Molecular Association of Bifunctional Organic Molecules: Infrared Cluster Spectroscopy and Quantum Chemical Conformational Analyses

From the existence of any liquid or solid to the behaviour of proteins in the human body, all depend on intermolecular interactions. These interactions determine most of the physical and chemical properties of matter. A deep and clear understanding of these interactions is crucial in many fields of study, from biochemistry to material science, for predicting the behaviour of various systems and designing new materials with desired properties.

Hydrogen bonds are one main class of intermolecular interactions that define all the extraordinary properties of water, stabilize DNA helices, and govern protein folding. Among systems capable of hydrogen bond formation, bifunctional molecules are less studied at high experimental and computational levels due to their structural and behavioural complexities. Investigating their self-association and microhydration mechanisms is an important step toward gaining a better understanding of larger and more complex molecular systems.

Matrix isolation is a technique for maintaining species in an inert host at cryogenic temperatures so that the guest species are trapped in a medium with minimal interactions. Coupled with infrared spectroscopy, this technique allows for the characterization of molecules and non-covalent interactions in molecular complexes.

This study investigates the non-covalent interactions involved in molecular association mechanisms of bifunctional molecules, using matrix-isolation infrared spectroscopy both in mid-IR and far-IR, together with systematic conformational search, and high-level density functional theory (DFT) and ab initio quantum computations.

The results of the hydrogen bond rearrangement study of monoethanolamine (MEA) reveal that in the self-association mechanism, a cyclic conformation stabilizes (MEA)₂ through two intermolecular hydrogen bonds, sacrificing the intramolecular hydrogen bond of the monomer. However, in the microhydration mechanism of MEA, the intramolecular hydrogen bond remains intact due to kinetic trapping, highlighting the ability of the neon quantum matrix to trap metastable conformations that cannot be observed in gas-phase studies.

Ethylene glycol (EG) homodimer, on the other hand, adopts a highly symmetric S₄ conformation in the self-association mechanism. The microhydration mechanism of EG, contrary to MEA−H₂O observations, shows a cooperative network of two intermolecular hydrogen bonds that replace the intramolecular hydrogen bond of the EG monomer.

These studies highlight the complexity and unpredictable nature of bifunctional molecules while also emphasizing the significance of studying large-amplitude OH torsional and librational modes in hydrogen bonding and non-covalent interactions. The OH torsional vibrations show more pronounced spectral shifts compared to bending and stretching vibrations, while the liberation modes are only expected as a result of complexation. Although these vibrational transitions appear in the technically and chemically demanding far-IR region, they can serve as sensitive and valuable probes for non-covalent interaction studies.

In addition to hydrogen bonding, this study explores weak van der Waals interactions in small amines, alcohols, and MEA with CO₂. It is demonstrated that the doubly degenerate bending mode of the CO₂ molecule lifts upon complexation, with the splitting being significantly larger in amines than in alcohols. Surprisingly, the association of MEA with CO₂ follows the splitting pattern observed in alcohols, indicating that CO₂ interacts primarily with the hydroxy group of MEA. This study introduces the splitting of the degenerate bending mode of CO₂ as a spectroscopic probe for investigating the behaviour of van der Waals complexes.