Ionic liquids from the bottom up: Local assembly motifs in [EMIM][BF ₄] through cryogenic ion spectroscopy

To clarify the intramolecular distortions exhibited by the complementary ions in the archetypal ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF₄], we report the vibrational spectra of the isolated ionic constituents and small aggregates cooled to about 10 K. Deuteration of bare EMIM⁺ at the C₍₂₎ position, the putative hydrogen bond donating group, establishes that the observed bulk red shift is too small (<10 cm^-1) for hydrogen bonding to be a dominant structural feature. We then analyze how the vibrational patterns evolve with increasing size to identify the spectral signatures of well-defined structural motifs in the growing assembly. Surprisingly, the main features of the bulk spectrum are already developed in the cluster with a single BF ₄^- anion sandwiched between just two EMIM⁺ cations. We suggest that this local motif, while not strongly hydrogen bonded, nonetheless induces considerable intensity in the C₍₂₎H stretches and is a robust feature in the local molecular structure of the liquid.