Azide Anion Interactions with Imidazole and 1-Methylimidazole in Dimethyl Sulfoxide

We chose imidazole (HIm) and 1-methylimidazole (1-MeIm) to probe their interaction with the azide anion. In dimethyl sulfoxide (DMSO), the formation of hydrogen-bonded pairs between the azide ion and HIm is clearly distinguishable from free azide ions in FT-IR spectra, allowing for accurate spectra differentiation. HIm can both donate and accept hydrogen bonds, forming hydrogen-bonded networks, while 1-MeIm can accept only hydrogen bonds, preventing hydrogen-bonded network formation. To differentiate the roles of hydrogen and nonhydrogen bonding interactions with azide at ultrafast time scales, we use femtosecond mid-IR pump-probe spectroscopy. From the time-resolved data analysis, we observe a clear difference in vibrational population relaxation times of the azide anion under the presence of HIm or 1-MeIm. With the addition of HIm, the vibrational population relaxation time of the azide anions decreases by a factor of 1.6 to 1.7, which we attribute to a more efficient energy dissipation pathway provided through the strong hydrogen bonds between the azide anion and HIm. Furthermore, we investigated the influence of alkali and alkaline earth metal counterions on the vibrational relaxation of azide ion pairs and found a dependence on the charge and size of the counterions. An assessment of the molecular distribution function from molecular dynamics simulation supports our results on the interactions between different ion species.