Theory meets experiment for elucidating the structure and stability of non-covalent complexes: water–amine interaction as a proof of concept†
Several gas-phase spectroscopic investigations have focused on a better understanding of the nature of weak, non-covalent interactions in model systems. However, their characterization and interpretation are still far from being satisfactory. A promising route to fill this gap is offered by strategies in which high-resolution rotational spectroscopy is deeply integrated with state-of-the-art quantum-chemical methodology to accurately determine intermolecular parameters and interaction energies, with the latter interpreted by means of powerful energy decomposition analyses (EDAs). As a proof of concept of this approach, we have selected the adducts formed by n-propylamine (PA) and iso-propylamine (IPA) with water. Among the stable structures computationally predicted, four (out of five) isomers of the PA–water complex and two isomers (trans and gauche) of the IPA–water adduct have been characterized with supersonic jet Fourier transform microwave spectroscopy. Starting from the experimental rotational constants for different isotopic species, computation of the corresponding vibrational corrections allowed a semi-experimental determination of the intermolecular parameters. Different EDAs point out that in all cases a strong O–H⋯N hydrogen bond is the primary interaction. Accurate computations indicate that the length and ramification of the alkyl chain do not significantly affect the water–amine interactions, which – on the contrary – modify the stability order of PA conformers with respect to the isolated systems.