High resolution rotational spectroscopy is used to map the conformational landscape and derive conformational relaxation pathways of citronellal.

The rotational spectrum of formamide₂–H₂O formed in a supersonic jet has been characterized by Fourier‐transform microwave spectroscopy. This adduct provides a simple model of water‐mediated interaction involving the amide linkages, as occur in protein folding or amide‐association processes, showing the interplay between self‐association and solvation. Mono‐substituted ¹³C, ¹⁵N, ¹⁸O, and ²H isotopologues have been observed and their data used to investigate the structure. The adduct forms an almost planar three‐body sequential cycle. The two formamide molecules link on one side through an N−H⋅⋅⋅O hydrogen bond and on the other side through a water‐mediated interaction with the formation of C=O⋅⋅⋅H−O and O⋅⋅⋅H−N hydrogen bonds. The analysis of the quadrupole coupling effects of two ¹⁴N‐nuclei reveals the subtle inductive forces associated to cooperative hydrogen bonding. These forces are involved in the changes in the C=O and C−N bond lengths with respect to pure formamide.

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 B₂ = 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 influence of a nitrogen atom in the backbone of a PAH was revealed by the hydrated clusters of phenanthrene and phenanthridine in a rotational spectroscopy study. Background image credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA) – ESA/Hubble Collaboration.