Recently, the CCSD(T) equilibrium (r_e) structures and the Semi-Experimental Equilibrium (r_e^SE) structures of several small molecules (hydrazoic acid, benzene, pyridazine, pyrimidine, thiophene, thiazole, etc.) demonstrated a new standard of agreement between theory and experiment (typically 0.001 Å for bond distances and 0.04^° for bond angles). In each of these examples, all or nearly all of the computed parameters fell within the 2σ statistical uncertainties of their corresponding experimental values. This agreement is typically possible by obtaining an r_e^SE structure based upon many isotopologues beyond single-isotopic substitution from many hundreds or thousands of transitions for each isotopologue. The resulting rotational constants are corrected using CCSD(T) calculations for the impact of the vibration-rotation interaction and for the electron-mass distribution. Additionally, we have found that such close agreement requires an r_e structure calculated at the CCSD(T)/cc-pCV5Z level further corrected to account for an incomplete basis set, untreated correlation, and relativistic effects. This talk will feature examples to demonstrate the agreement possible, current best practices, and the tools used to analyze these structures. Outstanding questions and future investigations will be discussed.

The methyl allyl disulfide-formaldehyde adduct were probed using Fourier transform microwave spectroscopy and quantum chemical computations. The low energy isomers of the adduct were sampled using the conformer-rotamer ensemble sampling tool (CREST) and geometrically optimized at the B3LYP-D3BJ/def2-TZVP level of theory. Although many isomers of the adduct were theoretically predicted to have close stability, only one isomer of the methyl allyl disulfide-formaldehyde adduct have been detected in the helium supersonic jet. Each observed transition exhibited multiple splitting arising from internal rotation of formaldehyde and methyl internal rotations. In the observed isomer, the non-covalent bonding distance between the carbon atom of formaldehyde and the nearest sulfur atom of methyl allyl disulfide has been found to be well within the corresponding sum of van der Waals radii, indicating the existence of a n → π* interaction. Also, formaldehyde acts as a lone pair donor forming a weak CH ... O hydrogen bond with methyl allyl disulfide. Detailed spectral analysis, spectroscopic and computational results will be presented.

1H-1,2,4-Triazole is a five membered aromatic heterocycle with 3 inequivalent nitrogen atoms. This molecule exists as an equilibrium of two tautomers; 1H-1,2,4-Triazole (C_s) and 4H-1,2,4-triazole (C_2v), with the 1H tautomer being dominant. 1,2,4-Triazole is predicted to exist in the atmosphere of Titan, forming in aerosols containing known constituents HNC and NH₃, where identification relies on accurate spectral information. The rotational spectrum of 1H-1,2,4-triazole was reported by Bolton et al.Bolton, K.; Brown, R. D.; Burden, F. R.; Mishra, A. The microwave spectrum and structure of 1,2,4 triazole. J. Mol. Struc. 1975, 27 (2), 261–266 but only the ground state rotational constants were determined. Here we present the 70-700 GHz spectrum of 1H-1,2,4-triazole with spectral frequencies comparable to telescopes such as ALMA. Analysis of this spectrum resulted in improved rotational constants and an accurate determination of the sextic and quartic centrifugal constants. We also obtained tentative least-squares fits of transitions for all vibrationally excited states below 1200 cm⁻¹, where the majority appear to be perturbed by Coriolis interactions. A partial structure determination of 1H-1,2,4-triazole, derived from three isotopologues, has been reported previously. Recent work on other heteroatomatic compounds has achieved impressive accuracy and precision in the determination of semi-experimental (r_e^SE) equilibrium structures. Using deuterium-enriched triazole samples, we have determined rotational constants for 27 isotopologues of 1H-1,2,4-triazole including multiple isotopic substitutions of all atoms. As a result, we have obtained a complete r_e^SE structure that is in full agreement with an r_e structure determined with high-level quantum chemistry. Here we will report on the spectroscopic and structural analysis of 1H-1,2,4-triazole.

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Footnotes:

Bolton, K.; Brown, R. D.; Burden, F. R.; Mishra, A. The microwave spectrum and structure of 1,2,4 triazole. J. Mol. Struc. 1975, 27 (2), 261–266,

The rotational spectra of pentafluoropyridine-CO and hexafluorobenzene-CO have shown unambiguously that substitution by fluorine atoms on the ring strongly influences the binding abilities of the aromatic ligand. Differently from their non-substituted counterparts, both molecules interact with CO forming a perpendicular lp···π interaction between the carbon and the ring. We report earlier the rotational spectroscopy studies performed with a Molecular Beam Fourier Transform Microwave spectrometer in which we have tested the binding abilities of the fluorine substituted aromatics molecules with CO. Now local energy decomposition analysis of the relative conformational energies shows the interplay between the non-covalent interactions which led to the final configuration preference of CO with respect to the aromatic rings in the supersonic expansion. Although the relative complexity of the non-covalent interactions made these molecular systems challenging to study, the progress in theoretical modelling have shown to physical explain the origin of the molecular binding.

Polycyclic aromatic hydrocarbons containing the nitro group (nitro-PAHs) are pollutants found in the atmosphere as the result of direct release from exhaust or of radical reactions on naphthalene involving hydroxyl radical (daytime), nitrate radical (night time) and nitrogen dioxide. Among them, the nitro-PAHs 1-nitronaphthalene (1NN) and 2-nitronaphthalene (2NN) are relevant as they are highly toxic and major environmental contaminants in urban areas. We have investigated 1NN and 2NN and their complexes with water by broadband rotational spectroscopy and determined their structures. Water primarily interacts with the -NO2 group forming an O-H…O hydrogen bond. Experimental observations are compared with predictions by theoretical methods to evaluate the performance of the latter. Our results contribute to understanding the microsolvation processes of atmospheric pollutants in the gas phase.

Here we present new measurements and calculations of line intensities in the 3 - 0 band of ¹²C¹⁶O. These experimental results and calculations exhibit unprecedented consistency and low uncertainty. Calibration-free agreement at the 1 permille level relative standard deviation level has been demonstrated between theoretical ab initio calculations and three sets of independent experiments, corresponding to a nearly twenty-fold reduction in uncertainty by comparison to literature values. The experimental techniques cover a broad range of rotational quantum numbers from J = 5 to 30, including three separate laser-based measurements of high-J lines performed at two institutions, along with independent Fourier transform spectroscopy measurements for J = 5 to 18. The most accurately determined intensity is that of the R23 transition determined to within 0.4 permille. The intensity of this transition is a possible intrinsic reference for evaluating and reducing biases in future spectroscopic determinations of molecular line intensities.

Hydrogen bonds (HBs) are important for a broad range of applications and play a fundamental role in structural chemistry and biology. HB interactions, dynamics and their directionality are discussed for almost one century and there is still a need for further experiments and theoretical investigations to fully encompass this complex interaction. Especially the experimental investigation of weak intramolecular HBs of isolated molecules in the gas phase remains challenging. Quantum chemical tools are needed to support high resolution THz and IR spectroscopies which can reveal the influence of intramolecular HBs on the rovibrational dynamics¹. In this work we focus on intramolecular HBs of oxygenated aromatic molecules. They are investigated through a combination of quantum theory of atoms in molecules QTAIM², non-covalent interactions NCI³, natural bond orbitals NBO⁴, and topological data analysis TDA⁵. We studied the influence of the substitutants, of the donor or acceptor groups and of the number of atoms included in the ring formed by the HB. We relate our findings with recent rovibrational measurements in catechol (1,2-dihydroxybenzene) and guaiacol. We provide an overview of the problems arising while studying weak intramolecular HBs stabilizing oxygenated aromatic compounds and we discuss the performance of the different quantum chemical tools.

[19]r2.5in Figure Methyl cyanide (CH₃CN) was among the first polyatomic molecules detected by radio-astronomical observations of the interstellar medium (ISM)[1]. As methyl cyanide has a proton affinity much larger than that of H₂, its protonated version (CH₃CNH⁺) is postulated to form efficiently via exothermic proton transfer from H₃⁺ to CH₃CN in the interstellar medium. In this talk, we present a comprehensive experimental and quantum-chemical study of the gas phase vibrational spectrum of CH₃CNH⁺ [2]. We employed the widely tuneable free electron lasers for infrared experiments (FELIX) coupled to a cryogenic ion trap instrument [3] for our measurements. The spectrum was recorded in the 300-1700 and 2000-3300 cm⁻¹ spectral regions using infrared predissociation (IRPD) action spectroscopy with neon as a weakly bound messenger atom. The assignment of the vibrational modes is based on anharmonic frequency calculations performed at the CCSD(T)/ANO2 level of theory. We demonstrate that the comparatively low-cost ANO0 basis-set provides accurate estimates on the influence of the weakly-bound neon atom as a tag in the IRPD experiments. The data presented here will support astronomical searches for the CH₃CNH⁺ ion in space.

[1] P.M. Solomon, K.B. Jefferts, A.A. Penzias and R.W. Wilson, Astrophys. J. 168 (1971) L107
[2] A. N. Marimuthu, F. Huis in’t Veld, S. Thorwirth, B. Redlich and S. Brünken, J. Mol. Spec. 379 (2021) 111477
[3] P. Jusko, S. Brünken, O. Asvany, S. Thorwirth, A. Stoffels, L. van der Meer, G. Berden, B. Redlich, J. Oomens and S. Schlemmer, Faraday Discuss. 217, (2019), 172

Although automated search algorithms are highly powerful tools to find out various structural isomers for a given elemental composition, human chemical intuition still delivers more geometries than automated searches once we know the initial geometries. This indirectly emphasizes the fact that creating an unbiased algorithm for structural isomerism is simply difficult. Moreover, it also outlines that an integrated approach is necessary between search algorithms and chemical intuition to further our knowledge of chemical space for any given elemental composition.