The salt 1,1,3,3-tetramethylguanidinium bis{(trifluoromethyl)sulfonyl}amide, [((CH3)(2)N)(2)C=NH2](+)[N(SO2-CF3)(2)](-) or [tmgH][NTf2], easily forms an ionic liquid with high SO2 absorbing capacity. The crystal structure of the salt was determined at 120(2) K by X-ray diffraction. The structure was found to be monoclinic, space group P2(1)/n with a = 11.349(2), b = 11.631(2), c = 11.887(2) angstrom, and beta = 90.44(3)degrees. Raman and IR spectra are presented and interpreted. The results are interpreted using ab initio quantum mechanics calculations that also predicted vibrational spectra. The relationship between the transoid (C-2 symmetry) structure of the [NTf2](-) ion and the conformationally sensitive bands is discussed.