 348 (1976)a W. Caminati, B. P. V. Eijck and D. G. Lister, J. Mol. Spectrosc., 90, 15 (1981)a O. Zakharenko, R. A. Motiyenko, L. Margulès, T. R. Huet., J. Mol. Spectrosc., 317, 41 (2015). J. T. Hougen, I. Kleiner, and M. Godefroid, J. Mol. Spectrosc. 163, 559 (1994)w

It is known that the barrier to internal rotation of the methyl groups in ethane (1) is about 1000 cm⁻¹.R. M. Pitzer, Acc. Chem. Res., 1983, 16, 207–210.f a C-C-triple bond is inserted between the methyl groups as a spacer (2), the torsional barrier is assumed to be dramatically lower, which is a common feature of ethinyl groups in general. To study this effect of almost free internal rotation, we measured the rotational spectrum of 3-pentyn-1-ol (3) by pulsed jet Fourier transform microwave spectroscopy in the frequency range from 2 to 26.5 GHz. Quantum chemical calculations at the MP2/6-311++G(d,p) level of theory yielded five stable conformers on the potential energy surface. The most stable conformer, which possesses C₁ symmetry, was assigned and fitted using two theoretical approaches treating internal rotations, the rho axis method (BELGI-C₁) and the combined axis method (XIAM). The molecular parameters as well as the internal rotation parameters were determined. A very low barrier to internal rotation of the methyl group of only 9.4545(95) cm⁻¹ was observed. r0pt Figure R. M. Pitzer, Acc. Chem. Res., 1983, 16, 207–210.I

Recent discovery of methyl isocyanate (CH₃NCO) in Sgr B2(N) and Orion KLD. T. Halfen, V. V. Ilyushin, L. Ziurys, Astrophys. J. Lett., 2015, 812, L5.^,J. Cernicharo, Z. Kisiel, B. Tercero, L. Kolesniková, I. R. Medvedev et. al., Astron. & Astrophys., 2016, 587, L4.akes methyl cyanate (CH_3OCN) a potential molecule in the interstellar medium.Methoxyamine (CH_3ONH_2) and its isomeric form N−methylhydroxylamine (CH_3NHOH) may be considered as a potential interstellar amines.R. T. Garrod, S. L. Widicus Weaver, E. Herbst, Astrophys. J., 2008, 682, 283.ure rotational transitions belonging to the ground state and several excited vibrational states were measured and analyzed up to 400 GHz. Rotational transitions revealed A−E J.Cernicharo, Z.Kisiel, B.Tercero, L.Kolesnikov, I.R.Medvedev et. al., Astron. Astrophys., 2016, 587, L4.mR. T. Garrod, S. L. Widicus Weaver, E. Herbst, Astrophys. J., 2008, 682, 283.PL.Kolesnikov, J.L.Alonso, C. Bermdez, E.R.Alonso, J.Cernicharo et. al., Astron. Astrophys., 2016, accepted.

Identification by the Rosetta mission that CH₃NCO is among the more plentiful molecules on the surface of the comet Churyumov-Gerasimenko stimulated rapid detection of this molecule in the interstellar medium.D.T.Halfen, V.V.Ilyushin, L.Ziurys, ApJ 812, L5 (1915).^,J.Cernicharo, Z.Kisiel, B.Tercero, et al., A&A 587, L4 (2016).n particular, we have been successful in detecting almost 400 lines of CH_3NCO in Orion^b by extending the KoputJ.Koput, J. Mol. Spectrosc. 115, 131 (1986).m−wave assignment to frequencies relevant to mm−wave radio−telescopes through measurement of the complete laboratory spectrum up to 363 GHz.^b,^th^th

Methyl rotors are sensitive indicators of the local and non-local electronic environment, so a study of methyl torsional barriers at different sites around the perimeter of a conjugated ring system may reveal electronic environment of the indole ring. Previously reported results^a are limited to microwave spectroscopic identification of the 1-methylindole, 2-methylindole, 3-methylindole, and 5-methylindole. The new measurements of 4-methylindole, 6-methylindole, and 7-methylindole complete the series. Over one thousand transitions were assigned in the 10.5 – 21 GHz frequency range, resolving both nuclear quadrupole coupling and methyl internal rotation for all seven species. Electronic structure calculations at MP2/6-311++G(d,p) level, torsional barrier calculations and molecular electrostatic potential maps at ωB97XD/6-311++G(d,p) level are used along with the experimental data. ¹⁴N nuclear quadrupole coupling tensor elements were used to determine the ionic character of the NH sigma bond, π bonding character of the nitrogen p_z orbital, and the amount of negative charge on the nitrogen of each methylindole. Fitted torsional barriers were compared with theoretical investigations of the origin of methyl torsional barriers to confirm that overall barrier arises from both hyperconjugative interactions and structural interactions of bonding and anti-bonding orbitals.

^aR. M. Gurusinghe, M. J. Tubergen. 69^th International Symposium of Molecular Spectroscopy, Urbana-Champaign, IL, 2014, RJ03.

Recently, a measurement of the rotational spectrum of DCOOCH₃ has been carried out in the frequency range from 0.85 to 1.5 THz at Jet Propulsion Laboratory (JPL) (Duan et al. 2015). These JPL data and the available spectroscopic millimeter- and submillimeter-wave data (Margulès et al. 2010 and references therein) of the ground state were analyzed using the Rho Axis Method (RAM) (Kleiner 2010). At present, a new analysis of JPL lines of DCOOCH₃ in the first excited v_t=1 torsional states is undertaken. This analysis may help the future identification of v_t=1 lines in the interstellar and circumstellar media as was carried out for the v_t=0 lines in Orion KL (Margulès et al. 2010). In this communication, the progress of this study is presented as well as a short outline of the spectral analyses accomplished for other methyl formate isotopologues. This research is supported by the FIS2014-53448-C2-2-P project (MINECO, Spain), the French PCMI (Programme National de Physique Chimie du Milieu Interstellaire), and the National Natural Science Foundation of China (Grant No. 11174098). Portions of this work is carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Government sponsorship is acknowledged.0.2cm C. Duan, M. Carvajal, S. Yu, J.C. Pearson, B.J. Drouin, I. Kleiner 2015, A&A, 576, A39
I. Kleiner 2010, J. Mol. Spectroc., 260, 1
L. Margulès, T.R. Huet, J. Demaison, M. Carvajal, I. Kleiner, H. Møllendal, B. Tercero, N. Marcelino, J. Cernicharo 2010, ApJ, 714, 1120

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This research is supported by the FIS2014-53448-C2-2-P project (MINECO, Spain), the French PCMI (Programme National de Physique Chimie du Milieu Interstellaire), and the National Natural Science Foundation of China (Grant No. 11174098). Portions of this work is carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Government sponsorship is acknowledged.

The microwave spectrum of 2,5-dimethylthiophene, a sulfur-containing five-membered heterocyclic molecule with two conjugated double bonds, was recorded using two molecular beam Fourier transform microwave spectrometers operating in the frequency range from 2 to 40 GHz. Highly accurate molecular parameters were determined. The rotational constants obtained by geometry optimizations at different levels of theory are in good agreement with the experimental values. A C_2v equilibrium structure was calculated, where one hydrogen atom of each methyl group is antiperiplanar to the sulfur atom, and the two methyl groups are thus equivalent. Transition states were optimized at different levels of theory using the Berny algorithm to calculate the barrier height of the two equivalent methyl rotors. The fitted experimental torsional barrier of 247.95594(30) cm⁻¹ is in reasonable agreement with the calculated barriers. Similar barriers to internal rotation were found for the monomethyl derivatives 2-methylthiophene (194.1 cm⁻¹) and 3-methylthiophene (258.8 cm⁻¹). A labeling scheme for the group G₃₆ written as the semi-direct product (C₃^I x C₃^I) (x C_2v was introduced.

The Fourier transform microwave spectra of the E and the Z isomer of butadienyl acetate have been measured in the frequency range from 2 to 26.5 GHz under molecular beam conditions. The most stable conformer of each isomer, in which all heavy atoms are located in a symmetry plane, was identified after analyzing the spectrum by comparison with results from quantum chemical calculations. The barrier to internal rotation of the acetyl methyl group was found to be 149.1822(20) cm⁻¹ and 150.2128(48) cm⁻¹ for the E and the Z isomer, respectively, which are similar to that of vinyl acetate B. Velino, A. Maris, S. Melandri, W. Caminati, J. Mol. Spectrosc. 2009, 256, 228^,H. V. L. Nguyen, A. Jabri, V. Van, and W. Stahl, J. Phys. Chem. A, 2014, 118, 12130A comparison between two theoretical approaches treating internal rotations, the rho axis method (using the program BELGI-C_s) and combined axis method (using the program XIAM), is also performed.Since several years we study the barriers to internal rotation of the acetyl methyl group in acetates, CH_3-COOR. Currently, we assume that all acetates can be divided into three classes. Class I contains , saturated acetates, where the torsional barrier is always close to 100 cm^-1. Examples are a series of alkyl acetates such as methyl acetate and ethyl acetate. Class II contains ,−unsaturated acetates where the C=C double bond is located in the COO plane. This is the case of vinyl acetate and butadienyl acetate. Finally, in class III with isopropenyl acetate and phenyl acetate as two representatives, ,−unsaturated acetates, in which the double bond is not located in the COO plane, are collected. There, we observed a barrier height around 135 cm^-1 H. V. L. Nguyen, A. Jabri, V. Van, and W. Stahl, J. Phys. Chem. A, 2014, 118, 12130.

Despite being an important prototype molecule for intramolecular proton tunnelling, the far-IR spectrum of the internally hydrogen-bonded species malonaldehyde (C₃O₂H₄) is not yet well understood. In the talk I gave at the ISMS meeting in 2015 I discussed the high-resolution spectra we obtained at the Canadian Light Source synchrotron in Saskatoon, Saskatchewan. These spectra include a number of fundamental vibrational bands in the 100-2000 cm⁻¹ region. In our efforts to analyze these bands we have noticed that our ground state combination differences show a large drift (up to an order of magnitude larger than our experimental error) away from those calculated using constants established by Baba et al.,T. Baba, T. Tanaka, I. Morino, K. M. T. Yamada, K. Tanaka. Detection of the tunneling-rotation transitions of malonaldehyde in the submillimeter-wave region. J. Chem. Phys., 110. 4131-4133 (1999)articularly in regions of high J (above 30) and low K_a (below 5). An examination of the previous microwave and far-IR studiesP. Turner, S. L. Baughcum, S. L. Coy, Z. Smith. Microwave Spectroscopic Study of Malonaldehyde. 4. Vibration-Rotation Interaction in Parent Species. J. Am. Chem. Soc., 106. 2265-2267 (1984)D. W. Firth, K. Beyer, M. A. Dvorak, S. W. Reeve, A. Grushow, K. R. Leopold. Tunable far-infrared spectroscopy of malonaldehyde. J. Chem. Phys., 94. 1812-1819 (1991) reveals that this region of J-K_a space was not represented in the lines that Baba et al. used to generate the values for their fitting parameters. By including our own measurements in the fitting, we were able to improve the characterization of the ground state so that it is now consistent with all of the existing data. This characterization now covers a much larger range of J-K_a space and has enabled us to make significant progress in analyzing our far-IR synchrotron spectra. These include an excited vibrational state at 241 cm⁻¹ as well as several states split by the tunnelling effect at higher wavenumber.

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T. Baba, T. Tanaka, I. Morino, K. M. T. Yamada, K. Tanaka. Detection of the tunneling-rotation transitions of malonaldehyde in the submillimeter-wave region. J. Chem. Phys., 110. 4131-4133 (1999)p P. Turner, S. L. Baughcum, S. L. Coy, Z. Smith. Microwave Spectroscopic Study of Malonaldehyde. 4. Vibration-Rotation Interaction in Parent Species. J. Am. Chem. Soc., 106. 2265-2267 (1984)

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The origin and strength of intra- and inter-molecular vibrational coupling is difficult to probe by direct experimental observations. However, explicitly including or not including some specific intramolecular vibrational modes to study intermolecular interaction provides a precise theoretical way to examine the effects of anharmonic coupling between modes. In this work, a full-dimension intra- and inter-molecular ab initio potential energy surface (PES) for H₂O−Ar, which explicitly incorporates interdependence on the intramolecular normal-mode coordinates of the H₂O monomer, has been calculated. In addition, four analytic vibrational-quantum-state-specific PESs are obtained by least-squares fitting vibrationally averaged interaction energies for the (ν₁,ν₂,ν₃)=(0,0,0),(0,0,1),(1,0,0),(0,1,0) states of H₂O to the three-dimensional Morse/long-range potential function. The resulting vibrationally averaged PESs provide good representations of the experimental infrared data, with RMS discrepancies smaller than 0.02 cm⁻¹for all three rotational branches of the asymmetric stretch fundamental transitions. The infrared band origin shifts associated with three fundamental bands of H₂O in H₂O−Ar complex are predicted for the first time and are found to be in good agreement with the (extrapolated) experimental values. Upon introduction of additional intramolecular degrees of freedom into the intermolecular potential energy surface, there is clear spectroscopic evidence of intra- and intermolecular vibrational couplings.J. Chem. Phys., 144, 014301 (2016)html:<hr /><h3>Footnotes: J. Chem. Phys., 144, 014301 (2016)

The rotational spectra of T-shaped cyanoacetylene carbon dioxide complex, HCCCN - CO₂, were measured using two Balle-Flygare type Fourier transform microwave (FTMW) spectrometers between 1.4 GHz and 22 GHz. The low J transitions were recorded using the low frequency FTMW spectrometer at the University of Arizona with a state-of-the-art resolution of “full width at half maximum” (FWHM)  1 kHz. The spectra above 4 GHz were recorded at Wesleyan University. Spectral hyperfine structures due to the ¹⁴N nuclear quadrupole coupling interactions can be fully resolved in low frequency bands. Since all K_a = 1 branches were not observed, this implies that HCCCN - CO₂ possesses a rigorous T-shaped structure. Assuming that A₀ is the same as that of HCN - CO₂, 11824 MHz, the spectroscopic constants of HCCCN - CO₂ are: B₀ = 794.59686(63) MHz, C₀ = 715.74488(60) MHz, ∆_J = 0.50067(18) kHz, ∆_JK = 120.892(12) kHz, δ_J = 0.04253(31) kHz, δ_K = 65.32(12) kHz, H_J = -0.00117(33) Hz, H_JK = 0.034876(21) kHz, H_KJ = -0.68254(73) kHz, χ_aa(¹⁴N) = -4.12873(78) MHz, χ_bb(¹⁴N) = 2.110(25) MHz, and χ_cc(¹⁴N) = 2.019(25) MHz.