We have examined the stereoselectivity of molecular recognition between two molecules of the anesthetic sevoflurane using broadband rotational spectroscopy. The transient axial chirality of sevoflurane is revealed upon the formation of the dimer, as two different diastereoisomers made of either homo- or heterochiral species are detected in a supersonic jet expansion. The conformational assignment was confirmed by the observation of eighteen different isotopologues in natural abundance (all possible (13)C's and two (18)O species of the homochiral form). The two clusters are formed in practically equal proportions (1.1 : 1), probably due to their similar hydrogen bonding topologies. In both clusters the complex is stabilized by a primary C-H···O hydrogen bond, assisted by weak C-HF interactions. This intermolecular binding regime is characterized by a mixture of electrostatic and dispersive interactions, midway between classical hydrogen bonds and van der Waals clusters.

Cooperativity between weak hydrogen bonds can be revealed in molecular clusters isolated in the gas phase. Here we examine the structure, internal dynamics, and origin of the weak intermolecular forces between sevoflurane and a benzene molecule, using multi-isotopic broadband rotational spectra. This heterodimer is held together by a primary C-H⋅⋅⋅π hydrogen bond, assisted by multiple weak C-H⋅⋅⋅F interactions. The multiple nonbonding forces hinder the internal rotation of benzene around the isopropyl C-H bond in sevoflurane, producing detectable quantum tunneling effects in the rotational spectrum.

We recorded the pulsed jet Fourier transform microwave spectrum of the 1 : 1 adduct of CH2ClF with formaldehyde. Formaldehyde is linked to CH2ClF through a C-H···Cl bond rather than a weak C-H···F hydrogen bond, with a H···Cl "bond length" of 2.918 Å. Two additional equivalent C-H···O contacts, with a H···O distance of 2.821 Å, characterize the complex. Tunnelling splittings due to the internal rotation of the formaldehyde moiety have been observed, which allowed estimating the barrier to the internal rotation of formaldehyde to be 125(10) cm(-1). The (35)Cl quadrupole coupling constants have been determined to be χaa = 31.131(7) MHz and χbb-χcc = -105.82(1) MHz.

The rotational spectrum of the adduct pyridine-methane shows that methane links to an aromatic molecule apparently through a C-H···π weak hydrogen bond. The shape and the internal dynamics behaviour of this complex are very similar to that of the van der Waals complexes involving aromatic molecules with rare gases.

The hydration of 2‐fluoropyridine with one water molecule has been studied with a pulsed jet Fourier transform microwave technique. The rotational spectra of five isotopologues of the 1:1 complex have revealed the formation of a single most stable isomer. The observed cluster is characterized by an O−H⋅⋅⋅N hydrogen bond and a C−H⋅⋅⋅O weak hydrogen bond, where water is coplanar to the aromatic ring and acts in a double role of proton donor and proton acceptor.