Non-covalent interactions play a key role in chemistry, physics and biology, being responsible for the three-dimensional structure of proteins, the physical properties of condensed phases, and the outcome of molecular recognition processes, to cite just a few examples. In this talk we will focus on intermolecular non-covalent interactions and present our results on several complexes investigated by chirped pulsed Fourier transform microwave spectroscopy and computational methods. A range of clusters, involving odorants, aromatic hydrocarbons, alcohols and water will be reviewed. The interplay between primary and secondary hydrogen bonds and/or dispersion forces will be discussed, as well as the performance of several theoretical methods in view of the experimental data.

The lowest energy structures for haloethylene-protic acid heterodimers result from an interplay between favorable electrostatic interactions and steric effects. For vinyl fluoride and 1,1-difluoroethylene complexes, steric effects predominate and the resulting planar structure has the acid binding across the double bond ("top"), while for trifluoroethylene, favorable electrostatics forces the complexes to adopt a sterically strained, but still planar structure with the acid at one end of the olefin ("side"). Substitution of a fluorine atom with chlorine allows a relaxation of steric requirements and the side binding motif to chlorine is observed in (Z)-1-chloro-2-fluoroethylene–acetylene despite the presence of the more electronegative fluorine atom. Similarly, in 2-chloro-1,1-difluoroethylene ethylene–acetylene, the acetylene prefers to adopt side binding to chlorine despite having the option of top binding to fluorine. In (E)-1-chloro-1,2-difluoroethylene, the possibilities are switched to top binding to chlorine and side binding to fluorine. Although the side binding arrangement to fluorine is predicted to be lower in energy than top binding to chlorine, a non-planar, crossed geometry, with the centers of the HCCH triple bond and ethylene double bond approximately aligned with each other is predicted as the global minimum when not correcting for basis set superposition error. When the correction is made, the non-planar geometry has an energy higher than both side and top binding. The microwave spectra of four isotopologues of the (E)-1-chloro-1,2-difluoroethylene-HCCH complex reveal that the side binding structure is indeed the one with the lowest energy. This structure is discussed in terms of the balance between electrostatics and sterics.

l0pt Figure Following previous experiments on sulfur hydrogen bondingM. Juanes, A. Lesarri, R. Pinacho, E. Charro, J. E. Rubio, L. Enríquez, M. Jaraíz, Chem. Eur. J., 2018, 24, 6564e have observed several oligomers of the aromatic thiols thiophenol, phenylmethanethiol and 2-phenylethanethiol in a jet-cooled expansion using broadband (chirped-pulsed) microwave spectroscopy (2-8 GHz).The homodimers of the three aromatic thiols are primarily stabilized by S-H ··· S hydrogen bonds and other weak interactions. The phenylmethanethiol and 2-phenylethanethiol dimers exhibit additional combinations of S-H ··· π, C-H ··· π or C-H ··· S interactions, while the two phenyl rings in the thiophenol dimer are stacked, displaying a π-π interaction. A single symmetric-top isomer of the thiophenol trimer has been observed, characterized by a C₃ symmetry topologically equivalent to that observed in the phenolN. A. Seifert, A. L. Steber, J. L. Neill, C. Pérez, D. P. Zaleski, B. H. Pate, A. Lesarri, Phys. Chem. Chem. Phys., 2013, 15, 11468nd anilineC. Pérez, I. León, A. Lesarri, B. H. Pate, R. Martínez, Judith Millán, J. A. Fernández, Angew. Chem. Int. Ed., 2018, 57, 15112rimers. The trimer structure combines S-H ··· S hydrogen bonds and C-H ··· π interactions. Accurate rotational parameters and supporting ab initio and DFT calculations will be reported at the symposium.

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M. Juanes, A. Lesarri, R. Pinacho, E. Charro, J. E. Rubio, L. Enríquez, M. Jaraíz, Chem. Eur. J., 2018, 24, 6564w N. A. Seifert, A. L. Steber, J. L. Neill, C. Pérez, D. P. Zaleski, B. H. Pate, A. Lesarri, Phys. Chem. Chem. Phys., 2013, 15, 11468a C. Pérez, I. León, A. Lesarri, B. H. Pate, R. Martínez, Judith Millán, J. A. Fernández, Angew. Chem. Int. Ed., 2018, 57, 15112t

The rotational spectrum of CH₃COSH-H₂O has been observed by pulsed-nozzle Fourier transform microwave spectroscopy. Spectra were also recorded for the CH₃COSH-D₂O and CH₃CO³⁴SH-H₂O isotopologues. A and E internal rotor states were observed for each species and the spectra were fit with the XIAM internal rotation program. Preliminary fits have resulted in a methyl internal rotation barrier of V₃ = 34.1290(28) cm⁻¹ for the parent CH₃COSH-H₂O complex, which is in reasonable agreement with the value predicted at the M06-2X/6-311++G(3df,3pd) level of theory (V₃ = 43.9 cm⁻¹). In comparison with the free syn-thioacetic acid monomer (V₃ = 76.300(12) cm⁻¹), the value of V₃ decreased by over half as a result of complexation with water.

Non-covalent inter- and intramolecular interactions involving aromatic rings are ubiquitous in chemical and biological processes. The presence of halogen atoms, fluorine in particular, can drastically change the binding properties of molecules and thus halogen or fluorine substitution are used to tune the properties of new materials, drugs or proteins. We report on a series of rotational spectroscopy studies performed with Molecular Beam Fourier Transform Microwave spectroscopy in which we have tested the binding abilities of fluorinated aromatic compounds, namely penta-fluoropyridine and hexafluorobenzene, with different ligands such as ammonia, carbon monoxide and halogenated methanes. The comparison to the analogous complexes formed by non-fluorinated aromatic moieties shows strickingly different structures. In the case of the fluorinated aromatic compounds the structures are dominated by π-hole interaction.

l0pt Figure Musk odorants are important notes in perfumery for their natural, animalistic and warm scent. Understanding how structure and odor relate is important for development of new musk classes, with scalable synthesis and higher biodegradability. However, musks are generally very flexible and difficult to crystallise, and their conformations are unknown. Here we present the conformational study of two widely used alicyclic musks, romandolide (C₁₅H₂₆O₄) and helvetolide (C₁₇H₃₂O₃), using chirped-pulse Fourier Transform Microwave (CP-FTMW) spectroscopy in combination with theoretical methods. Three conformations of romandolide and five conformations of helvetolide have been identified in the broadband spectra of romandolide and helvetolide, respectively. The observed conformers adopt horseshoe shapes and are stabilized by dispersion interactions between the side chain and the cyclohexane ring.

Recent examples of complexes formed between hydrogen chloride and chlorofluoroethylenes have revealed unexpected and surprising structures. To the contrary, the analysis of the microwave rotational spectrum of 1-chloro-1-fluoroethylene-hydrogen chloride shows that it has the "top binding to fluorine’’ structure typical of earlier dihaloethylene-HCl complexes. However, this result is not predicted by the MP2/6-311++G(2d, 2p) model chemistry on which we had been relying, even when corrected for basis set superposition error. The success of various model chemistries in predicting the experimental result is discussed.

Recently, an infrared study on the complex of dimethylfuran and methanol showed only a slight preference for methanol to bind via an OH…O hydrogen bond versus the OH…π bond.A. Poblotzki et al, Phys. Chem. Chem. Phys. 2016, 18, 27265.rom this work, a ‘blind challenge’ was undertaken to compare the quantum chemical and experimental results of the microsolvation of a furan ring in methanol, with varying degrees of methyl substitution on furan, having a focus on methanol binding sites.H. C. Gottschalk et al, J. Chem. Phys. 2018, 148, 014301.he hope of the challenge is to gauge how well experiment and theory can agree on site preference when the energy difference is near 1 kJ/mol, and ultimately to discern which theoretical models perform the best. To this end, we present the rotational spectrum and structural analysis of the dimethylfuran complex with methanol. The rotational spectrum was recorded on the Hamburg COMPACT (compact-passage acquired coherence technique) microwave spectrometer from 2−18 GHz.D. Schmitz et al, J. Mol. Spectrosc. 2012, 280, 77.he observed spectrum is complicated by the internal rotation of three methyl tops, two from the dimethylfuran and one from the methanol. Each methyl top adds additional torsional angular momentum to the rotating complex, which subsequently splits the rotational energy levels yielding a complex set of torsion-rotation transitions. The resulting methyl top tunneling parameters are essential to aid in the structure determination of the complex. These results will be compared with the furan methanol results to see the effect of methylation on site preference.

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Footnotes:

A. Poblotzki et al, Phys. Chem. Chem. Phys. 2016, 18, 27265.F H. C. Gottschalk et al, J. Chem. Phys. 2018, 148, 014301.T D. Schmitz et al, J. Mol. Spectrosc. 2012, 280, 77.T