The He-HCN complex is a weakly bound complex with binding energy of about 9 cm⁻¹. We have measured the the j=1 ← 0 internal rotation fundamental band of the He-HCN complex by millimeter-wave absorption spectroscopy and reported the potential energy surface (PES) to reproduce the observed transition frequencies.K. Harada, K. Tanaka, T. Tanaka, S. Nanbu, and M. Aoyagi, J. Chem. Phys. 117, 7041 (2002). In the present study, we have extended the measurement to the j=2 ← 1 internal rotation hot bands of the He-HCN and He-DCN complexes. In the analysis, the upper state of several observed transitions are found to be located above the "dissociation limit" (D₀). The rovibtrational levels with e label dissociate to the HCN molecule with j=0 and the He atom (D₀), while those with f label, due to the parity conservation, to the HCN molecule with j=1 and the He atom which is higher in energy by about 2.96 cm⁻¹ (2B_HCN) than D₀. The f levels are bound up to D₀ + 2B_HCN. The revised PES of He-HCN has a global minimum in the linear He-HCN configuration with a depth of 29.9 cm⁻¹ and has a saddle point at the anti-linear He-NCH configuration with a depth of 20.9 cm⁻¹. The ν_s intermolecular stretching first excited state and the j=2 internal rotation second excited state are determined to be located 9.1405 and 9.0530 cm⁻¹ above the ground state and very close to the calculated dissociation limit (D₀) of 9.32 cm⁻¹. Life times of several quasi-bound levels (both of e and f labels) and line widths of the related transitions are predicted for He-HCN and He-DCN from the revised PESs.

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Footnotes:

K. Harada, K. Tanaka, T. Tanaka, S. Nanbu, and M. Aoyagi, J. Chem. Phys. 117, 7041 (2002).

The light group 14 cations are found in a wide variety of environments, with, for example, C⁺ ions thought to play a key role in the chemistry of the interstellar medium,S. Petrie and D. K. Bohme, Mass Spec. Rev., 26, 258 (2007).hile Si⁺ ions are an important component of the upper atmosphere of the Earth due to their presence in meteoroids.J. M. C. Plane, J. C. Gómez-Martin, W. Feng, and D. Janches, J. Geophys. Res. Atmos. 121, 3718 (2016).e calculate accurate interatomic potentials for a singly charged carbon cationW. D. Tuttle, R. L. Thorington, L. A. Viehland and T. G. Wright, Mol. Phys. 113, 3767 (2015).^,W. D. Tuttle, R. L. Thorington, L. A. Viehland and T. G. Wright (in preparation).nd a singly charged silicon cationW. D. Tuttle, R. L. Thorington, L. A. Viehland and T. G. Wright, Mol. Phys. 115, 437 (2017).nteracting with the rare gas atoms helium, neon and argon. The RCCSD(T) method is employed, with basis sets of quadruple− and quintuple− quality, and the energies counterpoise corrected and extrapolated to the basis set limit at each point. In all cases, we consider the lowest electronic states of the M^+ atom, (^2P_J), interacting with the ground electronic state of the RG atom, (^1S_0), and compute potentials corresponding to the molecular terms, ^2

Π and 2

+, as well as the spin−orbit levels which arise: 2

3/2, 2

1/2 and 2

1/2

^+ W. D. Tuttle, R. L. Thorington, L. A. Viehland and T. G. Wright (in preparation).aW. D. Tuttle, R. L. Thorington, L. A. Viehland and T. G. Wright, Mol. Phys. 115, 437 (2017).i

The rovibrational spectrum of the open-shell complex Ar-NO was recorded in the 1870-1892 cm⁻¹range with a segmented rapid-scan pulsed supersonic jet infrared absorption spectrometer based on distributed-feedback quantum cascade lasers. Four b-type subbands were observed. The progress on the rotational analysis will be presented.

The need for a deep understanding of CO₂ interactions is significant given the importance of supercritical CO₂ (sc-CO₂) as a green solvent. Fluorinated compounds often have higher solubility in sc-CO₂ than their hydrocarbon analogs, and the reasons for this are not well understood. Investigations of dimers of one CO₂ molecule with a simple fluorinated hydrocarbon provide an initial step towards understanding the complex balance of forces that is likely to be present as a larger solvation shell of sc-CO₂ is built. The weakly bound dimer cis-1,2-difluoroethylene...CO₂ is the latest in a series of complexes of CO₂ with fluorinated ethylenes that has recently been studied using chirped-pulse (CP) Fourier-transform microwave spectroscopy. Unlike all previous members of the series, the observed structure of cis-1,2-difluoroethylene...CO₂ is nonplanar, with CO₂ sitting above the ethylene plane and crossed relative to the C=C bond. This nonplanar arrangement is consistent with predictions made using symmetry adapted perturbation theory (SAPT), where the dispersion energy of the nonplanar structure is significantly more favorable than for a structure where CO₂ lies in the same plane as the ethylene moiety. Observed transitions are doubled as a result of CO₂ tunneling between equivalent positions above and below the ethylene plane, leading to inversion of the μ_c dipole moment component. Observed transitions for the most abundant isotopologue have been fitted to a two state Hamiltonian to give an energy difference between tunneling states of ∆E ≈ 333 MHz, and analysis using Meyer's one dimensional model to determine the barrier to inversion is presently in progress.

m-Toluic acid is a photo-oxidation product of m-xylene, a chemical byproduct of the oil and gas industry, and is a common component of secondary atmospheric aerosol. Organic acids, such as m-toluic acid, are also thought to play an important role in the initial steps of aerosol formation, which involves formation of hydrogen bonded clusters with molecular species, such as water, ammonia, and sulfuric acid.
Somewhat surprisingly, the rotational spectrum of the m-toluic acid monomer has not been studied before. We have identified four stable conformers using ab initio calculations at the MP2/6-311++G(2df,2pd) level of theory. The two lowest energy conformers are rather close in energy and their rotational spectra were measured using a Balle-Flygare type microwave spectrometer. The structures and barriers to methyl internal rotation were determined.
We have identified four isomers of the monohydrate of m-toluic acid using ab initio calculations. Measurements of the microwave spectra of the two lowest energy isomers are underway with a newly constructed chirped pulse microwave Fourier transform spectrometer in the frequency range from 2 to 6 GHz. The spectra and analyses will be presented.

Supercritical (sc) CO₂ is a common industrial solvent for the extraction of caffeine, nicotine, petrochemicals, and natural products. The ability of apolar scCO₂ to dissolve polar solutes is greatly enhanced by the addition of a polar co-solvent, often methanol or ethanol. Experimental and theoretical work show that methanol interactions in scCO₂ are predominantly hydrogen bonding, while the gas-phase complex is an electron donor-acceptor (EDA) configuration. Ethanol, meanwhile, is predicted to form EDA complexes both in scCO₂ and in the gas phase, but there have been no experimental measurements to support this conclusion. Here, we report a combined chirped-pulse and cavity FTMW study of the ethanol-CO₂ complex. Comparison with theory indicates the EDA complex is dominant under our experimental conditions. We confirm the structure with isotopic substitution, and derive a semi-experimental equilibrium structure. Our results are consistent with theoretical predictions that the linearity of the CO₂ subgroup is broken by the complexation interaction.

Methane is exceptional in its solid-phase orientational disorder that persists down to 24 K. Only below that temperature does the structure become partially ordered, and full crystallinity requires even lower temperatures and high pressures. Not surprisingly, methane appears to freely rotate in most van der Waals complexes, although two notable exceptions are CH₄-HF and CH₄-C₅H₅N. Of interest to us is how alkane interactions affect the methane rotation. Except for CH₄-CH₄, rotationally-resolved spectra of alkane-alkane complexes have not been studied. To fill this void, we present the microwave spectrum of CH₄-C₃H₈ which is the smallest alkane complex with a practical dipole moment. The microwave spectrum of CH₄-C₃H₈ was measured using the Fourier Transform microwave spectrometer at Wesleyan University. In the region between 7100 and 25300 MHz, we observed approximately 70 transitions that could plausibly be attributed to the CH₄-C₃H₈ complex (requiring high power and the proper mixture of gases). Of these, 16 were assigned to the A-state (lowest internal rotor state of methane) and four to the F-state. The A-state transitions were fitted with a Watson Hamiltonian using nine spectroscopic constants of which A = 7553.8144(97) MHz, B = 2483.9183(35) MHz, and C = 2041.8630(21) MHz. The A rotational constant is only 1.5 MHz higher than that of Ar-C₃H₈ and, since the a-axis of the complex passes approximately through the centers of mass of the subunits, this indicates a similar relative orientation. Thus, we find that the CH₄ is located above the plane of the propane. The center-of-mass separation of the subunits in CH₄-C₃H₈ is calculated to be 3.993 Å, 0.16 Å longer than the Ar-C₃H₈ distance of 3.825 Å, a reasonable difference considering the larger van der Waals radius of CH₄. The four F-state lines, which were about twice as strong as the A-state lines, could be fitted to A, B, and C rotational constants, and further analysis is in progress.

The 1:1 complex of ammonia with pyridine has been characterized by using state-of-the-art quantum-chemical computations combined with pulsed-jet Fourier-Transform microwave spectroscopy. The computed potential energy landscape pointed out the formation of a stable σ-type complex, which has been confirmed experimentally: the analysis of the rotational spectrum showed the presence of only one 1:1 pyridine – ammonia adduct. Each rotational transition is split into several components due to the internal rotation of NH₃ around its C₃ axis and to the hyperfine structure of both ¹⁴N quadrupolar nuclei, thus providing the unequivocal proof that the two molecules form a σ-type complex involving both a N-H…N and a C-H…N hydrogen bond. The dissociation energy (BSSE and ZPE corrected) has been estimated to be 11.5 kJ·mol⁻¹. This work represents the first application of an accurate, yet efficient computational scheme, designed for the investigation of small biomolecules, to a molecular cluster.

N₂O₅ is an atmospheric species which serves as night-time sink for NO_x species. Its reconversion to NO_x products occurs through solvation in atmospheric aerosols. Detection of N₂O₅ and NO₃⁻ fragmentation products in such aerosols has previously utilized chemical ionization featuring halides (of which chlorine is ubiquitous in sea-spray aerosols). We examine the solvation behavior of N₂O₅ and the critical number of water molecules to form HNO₃ from N₂O₅ and water. We have been able to generate and spectroscopically characterize N₂O₅-halide ions formed from halide-water clusters. We observe X⁻N₂O₅ species whose spectra best correspond to a calculated (O₂NX)(ONO₂⁻) species.Funding for this work was provided by the NSF’s Center for Aerosol Impacts on Climate and the Environment.html:<hr /><h3>Footnotes: Funding for this work was provided by the NSF’s Center for Aerosol Impacts on Climate and the Environment.

The conformational behaviour of the hydrogen-bonded cluster ethanol dimer has been reinvestigated by chirped pulse Fourier transform microwave spectroscopy in the 2-8 GHz frequency region. Three new conformers (tt, tg+, and g-g+) have been identified together with the three (g+g+, g-t, and g+t) previously observed by Hearn et al. (J. Chem. Phys. 123, 134324, 2005) and their rotational and centrifugal distortion constants have been determined. By using different carrier gases in the supersonic expansion, the relative abundances of the observed conformers have been estimated. The monosubstituted ¹³C species and some of the ¹⁸O species of the most abundant conformers g+g+, g-t, and tt have been observed in their natural abundance, which led to the partial determination of their r_s structures, and the r₀ structure for the tt conformer. The six observed conformers are stabilized by the delicate interplay of primary O-H···O and secondary C-H···O hydrogen bonds, and dispersion interactions between the methyl groups. Density functional and ab initio methods with different basis sets are benchmarked against the experimental data.

The rotational spectra complexes of argon-urea, argon-thiourea and water-thiourea have been measured by chirped-pulse Fourier transform microwave spectroscopy from 2-18.5 GHz. The sample was produced via laser vaporisation of a rod containing copper and the organic sample as a stream of argon was passed over the surface and subsequently expanded into the vacuum chamber cooling the sample. Argon was found to bind to π system of the carbonyl bond for both the urea and thiourea complexes.