Density functional theory is used to examine the possible conformations of both homochiral (RR or SS) and heterochiral (RS or SR) homodimers of 3,3-difluoro-1,2-epoxypropane and 3-fluoro-1,2-epoxypropane as a guide in the search for their microwave spectra. Similar to the analogous homodimers of 3,3,3-trifluoro-1,2-epoxypropane, the lowest energy heterochiral dimers of these species contain an inversion center and are microwave silent. However, spectra are obtained for the lowest energy conformers of the homochiral dimers. Analysis of the spectra confirms the theoretically predicted geometries.

Structurally similar to previously characterized chiral tagging candidates, trans-2-fluoro-3-(trifluoromethyl)oxirane (tFTFO) is investigated, both theoretically and experimentally, as a potential chiral tagging molecule. With a strong, simple rotational spectrum, tFTFO shows promise for applications in chiral analysis through the conversion of enantiomers into spectroscopically distinct diastereomeric species through noncovalent attachment. Fourier transform microwave spectroscopy from 5.6 to 18.1 GHz is used to obtain the rotational spectrum of five isotopologues of tFTFO and four isotopologues of its heterodimer with the argon atom. Analysis of these spectra allows the determination of the structures for both species. Initial results show that the structure of the argon complex is distinct from the structures previously described for the 3,3,3-trifluoro-, 3,3-difluoro-, and 3-fluoro-1,2-epoxypropane species, suggesting that the addition of the fluorine atom to the ring carbon alters bonding in the epoxy moiety.

The rotational spectra of the propargyl alcohol (PA)–water complex has been studied using a pulsed nozzle Fourier transform microwave spectrometer. PA is multifunctional molecule having a hydroxyl group and an acetylenic moiety. We have observed a cyclic hydrogen bonded structure. The alcohol donates an H-bond to water and the acetylenic moiety accepts a weak O-H-π H-bond from water. Calculations show that the two lowest energy structures have this same structural motif and differ only in the position of the non-bonded H atom of water. Several isotopic substitutions were carried out to ascertain the position of the non-bonded hydrogen of H₂O. Rotational spectroscopy helps to differentiate between these two similar structures. Splitting of the rotational transitions was also observed, indicating the presence of internal motions of the H₂O fragment. The observed global minimum structure is compared with earlier results from IR spectroscopy in matrixSaini, J.; Viswanathan, K. S. J. Mol. Struct. 2016, 1118, 147–156.nd helium nanodropletMani, D.; Pal, N.; Smialkowski, M.; Beakovic, C.; Schwaab, G.; Havenith, M. Phys. Chem. Chem. Phys. 2019, 21, 20582–20587. We also compare the global minima of several alcohol-water complexes to understand the donor-acceptor capabilities of the OH groups in alcohol-water complexes.

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Saini, J.; Viswanathan, K. S. J. Mol. Struct. 2016, 1118, 147–156.a Mani, D.; Pal, N.; Smialkowski, M.; Beakovic, C.; Schwaab, G.; Havenith, M. Phys. Chem. Chem. Phys. 2019, 21, 20582–20587..

2-aminoacetophenone is a grape sweet odorant and foxy natural occurring compound. It can be found in fruits, milk and white wines for which it is responsible for a characteristic flavor within a few months of storage. It has also been found in some animal secretions and it plays a role in the metabolism of some bacteria.
Literature survey reveals that to the best of our knowledge no rotational investigations of aminoacetophenone in the isolated phase have been reported so far. So we investigated this system by supersonic jet broadband using chirped-pulse Fourier transform microwave spectroscopy (2-8 GHz).
Over 140 μ_a and μ_b transitions, ranging from J_up=1 to 6, were assigned to the monomer using a Watson S-reduced Hamiltonian implemented by the ¹⁴N nuclear hyperfine Hamiltonian. Rotational transitions of ¹³C and¹⁵N isotopologues have been observed in natural abundance, allowing for the determination of the molecular structure. Other lines in the spectrum revealed the presence of a water complex and its isotopologues.
The monomer structure is consistent with a planar molecule, except for the methyl CH bonds contribution, with an intermolecular O-HN hydrogen bond, while the structure of the complex involves one water molecule coordinated with the acetyl group through a OH-O hydrogen bond and secondary interaction between oxigen lone pair and methyl group.
Experimental results are in agreement with quantum mechanical calculations carried out at B3LYP and MP2 levels of theory.

l0pt Figure Over the years, great effort has been made in understanding small hydrogen-bonded systems due to their relevance in biological sciences, among others. In particular, the formic acid dimer has been long investigating because it represents a suitable model system to gain information about the proton donor-acceptor interactions. Very recently, the ground-state structure of the proton-bound formate dimer was characterized in helium nanodroplets by Thomas et al.Angew. Chem. Int. Ed. 2018, 57, 10615 – 10619howing that the lowest energy structure corresponds to a symmetrical motif in which the proton is equally shared between the carboxylates. Nevertheless, no reports are found for bigger hydrogen-bonded carboxylate complexes like trimers or tetramers. Here we report the spectrum of the anion formic acid trimer complex in the gas phase, utilizing vibrational spectroscopy of ions trapped in helium nanodroplets in the 400-2000 cm⁻¹ region. Isotopic labeling and anharmonic frequency calculations indicate that among several conformers a C2 symmetry motif may be the prevalent structure found in the gas phase.

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Angew. Chem. Int. Ed. 2018, 57, 10615 – 10619s

The hydrated radical anions CO₃⁻(H₂O)_1,2 and CO₄⁻(H₂O)_1,2 are important anions in the atmosphere. Quantitative models predict the steady state fractional abundance of CO₃⁻(H₂O)_0,1 to be in the range of 0.3–2.3 % of the total negative ion inventory, with CO₃⁻(H₂O) being predicted to be dominating up to an altitude of 11 km [1]. Direct sampling of ions in the boreal forest confirmed the presence of CO₃⁻(H₂O)_0,1 [2]. The CO₄⁻ ion is another radical anion that is derived from CO₂ and found with a fractional abundance of about 0.01 % [2]. The reactivity often depends strongly on the number of solvating water molecules [3], and therefore the hydration structure is key to understand their reactivity. Infrared multiphoton dissociation (IRMPD) spectroscopy is an excellent tool to collect information on the structure of ions. The spectra were measured in both the C-O and O-H stretch region at ion trap temperatures of 295 K and approx. 80 K [4]. The O-H stretching region exhibits broad spectra with additional maxima and shoulders beside the free O-H stretch frequency for all ions. Clear absorptions are observed in the C-O stretching region, but dissociation of CO₃⁻(H₂O)_1,2 was surprisingly inefficient, probably due to radiative cooling. While CO₃⁻(H₂O)_1,2 and CO₄⁻(H₂O) lose water upon dissociation, CO₄⁻(H₂O)₂ exhibits an additional dissociation channel with loss of CO₂. All experimentally measured infrared spectra are compared to calculated spectra within harmonic approximation and from analysis of molecular dynamics (MD) simulations. The comparison of experiment and theory indicates that multiple isomers contribute to the observed spectrum at finite temperatures. [1] H. Kawamoto and T. Ogawa, Planet. Space Sci. 34, (1986) 1229. [2] M. Ehn, et al., Atmos. Chem. Phys. 10, (2010) 8513. [3] C. van der Linde et al., Phys. Chem. Chem. Phys. 20 (2018) 10838. [4] M. G. Münst et al., J. Chem. Phys. 154, (2021) 084301.

Imine-based molecular motors and switchesL. Greb, A. Eichhöfer, J.-M. Lehn, Angew. Chem. Int. Ed., 2015, 54, 14345-14348ave been recently developed, and many of their properties remain largely unexplored. With potential to perform multi-step unidirectional rotations, these systems are an important addition to the existing nano-motor toolbox.S. Kassem, T. van Leeuwen, A. S. Lubbe, M. R. Wilson, B. L. Feringa, D. A. Leigh, Chem. Soc. Rev., 2017, 46, 2592-2621heir photochemically-induced switching processes foresee applications in the regulation of chemical reactions, as well as performing mechanical functions. Previous condensed-phase studies show that the energetic balance between isomers of such a molecular switch can be markedly different to that predicted from quantum chemistry calculations.C. H. Pollok, T. Riesebeck, C. Merten, Angew. Chem. Int. Ed., 2017, 56, 1925-192icro-solvation studies in supersonic jet conditions are thus an appealing route to investigate the origin of these population manoeuvres. Rotational spectroscopy allows us to study the 3D structure and conformational heterogeneities of these systems and their micro-solvated counterparts with great precision, since we can relate the pattern of rotational frequencies to the structure through the moments of inertia.S. R. Domingos, A. Cnossen, W. J. Buma, W. R. Browne, B. L. Feringa, M. Schnell, Angew. Chem. Int. Ed., 2017, 56, 11209-11212xperiments using chirped-pulse Fourier transform microwave spectroscopy in the 2-8 GHz range are ongoing. Preliminary results of our study will be presented and discussed.

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L. Greb, A. Eichhöfer, J.-M. Lehn, Angew. Chem. Int. Ed., 2015, 54, 14345-14348h S. Kassem, T. van Leeuwen, A. S. Lubbe, M. R. Wilson, B. L. Feringa, D. A. Leigh, Chem. Soc. Rev., 2017, 46, 2592-2621T C. H. Pollok, T. Riesebeck, C. Merten, Angew. Chem. Int. Ed., 2017, 56, 1925-192M S. R. Domingos, A. Cnossen, W. J. Buma, W. R. Browne, B. L. Feringa, M. Schnell, Angew. Chem. Int. Ed., 2017, 56, 11209-11212E

1- and 2-naphthol, hydroxy-derivatives of naphthalene, the simplest polycyclic aromatic hydrocarbon, are used as precursors in the dye, perfume, insecticide, and pharmaceutical industries.¹ Recently polyaromatic hydrocarbons (PAHs) such as naphthol have been examined for their complex intermolecular interplay within dimer structures.² To further build on previous PAHs studies we present a combined spectroscopic and theoretical study of 1- and 2-naphthol. The microwave spectra of the 1-naphthol and 2-naphthol monomers were measured using a chirped-pulse Fourier transform microwave spectrometer in the 2-6 GHz range. Spectroscopic assignments of the cis- and trans-conformers of 1-naphthol and 2-naphthol are supported by results from electronic structure calculations. Spectra of ten ¹³C isotopologues were identified and analyzed for all four species. This allowed for the determination of substitution structures and so-called semi-experimental effective structures. The theoretical results show that the OH group of cis-1-naphthol points   6^° out of plane, which is consistent with the difference in inertial defect between cis- and trans-1-naphthol. For cis-1-naphthol, NCI³ and QTAIM⁴ analyses resulted in a bond critical point between the hydroxyl group H-atom and the close H-atom in the neighbouring ring, suggesting a bonding interaction. To confirm or deny this hypothesis, local mode analyses were carried out to obtain a more in-depth assessment of the O-H bond strengths. Compared to trans-1-naphthol, the strength of the O-H bond is much stronger in cis-1-naphthol. This is reflected in a blue shift of the O-H stretching frequency and is consistent with the traditional notion of a steric repulsion rather than a bonding interaction. 1. H. Shindy, Chem. Int, 2016, 2, 2016.; 2. N. A. Seifert, A. S. Hazrah and W. Jäger, J. Phys. Chem. Lett., 2019, 10, 2836-2841.; 3. E. Johnson, S. Keinan, P. Mori-Sánchez and J. Contreras-García, J. Am. Chem. Soc., 2010, 2010, 132.; 4. R. F. Bader, Chemical Reviews, 1991, 91, 893-928.

MACE is demonstrated for the integration of experimental observables as constraints in molecular structure determination and for the systematic merging of multiple computational architectures. MACE is founded on simultaneously determining the equilibrium point between multiple experimental and/or computational agents; the returned state description (e.g., atomic coordinates for molecular structure) represents the intersection of each manifold and is not equivalent to the average optimum state for each agent. The moment of inertia, determined directly from microwave spectroscopy measurements, serves to illustrate the mechanism through which MACE evaluations merge experimental and quantum chemical modeling. MACE results reported combine gradient descent optimization of each ab initio agent with an agent that predicts chemical structure based on root-mean-square deviation of the predicted inertia tensor with experimentally measured moments of inertia. Successful model fusion for several small molecules was achieved as well as the larger molecule solketal. Fusing a model of moment of inertia, an underdetermined predictor of structure, with low cost computational methods yielded structure determination performance comparable to standard computational methods such as MP2/cc-pVTZ, and greater agreement with experimental observables.

The effects of substitution were investigated for the sulfonamides class of molecules, in particular those which contain the benzosulfonamide functional group. This group of molecules is of extreme interest in the biological field since many of them are active against a variety of diseases. In this work, structural investigations on the pharmacophoric group benzensulfonamide and the substitution effects have been performed through the studies of its derivatives benzensulfonamide itself, para-toluensulfonamide, ortho-toluensulfonamide and the bioactive molecule sulfanilamide. In all compounds, but in ortho-toluensulfonamide, the amino group lies perpendicular to the benzene plane with the amminic hydrogens eclipsing the oxygen atoms. In ortho-toluensulfonamide where a weak attractive interaction between the nitrogen lone pair and the methyl hydrogen atoms takes place, the amino group lies in the gauche orientation. These results show that such weak non-covalent interactions are able to change the conformational preferences of the pharmacophoric group. For all species, the ¹⁴N quadrupolar hyperfine analysis has been performed. This has provided crucial information for the unambiguous identification of the observed conformation and the structural parameters related to the position of the nitrogen atom. In addition, for ortho-toluensulfonamide, the vibration-rotation hyperfine structure related to the methyl torsion has been analyzed and the methyl group rotation barrier was determined.

Sabinene (C₁₀H₁₆-4-methylene-1-(1-methylethyl)bicyclo[3.1.0]hexane) is a bicyclic monoterpene with an exocyclic double bond. It is one of the main biogenic volatile organic compound. In Europe, it is emitted in the troposphere by trees such as Quercus ilex, Betula pendula, and Fagus sylvatica with an estimated flux of 20 Tg.yr⁻¹. It acts as precursor of secondary organic aerosols through several oxidation reactions. A first investigation of the low resolution rotational spectrum of sabinene was performed 40 years ago leading to the determination of the (B+C) combination of rotational constants of the parent species.[1] From this unique data, and within numerous assumptions, the dihedral angle ϕ defining the boat arrangement of the five-membered ring was determined (55^°), but it is not in agreement with the typical values observed in molecules based on bicyclo[3.1.0]hexane.[2] In this work, the rotational spectrum of sabinene along with its gas phase structure were reinvestigated theoretically and experimentally employing a combination of quantum chemical calculations and Fourier transform microwave spectroscopy coupled to a supersonic jet expansion in the 2-20 GHz frequency region. The spectra of the parent species and of all singly substituted ¹³C isotopologues have been analysed in natural abundance. The 11 sets of rotational constants allowed to determine the partial substitution and effective structures of sabinene. They are in fair agreement with the optimized ones at the B3LYP, M06-2X and MP2 levels associated with the 6-311++G(d,p) basis set. The dihedral angle ϕ was determined to be 26.9^°. [1] Z. Kisiel, A. C. Legon, Conformations of some bicyclic monoterpenes based on bicyclo [3.1.0] hexane from their low-resolution microwave spectra, J. Am. Chem. Soc. 100 (1978) 8166–8169. [2] E. J. Ocola, L. A. Wieding, S. Adams, J. Laane, Theoretical study of structures and ring-puckering potential energy functions of bicylo [3.1.0] hexane and related molecules, J. Phys. Chem. A 122 (2018) 5970–5977. The present work was funded by the French ANR Labex CaPPA through the PIA under contract ANR-11-LABX-0005-01, by the Regional Council Hauts de France, by the European Funds for Regional Economic Development (FEDER), and by the French Ministère de l'Enseignement Supérieur et de la Recherche. It is a contribution to the CPER research Project CLIMIBIO.

The rotational spectrum of neutral levodopa (L-dopa), an aminoacid precursor of neurotransmitters dopamine C. Cabezas, I. Pena, J. C. Lopez and J. L. Alonso, J. Phys. Chem. Lett., 2013, 4, 486–490. norepinephrine (noradrenaline) and epineophrine (adrenaline), has been investigated for the first time using a broadband chirped pulse FTMW spectrometer coupled with a laser ablation source (LA-CP-FTMW). The spectroscopic parameters derived from the analysis of the spectrum conclusively identify the existence of three conformers of levodopa. The ¹⁴N nuclear quadrupole coupling constants have been analyzed, probing the existence of stabilizing N-H-π interactions for the observed structures.

• Acknowledgments: The authors thank the financial fundings from Ministerio de Ciencia e Innovacion (CTQ2016-76393-P and PID2019-111396GB-I00), Junta de Castilla y Leon (VA077U16 and VA244P20) and European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013) / ERC-2013-SyG, Grant Agreement n. 610256 NANOCOSMOS, are gratefully acknowledged. E.R.A. acknowledges MINECO for a Juan de la Cierva postdoctoral fellowship and the Fundación Biofísica Bizkaia (Spain). M.S.N. acknowledges funding from the Spanish "Ministerio de Ciencia, Innovación y Universidades" under predoctoral FPU Grant (FPU17/02987).

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C. Cabezas, I. Pena, J. C. Lopez and J. L. Alonso, J. Phys. Chem. Lett., 2013, 4, 486–490.,

Vanillic acid is produced in significant quantities in forest fires and is used as a biomass burning tracer for softwood and hardwood combustion events. Vanillic acid has also been found in secondary organic aerosol and can be oxidized in aqueous atmospheric processes to humic-like substances. Hydrates of vanillic acid play undoubtedly important roles in the processes involved, and we present here a study of conformers of the vanillic acid monomer to lay the groundwork for subsequent studies of the mono- and di-hydrates. Computationally, 12 conformers were found within an energy range of 51 kJ/mol using the Conformer–Rotamer Ensemble Sampling Tool (CREST).¹ The two lowest energy conformers could then be identified in the spectrum recorded with a chirped pulse Fourier transform microwave spectrometer in the 2-6 GHz range. The deviation between experimental and theoretical rotational constants determined at the MP2/aug-cc-pVTZ and DFT B3LYP-D3(BJ)/def2-TVPZ levels of theory is less than 1%. No tunnelling splitting was observed which suggests a relatively high barrier to methyl internal rotation, in agreement with other, previously studied vanillin derivatives. Furthermore, no c-type transitions could be observed in agreement with computed zero c-dipole moment component of the lowest energy structures. From the theoretical structures, it is apparent that intra-molecular hydrogen bonds play a significant role in stabilizing the lowest energy conformers. To further explore the intra-molecular interactions in vanillic acid, QTAIM and NCI analyses were performed. ¹P. Pracht, F. Bohle, S. Grimme, Phys. Chem. Chem. Phys. 22, 7169-7192 (2020).

Recent algorithmsN.A. Seifert et. al., Journal of Molecular Spectroscopy 312, 13 (2015).L. Yeh, L. Satterthwaite, and D. Patterson, The Journal of Chemical Physics 150, 204122 (2019). have demonstrated context-free assignment of rotational constants-needing no knowledge of the chemical species other than the rotational spectrum. Efforts to date to subsequently determine molecular structure require further information including assignment of singly-substituted isotopologuesJ. Yan and D. Patterson. Submitted to the 75th International Symposium on Molecular Spectroscopy (2021).K. Mayer et. al., Proceedings of the 74th International Symposium on Molecular Spectroscopy (2019)., mass spectroscopy, and data miningM. Muckle, A. Mikhonin, D. McDaniel, and/or J. Neill, Proceedings of the 74th International Symposium on Molecular Spectroscopy (2019). We investigate two methodologies to resolve sign ambiguities of Kraitchman's substitution coordinates. The first methodology requires candidate rotational constants of doubly-substituted isotopologues. Given many such candidates, we have worked out how to determine 1) which candidates for singly- and doubly-substituted isotopologues are most probable, and 2) doubly-substituted atoms' relative position octant. This is realizable given resolution of doubly-substituted species in natural abundance, which is 10-100x order of magnitude above our instrument's present signal to noise. The second methodology requires precision measurement of the electric dipole moments and magnetic g-factors of both the parent and singly-substituted isotopologues. The magnetic g-factor is measured via application of a large magnetic field to our microwave spectrometer, as done by Flygare et. al. (1969). For a 6 carbon molecule, this could be realized given the ability to resolve a ≈ 10⁻⁵ percent difference between magnetic g-factors of the parent and singly-substituted isotopolog species, as well as resolve the electric dipole moment to 4 or 5 significant figures. Improving the capabilities of microwave spectrometers to within these thresholds would therefore enable context-free molecular structure determination.

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N.A. Seifert et. al., Journal of Molecular Spectroscopy 312, 13 (2015).

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J. Yan and D. Patterson. Submitted to the 75th International Symposium on Molecular Spectroscopy (2021).

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M. Muckle, A. Mikhonin, D. McDaniel, and/or J. Neill, Proceedings of the 74th International Symposium on Molecular Spectroscopy (2019)..