Understanding of the ability of the water molecule to form complex networks is relevant to a range of fields including materials science, oil and gas exploration,and medicine.
All forms of life on Earth depend on water. The ability of water to function as an almost universal solvent is largely defined by the ability of the water molecule to form complex hydrogen bonding networks involving water itself and organic molecules. Understanding of this unique ability is relevant to a range of scientific and industrials fields. Examples are materials science, gas hydrate inhibition in oil and gas exploration, and medicine. In the project, advanced low-temperature spectroscopy was utilized to study the ability of water to participate in various classes of weak intermolecular hydrogen bonds.
The macroscopic thermodynamic properties of condensed phases, the mechanical properties of functional materials, and the molecular organization of biological organisms result from the subtle interplay between different classes of non-covalent interactions on the molecular level. A range of intermolecular interactions were investigated by use of far-infrared and terahertz cluster spectroscopy at the nanoscopic scale for a series of weakly bound molecular hydrated cluster molecules embedded in solid cryogenic neon matrices below
4 K and/or isolated in supersonic jet expansions.
It has been demonstrated how the interaction strength, directionality and anharmonicity of intermolecular hydrogen bonds can be effectively probed directly via the class of large-amplitude hydrogen bond vibrational modes introduced upon complexation. These direct spectroscopic observables detected in the challenging far-infrared spectral region are shown to enable an accurate characterization of the conformational potential energy landscape spanned by the hydrogen-bonded subunits.
The main focus has been on the unexplored class of weak CH··O, CH··F and OH··π hydrogen bonds besides the stronger classical OH··O hydrogen bond, respectively. The ability of water to participate in various classes of weak intermolecular CH··O and OH··π hydrogen bonds was investigated for mixed cluster molecules of water and prototypical unsaturated hydrocarbons as ethylene and acetylene. The interaction strengths and potential energy minima were shown to be determined largely by the degree of sp-hybridization of the involved carbon atoms. The ability of water to participate in stronger, classical intermolecular OH··O hydrogen bonds was investigated for mixed cluster molecules of water and primary, secondary, tertiary, and fluorinated alcohols.
It was shown that water always is the donor for intermolecular OH··O hydrogen bonds with aliphatic alcohols. The ability of alcohols to be acceptors for intermolecular hydrogen bonds is significantly increased for tertiary alcohols compared with primary.
Further, the interaction between water and flexible alcohols with degrees of freedom for internal rotation was shown to be determined by cooperative interactions between strong OH··O and weak CH··O hydrogen bonds. Finally, fluorination of alcohols was shown to hamper the ability of the alcohol to function as an acceptor for OH··O hydrogen bonds. Fluorinated alcohols will function as donors of OH··O hydrogen bonds to water supported by cooperative secondary CH··F hydrogen bonds.
The performed low-temperature spectroscopic investigations were all complemented by high-level quantum chemical modelling, and the coupling of “first principles” theory and experimental observables provides insight into the nature of non-covalent interactions of relevance to more complex supra-molecular chemistry. Examples might be the formation of gas hydrates, molecular recognition and enzyme-substrate complex formation as well as the 3-dimensional folding mechanisms for biological macromolecules in aqueous solutions.
Illustration:
Animations of the two IR-active liberational modes to the ternary H2O(C2H4)2 complex. Left) The hindered rotation of the water molecule out of the mirror plane spanned by the two intermolecular OH··π hydrogen bonds. Right) The hindered rotation of the water molecule in the mirror plane spanned by the two intermolecular OH··π hydrogen bonds.