Spectroscopic Evidence for an Attractive Cation–Cation Interaction in Hydroxy‐Functionalized Ionic Liquids: A Hydrogen‐Bonded Chain‐like Trimer

We address the formation of hydrogen bonded domains among the cationic constituents of the ionic liquid (IL) 1‐(3‐hydroxypropyl)pyridinium tetrafluoroborate [HPPy][BF4] by means of cryogenic ion vibrational predissociation spectroscopy of cold (ca. 35 K) gas‐phase cluster ions and quantum chemistry. Specifically, analysis of the OH stretching bands reveals a chain‐like OH⋅⋅⋅OH⋅⋅⋅OH⋅⋅⋅BF4− binding motif involving the three cations in the cationic quinary cluster ion (HPPy+)3(BF4−)2. Calculations show that this cooperative H‐bond attraction compensates for the repulsive Coulomb forces and results in stable complexes that successfully compete with those in which the OH groups are predominantly attached to the counter anions. Our combined experimental and theoretical approach provides insight into the cooperative effects that lead to the formation of hydrogen bonded domains involving the cationic constituents of ILs. Like likes like: Cryogenic spectroscopy and quantum chemical calculations provide evidence for quinary complexes (HPPy+)3(BF4−)2 from ionic liquids, wherein cooperative hydrogen bonding is maximized through OH⋅⋅⋅OH⋅⋅⋅OH⋅⋅⋅BF4− binding motifs. Two isomers show triple contacts linking the cations to one of the anions.