We are happy to announce that Evangelos Drougkas successfully completed his PhD “Infrared spectroscopic analysis of hydrogen bonding in liquid-state associating substances”.
There is consensus among the modeling community that the primary factor for thermodynamic property profiles of associating substances are strong intermolecular interactions with an intense orientational character, commonly referred to as hydrogen bonding. Most advanced models incorporate special terms that involve a key structural parameter: the number of molecules that do not participate in hydrogen bond as donors, known as the free hydroxyl fractions. A traditional tool for the exploration of hydrogen bonding at the molecular level is infrared spectroscopy, a technique that studies the transitions between molecular vibrational states. Hydrogen bonding induces changes in the vibrational patterns of hydroxyl groups, generating complex spectra that encode orientational information using vibrations as a proxy.
There have historically been infrared spectroscopy methodologies that involve the extraction of free hydroxyl group fractions of a limited number of substances in the liquid phase, namely water and alcohols. While being sparse, free hydroxyl group fraction data have been utilized in the field of thermodynamics, however this has led to controversial trends in the results especially when compared to model predictions. After the recovery of relevant sources and a thorough discussion of the workings of the most prominent methodologies, it is demonstrated that they are faced with reproducibility issues, unclear and/or outdated assumptions as well as limited applicability to diverse chemical species.
Modern vibrational spectroscopy-based investigations are greatly benefited by quantum calculation techniques that offer great assistance in the prediction and interpretation of infrared spectra. Following the presentation of the key underlying physical principles of computational chemistry methods and theoretical aspects of infrared spectroscopy, a primary application is demonstrated on the quantification of the strength of hydrogen bonds in room temperature water/ethanol solutions. Specifically, it is observed in measurements of composition-varied water/ethanol-OD spectra that the strength of hydrogen bonds in which ethanol-OD participates as a donor shows a minimum for a specific ratio of water/ethanol-OD. A quantitative estimate for the hydrogen bond interaction enthalpy is provided with the use of quantum calculations on ethanol and water-ethanol clusters inside an implicit solvent cavity that simulates the effects of the bulk phase on the electronic properties of the system(s).
Returning to the subject of free hydroxyl group fractions, a new framework is proposed and applied on liquid methanol. This framework is an adaptation of the vibrational local mode theory that has successfully been applied for the study of gas-phase hydrogen-bonding systems. The framework is modified to include bulk phase effects through the implementation of an explicit solvation model at the quantum calculation stage. Near-infrared spectra of deuterated methanol analogues are measured at temperatures spanning from 10 to 50 °C, and after processing the contribution of free hydroxyl groups is isolated. Combined with the vibrational properties calculated through the local mode methodology, free hydroxyl group fractions for pure methanol are estimated and found to be significantly lower by approximately an order of magnitude than previously reported. This analysis brings new insights into the intricacies of association and hydrogen bonding lays the groundwork for the further development of a updated, generalized and adaptable approach towards the generation of valuable parameters of structure in important liquids.
Evangelos will continue as a Post Doc working under the supervision of Associate professor Xiaodong Liang and Professor Georgios Kontogeorgis on the ERC project “REMOTE”.