In this work, the interactions between a representative imidazolium-based ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]) and acetonitrile (CH₃CN) were investigated in detail using attenuated total reflection infrared spectroscopy (ATR-IR), hydrogen nuclear magnetic resonance (¹H NMR), and density functional theory calculations. The main conclusions are: (1) a number of species in the [BMIM][BF₄]–CH₃CN mixtures were identified with the help of excess infrared spectroscopy and quantum chemical calculations. The dilution process of the ionic liquid by acetonitrile was found to be the transformation from ion clusters to ion pairs. (2) The solvent molecules cannot break apart the strong Coulombic interaction between [BMIM]⁺ and [BF₄]⁻ but can break apart the ion cluster into an ion pair within the concentration range investigated. The strength of hydrogen bonds between the C–Hs of [BMIM]⁺ and the N of acetonitrile is enhanced during the dilution process. (3) The methyl group of CH₃CN locates above/below the imidazolium ring in the solution. These in-depth studies on the properties of the ionic liquid–acetonitrile mixed solvents may shed light on exploring their applications as reaction media in electrochemistry and chemical synthesis.

Excess infrared (IR) absorption spectroscopy, a new concept brought forward by applying the idea of excess thermodynamic functions to infrared spectroscopy, is shown to be a potential method to study hydrogen bonds. It can be applied to enhance spectral resolution of complexed IR bands, to evaluate nonideality of liquid mixtures, and to estimate selective molecular interactions. The sign of the excess infrared absorption coefficient is also of importance in providing information on molecular interactions. The results demonstrate that excess infrared absorption spectroscopy can unveil new information on hydrogen bonding in condensed phases.