Thermal Analysis and Raman Spectra of Different Phases of the Ionic Liquid Butyltrimethylammonium Bis(trifluoromethylsulfonyl)imide

The ionic liquid butyltrimethylammonium bis­(trifluoromethylsulfonyl)­imide, [C₄C₁C₁C₁N]­[Tf₂N], is a glass-forming liquid that exhibits partial crystallization depending on the cooling rate. Differential scanning calorimetry (DSC) indicates crystallization at <i>T</i>_c = 227 K, melting at <i>T</i>_m = 258 K, glass transition at <i>T</i>_g ∼ 191 K, and also cold crystallization at <i>T</i>_cc ∼ 219 K. Raman spectroscopy shows that the crystalline structure obtained by slow cooling is formed with [Tf₂N]⁻ in cisoid conformation, whereas [Tf₂N]⁻ in transoid conformation results from fast cooling. No preferred conformation of the butyl chain of the [C₄C₁C₁C₁N]⁺ cation is favored by slow or fast cooling of [C₄C₁C₁C₁N]­[Tf₂N]. Low-frequency Raman spectroscopy shows that crystalline domains developing in the supercooled liquid result in a glacial state made of a mixture of crystallites and amorphous phase. However, these crystalline structures obtained by slow cooling or cold crystallization are not the same because anion–cation interactions promote local structures with distinct conformations of the [Tf₂N]⁻ anion