Exploration of conformational landscapes and large amplitude motions in molecules and molecular clusters: a rotational spectroscopy approach

This work presents a series of gas-phase studies employing high-resolution rotational spectroscopy to probe molecular structure, dynamics, and non-covalent interactions. The technique is advanced beyond traditional structural determination to investigate complex conformational landscapes, large-amplitude motions, and subtle electronic effects. The conformational landscape of monomeric 3-fluorobenzylamine (3FBA) was resolved, revealing three distinct conformers. For two of them, interstate splittings provided direct evidence of quantum tunneling mediated by a large-amplitude amino-group flipping motion. For the global minimum, an effective structure was determined through the analysis of singly substituted (13C, 15N, 2H) isotopologues using Kraitchmann’s equations. This approach was extended to the micro-solvation of 3FBA, characterizing clusters with one to four water molecules. The interplay between intramolecular flexibility and intermolecular hydrogen bonding to multiple binding sites (F, NH2, and π-system) was elucidated, with conformational energies refined by high-level quantum calculations and Non-Covalent Interaction analysis. Additionally, the monomer of 3,4-ethylenedioxythiophene a key precursor of conductive polymers, was spectroscopically characterized for the first time. A comprehensive study of pentafluoroiodobenzene combined structural, electronic, and comparative analyses. Its molecular structure was derived from isotopic data, while the electron density distribution was examined by coupling the Extended Townes–Dailey model with Natural Bond Order calculations, quantifying the anisotropy of the iodine p-orbital population. This work contextualizes σ-hole and π-hole features arising from combined iodine and fluorine substitution, completing a systematic investigation of the pentafluorohalobenzene and halobenzene series and elucidating halogen-specific electronic effects on an aromatic scaffold. Finally, the microwave spectrum of the pentafluoroiodobenzene–water dimer was assigned, revealing a halogen bond between the iodine σ-hole and the oxygen lone pair.