In this presentation, the identification and characterization of alcohol and water tetramers and pentamers using Chirped Pulse Fourier-transform Microwave (CP-FTMW) spectroscopy is described. This talk will address calculating candidate cluster structures using ab initio techniques, fitting the observed lines to obtain experimentally derived rotational constants, and analyzing the splitting of these rotational states due to the internal rotation of methyl groups present in the clusters of interest. Continued work on the characterization of alcohol:water mixing will be discussed, as will other future targets of interest for this instrument.

Here we report the study on complexes between 2-decalone (C₁₀H₁₆O) and water, benzene, and phenol, respectively. The goal was to compare the interactions between the complex partners and the contributions of electrostatic and dispersion forces and which of them dominates when both forces are present. For that, a small hydrogen bond donor, water, was selected. Benzene is a good example for forming dispersion interactions, while in phenol both a phenyl ring and a hydrogen bond donor group are present and there could be competition between both forces. The complexes were studied in the gas phase in a cold and isolated environment generated by a supersonic expansion. The spectra were recorded using chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy in the 2-8 GHz frequency region. The sample consists of cis and trans 2-decalone, and complexes with both isomers were detected. In total five water complexes were identified with dominant electrostatic interactions between the complex partners. Three complexes with benzene were assigned showing mostly dispersion interactions. Another three complexes were identified with phenol. The analysis revealed that they have a higher percentage of electrostatic than dispersion forces and display a preference for a hydrogen bond when in competition with dispersion interactions. Figure

Difluorobenzaldehydes are used as the starting substrate for the synthesis of high-efficiency pesticides and pharmaceutical bioactive materials. The rotational spectra of difluorobenzaldehydes and of their hydrated complexes are measured by using a new-build 2–8 GHz chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy at Fudan University. Their precise structures have been determined from the rotational constants of the parent species combined with that of the 13C,18O and deuterated isotopologues. These results can benchmark theoretical methods for the structural optimization of weakly bound complexes. The effect of halogen substituents on intermolecular interactions is also discussed.

The broadband rotational spectra of N-methylimidazole…H₂O and 2-methylimidazole…H₂O have been recorded by Chirped Pulse Fourier Transform Microwave (CP-FTMW) spectroscopy in the frequency range 6.5-18.5 GHz. Each complex was generated by the co-expansion of the methylimidazole isomer and water in an argon backing gas. The spectra of five isotopologues of each complex have been assigned, allowing rotational constants (A₀, B₀ and C₀), centrifugal distortion constants (D_J and D_JK), nuclear quadrupole coupling constants (χ_aa and χ_bb−cc) and internal rotation parameters (V₃, ∠(i, b)) to be determined. In both N-methylimidazole…H₂O and 2-methylimidazole…H₂O, a hydrogen bond forms between H₂O acting as a hydrogen bond donor and the pyridinic nitrogen of the methylimidazole ring which is the hydrogen bond acceptor. In addition, there is a weak electrostatic interaction between the oxygen atom of H₂O and the hydrogen or CH₃ group attached to the C2 carbon of the methylimidazole ring. The (V₃) barrier to internal rotation of the CH₃ group has been determined for each complex. For N-methylimidazole…H₂O, the V₃ barrier is essentially unchanged from the monomer. For 2-methylimidazole…H₂O, there is a large increase in the barrier height (relative to the V₃ of CH₃ in the 2-methylimdizole monomer) which results from the interaction between the oxygen atom of H₂O and the CH₃ group.

Fluorinated alcohols have been widely used as co-solvents to study folding and unfolding behaviors of proteins and peptides. The detailed mechanism by which this happens is yet to be established. Recent molecular dynamics simulations suggested that clusters of the fluoroalcohol play an important role in the mechanism. In the current study, we applied jet-cooled chirped pulse Fourier transform microwave spectroscopy to probe structure and dynamics of 3,3,3-trifluoropropanol (TFP)¹ and its dimer. In comparison to 2,2,2-trifluoroethanol,² TFP is the smallest trifluoroalcohol molecule which exhibits folded conformations in its monomer form, thus serving as a prototype system for structural diversity associated with folding. The possible structural candidates of the TFP dimer were explored by using CREST, a recently developed conformational searching tool and nearly 70 stable binary conformers were identified. Rotational spectra of three low energy binary TFP conformers were assigned and their carriers identified. To help explain the observation of the binary conformers, a combined kinetic and thermodynamic conformational distribution model was developed to explain the non-observation of some lower energy conformations and to provide quantitatively explanation for the experimental conformational abundances. The study of the conformations of TFP and its dimer is a first and important step in understanding how TFP aggregates in bulk. 1. Marstokk, K.-M.; Møllendal, H.; Klika, K. D.; Fülöp, F.; Sillanpää, R.; Mattinen, J.; Senning, A.; Yao, X.-K.; Wang, H.-G.; Tuchagues, J.-P.; Ögren, M. Acta Chem. Scand. 1999, 53, 202; Heger, M.; Otto, K. E.; Mata, R. A.; Suhm, M. Phys. Chem. Chem. Phys. 2015, 17, 9899. 2. Xu, L.; Fraser, G. T.; Lovas, F. J.; Suenram, R. D.; Gillies, C. W.; Warner, H. E.; Gillies, J. Z. J. Chem. Phys. 1995, 103, 9541; Thomas, J.; Xu, Y. J. Phys. Chem. Lett. 2014, 5, 1850; Thomas, J.; Seifert, N. A.; Jäger, W.; Xu, Y. Angew. Chem. Int. Ed. 2017, 56, 6289.

Biogenic volatile organic compounds (BVOCs) are a class of molecules that have a noticeable effect on atmospheric chemical and physical processes. They are emitted naturally into the atmosphere mainly by plants and forests. An interesting family of this class is the monoterpenes (C₁₀H₁₆) and terpenoids (oxygenated terpenes) which are unsaturated hydrocarbons that are formed by the combination of two isoprene units (C₅H₈). These molecules are known to contribute to Secondary Organic Aerosol (SOA) and tropospheric ozone formation,D. Kotzias, J. L. Hjorth, H. Skov, Toxicol Environ Chem 1989, 20–21, 95–99^,T. Hoffmann, J. R. Odum, F. Bowman, D. Collins, D. Klockow, R. C. Flagan, J. H. Seinfeld, J. Atmos. Chem. 1997, 26, 189–222ainly through oxidation pathways. In addition, these molecules can form Hydrogen−bonded complexes with surrounding atmospheric molecules. The synergic combination of quantum chemical calculations and Fourier transform microwave spectroscopy (FTMW) in jet−cooled conditions, is a powerful tool to study the gas phase micro−solvation of atmospheric relevant molecules. Many hydrated complexes were studied using this approach in our group.E. M. Neeman, J. R. Avilés Moreno and T. R. Huet, Phys. Chem. Chem. Phys. 2021,23, 18137−18144^,E. M. Neeman, N. Osseiran and T. R. Huet, J. Chem. Phys. 2022, doi.org/10.1063/5.0084562e present herein a new approach, which employs the same theoretical-experimental approach to characterize complexes of terpenoids, i.e., alcohols and ketones, with H₂S, an atmospheric trace gas. The observed complexes are compared to their analog hydrates. The present work was funded by the ANR Labex CaPPA, by the Regional Council Hauts-de-France, by the European Funds for Regional Economic Development, and by the CPER CLIMIBIO.

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D. Kotzias, J. L. Hjorth, H. Skov, Toxicol Environ Chem 1989, 20–21, 95–99\end T. Hoffmann, J. R. Odum, F. Bowman, D. Collins, D. Klockow, R. C. Flagan, J. H. Seinfeld, J. Atmos. Chem. 1997, 26, 189–222m\end E. M. Neeman, J. R. Avilés Moreno and T. R. Huet, Phys. Chem. Chem. Phys. 2021,23, 18137−18144 E. M. Neeman, N. Osseiran and T. R. Huet, J. Chem. Phys. 2022, doi.org/10.1063/5.0084562W

The complex of benzofuran and diethyl disulfide has been investigated using Fourier transform microwave spectroscopy complemented by theoretical calculations. Two isomers have been observed, in which diethyl disulfide configures as gauche-gauche-gauche conformation sitting on the face of the benzofuran ring. The lone pair electrons of the sulfur atom points to the π-electron cloud of the benzofuran with a distance of about 3.6 Å, thus indicating a direct interaction between S and aromatic ring. NCIplot analysis suggests both observed isomers are stabilized by cooperative S…π, CH…π, and CH…O weak intermolecular interactions with total interaction energies of about 26 kJmol⁻¹ and is dominated by dispersion. Detailed spectroscopic and computational results will be presented.

Microwave spectroscopy allows for analysis of weakly-bound clusters in a mixture of difluoroethylene (DFE) and CO₂. The present study probes variations in interactions and orientations of DFE and CO₂ within weakly-bound clusters as cluster size increases. Four chirped-pulse FTMW spectra of DFE/CO₂ mixtures were obtained from 2-8 GHz, where the concentration of CO₂ was varied from 1% to 4%, with a constant DFE concentration of 1%. This experimental design allowed variation in intensity to be observed based on the variation of CO₂ concentration, where the pattern of intensity variation was used to identify transitions belonging to a particular cluster. In addition, patterns of intensity variation provided information about the size and DFE:CO₂ ratio of the cluster. Using these methods based on intensity variation analysis,H. Fino, R.A. Peebles, S.A. Peebles, C. West, B. Pate, International Symposium on Molecular Spectroscopy (Virtual), Talk FH12, June 25, 2021; R.A. Peebles, S.A. Peebles, P. Kannangara, H. Fino, International Symposium on Molecular Spectroscopy (Virtual), Talk FH13, June 25, 2021hree separate sets of transitions, each with unique intensity variation patterns, were extracted from the original raw spectra. Cluster composition was hypothesized based on further evidence from the intensity variation analyses, leading to compositions of (DFE)₁(CO₂)₂, (DFE)₁(CO₂)₃, and (DFE)₁(CO₂)₄. Fitted rotational constants for the spectra were compared to the results of ab initio calculations, which further supported hypothesized cluster compositions for the trimer, tetramer and pentamer. These results indicate that instead of forming a solvation shell around DFE, CO₂ molecules appear preferably to interact with other CO₂ molecules to form arrangements more closely resembling pure CO₂ clusters, with DFE on the outside of the cluster.

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Footnotes:

H. Fino, R.A. Peebles, S.A. Peebles, C. West, B. Pate, International Symposium on Molecular Spectroscopy (Virtual), Talk FH12, June 25, 2021; R.A. Peebles, S.A. Peebles, P. Kannangara, H. Fino, International Symposium on Molecular Spectroscopy (Virtual), Talk FH13, June 25, 2021t

Further spectral data of the O₂-H₂O van der Waals complex was obtained, expanding the range of transition lines for structural determination. Previous work was done in the 14-29 GHz rangeY. Kasai, E. Dupuy, R. Saito, K. Hashimoto, A. Sabu, S. Kondo, Y. Sumiyoshi, and Y. Endo. Atmos. Chem. Phys., 11, 8607-8612, 2011 Transitions have been measured as low as 11 GHz using a chirp pulse FTMW spectrometer. Working fits inclusive of these newfound transitions will be presented. Furthermore, current work with higher resolution cavity FTMW data utilizing Helmholtz coils on the complex will be discussed.

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Footnotes:

Y. Kasai, E. Dupuy, R. Saito, K. Hashimoto, A. Sabu, S. Kondo, Y. Sumiyoshi, and Y. Endo. Atmos. Chem. Phys., 11, 8607-8612, 2011.

Mandelic acid and its derivatives are useful as chiral synthons in the chemical and pharmaceutical industry because of their versatility. Their wide use in organic reactions makes them an important case to study their solute-solvent interactions. α-methoxy phenylacetic acid (AMPA), a methoxy-derived mandelic acid, can serve as a model to characterize the non-covalent interactions of such chiral solute with different solvents. The different functional groups in this chiral acid provide flexibility to the molecule, that conformational flexibility has been presented previously. Furthermore, the presence of a carboxylic acid and a methoxy group in AMPA provides good binding sites for solute-solvent interactions and thus serves as a good model system.
In this work, we investigate the microsolvation of AMPA in three different solvents using chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy. This technique coupled with the supersonic expansion reveals accurate structures of weakly bound complexes isolated in the gas phase. We chose three solvents that offer different functional groups and thus model more types of solute-solvent interactions. The three solvents were water, a small hydrogen bond donor partner, DMSO with a sulfoxide group, and phenol with a phenyl ring and a hydroxyl group. The preferred intermolecular interactions and the structural changes in complexes with three different solvents will be discussed.