Comparison of the local binding motifs in the imidazolium-based ionic liquids [EMIM][BF₄] and [EMMIM][BF₄] through cryogenic ion vibrational predissociation spectroscopy: Unraveling the roles of anharmonicity and intermolecular interactions

We clarify the role of the critical imidazolium C₍₂₎H position (the central C between N atoms in the heterocycle) in the assembly motif of the [EMIM][BF₄] ionic liquid by analyzing the vibrational spectra of the bare EMIM⁺ ion as well as that of the cationic [EMIM]₂[BF₄]⁺ (EMIM⁺ = 1-ethyl-3-methylimidazolium, C₆H₁₁N₂⁺) cluster. Vibrational spectra of the cold, mass-selected ions are obtained using cryogenic ion vibrational predissociation of weakly bound D₂ molecules formed in a 10 K ion trap. The C₍₂₎H behavior is isolated by following the evolution of key vibrational features when the C₍₂₎ hydrogen, the proposed binding location of the anion to the imidazolium ring, is replaced by either deuterium or a methyl group (i.e., in the EMMIM⁺ analogue). Strong features in the ring CH stretching region of the bare ion are traced to Fermi resonances with overtones of lower frequency modes. Upon incorporation into the EMIM⁺ ⋯ BF₄^- ⋯ EMIM⁺ ternary complex, the C₍₂₎H oscillator strength is dramatically increased, accounting for the much more complicated patterns derived from the EMIM⁺ ring CH stretches in the light isotopomer, which are strongly suppressed in the deuterated analogue. Further changes in the spectra that occur when the C₍₂₎H is replaced by a methyl group are consistent with BF₄^- attachment directly to the imidazolium ring in an arrangement that maximizes the electrostatic interaction between the molecular ions.