Hydrogen-Bond Duality Underlying the Enthalpy Anomaly in D₂O-Ethanol Mixtures

Thermodynamic anomalies in aqueous ethanol mixtures, such as the well-known enthalpy minimum at intermediate concentrations, have long eluded a clear molecular-level explanation. Here, we employ excess Raman spectroscopy (ERS) and two-dimensional correlation Raman spectroscopy (2DCRS) to track hydrogen-bond transformations in D₂O-ethanol mixtures. The spectra reveal two distinct types of hydrogen-bonding interactions formed between ethanol and water molecules near the ethanol volume fraction of 0.4, where the enthalpy minimum occurs. Density functional theory (DFT) calculations indicate that these two types of hydrogen-bond structures correspond to water-like (shorter, linear) and ethanol-like (longer, bent) interactions, respectively, and exhibit distinct electronic characters: one governed by charge-transfer (CT) interactions and the other by local excitation (LE) interactions. The stable coexistence of CT and LE type hydrogen bonds forms a highly coordinated and energetically favorable hydrogen-bonding network, providing direct microscopic insight into the enthalpic anomaly. This study establishes a combined spectroscopic-theoretical framework for resolving thermodynamic features in complex hydrogen-bonded liquids.