A great deal of attention has been given to noncovalent interactions involving π systems because of their widespread presence in biology as well as materials, where they are pivotal in determining the three-dimensional structures of, e.g., proteins and polymers or the selectivity of molecular affinity. Despite dramatic advances in our understanding over past decades, many aspects of π interactions have only recently been discovered, with many questions remaining. Rotational spectroscopy is arguably the most accurate high resolution molecular spectroscopic technique due to its high sensitivity to mass distributions of molecules and molecular complexes. Since the interaction sites and the relative arrangement of moieties can be determined without environmental bias, rotational spectroscopy allows describing the intermolecular forces at play and enables testing of quantum chemical methods. In this talk, with the recent rotational spectroscopic results we have obtained on π interactions, the comparisons between experimental and computaional data will be discussed.

The impact of gaseous nitrous acid (HONO) in atmospheric chemistry is well described, being a major source of OH radicals acting as a strong oxidantN. A. Saliba et. al. Geophys. Res. Lett. 27, 3229-3232, (2000) In standard conditions, HONO is in equilibrium with various nitrous oxides under rapid decomposition at daytime. However, results by Lammel and Cape describe a steady production of OH radicals by HONO in the atmosphere whose source might be the complex of HONO with waterG. Lammel, J. N. Cape, Chem. Soc. Rev. 25, 361-369, (1996) Recent experiments have revealed that HONO remains stable in an aqeous environment as the HONO · H₂O complex, supporting studies of its greater stability in environments with higher humidityD. Perner, U. Platt, Geophys. Res. Lett. 6, 917-920, (1979) In the present work, gaseous HONO · H₂O was generated in a laboratory scale and investigated with two molecular jet Fourier transform microwave spectrometers operating from 2 to 40 GHz. To guide the experimental observation, geometry optimizations were performed to obtain rotational constants using the standard coupled-cluster theory with single and double excitations. The HONO · H₂O spectrum has been assigned with the ¹⁴N quadrupole coupling taken into account. Further splittings by the ortho-hydrogens, resulting from spin-spin coupling interactions, could be fully resolved. Comparing the results to those of the dimethylamine-water complexM. J. Tubergen, R. L. Kuczkowski, J. Mol. Struct. 352/353, 335-344, (1994)onfirmed an absence of the water tunnelling motion. N. A. Saliba et. al. Geophys. Res. Lett. 27, 3229-3232, (2000). G. Lammel, J. N. Cape, Chem. Soc. Rev. 25, 361-369, (1996). D. Perner, U. Platt, Geophys. Res. Lett. 6, 917-920, (1979). M. J. Tubergen, R. L. Kuczkowski, J. Mol. Struct. 352/353, 335-344, (1994)c

This talk will discuss the design and performance of a novel high-throughput instrument for Chirped Pulse Fourier-transform Microwave (CP-FTMW) spectroscopy, and demonstrate its efficacy through the identification of the lowest energy conformers of the ethanol trimer and mixed water:ethanol trimers. Computational characterization of the target clusters will be described, as will experimental details and resulting conclusions as to the structure of the observed clusters. In addition, the increased speed of data collection and resulting sensitivity of the instrument will be addressed, with the new target species made available by these improvements.

Although many non-rigid molecules displaying a single LAM have been spectroscopically characterized, less results are available about non-rigid molecules displaying several LAMs, as they are theoretically more challenging. This is confirmed by a recent spectroscopic investigation of nitrotoluene^b which revealed that its 2-nitrotoluene isomeric species displays two LAMs corresponding to internal rotations of the CH₃ and NO₂ groups. In this investigation,^b because no approach accounting for two LAMs was available, the microwave spectrum of 2-nitrotoluene was analyzed using a simplified approach accounting only for the torsional motion of the CH₃ group. In this talk, the IAM water dimer formalism^c will be applied to 2-nitrotoluene. As this theoretical approach is designed for multidimensional tunneling in the high-barrier limit, it is well suited for this species. Once the equilibrium configurations and the tunneling paths are chosen, the IAM approach^c allows us to derive a fitting Hamiltonian accounting for the rotational dependence of the tunneling splittings, but not for their magnitude, which should be obtained fitting the spectroscopic data. In 2-nitrotoluene, there are six C₁ symmetry equilibrium configurations and two tunneling paths. The first and most feasible one corresponds to a 2π/3 rotation of the methyl group. The second one is the complicated geared internal rotation of both the CH₃ and NO₂ groups identified using quantum chemistry calculations.^b The results of the line position analysis of the available microwave data^b with the new IAM approach will be presented. It is hoped that the analysis results will be more satisfactory than with the simplified approach^b and this will provide us with a better understanding of the 2-nitrotoluene multidimensional potential energy surface.

Our previous work demonstrated that the measurement of pure rotational spectroscopy of “non-polar” dimer of formic acid can be achieved by means of asymmetric H-D substitution.[1] The concerted double proton transfer of the two hydroxyl hydrogens takes place between two equivalent minima and generates a tunneling splitting of 331.6(5) MHz. In this talk, I will discuss the double proton transfer over a phenyl ring in the complexes of formic acid dimer (FAD) with phenyl compounds. For example, in the FAD-fluorobenzene complex, the presence of fluorobenzene as a neighboring molecule does not quench the double proton transfer in the FAD but decreases its tunneling splitting to 267.608(1) MHz.[2] In the FAD-fluorobenzaldehyde complex, the protons transfer does not occur via tunneling, but produces two non-equivalent isomers. Our spectra show that the isotopic substitution at different atomic positions have different influences on the tunneling process. The experiments were carried out by using the CP-FTMW spectrometer in Hamburg and the new-build one in Shanghai. r0pt Figure [1] Angew. Chem. Int. Ed. 2019, 58, 859 –865. [2] Angew. Chem. Int. Ed. 2021, 60, 25674 –25679.

Molecular hydrogen plays a key role in our efforts to shift our energy production to renewable resources. Hydrogen is an important energy storage molecule which in the future may replace our fossilized fuels as a transportable energy source. To appropriately model hydrogen storage materials, an understanding of the fundamental binding to organic systems is required. In previous works only a few inorganic and metallic complexes with hydrogen have be investigated by rotational spectroscopy.Jäger, W. et al., J. Chem. Phys., 127 054305, 2007, DOI: 10.1063/1.2756534 ^,Obenchain, D., Frank, H., Pickett, H., Novik, S., J. Chem. Phys., 146 204302, 2017, DOI:10.1063/1.4983042his work aims to bridge the gap to large covalent organic frameworks (COF) by focusing on the microwave structure of hydrogen heterodimers with small aromatic ring systems.Geng, K., He, T., Liu, R. et al., Chem. Rev., 120 8814, 2020, DOI: 10.1021/acs.chemrev.9b00550n this work the binding sites of hydrogen to halogen benzaldehydes which serve as mimics COF monomers, specifically boronic ester based COFs, are studied. These volatile systems possess a large dipole moment and provide a method of increasing the complexity of the system by the introduction of quadrupolar nuceli to finally look at small boronic esters in hydrogen complexes. Of particular interest are the differences observed for the rotational spectrum of ortho− and para−hydrogen and its structural impact investigated by isotopic substitution. The significant differences between these two species demonstrate there are significant differences in binding strength of o−H₂ and p−H₂ which are experimentally observable. Broadband rotational spectra are presented, are supplemented with cavity Fourier transform microwave spectroscopy data to resolve the additional hyperfine splitting of o−H₂ (j = 1 Obenchain, D., Frank, H., Pickett, H., Novik, S., J. Chem. Phys., 146 204302, 2017, DOI:10.1063/1.4983042TGeng, K., He, T., Liu, R. et al., Chem. Rev., 120 8814, 2020, DOI: 10.1021/acs.chemrev.9b00550I

The vibrational predissociation spectra of Cl⁻H₂ and CN⁻H₂ are measured in regions between 450 and 3000 cm⁻¹ in an ion trap at different temperatures using the FELIX infrared free electron lasers. Strong differences between the vibrational spectra of the two para and ortho nuclear spin isomers X-(para-H₂) or X-(ortho-H₂), with X = Cl⁻ or CN⁻, are detected [1,2]. Above a certain temperature, the removal of the para nuclear spin isomer by ligand exchange to the ortho isomer is suppressed efficiently. Not only do the transition frequencies agree well with calculated spectra using an accurate quantum approach [3], also the line profile matches with the calculated bands. When comparing the absolute frequency positions of the measured and calculated vibrational bands one finds a redshift of about 5cm⁻¹ for the strongest band. [1] F. Dahlmann, P. Jusko, M. Lara-Moreno et al., Mol. Phys., submitted [2] F. Dahlmann, C. Lochmann, A. N. Marimuthu et al., J. Chem. Phys. Comm. 155, 241101 (2021) [3] M. Lara-Moreno, P. Halvick, and T. Stoecklin, Phys. Chem. Chem. Phys. 22, 25552–25559 (2020)

Due to its simplicity, molecular hydrogen perturbed by helium atom constitutes a great benchmark system for tests of ab initio quantum scattering calculations as a method of precise description of collisional effects in ultra-accurate experimental spectra. Here we present our recent cavity-enhanced measurements of H₂ lines perturbed by He. Our results exhibit an unprecedented subpercent agreement with fully quantum ab initio calculations. We investigate collisional line-shape effects that are present in highly accurate experimental spectra of the 3-0 S(1) and 2-0 Q(1) lines. We clearly distinguish the influence of six different collisional effects (i.e.: collisional broadening and shift, their speed dependences and the complex Dicke effect) on the shapes of H₂ lines. We demonstrate that if any of the six contributions is neglected, then the experiment-theory comparison deteriorates at least several times. We also analyze the influence of the centrifugal distortion on our ab initio calculations and we demonstrate that the inclusion of this effect slightly improves the agreement with the experimental spectra. In addition, we describe the theoretical calculations that were performed to obtain the subpercent agreement with experiment. In the analysis described here, we employed the state-of-the-art statistical model of the collision-perturbed shape of molecular lines. We obtained all the parameters of this model from quantum scattering calculations, and the dynamical calculations were performed on the most accurate potential energy surface (PES) to date.

The abundance of molecular hydrogen and atomic helium in the universe makes them an important system to study in various fields. A mixture of molecular hydrogen and helium is the main component of the atmospheres of gas giants in the Solar System and is predicted to be a dominant constituent of the atmospheres of some types of exoplanets. The hydrogen molecule is also the simplest molecule, the structure of which can be calculated from first principles, which makes it well suited for accurate tests of ab initio calculations. In particular HD molecule, despite its lower abundance than H₂ isotopologue is noticeable in spectroscopic studies due to the presence of its dipole moment. Studies show that in some cases the uncertainty of astronomical observations (f.e. measuring the D/H ratios) of hydrogen molecule spectra is dominated by the uncertainties of collisional parameters, including pressure broadening and pressure shift coefficients. We utilize the methodology of populating line-by-line spectroscopic databases with beyond-Voigt line-shape parameters P. Wcisło et al., J Quant Spectrosc Radiat Transf 2021;260:107477. doi: 10.1016/j.jqsrt.2020.107477 which is based on ab initio quantum scattering calculations and was first applied to the He-perturbed H₂. We report a comprehensive dataset of beyond-Voigt line-shape parameters (pressure broadening and shift coefficients, their speed-dependences, and the complex Dicke parameters) for all electric dipole and quadrupole transitions within the ground electronic state in He-perturbed HD that are present in HITRAN (11 575 lines) at temperatures spanning from 20 to 1000 K. We parametrize the temperature dependence of the line-shape parameters with double-power-law representation (DPL), recommended for the HITRAN database. In addition to the presentation of the calculations, we will discuss our latest experimental determination of collisional line-shape parameters for He-perturbed H₂ and its comparison with theoretical results.

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Footnotes:

P. Wcisło et al., J Quant Spectrosc Radiat Transf 2021;260:107477. doi: 10.1016/j.jqsrt.2020.107477,

Precision spectral measurement of simple molecules such as H₂ and their isotopes is one of the important research fields of spectroscopy. Combined with accurate calculations, allows us to test the fundamental quantum chemistry theory and to determine the fundamental physical constants such as the proton-to-electron mass ratio. Here we present the Doppler-free spectroscopy measurements of first overtone transition of HD at a temperature as low as 10K, measured the saturated absorption spectrum of the first overtone transition of HD and observed the Doppler free spectral of R₀ (2-0) for the first time. The line profile is different from the saturated absorption spectrum. We analyzed the line profile and it is expected to determine the transition frequency with 11 digits.