n KEMITEKNIK Supporting chemical thermodynamics: The role of infrared spectroscopy The use of molecular vibrations to probe structure in hydrogen bonding liquids. By Evangelos Drougkas, Georgios M. Kontogeorgis and Xiaodong Liang, Center for Energy Resources Engineering, DTU Kemitekni k, and Henrik G. Kjaergaard, Department of Chemistry, University of Copenhagen Hydrogen bonding compounds have a strong presence in many industrial and chemical processes. To tackle the challenge that many thermodynamic models face in the description of their properties, novel experimental data of their structure are of great importance to the chemical engineering community. Vi - brational spectroscopy is an accessible yet invaluable tool for inferring the microscopic picture of hydrogen bonding, and its pairing with profound theoretical tools can help revolutionize chemical thermodynamics. Describing the ”network” A vast range of industrial applications involves hydrogen bonding substances, and the accurate prediction of their physical properties lays the groundwork for the design of advanced and efficient processes. Among them, water has arguably the largest number of applications, but despite its commonness a conclu - sive model that can capture its unique thermophysical property profiles remains as the “holy grail” of modern applied thermo dynamics. Specifically, traditional equations of state fail to account for water’s anomalous properties, such as its maximum density or minimum heat capacity as a function of temperature. There is consensus among the modeling community that the primary factor for water’s peculiar property profiles is its strong intermolecular interactions with an intense orientation - al character, commonly referred to as hydrogen bonding [1]. Most advanced models incorporate special terms that involve a key structural parameter: the number of molecules that do not participate in hydrogen bonds as donors, known as the free hydroxyl fractions. A traditional and highly accessible tool for the analysis of hydrogen bonding at the molecular level is infrared spectroscopy, a common technique that studies the transitions between molecular vibrational states. Hydrogen bonding induces changes in the vibrational patterns of hy droxyl groups, generating complex spectra that encode orienta - tional information using vibrations as a proxy. From spectra to free hydroxyl fractions The analysis of temperature-varied vibrational spectra for the estimation of the degree of hydrogen bonding has been the sub- ject of multiple works over the past 80 years. Among them, one empirical approach has survived the test of time, having been greatly utilized by the thermodynamics community. The works Figure 1. Linear tetramolecular cluster of methanol inside an explicit solvent cavity. of Werner Luck have provided free hydroxyl fraction data for a wide temperature range for methanol, ethanol and water [2]. While these important data have found use in the calibration of the association terms of equations of state, they have surpris - ingly not been replicated or consolidated in later studies. Contemporary computational means enable the investigation of the free hydroxyl group fraction problem, challenging the previous attempts. Taking methanol as a base case, quantum mechanical techniques are used for the geometry optimization of several methanol clusters. The vibrational properties of the hydroxyl stretching modes are determined with the local vibra - tional mode methodology [3], a highly customizable approach that relies on first principles and allows for the efficient calcula - tion of spectroscopic peak positions and intensities. A consideration commonly encountered during the appli - n Deuteration The substitution of methanol’s methyl hydrogen atoms with deuterium leads to a shift to the peaks related to methyl-related vibrational transitions, simplifying the statistical treatment of the spectra specifically for the treat - ment of the hydroxyl bands. 20 Dansk Kemi, 107, nr. 1, 2026 -
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