Weak n→π* and C–H⋯O interactions determine the structure of pyridine–acetaldehyde adduct. The n→π* distance oscillates with the methyl group internal rotation which acts as a sort of molecular balance to explore the n→π* interaction energy.

The prevalence of the trans-COOH form in a hydrated acid complex is shown for the first time in o-anisic acid.

Four conformers of the heterodimer o-anisic acidformic acid, formedi nas upersonic expansion,h ave been probed by Fourier transform microwave spectroscopy.T wo of theseformshave the typical double intermolecularhydrogen-bond cyclic structure. The other two show the o-anisic acid moiety bearing a trans-COOH arrangement supported by an intramolecular OÀH···Ob ond to the neighbor methoxy group. In these conformers, formic acid interacts with o-anisic acid mainly through an intermolecular OÀH···O hy-drogen bond either to the OÀHortothe C=Omoieties, reinforced by other weaki nteractions. Surprisingly,t he most abundantc onformer in the supersonic expansion is the complex in which the o-anisica cid is in trans arrangement with the formic acid interacting with the OÀHg roup. Such a trans-COOH arrangement in which the intramolecular hydrogen bond dominates over the usually observed double intermolecular hydrogen bond interactionh as never been observed previously in an acid-acid dimer.Supporting information and the ORCID identification number(s) for the author(s) of this articlecan be found under: https://doi.

The rotational spectrum of the weakly bound complex pentafluoropyridine⋅⋅⋅formaldehyde has been investigated using Fourier transform microwave spectroscopy. From the analysis of the rotational parameters of the parent species and of the 13C and 15N isotopologues, the structural arrangement of the adduct has been unambiguously established. The full ring fluorination of pyridine has a dramatic effect on its binding properties: It alters the electron density distribution at the π‐cloud of pyridine creating a π‐hole and changing its electron donor‐acceptor capabilities. In the complex, formaldehyde lies above the aromatic ring with one of the oxygen lone pairs, as conventionally envisaged, pointing toward its centre. This lone pair⋅⋅⋅π‐hole interaction, reinforced by a weak C−H⋅⋅⋅N interaction, indicates an exchange of the electron‐acceptor roles of both molecules when compared to the pyridine⋅⋅⋅formaldehyde adduct. Tunnelling doublets due to the internal rotation of formaldehyde have also been observed and analysed leading to a discussion on the competition between lone pair⋅⋅⋅π‐hole and π⋅⋅⋅π stacking interactions.

Thalidomide is a drug that presents two enantiomers with markedly different pharmacological and toxicological activities. It is sadly famous due to its teratogenic effects mostly due to the preferential docking...