The processes that govern aggregation at the molecular level are loosely established. Molecular recognition is mediated via a delicate balance between the prevailing intermolecular interactions at play, hydrogen bonding and dispersion interactions. Using high-resolution broadband rotational spectroscopy and supersonic jets, we studied the styrene oxide dimer. Due to its chirality, these dimers exist as enantiomeric and diastereomeric pairs, with the diastereomers being directly differentiable via their rotational spectra. Interestingly, the three most stable styrene oxide dimers are stabilized by three intermolecular contacts, arising from two CH-O and π-π interactions. The phenyl groups show a similar arrangement as in the case of the parallel-displaced benzene dimer. The next set of dimers, which are slightly higher in energy, is stabilized by two CH-π interactions each. The interplay between hydrogen bonding and dispersion on the formation of these homo- and heterodimers as well as their prospects for chiral tagging will be discussed.

3,3,3-Trifluoro-1,2-epoxypropane [2-(trifluoromethyl)-oxirane, or TFO] has shown promise as a tag for chiral analysis through conversion of enantiomers into spectroscopically distinguishable diastereomers via the formation of non-covalently bound heterodimers. We demonstrate the suitability of this method through characterization of the microwave rotational spectrum of complexes formed between TFO and styrene oxide (SO). Molecular dynamics calculations are used to quickly identify possible heterodimer conformations which are then optimized and evaluated using density functional theory. Using a mixture of racemic samples of both species, we observe and assign spectra for the lowest energy conformers of both homochiral (RR/SS)-TFO-SO and heterochiral (RS/SR)-TFO-SO. Had a single enantiomer of TFO been used, say (R), the spectra are sufficiently distinct and sufficiently well predicted by theory that (RR)-TFO-SO and (RS)-TFO-SO are readily identified and separately analyzed.

Continuing our efforts in characterizing small molecules for use as potential chiral tags for the conversion of enantiomeric molecules into spectroscopically distinct diastereomeric complexes for chiral analysis, we examine the microwave spectrum and molecular structure of 3-fluoro-1,2-epoxypropane. Although this species has a lower vapor pressure than the trifluoro- and difluoro- analogues previously reported at this meeting, it is still relatively easy to incorporate into a free jet expansion by flowing a carrier gas over a heated liquid sample. In common with the structurally similar trifluoro- and difluoro- species, it has a simple, hyperfine-free rotational spectrum. This spectrum has been obtained for the most abundant and four singly-substituted isotopologues, all in natural abundance, and the structure of the molecule determined. Multiple minima of similar energies are predicted for the complex of 3-fluoro-1,2-epoxypropane with argon, and progress on assigning and analyzing the spectra of these complexes will be reported.

r0pt Figure Following previous investigation of the monohydrateM. Juanes, W. Li, L. Spada, L. Evangelisti, A. Lesarri, W. Caminati, Phys.Chem.Chem.Phys., 2019, 21, 3676.nd homodimer of cyclohexanolM. Juanes, I. León, R. Pinacho, J. E. Rubio, W. Li, L. Evangelisti, W. Caminati, A. Lesarri, Comm. WK09, 73rd ISMS (Urbana-Champaign), 2018. we have examined the non-covalent interactions in the saturated six-membered rings of cyclohexylamine (CA) and cyclohexanethiol (CT), using chirped-pulsed and cavity Fourier-transform microwave spectroscopy (2-18 GHz). Water behaves as proton donor to both CA and CT, generating two isomers in CT ··· H₂O (equatorial-gauche and equatorial-trans) and a single isomer in CA ··· H₂O (equatorial gauche-trans). Torsional splittings were observed for CT ··· H₂O, while nuclear quadrupole coupling effects were resolved for CA ··· H₂O. The predicted hydrogen bond distances in the hydrated amine and thiol are enlarged with respect to the alcohol (r_{O H···S})=2.47Å and (r_O−H···N)=1.91Å vs (r_O−H···O)=1.88Å). Work on the CA₂ and CT₂ dimers will also be reported. In these dimers the hydrogen bonds are characterized by r_O−H···O=1.88Å and r_O−H···N=1.91 Å. Rotational data and supporting ab initio calculations will be presented for the investigated species.

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Footnotes:

M. Juanes, W. Li, L. Spada, L. Evangelisti, A. Lesarri, W. Caminati, Phys.Chem.Chem.Phys., 2019, 21, 3676.a M. Juanes, I. León, R. Pinacho, J. E. Rubio, W. Li, L. Evangelisti, W. Caminati, A. Lesarri, Comm. WK09, 73rd ISMS (Urbana-Champaign), 2018.,

The large diadamanthyl ether (DME, C₂₀H₃₀O) molecule presents a good model to study the interplay between dispersion and hydrogen bond interactions. In this work, we have studied different diadamanthyl ether complexes where the sizes and the steric complexity of the partners were systematically changed. In addition, their contributions to dispersion and hydrogen bond interactions were different to create a broad overview. On the one side, we have analyzed the structures and different interactions taking place between DME and a series of alcohol aggregates, that is, water (finding clusters up to 3 molecules of water), ethanol and tert-butanol. On the other side, we have studied the complexes formed between DME and benzene, hexafluorobenzene and phenol. All these structures have been studied in the gas phase under the cold and isolated conditions of a supersonic expansion. To analyze them, we have used the highly sensitive and high resolution chirped pulse Fourier transform microwave (CP-FTMW) spectroscopy in the 2-8 GHz region. Quantum chemical calculations were performed to aid the analysis of the experimental data. The results obtained can help to understand the influence of the interplay between dispersion and hydrogen bond interactions on the formation and stabilization of the structures of large complexes.

r0pt Figure Water is present in the air with a concentration of up to 4 %, and also in the olfactory mucosa. Odorants are thus very likely to interact with water before reaching olfactory receptors. We have investigated the complexes of cyclooctanone with water as a first step towards understanding how larger macrocyclic odorants interact with water. Two complexes of the most abundant conformer of cyclooctanone with one water molecule, and two complexes with two water molecules have been characterised using chirped-pulse Fourier Transform Microwave (CP-FTMW) spectroscopy. In the cyclooctanone-H₂O complexes, water forms a O–H...O hydrogen bond with the carbonyl oxygen of cyclooctanone and two O...H–C hydrogen bonds with the –CH₂ groups in the cyclooctanone ring. In the cyclooctanone-(H₂O)₂ complexes, the second molecule of water binds primarily to the first molecule of water through hydrogen bonding, whilst also forming secondary interactions with the –CH₂ groups in the ring. The observation of all ¹³C isotopic species of the cyclooctanone ring in the complexes in their natural abundances, and of the ¹⁸O species using isotopically enriched water allowed us to calculate the experimental structures of the complexes.

Herein, the pure rotational spectra of benzophenone complexed with up to three water molecules were observed by using chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy. Benzophenone offers dispersion interaction sites through the phenyl rings and a hydrogen bond acceptor through the carbonyl oxygen, allowing us to study the competing binding sites for water molecules. The theoretical calculation combined with the isotopic substitution measurement provides the unambiguous structural information of the complexes. In benzophenone-(H₂O)_1,2 clusters, the water molecules are located at one side of a phenyl group, where benzophenone and water molecules form a ring. Water monomer or dimer links with the carbonyl group through an OH…O hydrogen bond and links with the phenyl group through a CH…O weak hydrogen bond. The benzophenone-(H₂O)₃ complex is of interest as the water trimer was found located at the top of one phenyl group with the cooperativity of the hydrogen bond net. The water trimer forming an open loop through two hydrogen bonds is locked by benzophenone through one OH…O, one OH…π, and one CH…O hydrogen bonds, respectively.

l0pt Figure We are using rotational spectroscopy to examine sulfur hydrogen bonding in a series of thiophene and furan mercapto derivativesM. Juanes, A. Lesarri, R. Pinacho, E. Charro, J. E. Rubio, L. Enríquez, M. Jaraíz, Chem. Eur. J., 2018, 24, 6564 in order to compare their aggregation properties with those of the equivalent alcohols. Here we report on the dimers and monohydrates of 2-thiopheneethanol (2TE) and 3-thiopheneethanol (3TE), isolated in a jet-cooled expansion. Two isomers of (2TE)₂, three isomers of (3TE)₂ and the two monohydrates 2TE···H₂O and 3TE···H₂O were observed using chirped-pulsed and cavity Fourier transform microwave spectroscopy (2-18 GHz). The dimers are primary bound in all cases by the stronger O-H···O interaction (r_O−H···O ca. 1.88 Å) originated by the alcohol groups. In the monohydrates water behaves as a proton acceptor, and the spectrum shows evidence of torsional tunneling. Rotational parameters and supporting ab initio calculations will be reported.

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Footnotes:

M. Juanes, A. Lesarri, R. Pinacho, E. Charro, J. E. Rubio, L. Enríquez, M. Jaraíz, Chem. Eur. J., 2018, 24, 6564,