Structure and vibrational spectroscopy of lithium and potassium methanesulfonates

In this work, we have determined the structures of lithium methanesulfonate, Li(CH₃SO₃), and potassium methanesulfonate, K(CH₃SO₃), and analysed their vibrational spectra. The lithium salt crystallizes in the monoclinic space group C2/m with two formula units in the primitive cell. The potassium salt is more complex, crystallizing in I4/m with 12 formula units in the primitive cell. The lithium ion is fourfold coordinated in a distorted tetrahedron, while the potassium salt exhibits three types of coordination: six-, seven- and ninefold. Vibrational spectroscopy of the compounds (including the ⁶Li and ⁷Li isotopomers) confirms that the correlation previously found, that in the infrared spectra there is a clear distinction between coordinated and not coordinated forms of the methanesulfonate ion, is also valid here. The lithium salt shows a clear splitting of the asymmetric S-O stretch mode, indicating a bonding interaction, while there is no splitting in the spectrum of the potassium salt, consistent with a purely ionic material.