Unique Formation of a Gauche Conformer in the Crystalline State of 1-Butyl-2,3-dimethylimidazolium Tetrafluoroborate at High Pressures

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This study is focused on pressure-induced Raman spectral changes in the ionic liquid (IL) 1-butyl-2,3-dimethylimidazolium tetrafluoroborate ([C 4 dmim][BF 4 ]) at ∼8 GPa. The sample comprises a methylated C2–H group in the imidazolium cation. As a counterpart of [C 4 dmim][BF 4 ], a sample with an unmethylated C2–H group in the imidazolium cation, i.e., 1-butyl-3-methylimidazolium tetrafluoroborate ([C 4 mim][BF 4 ]), was compared with [C 4 dmim][BF 4 ]. Visual observations and Raman spectral changes show crystallization of [C 4 dmim][BF 4 ] at ∼1.3 GPa. However, the unmethylated [C 4 mim][BF 4 ] sample exhibited a pressure-induced glass formation similar to that of most ILs reported in the literature. Therefore, only C2–H-methylation leads to the opposite results for [C 4 dmim][BF 4 ] and [C 4 mim][BF 4 ]. The results of this study also highlight that a gauche conformer forms in the crystalline state of [C 4 dmim][BF 4 ] upon applying pressure. This is unexpected, because trans conformers are typically the major conformers when most imidazolium-based ILs crystallize at high pressures. We analyze this unexpected behavior in terms of the local structural changes in the [C 4 dmim] cation using molecular dynamics calculations.