In the last decade, experiment and theory have expanded our vision of non‐covalent interactions (NCIs), shifting the focus from the conventional hydrogen bond to new bridging interactions involving a variety of weak donor/acceptor partners. Whereas most experimental data originate from condensed phases, the introduction of broadband (chirped‐pulse) microwave fast‐passage techniques has revolutionized the field of rotational spectroscopy, offering unexplored avenues for high‐resolution studies in the gas phase. We present an outlook of hot topics for rotational investigations on isolated intermolecular clusters generated in supersonic jet expansions. Rotational spectra offer very detailed structural data, easily discriminating the isomeric or isotopic composition and effectively cancelling any solvent, crystal, or matrix bias. The direct comparison with quantum mechanical predictions provides insight into the origin of the inter‐ and intramolecular interactions with much greater precision than any other spectroscopic technique, simultaneously serving as test‐bed for fine‐tuning of theoretical methods. We present recent examples of rotational investigations around three topics: oligomer formation, chiral recognition, and identification of halogen, chalcogen, pnicogen, or tetrel bonds. The selected examples illustrate the benefits of rotational spectroscopy for the structural and energetic assessment of inter‐/intramolecular interactions, which may help to move from fundamental research to applications in supramolecular chemistry and crystal engineering.

Two for a tango: the rotational spectrum of a cyclohexanol–water dimer evidences a concerted motion of the water molecule and the hydroxyl group of the ring.

We used jet-cooled broadband rotational spectroscopy to explore the balance between π-stacking and hydrogen-bonding interactions in the self-aggregation of thiophenol. Two different isomers were detected for the thiophenol dimer, revealing dispersion-controlled π-stacked structures anchored by a long S–H···S sulfur hydrogen bond. The weak intermolecular forces allow for noticeable internal dynamics in the dimers, as tunneling splittings are observed for the global minimum. The large-amplitude motion is ascribed to a concerted inversion motion between the two rings, exchanging the roles of the proton donor and acceptor in the thiol groups. The determined torsional barrier of B 2 = 250.3 cm –1 is consistent with theoretical predictions (290–502 cm –1 ) and the monomer barrier of 277.1(3) cm –1 . For the thiophenol trimer, a symmetric top structure was assigned in the spectrum. The results highlight the relevance of substituent effects to modulate π-stacking geometries and the role of the sulfur-centered hydrogen bonds.

The hydrogen bonds involving sulfur in the furfuryl mercaptan monohydrate are compared with the interactions originating from the hydroxyl group in furfuryl alcohol. The dimers with water were created in a supersonic jet expansion and characterized using microwave spectroscopy and supporting molecular orbital calculations. In furfuryl alcohol-water, a single isomer is observed, in which the water molecule forms an insertion complex with two simultaneous hydrogen bonds to the alcohol (O-H⋅⋅⋅O ) and the ring oxygen (O -H⋅⋅⋅O ). When the alcohol is replaced by a thiol group in furfuryl mercaptan-water, two isomers are observed, with the thiol group preferentially behaving as proton donor to water. The first isomer is topologically equivalent to the alcohol analog but the stronger hydrogen bond is now established by water and the ring oxygen, assisted by a thiol S-H⋅⋅⋅O hydrogen bond. In the second isomer the sulfur group accepts a proton from water, forming a O -H⋅⋅⋅S hydrogen bond. Binding energies for the mercaptan-water dimer are predicted around 12 kJ mol weaker than in the alcohol hydrate (B3LYP-D3(BJ)). The non-covalent interactions in the furfuryl dimers are dominantly electrostatic according to a SAPT(0) energy decomposition, but with increasing dispersion components in the mercaptan dimers, which are larger for the isomer with the weaker O -H⋅⋅⋅S interaction.

Water forms weak H-bonds with thenyl compounds, simultaneously retaining internal mobility in the dimer.