Local Interactions and Dynamics in Aqueous Imidazole Probed by Vibrational and NMR Spectroscopy

Imidazole is a small molecule with fundamental importance in biology, chemistry, and technology. It is found as a building unit in larger chemical systems, like amino acids, protic ionic liquids, and modern covalent organic frameworks. Water is an even more common substance of at least equal importance, not least as the charge carrier in proton conducting materials. Aqueous imidazole is a peculiar liquid system whose local structure, governed by a network of hydrogen bonding, has attracted the attention of several researchers. In this experimental work, multiple techniques, mainly vibrational and NMR spectroscopy, have been used to study aqueous imidazole solutions with the mole fraction of imidazole varying from zero to unity. Local interactions and molecular dynamics are investigated, providing results that complement previous knowledge of this molecular system. Distinct changes in the Raman spectra mark the composition with a 1:1 water/imidazole ratio as the one beyond which imidazole is solvated, whereby the local structure of crystalline imidazole is disrupted. Upon further dilution in water, the local structure changes marginally, as revealed by a careful peak fit analysis and in agreement with previous results from computational methods. Also, our experimental results help discriminate between two computational models previously used, suggesting that hydrogen bonded chains of imidazole are more frequent than other dimer forms.