The synthesis and XRD investigation of hexafluorosilicate salt with 1-propy-3-methylimidazolium ([Pmim]+) cation is described. Analysis of crystal packing has revealed that unexpectedly high m.p. (210oC) of salt is resulted from the presence of extremely short interionic C(Im)H…F contacts in the crystal (1.94 – 2.42Å). The absence of strong C-H…F interaction for alkyl radicals lead to high mobility of substituents and resulted to phase transition of the order-disorder type. The total energy of the CH…F interactions in the hypothetical [SiF6(Pmim)6]+4 cluster according to DFT calculation and topological analysis of the electron density distribution attains ca. 33 kcal/mole.