Rotational spectroscopy of non-covalently bound complexes of medium size organic molecules

The conformational space of non-covalently bound complexes of medium size organic molecules is shaped by competing interactions occurring within the molecules or with the partners. It usually presents a high number of low energy conformations very close in energy with shallow potential energy barriers through which the molecular system can tunnel. The conformational preferences of non-covalently bound complexes can be studied to a very high degree of accuracy by free jet rotational spectroscopy1 and from the detailed structural and dynamical data that can be obtained, the site and geometry of the interaction and information on the binding energy can be inferred without ambiguity. The questions usually addressed are: which is the preferred binding site, which type of interactions are established, and whether any conformational change takes place in the monomers upon complexation. Answers to these questions allow insight into the molecular interaction process at the molecular level, bridging the gap between gas-phase and bulk properties. Chosen examples of published and unpublished results of complexes of medium-size organic molecules with different partners formed in a supersonic expansion and characterized by rotational spectroscopy will be discussed. The partner molecules are held together by hydrogen bonds, weak hydrogen bonds and lone-pair--hole interactions. It will be shown how non-bonding interactions compete to shape the conformational space of the complexes, the structural changes brought to the conformers of the monomers by complexation and how these interactions can be drastically changed through atomic or functional group substitution.