Vibrational assignments, conformational analysis, and molecular structures of $$\left[ {\text{C}_{\text{n}} \text{mim}} \right]\left[ {\text{NTF}_{\text{2}} } \right]$$CnmimNTF2 (n = 2, 4, 6, and 8) imidazolium-based ionic liquids: a combined experimental and quantum chemical approach