Energies, geometries, and frequencies of normal vibrations have been calculated by quantum-chemical methods for different conformers of a bis(trifluoromethylsulfonyl)imide anion (NTf2-), 1-alkyl-3-methylimidazolium cations ([C(n)mim](+), n = 2, 4, 6, 8), and [C(n)mim]NTf2 ionic pairs. The assignment of frequencies for NTf2-, [C(2)mim](+), and [C(4)mim](+) in the vibrational spectra of ionic liquids have been performed. Thermodynamic properties of [C(n)mim]NTf2, [C(n)mim](+), and NTf2- in the gas state have been calculated by the statistical thermodynamic methods. The resulting entropies are in satisfactory agreement with the values obtained from the experimental data previously reported in literature.