Hydrogen-bond networks and proton transfer in choline geranate: insights from IR spectroscopy

Choline-geranate (CAGE) systems have attracted considerable interest due to their remarkable performance in biomedical and drug-delivery applications, yet their microscopic structure remains incompletely understood. In particular, the nature of the interaction between geranate (GE) and geranic acid (AGE) in CAGE formulations with excess acid has been the subject of ongoing debates, with spectroscopic evidence suggesting proton transfer but not resolving whether the proton is symmetrically shared or dynamically exchanged between the carboxylate groups. In this work, we combine experimental attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy with theoretical vibrational spectra derived from ab initio molecular dynamics (AIMD) simulations to elucidate the hydrogen-bond network and local structural organization in CAGE at 1 : 1 and 1 : 2 choline-to-acid ratios. The simulations are performed at a representative water content of 8 wt%, while the experimental IR spectra are recorded over a range of water contents to assess the effect of hydration. Structural analysis of the AIMD trajectories reveals a dynamic hydrogen-bond network in which proton transfer between AGE and GE occurs intermittently, consistent with transient proton exchange within an asymmetric hydrogen-bonding environment, rather than a symmetrically shared configuration. The calculated infrared (IR) spectra reflect this behavior through the coexistence of vibrational signatures characteristic of both carboxylate and carboxylic acid groups. Experimental IR spectra corroborate these findings, showing persistent carboxylate and carboxylic acid features across compositions and water contents, with no evidence for a delocalized proton-bound dicarboxylate species. Together, the combined experimental and theoretical results provide a consistent structural picture of CAGE, reconciling previously reported nuclear magnetic resonance observations of proton transfer with the vibrational signatures observed in IR spectroscopy and clarifying the role of proton dynamics in these systems.